Influence of the Freezing Process on Quality Retention of Frozen Tomato Slices
THESIS
Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in
the Graduate School of The Ohio State University
By
Qinfan Zhou
Graduate Program in Food, Agricultural & Biological Engineering
The Ohio State University
2016
Master's Examination Committee:
Dr. Dennis R. Heldman, Advisor
Dr. Sudhir K. Sastry
Dr. Christopher Simons
Copyrighted by
Qinfan Zhou
2016
Abstract
Tomato (Lycopersicon esculentum Mill.) is one of the most important economic plants worldwide. Frozen vegetables and fruits have gained popular nowadays. However, the fragile and complex structure of the tomato is very sensitive to ice crystal formation of water during phase change. The overall objective of this investigation was to optimize the freezing conditions required to achieve maximum retention of the quality attributes of frozen-thawed tomato slices.
Fresh-cut Roma tomato were sliced into pieces with the thickness of 4.5 mm and pre-treated with CaCl2 solutions in concentrations ranging from 0.4 to 4.0 g/100g. The tomato slices were then frozen in either a dry-ice ethanol bath, liquid nitrogen or an air blast freezer to achieve a significant range of “time-to freeze”. The various cold media were used to reduce the geometric center temperature of the slices from 25℃ to -18°C.
The frozen tomatoes slices were stored in freezer for temperature equilibration at -18 °C for 12 hours before thawing at ambient temperature for measurement of texture and mass loss.
The results from the experimental measurements indicated that reducing the
“time-to-freeze” (increasing freezing rates) was not sufficient to increase the hardness retention of tomato slices. However, the calcium chloride solutions with calcium concentration ranging from 2.4% to 4.0% improved the hardness retention of the frozen- thawed tomato slices significantly from 48.6% to more than 100.0%, which is
ii comparable to the fresh ones. When the maximum hardness retention was obtained, the total moisture loss increased and added up to 11.5% of the initial moisture content.
Regarding the demands of maximizing the hardness retention and minimizing the total mass loss, the optimal processing condition was freezing at 45 second “time-to-freeze” after pretreatment with around 2.4% calcium solutions for 3.5 minutes.
iii Acknowledgments
First and foremost, I would like to express my sincerest gratitude to my advisor
Dr. Dennis R. Heldman for his intelligent and patient guidance, discipline, and warm encouragements and help during my study and research in the Ohio State University. His expertise, insight and enthusiasm on food science and engineering inspired me to continue my career in food processing filed. I would like to express my heartfelt thanks to
Dr. Sastry, whose lecture of food engineering and insight of research method inspired me in both research and academia life. I also greatly appreciate the help of Dr. Simons, who led me to a fantastic world of sensory and hedonic properties of food and encouraged me to approach the food processing from a different perspective.
I would also like to thank the past and current members of the Heldman lab group: David Phinney, Cheryl Wick, John Frelka, Sravanti Paluri, Helen Bunker,
Mengyuan Fan, Ariella Feldman, Yunqi Huang, James Stone, Erica Cramer, Sangeetha
Krishnaswamy, Anita Wickramasinghe and Paul Park, for their precious friendship in the group. Special thanks go to David Phinney, Yunqi Huang, John Frelka and Anita
Wickramasinghe, for their encouragements and helps during my experiment and thesis writing. None of this research would be possible if they didn’t help me.
To my beloved mother and sister, I sincerely express my gratitude for their encouragement and support for my education.
iv Vita
September 30, 1991 ...... Born, Jiangsu, China
2013...... B.S. of Food Science & Engineering, South
China University of Technology
Fields of Study
Major Field: Food, Agricultural & Biological Engineering
v Table of contents
Abstract…………………………………………………………………………………..ii Acknowledgements……………………………………………………………………….iv Vita………………………………………………………………………………………..v Table of Contents…………………………………………………………………………vi List of figures……………………………………………………………………………...x List of tables………………………………………………………………………………xi
Introduction ...... 1
Literature Review ...... 5
2.1 Tomatoes ...... 5
2.1.1 Physiology characteristics of tomato fruits...... 5
2.1.2 Chemical composition of tomatoes...... 6
2.1.2.1 Lycopene...... 7
2.1.2.2 Ascorbic acid...... 8
2.1.2.3 Phenolics...... 8
2.1.3 Tomato Processing ...... 8
2.1.3.1 Processed tomato products...... 8
2.1.4 Critical quality attributes for processed tomato products ...... 9
2.1.4.1 Color and appearance ...... 9
2.1.4.2 Flavor ...... 10
vi 2.1.4.3 Texture ...... 10
2.1.4.4 Nutritional level ...... 15
2.2 Mechanism and Development of Freezing Processing ...... 16
2.2.1 Ice crystal formation during freezing ...... 16
2.2.2 Changes of chemical reactions and microbial growth in frozen food system ...... 18
2.2.3 Modern freezing system and freezing system design ...... 20
2.2.4 Frozen food quality control ...... 21
2.2.4.1 Quality control in freezing process ...... 21
2.2.4.2 The effect of storage conditions on the quality of frozen food ...... 23
2.2.4.3 The effect of thawing methods on the quality of frozen food ...... 26
2.3 Frozen fruits and vegetables ...... 28
2.3.1 Typical freezing processing for frozen fruits and vegetables...... 28
2.3.2 Quality of frozen fruits and vegetables...... 30
Materials and Methods ...... 32
3.1 Sample Preparation ...... 32
3.1.1 Tomato samples ...... 32
3.1.2 Preparation of calcium chloride solution ...... 33
3.2 Sliced tomato calcification...... 33
3.3 Freezing ...... 34
3.4 Storage (Temperature equilibrium)...... 35 vii 3.5 Thawing ...... 35
3.6 Moisture content and mass change determination ...... 36
3.6.1 Moisture content measurement for fresh tomato slice ...... 36
3.6.2 Mass change measurement for calcium treated tomato slice ...... 36
3.6.3 Mass loss measurement after thawing ...... 36
3.6.4 Dry matter determination for frozen tomato slices ...... 37
3.7 Texture profile analysis method ...... 37
3.8 Initial Freezing Point Prediction ...... 38
3.9 Calculation of ice mass fraction ...... 41
3.10 Statistical Analysis ...... 42
Results and Discussion ...... 43
4.1 The effect of time to freeze on the texture and water retention of frozen tomatoes without treatment ...... 44
4.2 The mass change of the fresh cut tomato slices during calcium pretreatment ...... 49
4.3 The effect the calcium chloride pretreatment on the hardness of fresh tomato slices without freezing ...... 53
4.4 The effect of the calcium treatment of the mass loss of the tomato slices after freezing and thawing ...... 57
4.5 The effect of the calcium treatment of the hardness retention of the tomato slices after freezing and thawing ...... 59
Conclusions & Future Work ...... 64 viii 5.1 Conclusion ...... 64
5.2 Future work ...... 65
Reference ………………………………………………………………………………..66
Appendix A………………………………………………………...... 77
ix List of Figures
Figure 2.1 A brief scheme of major structures of a tomato fruit...... 5
Figure 2.2 A schematic plot of product temperature profile during freezing (Nesvadba, 2008). . 17
Figure 3.1 Color classification requirements in tomatoes...... 32
Figure 4.1 An example of temperature history at the center of a tomato slice when frozen in ethanol bath at -25 ºC...... 44
Figure 4.2 Hardness retention of frozen tomato slices without pretreatment at rapid freezing (45- second “time-to-freeze”) and slow freezing (1800-second “time-to-freeze”)...... 47
Figure 4.3 The mass change after pretreated with calcium chloride solutions at different concentration for 3.5 minutes...... 51
Figure 4.4 Hardness retention of tomato slice before freezing and after pretreated with different
CaCl2 solutions...... 54
Figure 4.5 Mass loss during thawing of tomato slices frozen at same 45-second “time-to-freeze” after pretreated with different CaCl2 solutions...... 58
Figure 4.6 Hardness retention of tomato slices frozen at same freezing rate after pretreated with different CaCl2 solutions...... 60
Figure A.1 Temperature history at the center of the tomato slice when frozen at a range of “Time to freeze” without calcium treatment……………………………………………..77
x List of Tables
Table 2.1 Biophysical and biochemical properties of PME and PG purified from tomatoes...... 14
Table 2.2 Typical surface heat transfer coefficient (h) for different freezing methods (George,
1997)...... 21
Table 3.1 Texture analyzer setting for puncture test of tomato slices ...... 38
Table 3.2 Molarity, average content and molecular weight of critical components of tomato for initial freezing point prediction...... 40
xi Introduction
Tomato (Lycopersicon esculentum Mill.), botanically a kind of fruit, is one of the most important economic plants worldwide (Peng et al., 2008). United States Department of Agriculture (USDA) Economic Research Service (ERS) (2016) reported that the
United States is the second largest tomato-producing country in the world, contributing more than 2 billion dollars in annual farm cash receipts. Tomato is a major source of antioxidants in human diet, providing carotenoids and flavonoid (Pernice et al., 2010).
The flavonoids are a family of phenolic compounds, including anthocyanin, hydroxycinnamic acid, rutin, naringenin, etc (Stewart et al., 2000; Mertz et al. 2009).
Tomatoes are consumed both freshly and processed in the North America, processed tomatoes contributing 75% (Pernice et al., 2010). The processed tomatoes include canned diced or whole tomatoes, dehydrated tomatoes, tomato sauces, etc. Bulk productions of diced tomatoes are thermally processed, aseptically packaged and added with calcium to improve the hardness for further production like pizzas (Anthon, Blot, & Barrett, 2005).
Generally, the thermal treated tomato products in market is blanched, peeled and processed in high temperature, resulting in lack of “fresh” flavor because of loss of nutrients. For example, in tomato paste processing, 30% decreasing in beta-carotene after hot break (93 ℃ for 5 mins) and almost 20% lycopene decreasing after sterilization at 100 ℃ for 3~5 mins were observed (Koh et al., 2012). Meanwhile, frozen fruits and 1 vegetables have gained popular in modern society as high quality and easily prepared food. The freezing processing can inhibit microbial growth, chemical reactions and cellular metabolic reactions without damaging the “fresh” perception (Delgado and Sun,
2001). The quality of the high quality frozen tomatoes are expected to obtain more market share with the development of freezing technologies.
While the freezing and frozen storage technology is theoretically an ideal method to keep the high quality characteristics of the tomatoes, no commercialized frozen tomatoes are found in the U.S.A market yet. One of the big challenges is that the tomato structure is fragile and sensitive to freezing process because of high moisture content and complex structure. Dramatic quality loss of the tomatoes after freezing, storage and thawing has been observed in the past decades, including discoloration due to carotenoid oxidation, nutrients degradation and turgor loss (Biacs and Wissgott, 1997; Lisiewska &
Kmiecik, 2000; Levine, 1973 & Seynave, 1972). However, the development of machine harvest in the United States in the last 60 years makes the increase in planting processing
(Barringer et al., 2003), the tomatoes smaller than those on fresh market and with more solid contents and firmer texture. The tomatoes in today’s market are expected to have better quality characteristics for frozen products (Lisiewska and Kmiecik, 2000).
In fresh market, hardness are significant quality characteristics directly perceived by the consumers for fruits and vegetables (Oey etc., 2008). Considering the characteristic of the frozen products, mass retention after thawing might be the other factors which will affect the consumers’ preference for direct consumable frozen tomatoes. Mass retention after freezing could also be an important factor which should be considered for feasible manufacture and marketing. Previous studies have shown positive
2 correlation between the freezing rate and texture retention of the frozen fruits because of less structure damage resulting from smaller ice crystal size and less water immigration
(Singh and Heldman, 2014). Meanwhile, particular pretreatments for frozen fruits and vegetables for better texture, color and flavor retention, such as the addition of sugars and other solutes, is widely used in food processing. The solutes will cause freezing point depression resulting in less water freezing intercellular and reducing structure damage
(Dermesonlouoglou, 2007). The solutes could also exclude oxygen out and inhibit oxidation reaction to improve color and flavor retention. In this investigation, the effects of freezing rates (defined as “time-to-freeze”) and calcium pretreatment conditions are explored to maximum hardness and mass retention.
From the economic perspective, compared with other conventional preservation method, including dehydration and canning, the overall energy consumption of freezing process and storage is close to the conventional ones but less time consuming (Harris and
Kramer, 1975).
To figure out the influence of freeze method and pretreatments on the quality attributes of frozen tomato slices will help to develop a commercialized frozen tomato products either suitable for further processing or direct consumption. It could be a promising way to preserve the perishable fruits and providing both the developed and developing countries with a new and easy prepared frozen food. The investigation can also be applied for development of other frozen fruits, which has similar structure and thermo-physical properties.
3 To achieve the goals mentioned above, the objectives of this investigation include:
a) To evaluate the influence of time-to-freeze on the quality attributes retention of frozen tomato pieces.
b) To evaluate the influence of a calcium chloride pre-treatment on the quality attributes of frozen tomato pieces.
c) To identify the optimum combination of pretreatment and time-to-freeze for retention of the desired quality attributes of frozen tomato pieces.
4 Literature Review 2.1 Tomatoes
2.1.1 Physiology characteristics of tomato fruits.
The tomato belongs to the genus of Lyscopersicum of the family Solanaceae, botanically a fruit considered as a berry but legally a vegetable (Davies, Hobson &
McGlasson, 1981; Ludford, 1995).
The tomato fruit has relatively a complex botanical structure, including epidermis, pericarp, vascular bundle, placenta, seeds, locular cavity and funiculus. As shown in
Figure 2.1, the pericarp contains three parts: outer wall; radius wall, septa; and inner wall, collumella (Davies, Hobson & McGlasson, 1981; Ludford, 1995). The vascular bundle is a part of the transport system.
Figure 2.1 A brief scheme of major structures of a tomato fruit.
5 From the microscopic point of view, the tomato fruit cell share the typical common properties with other plant cells. Plant cells are eukaryotic cells. The cell wall outside the cell membrane is the most distinctive structure of plant cells. Plan cell walls, composed of carbohydrate, protein, and aromatic compounds, are critical to plant growth and development (Caffall & Mohnen, 2009). Polysaccharides, including cellulose, hemicellulose and pectin, is the major component (~90%) of the primary fruit cell wall
(Caffall & Mohnen, 2009), composing the cellular skeleton which supports the cell shape by adjusting the cell volume due to the turgor pressure (Chylińska, Szymańska-Chargot
& Zdunek, 2014). The turgidity of the fruit cell is critical to the texture perception when consumed directly. The primary cell wall also considered to contribute to cell adhesion and signal transduction (Caffall & Mohnen, 2009).
Different cultivars have been developed with distinctive differences for commercial utilization or fresh consumption. For tomatoes in fresh market, flavor, color, texture and taste will be significant factors affecting consumer perception. On the other hand, desired rheological properties is important for tomatoes utilized in intended further processing (Madhavi & Salunkhe, 1998). As the development of the modern mechanical harvesting, the tomatoes with harder texture, well-colored and uniform ripening are more prepared nowadays for further processing (Ware & McCollum, 1980).
2.1.2 Chemical composition of tomatoes.
As a crop, the year-round chemical compositions of tomatoes varied due to cultivars, planting environment (sunshine exposure, raining and soil nutrients), maturity stage, etc. However, a reprehensive data of the nutrient levels of a typical tomato fruit can
6 still be retrieved from the United States Department of Agriculture (USDA) website.
According to USDA National Nutrient Database for Standard Reference 28 (2015), the year round composition contents of the red, raw tomatoes are described as follows:
94.52% water, 0.88% protein, 0.2% lipid, 1.2% fiber, 3.89% carbohydrate and 0.50% ash. Quercetin, kaempferol, and myrcetin are the major flavonols in tomatoes (Wu and
Burrell, 1958); 0.6mg, 0.1mg and 0.1mg per 100g tomato, respectively.
Tomato fruit is great dietary resource for a range of antioxidant, protective agents to delay or prevent harmful oxidation in human body, including carotenoids, ascorbic acid and phenolics (Peng, Zhang, & Ye, 2008). Generally, the antioxidants prevented or delayed biomolecules from oxidation by preventing the initiation or terminating the chain reactions of free radicals (propagation) (Halliwell, 1995).
2.1.2.1 Lycopene.
The lycopene content is reported to vary from 18.2 to 111.9 mg/kg fresh weight.
Lycopene, the predominant component of carotenoids in tomato fruited as well as in human plasma, is reported by Agarwal and Rao (2000) to be associated with an inhibitory potential for chronic diseases though multiple mechanisms, such as cancer and cardiovascular disease, main killers for human health. Additionally, according to
Agarwal and Rao (2000), the lycopene from the processed tomato products (estimated as
25 mg per day) accounts for 50% of the total daily intake for consumers, which indicating the significant role played by tomatoes in human dietary. On the other hand, lycopene is the natural fat-soluble pigment providing redness of the tomato tissue (Clinton, 1998).
7 2.1.2.2 Ascorbic acid.
The mean content of ascorbic acid in tomatoes from six cities was reported as
15.3 ± 5.43 mg/100g fresh tomato in 1994, with the range from 10.2 to 25.3 mg/100g fresh tomato (Klein & Perry, 1994). Klein and Perry also suggested that the tomato is an excellent source of vitamin C compared with other vegetables investigated, such as carrots, onions and corns.
2.1.2.3 Phenolics.
According to Martı́nez-Valverde (2002) et al, flavonoids and hydroxycinnamic acids are the most investigated phenolics. Martı́nez-Valverde, Periago, Provan and
Chesson (2002) analyzed nine commercial varieties of tomato for phenolic compounds profile, discovered the most abundant flavonoid was quercetin and most abundant hydroxycinnamic was chlorogenic acid. The concentration of quercetin was ranging from
7.19 to 43.59 mg/kg fresh weight, for chlorogenic acid from 14 to 32mg/kg fresh weight
(Martı́nez-Valverde et al., 2002).
2.1.3 Tomato Processing
2.1.3.1 Processed tomato products
Due to the nutrient abundance of the tomato, ripe fruit tomatoes are consumed both fresh and as further processed products, such as tomato juice, tomato puree, paste, ketchup, sauce and canned diced or whole tomatoes (Thakur, Singh & Nelson, 1996).
The preparation stage include a series of unit operations: dry sort and grading, washing (soaking and spray rinse), final sorting and trimming, coring, peeling and final inspection (Gould, 2013). Most processed tomato products are peeled although it is an
8 optional processing. Steam peeling, lye peeling, infrared peeling are the three major methods used in tomato processing industry (Gould, 2013).
Thermal treatment is one critical unit processing for processed tomato products
(Colle, Lemmens, Buggenhout, Van Loey, Hendrickx, 2010), which is necessary for microbial population reduction and pectic enzyme inactivation. In tomato juice manufacturing, the diced tomatoes will go through either hot break (temperature increased to 82 ºC rapidly) or cold break (mild heating) (Thakur, Singh, & Nelson, 1996).
For solid based tomato products, including the paste and puree, after the break processing, different amount of water is removed using vacuum evaporator or by steam coils from the tomatoes and the paste and puree are formed after the juice extraction
(Thakur, et al., 1996).
2.1.4 Critical quality attributes for processed tomato products
2.1.4.1 Color and appearance
As mentioned before, the carotenoids are the major contributor of the red color of the tomato fruits. Theoretically, there are two types of carotenoids, cis- and trans-, which indicates that the existence of isomers in food matrix. However, in most foods, carotenoids (including lycopene) exist only in the most heat resistance form: trans- configuration (Clinton, 1998). But some carotenoids, such as epoxycarotenoids, can still be affected by heat processing (De Ancos, Sánchez‐Moreno, Pascual‐Teresa, &Cano,
2012). Lycopene is quite stable under 100 ºC, but when processed at higher temperature for prolong time, lycopene trends to have more unstable cis-isomers, which is less reddish. The trans-cis isomerization of lycopene can also be caused by other factors in
9 heating processing, such like light, oxygen and mental ions in the processing facility (Shi
& Maguer, 2000).
The degradation of pigments is believed to be responsible for the color change during tomato products processing. Hunter values, including lightness (L), redness (a), yellowness (b) or the hue angle (h*) is usually used to represent color quality of processed tomato products (Askari, Emam-Djomeh, & Tahmasbi, 2009). Stinco et al. (2013) evaluated the color difference between the fresh tomatoes and processed products (juice, canned diced tomatoes, ketchup, etc) from the local market using digital image analysis and spectroradiometry and a significant decrease (p < 0.05) in lightness (L), hue (h*) 0 and a significant increase of redness (a) is reported in processed products.
2.1.4.2 Flavor
Despite the influence of the raw material of the processed tomato products, which is the fresh tomato, the effect of the processing conditions on the flavor profile of the processed tomato products is important for processing optimization and quality control.
A significant loss of tomato volatiles in the process of tomato concentration was reported
(Buttery, Teranishi, Ling &Turnbaugh, 1990), the almost complete loss of (Z)-3-hexenal, the major contributor of fresh tomato aroma was discovered in tomato paste.
2.1.4.3 Texture
Just like color, the texture, especially the firmness, is a quality attribute can be evaluated by the consumer directly even without tasting and can affect the consumers’ preference. During chewing, a series of texture characteristics can be felt by human mouth, and this kind of profile can be analyzed using instrumental methods.
10 The insoluble solids, which derived from the 3-demensinal cell wall, is the major contributor of the texture perception of fruit and vegetables. As mentioned before, the major components of the plant cell wall is pectin, cellulose and hemicellulose. Pectin is galacturonic acid-rich polysaccharide, derived from hexoses, pentoses and their derivatives (Paliyath et al., 2012), is the major component of the primary cell wall matrix of tomatoes (Jarvis, 1984).When the tomato fruit is ripening, a decrease of the hardness of the tomato fruit will be observed. There is an assumption proposing that the pectin is the target of the development modifications for the plants, which is responsible for the cell wall softening during ripening (Fischer & Bennett, 1991). One supportive evidence is that the pectin contents varied in different maturation stages of tomato fruits (Madhavi &
Salunkhe, 1998) while the positive correlation of the increase in soluble pectin and the softening of the tomato fruits was discovered (Luh, Villarreal, Leonard & Yamaguchi,
1960).
Homogalacturonan (HGA), rhamnogalacturonan-I (RG-I) and rhamnogalacturonan-II (RG-II) are the three pectin polysaccharides domains in the primary cell wall (Ridley, O’ Neill & Mohnen, 2001), which are crosslinked by covalent linkage. More specifically, both RG-I and RG-II are thought to be attached to HGA, but no evidence has been shown for a direct attachment between RG-I and RG-II ((Ridley,
O’ Neill & Mohnen, 2001). HGA is the best explored pectin matrix regarding structure- function correlation. One outstanding property of HGA as a high-ester pectin is its ability to cross link with calcium to form calcium gels by removal of methyl ester groups to maintain the integrity of the pectin network (Jarvis, 1984). The removal of methyl ester groups from the HGA is catalyzed by pectin methyl esterase (PME), which will expose
11 acidic residues for asscociation with other HGA chains by calcium cross-link bridges and furtherly form pectin network (Willats, McCartney, Mackie, & Knox, 2001).
Although the details of the fine structures of the pectin matrix are still not thoroughly explored due to the complexity, it is believed that the pectin matrix has interactions with other macromolecules in plant cell, e.g., Forster et.al (1996) discovered that the RG-I is associated with cellulose via NMR studies (Foster, Ablett, MaCann &
Gidley, 1996).
While the mechanism of the cell wall modification during ripening remains unclear, a generally accepted assumption is that the softening of the tomato fruit could be related to the activities of the pectin degradation enzymes: polygalacturonase (PG) and pectin methyl esterase (PME) (Gross & Wallner, 1979). As mentioned before, PME is critical for formation of supramolecular pectin gels for its ability to remove methyl ester groups from HGA (Willats, et al., 2001). However, the de-esterification sites can be modified by other enzymes to cause the backbone cleavage (Dumville & Fry, 2000). PG belongs to the glycosyl hydrolases, and catalyzese the depolymerization of the pectin and progressive dissolution of polyuronide (Parkin, 2008). PG is able to hydrolyze the α-1,4-
D galacturonan linkage in HGA. The correlation between the presence of PG activity and the softening of the tomato fruit during ripening is well documented.
The biophysical and biochemical properties of the PME and PG, including molar mass, isoelectric points (pIs) and kinetic properties diverse among the different fruit sources (Duvetter, Sila, Van Buggenhout, Van Loey, & Hendrickx, 2009).
12 Table 2.1 shows the properties of PME and PG extracted from tomatoes. There are two types of PG, PG-1 and Pg-2 existing in tomatoes with distinguishable molar mass, 100 kDa and 42 kDa, respectively (Tucker, Robertson, & Grierson, 1980).
13 Table 2.1 Biophysical and biochemical properties of PME and PG purified from tomatoes.
Enzyme Molar mass Isoelectric Optimum pH Reference type (kDa) points (pI) PME 33.6 > 9.3 8.0 Duvetter, et al., (2009) PG-1 100 8.6 4.5 Ali & Brady (1982); Tucker, et el.(1980). PG-2 44 9.4 4.5 Ali & Brady (1982); Tucker, et el.(1980).
The activities of the PG and PE have been reduced in Low-PG and Low-PE tomatoes to produce firmer fruits with desired characteristics through gene modification technology while significant increases in soluble solids observed in both lines of tomatoes compared with unmodified ones.
Regarding the characteristics of the pectin matrix, especially the HGA, to enhance the texture profile of the processed tomato products, adding calcium to the processed tomato products has been proven to be helpful to enhance the hardness of the products.
Calcium chloride is approved by FDA as GRAS (Generally Recognized as Safe) at maximum level of 0.2 percent for processed fruit and fruit juices and 0.4 percent for processed vegetables and vegetables juices (Food and Drugs, 1996). Since the tomato fruits have a low transpiration rate, the normal range of calcium content in a raw and ripe tomato is relatively low (Grange, 1995), which is 4.0 to 21mg per 100 g fresh tissue
(Davies, Hobson & McGlasson, 1981) and the year round average level is 10mg per 100g fresh tissue (USDA, 2015). Vacuum infusion or immersion in calcium solutions are two common ways to apply postharvest calcium treatments for vegetables and fruits (Verdini etc., 2008). Besides the benefits resulting from osmosis, the calcium chloride
14 pretreatment has advantages of protecting texture characteristics of processed vegetables and fruits. Calcium chloride is not only a firming agent but also an anti-microbial agent.
Pinherio and Almeida (2008) revealed that the calcium chloride treatment resulted in significant decrease of the water soluble pectin in turning and ripe tomatoes.
Sirijariyawat and Sharoenrein (2014) studied the correlation between water soluble pectin content and calcium chloride treatments of 4 fresh and frozen-thawed fruits (apple, mango, cantaloupe and pineapple). Similar to Pinherio and Almerida’s study, a negative correlation between water-soluble content and calcium treatment was found except for pineapple in Sirijariyawat and Sharoenrein’s study.
2.1.4.4 Nutritional level
Compared with color and appearance, flavor and texture, the nutritional level of the tomato products is a factor that cannot be perceived intuitively by the consumers
(Barrett, Beaulieu & Shewfelt, 2010) However, nutritional level of the processed products have been valued by the industry due to the consumers’ increasing awareness of significance of nutrients contents (Barrett et al., 2010). On the other hand, the interactions between the nutritional level and the color, flavor and texture can always be observed for tomato products (Barrett et al., 2010). For example, the carotenoids and flavonoids are natural pigments as well as an important nutrient.
The effect of heating processing on the antioxidant activity of tomatoes, such as ascorbic acid and lycopene retention is also well documented. Dewento, Wu, Adom and
Liu (2002) evaluated the ascorbic acid content in tomatoes and discovered a decrease of
10.53, 15.79 and 28.95% compared to the fresh tomato slurry after heating the homogenized tomato slurry at 88 ºC for 2, 15 and 30 minutes. However, the total
15 antioxidant activity of the tomato after heated for 30 minutes increased by 62.09% in vitamin equivalent compared with the fresh one with a total trans-lycopene content increase (Dewento et al., 2002). The thermal processing also promotes the cis-isomers of lycopene, which is more bioavailable than all-trans isomers (Shi & Maguer, 2000). The result indicates the potential of thermal processing on enhancing the bioaccess of antioxidants in tomatoes.
2.2 Mechanism and Development of Freezing Processing
Freezing is a widely used preservation method for food in modern society.
Actually, ancient people are believed to use cold air or ice for food preservation as early as 1000 B.C. to 500 B.C. (Heldman & Nesvadba, 2010). In early 1800, large scale food freezing became practical because of the development if food freezing and mechanical refrigeration. In 1925, the pioneer of frozen food industry- Clarence Birdseye invented double belt freezer in 1925 making a breakthrough in the modern frozen food industry and commercialized retail frozen food in 1930s (Heldman & Nesvadba, 2010).
2.2.1 Ice crystal formation during freezing
Freezing is a processing involving removal of heat from the product and development of ice crystals within the products. When the food is contacted with the freezing media, heat starts to transfer from the food to the media.
16
Figure 2.2 A schematic plot of product temperature profile during freezing (Nesvadba, 2008).
T0: the starting temperature; Tf: the initial freezing temperature; Ts: super cooling temperature; Te: equilibrium temperature; Plateau B-C: freezing plateau.
Figure 2.2 illustrated a typical temperature history of food products during freezing. An obvious temperature decline involving removal of latent heat and sensible heat in the food product can be observed. Nucleation and growth (propagation) are the two stages for ice crystal formation (Nakagawa, Hottot, Vessot, & Andrieu, 2006). After contacting with the freezing media, the temperature of the product starts to decrease gradually with a faster rate on the product surface than within the product. The surface of the product may show supercooling (Ts) before reaching the initial freezing point. The first ice crystal forms at the Tf , which is lower than the freezing point of pure water, then started to grow and to spread in volume. The growth of the seed or nucleus is called nucleation. Nucleation can be homogenous at lower sub-zero temperatures in 17 homogenous partical-free liquids or heterogeneous at higher sub-zero temperatures
(Nesvadba, 2008). After the nucleation happens, the crystal starts to grow, the system temperature will decrease slowly until the latent heat is released. The temperature of the frozen food will be stable during the release of the latent heat (shown as the plateau in
Figure 2.2). After the latent heat is released, the product temperature is lowered to the equilibrium temperature and most sensible heat is removed (Silva, Gonçalves, &
Brandão, 2008).
Since ice crystal formation during freezing has strong impact on the final quality of frozen food, novel technologies have been developed to modify and control the ice crystal formation. Among the advanced technologies, high pressure shift freezing (PSF) and power ultrasound assisted freezing have been studied in recent years (Li & Sun,
2002; Zheng & Sun, 2006). The mechanisms and advantages of the novel freezing methods will be introduced when the quality control in freezing process is discussed.
2.2.2 Changes of chemical reactions and microbial growth in frozen food
system
As the food matrix is complex, freezing processing and frozen storage may result in complicate physical and chemical interactions among the substances in the food
(Berry, Fletcher, McClure, Wilkinson, 2008). However, the temperature decline and transformation from water to ice crystals are the most significant change we can observe in the frozen food.
Water is a major component for animal and vegetable tissues, contributing more than 80% of total weight. Therefore, the water is also a dominant composition of the common food matrix derived from the animal and plants (Nesvadba, 2008). As a reactant
18 or medium for quality degradation or microbial growth related biological or chemical reactions, water plays a significant role in food quality and safety (Singh & Heldman,
2014). Water activity (aw), is a widely accepted expression of water content in food when the correlation between water content and microbial growth or quality degradation reactions are intercepted (Slade, Levine, & Reid, 1991). aw is defined as the ratio of partial vapor pressure of water in the food matrix divided and the pure water vapor pressure at the same temperature. Freezing can remove the water and reduce aw in the food matrix by forming ice crystals. Microbial growth in food are believed to cease when aw is below 0.6 (Chieh, 2012). The low aw of the frozen food is a key parameter that ensure food safety. On the other hand, when the ice crystals form, the solutes in the food is concentrated and the freezing point is depressed, which is called “Freeze concentration”. Boonsupthip and Heldman (2007) proposed a model to predict the frozen water fraction and initial freezing point using product compositions based on the concentration and molecular weight. The proposed model indicates that the food solutes with molal concentration of 50 μmol/100g food or higher will make a significant contribute to unfrozen water fraction and freezing depression (Boonsupthip & Heldman,
2007). The “freeze concetration” may accelerate reactions, such as protein denaturation, lipid oxidation or colloidal structure destruction (Berk, 2009).
Despite the advantage of lower water activity in food matrix after freezing processing, the constant low temperature will slow down the rate constant of the chemical and enzymatic reactions related to food deterioration and spoilage from the thermal dynamic aspect (Goff, 1992). From the food safety aspect, the freezing temperature will cause lethal injury to the microorganism.
19 2.2.3 Modern freezing system and freezing system design
Based on a general principle in Physical Chemistry that the rate of chemical reactions and biological processes will be slowed down due to the molecular mobility is decreased at low temperature, preserving food in low temperature (refrigeration, chilling and freezing) has become a popular food processing in food industry after World War II
(Berk, 2009). The common freezing equipment in frozen food industry includes air-blast freezers (tunnel freezers, belt freezers, fluidized bed freezers, etc.), contact freezers
(immersion freezers, indirect contact freezers, plate freezers and contact belt freezers) and cryogenic freezers (liquid nitrogen freezers and liquid carbon dioxide freezers). The freezing method and freezing system design depend greatly on the characteristic of the raw material, product load, expected condition at the exit, etc.
Prediction of freezing time according to the thermos-physical properties of the food matrix is a complex and key step in freezing system design. The freezing processing will affect the thermo-physical properties of the food, including density, thermal conductivity, heat content (enthalpy), and apparent specific heat, as the product temperature decreases (Heldman & Lurd, 2007). The models to predict the thermo- physical changes and the comparison between the predicted value and experiemental data have been well documented by researchers (Cleland & Earle; 1982; Hsieh, Lerew, &
Heldman, 1977; Pham, 1996). Another important factor used in those models are surface heat transfer coefficient, which brings great uncertainty to the prediction because of difficulties to quantify the boundary conditions. However, typical surface heat transfer coefficient values are available for reference. Table 2.2 shows the typical heat transfer coefficient for different freezing methods. 20 Table 2.2 Typical surface heat transfer coefficient (h) for different freezing methods (George, 1997).
Freezing Method h (W/m2K) Ultra-rapid freezing (cryogenic) 100-140 Rapid freezing(fluidised-bed freezer/plate freezer) 80-120 Normal freezing(air blast) 26-30 Slow freezing (air blast in cartons) 17-20 Very slow freezing (still air)/ 5-10
2.2.4 Frozen food quality control
Freezing processing are believed to preserve the quality attributes, including taste, texture, appearance and nutrition value better than other thermal processing. The quality of frozen food could be affected by the pre-freezing treatment, freezing processing, frozen storage and distribution, and thawing (Goff, 1992).
2.2.4.1 Quality control in freezing process
As mentioned before, during freezing, ice will nucleation and propagate. The freezing rates will affect the ice crystal structure, therefore affect the quality of the frozen food. According to Hayes at el. (1984), freezing rate is defined as the velocity of the movement of the ice-water freezing front (Nesvadba, 2008). From a more direct perspective, Ngapo et al. (1999) express the freezing rates as the characteristic freezing time (tcf) through the temperature range from -1 to -7 ºC, in which majority ice crystals formed (Li & Sun, 2002) . In our study, the freezing rate is expressed as “time-to-freeze”, which is the period of time when the temperature of the geometry center decreased from room temperature to the target frozen temperature -18C.
21 The freezing rate will influence the size, shape and distribution of the ice crystals.
Generally, in slow freezing, fewer larger crystals are produced. On the contrary, more and smaller ice crystals form in fast freezing. Furthermore, in slow freezing or with high water permeability, large crystals in extracellular can cause dehydration of the cells through osmosis; in fast freezing or with low water permeability, the “freezing concentration” develops fast extracellular, then ice crystal can form intercellular at critical super cooling temperature and cell damage happens (Reid, 1997) .
Although fast freezing is more advanced than the slow freezing regarding structure damage, freeze- cracking could happen in fast freezing. Fast freezing is achieved by increasing the temperature gradient between the food and the freezing medium, causing the increase of internal stress within food matrix (Kim & Hung, 1994).
The internal stress can lead to cracking, which can be observed on the product surface.
There are two types of cracking, a) surface only cracking; b) inside originating cracking, then progress to the surface (Hung, .
In recent years, inspired by the demanding of producing high quality frozen food, power ultrasound and high pressure has been applied to assist traditional freezing processing to modify or control the ice crystal formation. In power ultrasound assisted freezing, cavitation is proven to help with the quality improvement by two mechanisms: a) creating gas bubbles to promote ice nucleation (increase numbers of nucleation), so that leading to smaller and more even nucleation distribution; b) forming microstreaming to enhance heat and mass transfer for shorter freezing time (Zheng & Sun, 2006). In high pressure shifted freezing (HPSF), the high pressure will affect the phase change in freezing, and the initial freezing point is lower than the freezing point in atmosphere
22 pressure. The melting point of water will be decreased down to -21 ºC at 210 Mpa
(LeBail, Chevalier, Mussa, & Ghoul, 2002). The HPSF can increase the ice nucleation rate and induce homogenous ice nucleation, therefore helps to retain microstructure of the food (LeBail et al., 2002; Otero, Martino, Zaritzky, Solas, & Sanz, 2000). Since the thermal gradient is reduced with help of the high pressure, the HPSF also helps to minimize the possibility of freeze-cracking during fast freezing (Otero et al., 2000).
2.2.4.2 The effect of storage conditions on the quality of frozen food
The “freezing concentration” phenomenon mentioned above will result in an unfrozen phase in the frozen food, causing the diffusion-controlled deterioration reactions continue occur in the food matrix even at very low temperature during storage (Goff,
1992). As the existence of the freezing point depression, the frozen food may not completely frozen at the conventional -18 ºC storage temperature (Goff, 1992). The continued reactions will be a significant factor responsible for the limited shelf-life of frozen food.
However, besides the effect of “freezing concentration”, the second order glass transition is another physical change playing a key role in the frozen food quality, which happens at the temperature called glass transition temperature (Tg). According to Goff
(1992), the glass transition will happen in maximally freezing-concentrated solutes via second order glass transition with no latent heat release (Nesvadba, 2008). When the glass transition happens, the system is at an extremely high-viscosity state with extremely low diffusion rate. The food matrix stored at the temperature below the Tg will stay relatively stable because of the very limited mobility of the water phase in its glassy state.
However, Tg is not a constant number but reported to decrease when freezing rates
23 increases (Hsu, Heldman, Taylor and Kramer, 2003). Therefore, it is difficult to achieve maximum freezing-concentration glassy state in rapid freezing and during frozen storage.
As the advantages of rapid freezing are significant for quality control during freezing process, it is worthy of exploring the optimal freezing conditions to maximum quality retention by finding a balance between taking the advantages of more rapid freezing and achieving higher Tg (Hsu et,al, . 2003)
Since the shelf-life of frozen food is limited, it is important to predict the shelf- life to make assurance of the safety and acceptable quality retention of the frozen food.
The Arrhenius model is widely used and adequate to predict the shelf-life of frozen food by calculating the quality loss because of chemical or biological reactions, such as the non-enzymatic browning and loss of vitamins (Berk, 2009), based on the storage temperature. Common deterioration related physico-chemical and biological changes observed in frozen food includes water diffusion, ice recrystallization, protein cross- linking, protein-lipid interaction, lipid oxidation and other oxidative changes, enzymatic reactions, pigments, vitamins and flavor degradation (Berk, 2009; George,1992; Zaritzky,
2008).
As the temperature fluctuates in the storage system, the temperature gradients exist between the surface and within the food product and between the package room and the product, which will lead to the relatively irreversible water migration from the interior to the surface of the product (Zaritzky, 2008). Undesirable weight loss and freezing burn are the major two product quality loss observed in packaged frozen food due to water migration.
24 During storage, the ice crystals formed during freezing processing will go through metamorphic changes, including change in decreased number, increased size etc., in the food matrix (Zaritzky, 2008; Sutton, Lips, Piccirillo, & Sztehlo, 1996). The recrystallization will weaken the advantages of fast freezing over slow freezing regarding damage to the food structure. According to Sutton et al. (1996), the ice recrystallization rate in fructose solutions decreases when ice phase volume decreases and storage temperature decreases, which indicates that the undesired recrystallization could be reduced by selecting proper storage temperature.
For the quality attributes related reactions, in general, the low storage temperature can slow down the reaction rate; on the other hand, the concentration in the unfrozen phase during freezing will speed up the reactions. Enzymes are not inactivated in low temperature, for example, hydrolases may remain active, which will catalyze reactions responsible for off-flavor, texture change or enzymatic browning (Zaritzky, 2008). The protein denaturation caused by freezing processing and frozen storage are mainly a result of the ice formation, ice crystallization, salt concentration because of “freeze- concentration”, oxidation, etc. (Zaritzky, 2008). The changes in certain quality attributes, such as viscosity change, gelation formation, etc. can be tested to detect the protein de- functionality (Xiong, 1997; Zaritzky, 2008). Lipid oxidation can be initiated in both enzymatic and non-enzymatic reactions, which is also the trigger for pigment and flavor degradation (Zaritzky, 2008).
For microbial population in frozen food, since the storage temperature is always below the limitation of microbial growth (approximately -10 ºC), microbial spoilage is
25 usually not a problem (Zaritzky, 2008). Cold shock associated with Gram-negative bacteria, cell dehydration (low water activity) and extracellular solutes concentration also contribute to the low microbial load in frozen foods (Calcott & MacLoed, 1974). On the other hand, the temperature needs to be controlled well for the bioactivity for the microbial for certain products, such like frozen yogurt or frozen dough.
Since the quality of frozen food in storage is mainly depend on the storage temperature, it is necessary to monitor the temperature in the cold chain. In the United
States, the -18 ºC or colder is recommended for frozen food storage. The practical storage life (PSL) is a commonly used term to describe the period of storage time for suitable consumption with acceptable quality attributes retention since the product is initially frozen (Singh & Heldman, 2009; Zaritzky, 2008). The PSL could be predicted based on the initial quality of the raw materials, processing and packaging, and storage temperature and duration (Heldman & Lai, 1983; Zaritzky, 2008).
2.2.4.3 The effect of thawing methods on the quality of frozen food
Many frozen food stuffs need to be thawed before consumption or further processing. Although both freezing and thawing involve phase change, thawing is not simply a reverse of freezing process (Reid, 1997; Singh & Heldman, 2009).
In industrial application, the frozen materials can be thawed via pressure-shift thawing: melt the ice crystal below -15 ºC under high pressure (200-400 MPa)
(Nesvadba, 2008). In most cases, the frozen products will be thawed by the consumers at home at the end of the supply chain (Nesvadba, 2008).
26 Ideally, the thawed products should be comparable to fresh ones, considering the reduced quality loss compared with other processing (Nesvadba, 2008). However, the food with fragile structures may suffer from structure damage during freezing and thawing, especially for home thawing. Irreversible structure due to the expansion of ice crystal volume and protein cross-linking because of “freeze-concentration” will lead to the drip loss in meat products. The moisture loss during the thawing will cause the decrease in appearance appealing, nutrition level and total weight of the product. From the aspect of food safety, the thawing process may also be favorable for microbial growth while the risk of microbial during frozen storage is limited (Laguerre, 2008).
The effect of thawing methods on the quality of the thawed food may vary depending on the raw materials, freezing processing and storage conditions. But in general, the thawing method would not have dramatic effect on the quality attributes, especially for the texture. Vail et al. studied the effect of thawing methods (at room temperature, refrigeration temperature and oven temperature) on cook loss, shear force and press fluid of frozen beefsteaks and pork roasts, and the result of statistical analysis indicated the effect of thawing method was not significant on the interested quality attributes (Vail, Jeffery, Forney, & Willey, 1943). Similarly, Boonsumrej et al. found the tiger shrimp thawed by microwave, in refrigerator, and freeze–thaw cycle had similar salt-soluble protein and cutting force; however, the microwave thawed samples had thiobarbituric acid content (Boonsumrej, Chaiwanichsiri, Tantratian, Suzuki, &Takai,
2007).
For safety issue, thawing in refrigeration is recommended to avoid high temperature in the surface or exposed to microbial growth favorable temperature for a
27 long-time. However, Ersoy et al. found that the yeast count of the refrigerator–thawed eels increased compared with the samples thawed in water, at ambient temperature and in microwave oven (Ersoy, Aksan, & Ozeren, 2008). The optimal thawing temperature should be determined individually for different types of food.
2.3 Frozen fruits and vegetables
From 2009 to 2014, overall annual fruit per capita consumption in the United
States decreased by 6% and vegetables by 7%; while annual In-home per capita consumption of frozen fruit increased from 2 to 4 Kg, and from 40 to 46 Kg for frozen vegetables from 2004 to 2014 (Foundation, 2015) . Furthermore, for the all home fruit and vegetable consumed in 2014, frozen fruits accounts for 1.6% and frozen vegetable account for 14%, respectively (Foundation, 2015). The future market of frozen fruits and vegetables could be promising in modern society with its advantage of easy storage, preparation and high quality.
2.3.1 Typical freezing processing for frozen fruits and vegetables.
Since the fruits and vegetables are prone to nutrition and organoleptic degradation, postharvest storage is critical for quality retention, such as nutrients retention, color or texture retention. The quality of the frozen fruits and vegetables also depends greatly on the ripeness stage (Lisiewska et al.1999), respiration after harvesting, enzyme activity (Alonso et al. 1997); pre-freezing treatment, microbial load and freezing processes.
Typical freezing processing for frozen fruits and vegetables include pre-freezing and freezing processing. Grading, cleaning, sorting, dicing or slicing, and blanching are
28 the major operations in the pre-freezing stage. For the safety issue, the disinfectant washing and thermal processing (blanching) can help to inactivate microbial load before freezing to avoid final product contamination. Blanching to inactivate enzymes is an important operation to minimize deteriorative reactions. However, since blanching is a heat treatment, the fruits and vegetables could suffer nutrition loss, pigment degradation and texture soften during blanching (Silva et al., 2008). To improve the quality of post- blanching products, Quintero-Ramos et al. reported that blanching in calcium chloride solution could help to increase the firmness of fruits and vegetables with the help of the activation of pectinmethylesterase (Quintero-Ramos et al., 2002). On the other hand, low- temperature longtime blanching have proven to have less quality loss than high- temperature short time blanching (Shivhare, Gupta, Basu, & Raghavan, 2009).
The most commonly used methods for freezing of fruits and vegetables are air- blast and multi-plate freezers, while air fluidizing is used for small-size IQF products.
Considering the demand of longer-shelf life and high quality frozen fruits and vegetables products with high values, cryogenic IQF is becoming more popular despite the high cost
(Silva et al., 2008). The freezing time varies depending on the dimension, shape, and compositions of the raw materials. Due to the high moisture content, the damage to the cell structure of the fruits and vegetables resulting from the ice crystal formation is worse than in meat products (Silva et al., 2008).
The effect of the freezing rates on the quality of frozen fruits and vegetables are not well documented. Generally, for freezing fruits and vegetables, the ice crystals form in the extracellular first and gradually develop to the cytoplasm when the membrane lost permeability (Silva et al., 2008). During thawing processing, the damage to the cell
29 structure due to the ice crystal formation allows water re-absorb into the intracellular. As a result, turgidity of the cell and the texture of the product is significantly affected by the freezing processing.
Although freezing is an ideal for long-term preservation method of fruits and vegetables, the texture changes due to the ice crystal growth is a major quality loss. The addition of sugars and other solutes will cause further freezing point depression resulting in less water freezing intercellular and less structure damage. The solutes can also exclude oxygen out and inhibition of oxidation reaction, which is responsible for the flavor and color change. Driven by the demand of improving quality retention of frozen fruits and vegetables. A series of innovation controlling strategies have also been development for control ice crystal nucleation and growth, including inhibition and control of nucleation, control of growth, exploitation of glassy state, bacterial ice nucleation (Silva et al., 2008).
Air-tight package is recommended for frozen fruits and vegetables to avoid irreversible moisture migration and a series of deterioration related to moisture migration
(Silva et al., 2008).
2.3.2 Quality of frozen fruits and vegetables.
Ideally, the thawed products should be comparable to fresh ones, considering the reduced quality loss compared with other processing (Nesvadba, 2008). However, the fruits and vegetables with fragile structures and high moisture content may suffer from structure damage during freezing and thawing. The moisture loss is another significant quality loss for frozen fruits and vegetables during storage. However, for the products
30 need to be cooked, the nutrition level could be paid more attention by the consumers than the appearance and texture.
Bouzari, Holstege and Barrett (2015) studied the impact of refrigerated and frozen storage on the mineral, fiber and total phenolic retention in two fruits and six vegetables and discovered that there was no significant difference between majority fresh
(refrigerated for 10 days) and frozen (blanched, frozen and stored for 90 days) products for all interested attributes. The results indicated that the freezing processing and frozen storage could be an alternative method for fresh consumption in culinary practice or home consumption (Bouzari, et al., 2015).
The effect of storage temperature and time period on the chemical composition and organoleptic quality of frozen tomato cubes has also been investigated. Micra RS tomatoes with a proper sugars to acids ratio, were selected for freezing in the form of slices or cubes (Lisiewska and Kmiecik 2000). After sorting, washing, drying and cutting, the tomato dices were frozen in Feutron blast freezer at −40°C. The products frozen to
−20 and −30°C at the center of the cube were stored for a period of 12 months (Lisiewska
& Kmiecik, 2000).The results showed that, even after 6-months storage at −20°C and after 12 months at −30°C the tomatoes were still good for salad. Compared with the raw material, 26% vitamin C, 7%carotenoids, 7% beta-carotene and 15% lycopene were lost right after freezing. After 12- month storage at −30°C, 26% lycopene were degraded
(Lisiewska and Kmiecik 2000). Lycopene is a significant nutrient in the tomato, but the effect of the freezing process on the lycopene has not well investigated (Rickman, Bruhn,
& Barrett, 2007).
31 Materials and Methods
3.1 Sample Preparation
3.1.1 Tomato samples
Fresh Roma tomatoes were obtained from the local market in Columbus, OH.` , and stored at ambient temperature and used as soon as possible. The tomatoes were inspected by visual observation based on the “Color classification requirements in tomatoes-United States Standards for Grades of Fresh Tomatoes” (Figure 3.1). Only the tomatoes at the same ripeness stage – “Red”, which is described as “more than 90 percent of the surface in the aggregate shows red color”, were used in this investigation.
Figure 3.1 Color classification requirements in tomatoes.
32 The tomatoes were handled carefully and washed using tap water. After gentle water removal with paper towel, the tomatoes were sliced with electric slicer (Chef’s
Choice® -615, Edgecraft Co., Avondale, CA) to desired thickness at 4.5mm. Each slice which was processed further was paired comparison with fresh tomato slice from the same fresh tomato for the evaluation of quality attributes.
The fresh tomato slices were either dried in the oven for moisture content measurement or tested in the texture analyzer machine for texture profile analysis immediately after slicing.
The tomato slices were immersed in the calcium chloride solutions at different concentrations in ambient temperature for 3.5 minutes. After the geometry center temperature reach -18℃, the frozen tomato slices were pulled out immediately from the freezing media or equipment and stored in the commercial refrigerator at -20 ℃ for 12 hours as temperature equilibrium before drip loss and texture analysis.
3.1.2 Preparation of calcium chloride solution
Calcium chloride solutions was prepared from food grade calcium chloride dehydrate crystal. When the solutions prepared, the calcium chloride dehydrate was weighed using analytical and added into the deionized water to prepare solutions with concentration of 0, 0.4, 0.8, 1.2, …, 3.6, 4.0 g/ 100g deionized water. A magnetic stirrer was used to help with the dissolve of the calcium chloride in the water.
3.2 Sliced tomato calcification
The tomato slice was immersed in 200 ml of 0, 0.4, 0.8, 1.2, …, 3.6, 4.0 % (w/w) calcium chloride solutions in a 500 mL beaker for 3.5 minutes (Rao & Barringer, 2005;
Rao & Barringer, 2006; Floros, Ekanayake, Abide, & Nelson, 1992) at room temperature
33 (~25 ºC ) and drained in a funnel for 1 minutes to remove excessive solution on the tomato surface prior to freezing.
3.3 Freezing
After the pretreatment, the tomato slices were frozen in the liquid nitrogen, dry- carbon dioxide cooled ethanol bath and air blast freezer to achieve a range of characteristic freezing time; the time required for temperature to decrease from the initial freezing temperature to the temperature when 80% of the water is frozen. Freezing rates were presented as “time to freeze”, which was defined as the time used to reduce the geometric center temperature of the slices from room temperature to -18℃. A grounded copper-constantan T-type thermal couple was put through the radius direction into the center of the tomato slice and the time-temperature history of the tomato at the center was recorded using a Multilogger Thermometer (HHH506RA, Omega Engineering Inc.,
Stamford, CT) for further analysis to obtain time to freeze.
For the cryogenic freezing, the tomato slice was put into a zip lock bag individually to prevent direct contact with the ethanol medium, and the air was out of the bag to improve the heat transfer between the ethanol bath and the tomato slice.
The tomato slices were frozen in a dry-ice cooled ethanol bath ((Decon
Laboratories, Inc., King of Prussia, PA) in a 4300-mL capacity cylindrical wide mouth cryogenic storage Dewar (#8642, Pope Scientific, Inc., Saukville, WI) with mesh protective covering. The ethanol bath was cooled to different temperature to generate a range of time to freeze and monitored with thermometer. The fluctuation of the bath temperature was controlled with 1 ºC. Adhesive tapes was used to fix the wire of the
34 thermocouple in the same place. The bag with tomato slice was put in the same depth of the bath to minimize the effect of nuisance factors on the heat transfer. The time increment of temperature logging for both tomato slices and bath was set to be 1 second.
3.4 Storage (Temperature equilibrium)
After the geometry center temperature reach -18℃, the frozen tomato slices were pulled out immediately from the freezing media or equipment and stored in the commercial refrigerator at -20 ℃ for 12 hours as temperature equilibrium before mass loss and texture analysis.
3.5 Thawing
Individual frozen sample will be thawed in the room temperature in the weigh boat for around 90 minutes until the temperature at the center increased to 20 ºC. During thawing, the tomato slice was put on a pizza spacer to allow dripping of the extra moisture content from the bottom surface of the tomato slice. The thermal couple was inserted in the geometry center of the tomato slice to monitor the temperature increase.
The weigh boat containing the tomato slice was covered by another weight boat to form a cell to avoid ambient environment influence during the thawing process, such as uncontrolled air flow.
35 3.6 Moisture content and mass change determination
3.6.1 Moisture content measurement for fresh tomato slice
Moisture content measurement was conducted on both fresh and frozen-thawed tomato slices. The original weight of each fresh tomato slice was determined immediately after slicing using analytical balance and recorded as for further interpretation, regardless of the further treatment of the slice. The same fresh tomato slice was dried in the oven (40 GC, Quincy Lab Inc., Chicago, IL) for 5 hours at 105 (±1) ºC. After drying and temperature equilibrium to room temperature, the dry matter of the tomato slice was determined using the same analytical balance and recorded as .
3.6.2 Mass change measurement for calcium treated tomato slice
For the frozen tomato slices with calcium treatment, additional weight determinations were conducted. The original weight of the tomato slice was determined using the same method mentioned above. After slicing, the group of the “calcium pretreated tomato slice” were immersed and drained in the funnel for 1 minutes. Then the weight of the tomato slice was determined and recorded as .
3.6.3 Mass loss measurement after thawing
After the temperature equilibration in the refrigerator at temperature fluctuating around – 20 ℃, the samples were thawed in the room temperature as mentioned before until the temperature at the center increased to 20℃. The weigh boat containing the tomato slice was covered by another weight boat to form a cell to avoid ambient environment influence on the thawing process. The weight of the two empty weigh boats and the weight of the cell with tomato slice inside were recorded to calculate the initial weight ( ) of the frozen tomato slice for reference. After thawing, the tomato slices 36 were handled carefully and wiped with Kim wipes cautiously to avoid undesired human error. Then the weight after thawing was determined and recorded as . The original weight of the tomato slice before thawing was measured as .
The mass loss (%) is calculated as:
− (% ) = ∗ 100% ∗ (%)
3.6.4 Dry matter determination for frozen tomato slices
The same thawed tomato slice was dried in the oven for 5 hours at 105 (±1) ºC.
After drying and temperature equilibrium to room temperature, the dry matter of the tomato slice was determined using the same analytical balance and recorded as .
3.7 Texture profile analysis method
After temperature equilibrium for 24 hours and thawing, the hardness retention of thawed tomato slices was determined using a TA-XT2 texture analyzer (Stable Micro
System, Godalming, Surrey, UK) equipped with a 50-kg load cell at room temperature.
The two-cycle puncture test was used here and the peak force of the first circle was recorded as the hardness of the sample. A 50-kg load cell was used for cell calibration and a 2-mm-diameter flat-tipped cylindrical probe was used for measurement of mesoarp in the direction of parallel to the main axis of the tomato. The sample was punctured to reach 35% strain of the initial height using a at a test speed of 0.3mm/s. Three different locations (same botanical parts) of the tomato slices will be tested to get 3 measurements
37 for each slice. Due to the high variety of the hardness of the fresh tomatoes, a systematic sampling design is used here to minimize the interference of the hardness difference of the raw material. Three slices from the same tomato were measured as replicates for each processing condition. The test parameters for texture are listed in Table 3.1.
Table 3.1 Texture analyzer setting for puncture test of tomato slices
Pre-test speed Test speed Post-test speed Distance (strain)
2.0 mm/s 1.0 mm/s 2.0 mm/s 35% strain
3.8 Initial Freezing Point Prediction
The prediction of the initial freezing point of tomato slice is based on the composition of the composition table of tomato in Food Composition and Nutrition Tables (7th Edition) (Souci, Fachmann, & Kraut, 2008). The composition contents used to predict the initial freezing point of the tomato slice are listed in
38 Table 3.2. The prediction model is based on the following assumptions
(Boonsupthip and Heldman, 2007):
a) Only the components that contributing more than 50 µmol/100g will have
significant influence on the initial freezing point
b) Lipids will not affect the initial freezing point significantly
c) The molecular weight of the proteins, carbohydrates and lipids are considered
as 50 kDa.
39 Table 3.2 Molarity, average content and molecular weight of critical components of tomato for initial freezing point prediction.
Food Component Quantity Average MW Molality Content (µmol/100g) Water g 94.2 18.02 / Proteins g 0.95 50k / Carbohydrates g 2.60 50k / Lipids g 0.21 / / Ashes g 0.61 / / Fibers g 0.95 50k / Minerals and Element Sodium, Na mg 3.3 22.99 143.54 Magnesium, Mg mg 11 24.31 452.49 Phosphate, P mg 22 30.97 710.36 Chloride, Cl mg 30 35.45 846.26 Potassium, K mg 235 39.10 6010.23 Calcium, Ca mg 8.9 40.08 222.06 Nitrate mg 5.0 62.00 80.65 Carbohydrates Monosaccharide mg 2450 180.07 13605.82 Disaccharide mg 84 342.11 245.54 Acid & Base Lactic acid mg 60 90.08 666.07 Malic acid mg 51 134.10 380.31 Ascorbic acid mg 19 176.10 107.89 Citric and Isocitric acid mg 325 192.10 1691.83 Acetic Acid mg 8 60.25 132.78 Oxalo acetic acid mg 24 132.07 181.72
The average experimental initial freezing point of tomatoes was -0.89 ±0.10 C, which was significantly different from the predicted value (p < 0.05). The equation used by Boonsupthip and Heldman (2007) is listed as below:
Equation 3.1 Prediction of initial freezing temperature.
( − ) 1 1 = − ln [ ] − + ∑ 40 Where is the melting point of the pure solvent which is 273.15K; R is the ideal gas constant (8.314 KJ /Kg*mol*K); is molecular weight of water; is the mass enthalpy change for fusion or latent heat of fusion for water (336. 64KJ/Kg); is the water molecular weight; component I are the dry solid components which will affect the freezing temperature significantly, including minerals and element, Mono- and di- saccharides, acid and base; can be calculates using the following equation:
Equation 3.2 Equation for calculation.
= ∗ ∗
Where is is an experimental coefficient for specific types of food (0.18 to 0.25 for vegetable; is the total mass fraction of proteins, carbohydrates, lipids, ashes, and fibers; is the total mass of the food (100g).
3.9 Calculation of ice mass fraction
The mass fraction of ice in the tomato slice was calculated based on Tchigeov’s empirical relationship (Fricke & Becker, 2001):
Equation 3.3 Tchigeov’s empirical relationship for ice mass fraction prediction.
1.105 = 0.7183 [1 + ] ln − +1
Where is the mass fraction of ice, is the initial freezing point of food, t is
the temperature of the food, is the initial water content of the food.
41 3.10 Statistical Analysis
The quality attributes data of frozen tomato slices and the fresh tomatoes from the same batch of tomatoes will be compared by paired t-Student test (O’Mahony 1986). The influence of “time-to-freeze” and different pretreatments on the quality changes of frozen tomato slices will be evaluated by comparison with fresh ones using One-Way ANOVA and the least significant difference (LSD) test with a 95% confidence interval (p≤0.05) using SPSS.22 (IBM, Inc,. USA) statistics software.
42 Results and Discussion
The objective of this investigation was to determine the effect of freezing processing conditions, including “time-to-freeze” and calcium chloride pretreatment, on the texture and mass retention of frozen and thawed tomato slices. The different “time-to- freeze” were achieved by modifying the freezing medium or freezing methods. For the calcium treatment, the solutes concentration was the parameter to be studied. The interaction between “time to freeze” and calcium treatment was also studied. For slow freezing (~1800 seconds), the tomato slice was frozen in a manual defrost freezer. For fast freezing (~ 45 seconds) to explore the effect of the calcium treatment on the quality retention of the frozen tomato sample, the tomato slice was frozen in a dry-ice cooled ethanol bath at -40 ± 1 ºC.
43 4.1 The effect of time to freeze on the texture and water retention of frozen tomatoes without treatment 30 25 20 15 10 5 0 -5
Temperature(ºC) -10 -15 -20 -25 0 20 40 60 80 100 120 Time (s)
Figure 4.1 An example of temperature history at the center of a tomato slice when frozen in ethanol bath at -25 ºC.
Figure 4.1 shows the temperature history of a tomato slice in the ethanol bath at -
25 ºC decreasing from room temperature (around 25 ºC) to target -18ºC at the center. The time increment for the temperature monitor was set as 1 second in this relatively fast freezing. The center temperature reached sub-zero in 20 seconds. Then the super cooling was observed until the first ice crystal form at the initial freezing point. The super cooling stage might not be monitored in fast freezing as the temperature decreased rapidly. As the latent heat was removed gradually, the removal of sensible heat started and the temperature decrease in a much greater rate. The total “time to freeze” was 110 seconds, which could be expressed as 23.4 ºC decrease per minute.
44 The initial freezing point of the tomato slice shown in Figure 4.1 was around -0.9
ºC, which is close to but lower than the predicted value of -0.65 ºC reported by
Boonsupthip and Heldman (2007). The average experimental initial freezing point of tomatoes was -0.89 ±0.10 C, which was significantly different from the predicted value
(p < 0.05) reported by Boonsupthip and Heldman (2007).
Since the initial freezing point model was based on concentration and molecular weight of the product composition, the difference between the experimental and predicted initial freezing point could result from the variety in food composition between the tomatoes in the table and the tomatoes used in measurement of initial freezing point
(Boonsupthip and Heldman, 2007). Another possible reason was the potential systematic error during the experimental measurement for the inhomogeneous structure of tomato.
The comparison between the experimental initial freezing point and the value from the literature indicating the sufficient of the temperature monitor in this investigation.
The initial moisture content of the tomatoes used in this investigation was measured as 94.52± 0.03 %. According to Equation 3.3, the mass fraction of ice in the tomato slices was calculated as 83.79% when the temperature of the tomato slices decreased to -18 C during freezing.
To explore the effect of “time-to-freeze” on the hardness retention of the tomato slices after freezing and thawing, the tomato slices were frozen fresh without any pre- treatment. Figure 4.2 shows the hardness retention of the thawed tomato slices frozen at different freezing rates. The freezing time varied from 40 seconds to 1800 seconds. A range of “time-to-freeze” were generated by modifying the temperature of freezing media or freezing method. The freezing rates here is defined as “time to freeze”, which is the 45 time for the temperature at the geometry center of the tomato slice to drop from room temperature (~ 25 ºC) to target frozen storage temperature (-18 ºC). In this study, the hardness retention of thawed tomato slices without treatment varied around 10 % to 50% compared with the fresh samples at different “time-to-freeeze” (from -1.43 ºC to 64.5 ºC decrease per minute). In cryogenic freezing, reducing the “time-to-freeze” from 320 seconds to less than 40 seconds does not improve the texture retention significantly. In extremely quick freezing (liquid nitrogen freezing), the temperature difference between the out pericarp of the tomato slice and the inner structure even caused crack of the tomato slice and destruction of tomato structure to make the thawed sample softer (~20% hardness retention). Statistical analysis showed no significant correlation between the hardness retention and “time-to freeze” Similar results were reported by Gradziel that
80% firmness loss was detected for thawed tomatoes after freezing and frozen storage at -
10 ºC and at -40 ºC for 5 days.
46 30 a 25
20
15 b 10 Hardness retention (%)Hardness 5
0 45 (Rapid) 1800 (Slow) Time to freeze (second)
Figure 4.2 Hardness retention of frozen tomato slices without pretreatment at rapid freezing (45-second “time-to-freeze”) and slow freezing (1800-second “time-to-freeze”). a, b; letters indicate statistically significant difference (P < 0.05).
As shown in Figure 4.2, when the tomato slices were frozen at the “time to freeze” around 45 seconds (approximately 57.3 C decrease per minute), the hardness retention of the frozen tomato samples was significantly increased than the ones from the slow freezing (p <0.05). However, in more rapid freezing, the damage to the tomato structure was not better in slower freezing. Mohr and Stein (1969) reported similar results that the tomato samples frozen at freezing rate of 10 to 100 C decrease per minute were more stable than others.
The statistical analysis which indicated that the effect of the “time-to-freeze” on the hardness retention was statistically different. However, the hardness retention of the frozen tomato slices from the rapid freezing method was 25.24%, which indicating the
47 effect of “time to freeze” was not practically significant because the hardness retention was still not acceptable in rapid freezing.
As shown in Figure 4.2, the advantage of the fast freezing over the slow freezing was not practically significant regarding the hardness retention. This relatively low hardness retention for both fast freezing and slow freezing indicated the structure damage was dramatic in both freezing method. The damage could be the result of the high moisture content and fragile cell structure. Turgor pressure of a plant cell is the pressure from cell content inside the plant cell pushing against the cell wall. Turgor of the tomato slice is a major factor contributing to the expected texture profile when the tomato products are consumed raw. Turgor is closely related to the function of the semi- permeability of the tomato cell (Brown, 1977). When the water transfers from the fruit cell into extracellular, the fruit cell could lose the turgidity. From the texture profile perspective, the loss of turgidity will lead to loss of crispness and rigidity, which will affect the “freshness” perception. As mentioned before, the fragile tomato cells was less resistant to the volume expansion of the ice crystals. The ice crystals formed and expanded both intercellular and extracellular, which will break the cell structure, therefore made the cell wall damaged and lose semi-permeability. The tomato cell shrink away from the cell wall and the texture became soft. Furthermore, the freezing cell destruction that the cell prone to was irreversible (Sun & Li, 2003) and the cell content would leak out during thawing. Significant changes in moisture content and total weight might be observed. The loss of turgidity was closely related to the water loss.
Another reason could be the more complex inner structures of tomato fruits, which made the heat transfer during freezing and thawing less efficient. The insufficient
48 heat transfer could cause undesired ice recrystallization and water migration. Fast freezing combined with fast thawing was reported as the most proper way to keep the texture of the frozen fruits. The thawing method here could be another factor which influence the texture retention of the thawed tomato slice.
In early frozen fruits and vegetable industry, the frozen fruits and vegetables were prepared for further processing like baking or ice cream, for which, texture was not a key quality attribute for quality control (Brown, 1977). The hardness retention here was calculated based on the hardness of the fresh ones. When the frozen products are consumed raw, the consumers will compare the quality characteristics to the fresh ones, instead of the processed ones, like canned or dried. Therefore, the “desired hardness” of the frozen tomato slice ready for direct consumption should be close to the hardness of the tomatoes when consumed raw. As shown in Figure 4.2, the hardness retention of the frozen tomatoes after thawing was relatively low (20%~50%) and not comparable to fresh samples even at the optimal “time-to-freeze”.
From Figure 4.2, a conclusion could be drawn that optimization of “time-to- freeze” was not adequate to maintain the acceptable hardness retention of the frozen tomato slices. Therefore, it is necessary to figure out a potential method to enhance the hardness of the frozen tomato slice in this project.
4.2 The mass change of the fresh cut tomato slices during calcium pretreatment
To explore the correlation between the mass loss and the texture retention of the frozen tomato slice, the effect of the calcium treatment on the mass change of the fresh tomato samples was determined. Calcium chloride solutions of certain concentrations are
49 selected to explore the mass change during the calcification process. The tomato slice pretreated in the de-ionized water is the control sample. Since the hardness retention of the frozen/thaw tomato slices were significantly improved by increasing the concentration of calcium chloride solution from 2% to 2.4%, calcium chloride at the concentration of 1.2%, 2.0%, 2.4% and 3.6% were used to capture the possible significant change. All the mass change was present as related to the original moisture content of the fresh tomato slice. The tomato slices were treated with calcium solutions for 3.5 minutes.
Figure 4.3 illustrated the mass change of the tomato slices immediately after the calcium treatment. The mass change showed that the concentration of the calcium chloride solutions had significant influence on the mass change of the tomato slices.
More specifically, as the concentration of the calcium chloride increased, the mass change of the tomato slices switched from positive to negative. When the concentration of the calcium chloride was around 2.0%, the mass of the tomato slice was kept almost the same as original.
50 5 4 3 2 1 0 Mass change -1
(% Original water content) waterOriginal(% -2 -3 0 1.2 2.4 3.6
CaCl2 % (g/100g)
Figure 4.3 The mass change after pretreated with calcium chloride solutions at different concentration for 3.5 minutes.
Osmosis mass transfer happened during the calcium pretreatment. Osmosis is a transport phenomenon involving the movement of certain solutes through a permeable membrane from a region of higher solute concentration to a region of a lower solution concentration. Simultaneously, the water will flow from a region of lower solute concentration to the higher one, which is called osmosis dehydration and could cause weight loss and shrinkage of the product. The driving force is the difference in chemical potential across a semi-permeable membrane. Osmotic dehydration, which allows loss of water content and gaining solid content, is a technology that been used in industry to concentrate fruits or vegetables (Torreggiani, 1992). In this study, when the tomato slices were immersed in the de-ionized water, the water flowed from the water into the tomato slice matrix. On the contrary, when the tomato slice was immersed in the calcium
51 solutions, the water transferred from the tomato matrix into the calcium chloride solutions; meanwhile calcium uptake into the tomato matrix happened Generally, both water loss and solid content uptake depended on the operating conditions, cellular tissue type and raw material conditions (i.e. with or without certain pretreatment. From the mass transfer perspective, the mass transfer rate has a positive correlation with the contact surface between the food and the pretreatment solution, treatment temperature and concentration gradient (Spiazzi & Mascheroni, 1997). Since the tomatoes with similar size and only the centered three pieces of the single tomato were used, the contact surface was not a significant factor affecting the mass transfer in this study. As all the samples were pretreated under room temperature for 210 seconds, the concentration gradient was the major factor affecting the mass transfer rate between the food matrix and the environment, which was the fresh-sliced tomato and calcium chloride solutions in this study. The living tomato cells allowed both mass transfer of water and solutes, which means the mass transfer here was complex. Raoult-Wack and Guilbert (1990) proposed a term to describe the phenomenon as “dewatering-impregnation soaking in concentrated solutions” instead of “osmotic dehydration”. To simply the model, water loss was represented for the all the dripped out cell content. Similarly, the results in Figure
4.3were combined both water loss and calcium uptake, which means, the ration of the two were what we observed here. Theoretically, water loss was favored by using highly concentrated solutes and solutes with higher molecular weight at the cost of solid intake
(Spiazzi & Mascheroni, 1997). When the tomato slice was treated with calcium of higher concentration, the dehydration of intracellular water and other content became weigh more than the calcium gain, then a significant weight loss was observed. Both the
52 dehydration and calcium intake could affect the texture of the tomatoes as mentioned above. Dehydration made the cell structure less fragile and less susceptible to break down during freezing. The moisture loss made the cell content concentrated, which lead to less ice crystals form intracellular in slow freezing (Bolin & Huxsoll, 1993) and less osmotic cell water migration from intracellular into extracellular in fast freezing (Mazur,
1984). However, the effect of the osmosis dehydration on the texture retention was not simply positive correlation. Bolin and Huxsoll (1993) also found similar results in their research of improving the freeze/thaw texture of pears by partial drying. The results in their study show that a minimum weight loss of 20 to 30% of original weight was necessary to improve the firmness of the frozen pear cuts after thawing and low degree of the osmotic dehydration may make the structure of the freeze/thaw pear cuts softer (Bolin
& Huxsoll, 1993).
4.3 The effect the calcium chloride pretreatment on the hardness of fresh
tomato slices without freezing
Calcium treatment has been widely used in fruit and vegetables processing to enhance the texture retention of thermal processed tomato products. Calcification has been proven to be helpful for texture improvement of tomato product by reducing the hardness loss during blanching or improving the firmness of final products like canned tomatoes. Based on the documented successful application, the effect of calcium pretreatment for frozen tomato slices was investigated in this study.
The preliminary results indicated that the calcium treatment could be a potential method to improve the hardness retention of frozen tomato samples, considering the wide
53 application and cost. To reveal the correlation between the calcium treatment and the original hardness of the tomato slice before freezing, 4.5mm tomato slices were immersed in the 0%, 0.4%, 0.8,1.2%, 1.6% 2.0%, 2.4%, 2.8%, 3.2% , 3.6%, 4.0% and
4.4% calcium chloride solutions for 3.5 minutes at ambient temperature. Since As mentioned before, the tomato tissue frozen at 10 to 100 C decrease per minute could be more stable and the hardness of the tomato slices were more acceptable at around 45- second “time-to-freeze”, all the tomato slices were frozen at the same freezing rates of
45-seconds “time to freeze” to obtain more stable and harder frozen tomato samples.
The hardness of the samples were measured using TPA after drained for 1 minute in the tunnel to remove excessive water on the sample surface.
140 130 120 110 100 90 80 70 60 50 40 30
Hardness Hardness retention (%) 20 10 0 0 0.4 1.2 2 2.8 3.6 4.4
CaCl2% (g/100g)
Figure 4.4 Hardness retention of tomato slice before freezing and after pretreated with different CaCl2 solutions.
54 Figure 4.4 showed that without freezing and thawing process, the effect of the calcium treatment on the hardness of the fresh tomato slices was not significant (p>0.05).
When the fresh tomatoes were treated with deionized water, the hardness even slightly decreased.
Luna-Guzman and Barret (2000) studied the effect of the calcium chloride and calcium lactate on the firmness of the fresh-cut cantaloupes during storage. The fresh-cut cantaloupes were immersed in the solutions of 2.5% calcium chloride or calcium lactate for 1 minute. No significant increase was observed for the firm fresh-cut cantaloupes with calcium treatment compared to the fresh slices initially. But the calcium treatment helped to maintain the texture throughout storage (Luna-Guzman & Barret, 2000). The results in this study were similar to Luna-Guzman and Barret’s (2000) results. No significant pattern was observed regarding the correlation between concentration of calcium and the hardness.
In higher calcium chloride solution, the calcium ion in the solutions was expected to diffuse from the outer pericarp of the tomato to the inner tissue because of osmic pressure during the pretreatment. According to National Nutrient Database, the year round average calcium content of the ripe tomatoes is 10 mg Ca per 100g fresh tomatoes.
For the 0.4% calcium chloride solution, the concentration of the calcium in the solution is
14.44 mg/100g solutions. Tomato slices treated with calcium chloride were expected to be subjected to osmotic dehydration and calcium uptake. Theoretically, the calcium uptake after immersion will increase as the concentration of the calcium chloride solutions increases. This could also be a possible explanation for the results shown in
Figure 4.4, regarding the effect of dehydration on the tomato cell structure and influence
55 of the calcium uptake on increase in the turgor pressure in the tomato cell and the formation of the pectin net, which could increase the hardness of the tomato slice. The decrease in the hardness for the sample could be due to the leakage of calcium and other cell contents to the de-ionized water and the moisture uptake.
Conway et al. treated apple with calcium by pressure infiltration and observed that the calcium concentration of the cell wall increased steadily from 0.055% to 0.190% as the concentration of the applied calcium chloride increased from 0% to 4% (Conway,
Gross, & Sams, 1987; Conway, Sams, McGuire, & Kelman, 1992). However, the calcium uptake was not observed as proportional to the concentration of calcium chloride
(Conway et al., 1992). Conway proposed that might be a result of the limitation of the available biding site of the pectin for the calcium ion. Although the calcium content analysis was not measured in this study, the calcium uptake could be reflected by the hardness of the tomato slice. However, the correlation between the calcium uptake and hardness retention of the frozen tomato slices may not be revealed until the texture of the frozen tomato slices was analyzed after thawing.
In a word, the calcium treatment of the tomato slices did not significantly decrease or increase the hardness of the tomato slices by keeping the hardness at least
90% before freezing, indicating the calcification is a potential pretreatment worthy of exploring.
56 4.4 The effect of the calcium treatment of the mass loss of the tomato slices after
freezing and thawing
Experiments were also conducted to discover the influence of the calcium ion on the mass change of the frozen tomato slices. The tomato slices were immersed in the
0.4%, 0.8%, 1.2%, 1.6%, 2.0%, 2.4%, 2.8%, 3.2%, 3.6% and 4% calcium chloride solutions before freezing in the dry-carbon dioxide cooled ethanol bath at the same freezing rate (45-second freezing time). After thawing in the weigh boat in the ambient environment, the thawing mass change of the thawed tomato slice was determined by measuring the weight of the frozen tomato slice before and after thawing. The tomato slices without calcium pretreatment was frozen as controlled sample. The same batch of frozen tomato slices were used for both hardness retention and mass change measurement to reveal possible correlation between mass loss and hardness retention.
57 14 13 b b b b b 12 11 10 9 8 7 6 a 5 a a a a a 4 3 (% initial content) waterinitial (% Mass loss duringthawing Mass loss 2 1 0 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0
CaCl2 (g/100g)
Figure 4.5 Mass loss during thawing of tomato slices frozen at same 45-second “time-to- freeze” after pretreated with different CaCl2 solutions. a, b; letters indicate statistically significant difference (P < 0.05).
Compared to the original weight of the tomato slices, the mass change increased from approximately 2.19 % to approximately 11.50% as the concentration of the calcium solutions increased.
The thawing mass loss presented in Figure 4.5 was determined by calculating the difference in the mass of the pre-thawing and post-thawing tomato samples and expressed as the percentage of the initial moisture content of the tomato (94.52± 0.03 %). As mentioned before, moisture migration could happen during the storage. The ice crystals formed in the surface of the product during storage and a whitening appearance was observed when the frozen tomato samples were pulled out of the storage package. The moisture loss from the tomato slice sample to the storage package could happen and were not captured when the thawing mass loss was measured. Considering the short storage period of storage time and the same storage conditions for all the frozen tomato samples, 58 the differences in moisture loss to the storage package among the different tomato slice samples was considered negligible.
4.5 The effect of the calcium treatment of the hardness retention of the tomato
slices after freezing and thawing
Figure 4.6 illustrated the hardness retention of the frozen tomato slices with calcium pretreatment. The tomato slices were immersed in the 0%, 0.4%, 0.8%, 1.2%,
1.6%, 2.0%, 2.4%, 2.8%, 3.2%, 3.6% and 4% calcium chloride solutions before freezing in the dry-carbon dioxide cooled ethanol bath at the same freezing rate (~45 seconds of time to freeze). The hardness was measured after mass loss measurement. The hardness retention was defined as the ration of the hardness of the processed tomato slices to the hardness of the fresh ones. To minimize the influence of the original hardness variety among the tomatoes on the analysis, systematic sampling method was used here, which means, the hardness analysis and mass analysis were measured for the tomato slices from the same single tomato.
59 140 a 130 120 110 a 100 b ab 90 abc 80 bcd 70 60 cd bcd cd e 50 d 40 30 Hardness Hardness retention (%) 20 10 0 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 CaCl2 concentration (g/100g)
Figure 4.6 Hardness retention of tomato slices frozen at same freezing rate after pretreated with different CaCl2 solutions. a, b, c, d, e; letters indicate statistically significant difference (P < 0.05).
Compared with the hardness retention of the tomato slices frozen at the same
“time to freeze” without calcium pretreatment (see Figure 4.2), the calcium chloride helped to improve the hardness retention from 28.7 % to 48.9 % even after thawing.
Calcium chloride solutions with concentration ranging from 2.0% to 4.0% improved the hardness retention from 48.6% to 100% (Figure 4.6). Compared with the control samples, the effect of the high concentration calcium pretreatment on the hardness retention of frozen-thawed tomato slices is significant (p<0.05).
The maximum hardness retention was observed when the maximum mass change was retained (shown in Figure 4.5 and Figure 4.6). The mass change varied from 2.19% to 11.50%. A potential positive correlation between the hardness retention and mass change was observed. The compositions of the solution dripped out from the tomato slice 60 was complex, since the cell contents dripped out because of the function loss of the cell wall during freezing and thawing. However, water could still be the major component of the dripped juice. So the mass change could be a sufficient indicator of the correlation of between the moisture loss and the calcium treatment. When the water content dripped out from the tomato cell and the tomato cell shrinked from the cell wall, a hardness retention was captured by the TPA hardness retention. However, this increase in hardness retention could be different from the texture change evaluated in sensory test, which might be explained by other texture attributes.
On the other hand, calcium pretreatment was the other significant factor affecting the hardness retention. As shown in Figure 4.4, the calcium treatment did not improve the hardness of the fresh tomato slices without freezing. The reason for this observation could be that the initial hardness of the tomato was high enough, the cell structure was well maintained and the effect of the calcium ion in integrity tomato structure was limited. However, once the freezing caused damage to the tomato cell wall, the effect of the calcium could be observed by increasing the hardness retention. HGA as a high-ester pectin was cross linked with calcium to form calcium gels by removal of methyl ester groups to maintain the integrity of the pectin network. The calcium pretreatment provided the tomato matrix with more available calcium ion and enhanced the formation of the pectin network, which made the tomato cell more resistant to freezing damage and improved the hardness of the products after thawing. There was also a possibility that the more mass loss resulting from pectin net was formed because of more calcium up take in the higher concentration of calcium chloride, which lowered the binding capacity of
61 water in the food matrix. The calcium pretreatment could have affected the pH value of the food matrix, which also affect the water holding in the food matrix.
Comparing the result from Figure 4.5 and Figure 4.6, a significant difference between the quality attributes of the tomato slices pretreated using 2.0 % and 2.4% calcium chloride was observed, probably resulting from that the critical calcium concentration for pectin network formation was achieved after immersing the tomato slices in higher calcium chloride. There is also a possibility that the effect of the calcium treatment with low calcium concentration was limited and the improvement of texture retention was not detectable. However, when the effect of increasing calcium uptake and more moisture loss was combined, a detectable improvement was captured by the TPA analysis. Therefore, if the increment of the calcium concentration is smaller, it is probably that a gradual increase in hardness retention can be detected by TPA analysis.
Since the calcium uptake should depend on three factors: treatment temperature, immersion time and calcium concentration in the treatment solutions. In this study, the treatment temperature and immersion time were controlled as same for all samples.
Floros et al. (1992) studied the calcification of diced tomatoes and discover that the calcium uptake into the tomato dices increased as the contacting time increases but not significantly changed due to temperature change. Floros et al. (1992) also discovered that when the tomato dices were immersed in the 0.43 % calcium chloride solutions at ambient temperature (35 C) for 3.5 minutes, the maximum calcium uptake was achieved and total calcium content was below the maximum legal limit (800 µg/ g). Therefore, the immersion time of 3.5 minutes should be sufficient to discover the effect of the calcification on the tomato slice structure.
62 Ideally, in the higher concentration of calcium chloride, the tomato slices were supposed to absorb more calcium ion and the tomato slices pre-treated with the solutions with higher calcium concentration should have got higher hardness retention as well.
However, the effectiveness of the calcium on improving the hardness of the frozen tomato slices after thawing was not significantly improved when the concentration of the calcium chloride solutions increased (p>0.05). This result could be a result of the limitation of the calcium uptake due to the number of the accessible binding sites as the calcium could enhance the stability of the cell wall by interacting with the pectin matrix.
Furthermore, the immersion time of the tomato slices were all 3.5 minutes. The immersion time may not be sufficient for the diffusion of the calcium into the inner site of the pectin or diffusion into inner structure of the tomato slices, which may be another explanation of the limited effectiveness of the calcium ion. But the hardness retention of the frozen-thawed tomatoes was comparable to the hardness of fresh ones after the calcification. So the calcification processing here was considered as sufficient.
Based on the quality and mass retention of the frozen-thawed tomato slices, the tomato slices treated with 2.4% calcium chloride solution can provide with the hardness retention comparable to the fresh ones and more acceptable mass retention.
63 Conclusions & Future Work
5.1 Conclusion
The results of this investigation indicated that decreasing “time to freeze” cannot be used to preserve the physical structure of tomatoes, as indicated by TPA hardness measurements. Reducing the time-to-freeze by using cryogenic refrigerants is not a sufficient method to freeze tomato slices.
The use of calcium chloride as a pre-treatment before freezing provides an opportunity for retention of physical structure. By immersion of tomato slices in a 2.4% calcium chloride solution, the retention of TPA Hardness could be improved from less than 50% to nearly 100%, when compared to unfrozen tomato slices
The application of the calcium chloride treatment causes changes in water content of the tomato slices depending on concentration of calcium chloride. As the concentration increased, the amount of water retained in the tomato decreased. Although the maximum hardness retention occurred after a 2.4% calcium chloride pretreatment, the same pretreatment resulted in nearly 10% loss of water. The calcium chloride pretreatment of
2.4% seemed to provide the benefits of reduced structural change, while maintaining loss of water at below 10%.
64 In summary, the maximum retention of TPA Hardness in tomato slices was obtained with a time-to-freeze of 45 sec, a calcium chloride pretreatment of 2.4 % for 3.5 min, and with less than 10% water loss.
5.2 Future work
The effect of calcium treatment on the hardness retention is significant and critical in this study. The actual solid uptake during pretreatment need further analysis to reveal the direct correlation of calcium uptake and the hardness of the frozen-thawed tomatoes.
Microscopy can be applied to observe fine structure change during freezing and thawing and reveal the correlation of the calcium pretreatment and the tomato cell structure.
Color, another important quality attribute which will affect the consumer behavior, need to be studied under different freezing and pretreatment conditions.
From the quality prospective, nutritional level determination for frozen tomatoes under different processing conditions and different storage time can be conducted to test the nutrition degradation during freezing and storage. Shelf-life studies of the frozen tomato slices under different storage conditions on microbial load and quality retentions is critical for further commercialization. Sensory testing need to be conducted to compare the quality attributes of frozen tomato slices to other marketed processed and fresh tomato products. Since the calcium pretreatment may lead to the bitterness of the product, the sensory testing is also necessary for reference of optimization of the calcium pretreatment regrading of the taste of the final products.
65 References
Agarwal, S., & Rao, A. V. (January 01, 2000). Tomato lycopene and its role in human health and chronic diseases. Cmaj: Canadian Medical Association Journal = Journal De L'association Medicale Canadienne, 163 (6), 739-744.
Ali, Z. M., & Brady, C. J. (1982). Purification and characterization of the polygalacturonases of tomato fruits. Functional Plant Biology, 9(2), 155-169. doi:10.1071/PP9820155
Alonso, J., Canet, W., & Rodriguez, T. (1997). Thermal and calcium pretreatment affects texture, pectinesterase and pectic substances of frozen sweet cherries. Journal of Food Science, 62(3), 511-515. DOI: 10.1111/j.1365-2621.1997.tb04418.x
Anthon, G. E., Blot, L., & Barrett, D. M. (2005). Improved firmness in calcified diced tomatoes by temperature activation of pectin methylesterase.Journal of Food Science, 70(5), C342-C347. DOI: 10.1111/j.1365-2621.2005.tb09964.x
Askari, G. R., Emam‐Djomeh, Z. A. H. R. A., & Tahmasbi, M. (2009). Effect of various drying methods on texture and color of tomato halves. Journal of Texture studies, 40(4), 371-389. DOI: 10.1111/j.1745-4603.2009.00187.x
Barrett, D. M., Beaulieu, J. C., & Shewfelt, R. (2010). Color, flavor, texture, and nutritional quality of fresh-cut fruits and vegetables: desirable levels, instrumental and sensory measurement, and the effects of processing. Critical reviews in food science and nutrition, 50(5), 369-389. Doi: 10.1080/10408391003626322
Berk, Z. (2009). Food process engineering and technology (pp 391-411). Amsterdam: Academic.
Berry, M., Fletcher, J., McClure, P., Wilkinson, J. (2008). Effects of freezing on Nutritional and Microbial Properties of food. In: J. A. Evans. Frozen Food Science and Technology (pp 26-50). Oxford, UK: Blackwell Publishing Ltd.
Biacs, P., & Wissgott, U. (January 01, 1997). Investigation of colour changes of some tomato products during frozen storage. Food / Nahrung, 41, 5, 306-310. DOI: 10.1002/food.19970410512
Bolin, H. R., & Huxsoll, C. C. (1993). Partial drying of cut pears to improve freeze/thaw texture. Journal of Food Science, 58(2), 357-360. DOI: 10.1111/j.1365- 2621.1993.tb04274.x
66 Boonsumrej, S., Chaiwanichsiri, S., Tantratian, S., Suzuki, T., & Takai, R. (2007). Effects of freezing and thawing on the quality changes of tiger shrimp (Penaeus monodon) frozen by air-blast and cryogenic freezing. Journal of Food Engineering, 80(1), 292-299.
Boonsupthip, W., & Heldman, D. R. (2007). Prediction of frozen food properties during freezing using product composition. Journal of food science, 72(5), E254-E263.
Bouzari, A., Holstege, D., & Barrett, D. M. (2015). Mineral, fiber, and total phenolic retention in eight fruits and vegetables: A comparison of refrigerated and frozen storage. Journal of agricultural and food chemistry, 63(3), 951-956. DOI: 10.1021/jf504890k
Brown, M. S. (1977). Texture of frozen fruits and vegetables. Journal of Texture Studies, 7(4), 391-404. DOI: 10.1111/j.1745-4603.1977.tb01147.x
Buttery, R. G., Teranishi, R., Ling, L. C., & Turnbaugh, J. G. (1990). Quantitative and sensory studies on tomato paste volatiles. Journal of Agricultural and Food Chemistry, 38(1), 336-340. DOI: 10.1021/jf00091a074
Caffall, K. H., & Mohnen, D. (2009). The structure, function, and biosynthesis of plant cell wall pectic polysaccharides. Carbohydrate research, 344(14), 1879-1900. doi:10.1016/j.carres.2009.05.021
Calcott, P. H., & MacLeod, R. A. (1974). Survival of Escherichia coli from freeze-thaw damage: a theoretical and practical study. Canadian journal of microbiology, 20(5), 671-681. Retrieved from http://www.nrcresearchpress.com/doi/pdf/10.1139/m74-103
Chylińska, M., Szymańska-Chargot, M., & Zdunek, A. (2014). Imaging of polysaccharides in the tomato cell wall with Raman microspectroscopy.Plant methods, 10(14), 1.doi: 10.1186/1746-4811-10-14
Chieh, C. (2012). Water Chemistry and Biochemistry. In: B. K. Simpson, L. M.L. Nollet, F. Toldra, S. Benjakul, G. Paliyath and Y.H. Hui. Food biochemistry and food processing (pp 84-106). Ames, Iowa: Wiley-Blackwell.
Cleland, A. C., & Earle, R. L. (1982). Freezing time prediction for foods—a simplified procedure. International Journal of Refrigeration, 5(3), 134-140. doi:10.1016/0140-7007(82)90092-5.
Clinton, S. K. (February 01, 1998). Lycopene: Chemistry, Biology, and Implications for Human Health and Disease. Nutrition Reviews, 56 (2), 35-51.doi: 10.1111/j.1753- 4887.1998.tb01691.x
Colle, I., Lemmens, L., Van, B. S., Van, L. A., & Hendrickx, M. (January 01, 2010). Effect of thermal processing on the degradation, isomerization, and 67 bioaccessibility of lycopene in tomato pulp. Journal of Food Science, 75 (9), 753- 759.
Conway, W. S., Gross, K. C., & Sams, C. E. (1987). Relationship of bound calcium and inoculum concentration to the effect of postharvest calcium treatment on decay of apples by Penicillium expansum. Plant disease, 71(1), 78-80. Retrieved from http://www.apsnet.org/publications/PlantDisease/BackIssues/Documents/1987Art icles/PlantDisease71n01_78.pdf
Conway, W. S., Sams, C. E., McGuire, R. G., & Kelman, A. (1992). Calcium treatment of apples and potatoes to reduce postharvest decay. Plant disease, 76(4), 329-334. Retrieved from http://www.apsnet.org/publications/PlantDisease/BackIssues/Documents/1992Art icles/PlantDisease76n04_329.PDF
Davies, J. N., Hobson, G. E., & McGlasson, W. B. (1981). The constituents of tomato fruit—the influence of environment, nutrition, and genotype. Critical Reviews in Food Science & Nutrition, 15(3), 205-280. DOI: 10.1080/10408398109527317
De Ancos, B., Sánchez‐Moreno, C., Pascual‐Teresa, D., & Cano, M. P. (2012). Freezing preservation of fruits. In N. K. Sinha, J. S. Sinhu, J. Barta, J. S. B. W, & M. P. Cano. Handbook of Fruits and Fruit Processing (Second Edition) (pp103-119). Oxford, UK: Wiley- Blackwell.
Delgado, A. E., & Sun, D. W. (2001). Heat and mass transfer models for predicting freezing processes–a review. Journal of Food Engineering, 47, 3, 157-174. doi:10.1016/S0260-8774(00)00112-6
Dermesonlouoglou, E. K., Giannakourou, M. C., & Taoukis, P. (2007). Stability of dehydrofrozen tomatoes pretreated with alternative osmotic solutes. Journal of Food Engineering, 78, 1, 272-280. doi:10.1016/j.jfoodeng.2005.09.026
Dewanto, V., Wu, X., Adom, K. K., & Liu, R. H. (2002). Thermal processing enhances the nutritional value of tomatoes by increasing total antioxidant activity. Journal of agricultural and food chemistry, 50(10), 3010-3014. DOI: 10.1021/jf0115589
Dumville, J. C., & Fry, S. C. (2000). Uronic acid-containing oligosaccharins: their biosynthesis, degradation and signalling roles in non-diseased plant tissues. Plant Physiology and Biochemistry, 38(1), 125-140. Doi: 10.1016/S0981- 9428(00)00163-7
Duvetter, T., Sila, D. N., Van Buggenhout, S., Jolie, R., Van Loey, A., & Hendrickx, M. (2009). Pectins in processed fruit and vegetables: Part I—Stability and catalytic activity of pectinases. Comprehensive reviews in food science and food safety, 8(2), 75-85. DOI: 10.1111/j.1541-4337.2009.00070.x\
68 Ersoy, B., Aksan, E., & Özeren, A. (2008). The effect of thawing methods on the quality of eels (Anguilla anguilla). Food chemistry, 111(2), 377-380.
Fischer, R. L., & Bennett, A. B. (1991). Role of cell wall hydrolases in fruit ripening. Annual review of plant biology, 42(1), 675-703. Retrieved from http://www.annualreviews.org/doi/pdf/10.1146/annurev.pp.42.060191.003331.
Floros, J. D., Ekanayake, A., Abide, G. P., & Nelson, P. E. (1992). Optimization of a diced tomato calcification process. Journal of food science, 57(5), 1144-1148. DOI: 10.1111/j.1365-2621.1992.tb11284.x
Food and Drugs, 21 CFR 184.1193 (1996).
Foster, T. J., Ablett, S., McCann, M. C., & Gidley, M. J. (December 06, 1996). Mobility- resolved 13C-NMR spectroscopy of primary plant cell walls. Biopolymers, 39, 1, 51-66. Retrieved from http://onlinelibrary.wiley.com/doi/10.1002/(SICI)1097- 0282(199607)39:1%3C51::AID-BIP6%3E3.0.CO;2-U/epdf.
Fricke, B. A., & Becker, B. R. (2001). Evaluation of thermophysical property models for foods. HVAC&R Research, 7(4), 311-330. DOI:10.1080/10789669.2001.10391278
George, R. M. (1997). Freezing System. In: M.C. Erickson & Y. C. Huang. Quality in Frozen Food (pp 1-9). Boston, MA: Springer US.
Goff, H. D. (1992). Low-temperature stability and the glassy state in frozen foods. Food research international, 25(4), 317-325. doi:10.1016/0963-9969(92)90128-R
Gould, W. A. (1992). Tomato production, processing & technology. Baltimore, MD.: CTI Publications. Retrieve from http://www.sciencedirect.com/science/book/9781845695996.
Grange, R. I. (1995). Water relations and growth of tomato fruit pericarp tissue. Plant, Cell & Environment, 18(11), 1311-1318. DOI: 10.1111/j.1365- 3040.1995.tb00190.x
Gross, K. C., & Wallner, S. J. (1979). Degradation of cell wall polysaccharides during tomato fruit ripening. Plant Physiology, 63(1), 117-120.
Halliwell, B. (1995). Oxidation of low-density lipoproteins: questions of initiation, propagation, and the effect of antioxidants. The American journal of clinical nutrition, 61(3), 670-677.Retrieved from http://ajcn.nutrition.org/content/61/3/670S.short
69 Hayes, L. J., Diller, K. R., Lee, H. S., & Baxter, C. R. (1984). On the definition of an average cooling rate during cell freezing. Cryo-Letters, 5(2), 97-110.
Heldman, D.R., Lai, D.J.,( 1983). A model for prediction of shelf-life for frozen foods. Proceedings of the 16th International Congress Refrigeration Commission C2, 427–433.
Heldman, D.R. & Nesvadba, P. (2010) Frozen food: History. In; D.R. Heldman & C. I. Moraru (eds). Encyclopedia of Agricultural, Food, and Biological Engineering (2nd Edition) (pp 651-653). Boca Raton, FL: CRC Press.
Hsieh, R. C., Lerew, L. E., & Heldman, D. R. (April 01, 1977). Prediction of freezing times for foods as influenced by product properties. Journal of Food Process Engineering, 1(2), 183-197.Doi: 10.1111/j.1745-4530.1977.tb00177.x
Hsu, C. L., Heldman, D. R., Taylor, T. A., & Kramer, H. L. (2003). Influence of cooling rate on glass transition temperature of sucrose solutions and rice starch gel. Journal of food science, 68(6), 1970-1975. DOI: 10.1111/j.1365- 2621.2003.tb07003.x
Hung, Y.C. (1997). Freeze-cracking. In: M.C. Erickson & Y. C. Huang. Quality in Frozen Food (pp 92-100). Boston, MA: Springer US.
Jarvis, M. C. (April 01, 1984). Structure and properties of pectin gels in plant cell walls. Plant, Cell & Environment, 7(3), 153-164. Doi: 10.1111/1365- 3040.ep11614586
Kim, N. K., & Hung, Y. C. (1994). Freeze-cracking in Foods as Affected by Physical Properties. Journal of Food Science, 59(3), 669-674. DOI: 10.1111/j.1365- 2621.1994.tb05590.x
Klein, B. P., & Perry, A. K. (1982). Ascorbic acid and vitamin A activity in selected vegetables from different geographical areas of the United States. Journal of Food Science, 47(3), 941-945.doi: 10.1111/j.1365-2621.1982.tb12750.x
Koh, E., Charoenprasert, S., & Mitchell, A. E. (January 01, 2012). Effects of industrial tomato paste processing on ascorbic acid, flavonoids and carotenoids and their stability over one-year storage. Journal of the Science of Food and Agriculture, 92, 1, 23-8. DOI: 10.1002/jsfa.4580
Laguerre, O. (2008). Consumer Handling of Frozen Foods. In: J. A. Evans. Frozen Food Science and Technology (pp 325-346). Oxford, UK: Blackwell Publishing Ltd.
LeBail, A., Chevalier, D., Mussa, D. M., & Ghoul, M. (2002). High pressure freezing and thawing of foods: a review. International Journal of Refrigeration, 25(5), 504- 513. doi:10.1016/S0140-7007(01)00030-5
70 Levine, M. B. (1973). Studies on the freezing and thawing properties of tomatoes. Cornell University.
Li, B., & Sun, D. W. (2002). Effect of power ultrasound on freezing rate during immersion freezing of potatoes. Journal of Food Engineering, 55(3), 277-282. doi:10.1016/S0260-8774(02)00102-4
Li, B., & Sun, D. W. (2002). Novel methods for rapid freezing and thawing of foods–a review. Journal of food engineering, 54(3), 175-182. doi:10.1016/S0260- 8774(01)00209-6
Lisiewska, Z., & Kmiecik, W. (2000). Effect of storage period and temperature on the chemical composition and organoleptic quality of frozen tomato cubes. Food Chemistry, 70(2), 167-173. doi:10.1016/S0956-7135(99)00110-3
Lisiewska, Z., Kmiecik, W., & Gȩbczyński, P. (1999). Effect of maturity stages on the content of ash components in raw, frozen and canned broad beans. Food chemistry, 67(2), 155-162. doi:10.1016/S0308-8146(99)00112-0
Ludford, P. M. (1995). Postharvest Hormone Changes in Vegetables and Fruit. In P. J. Davies (Eds). Plant Hormones (pp 725-750). Dordrecht, Netherlands: Kluwer Academic.
Luh, B. S., Villarreal, F., Leonard, S. J., & Yamaguchi, M. (1960). Effect of ripeness level on consistency of canned tomato juice. Food Technology, 14(12), 635-639.
Luna-Guzmán, I., & Barrett, D. M. (2000). Comparison of calcium chloride and calcium lactate effectiveness in maintaining shelf stability and quality of fresh-cut cantaloupes. Postharvest Biology and Technology, 19(1), 61- 72.doi:10.1016/S0925-5214(00)00079-X
Luterotti, S., Bicanic, D., Marković, K., & Franko, M. (2015). Carotenes in processed tomato after thermal treatment. Food Control, 48, 67-74. doi:10.1016/j.foodcont.2014.06.004
Ma, W. H., & Barrett, D. M. (2001). Effects of Raw Materials and Process Variables on the Heat Penetration Times, Firmness, and Pectic Enzyme Activity of Diced Tomatoes (Halley Bos 3155 cv). Journal of food processing and preservation, 25(2), 123-136. DOI: 10.1111/j.1745-4549.2001.tb00448.x
Madhavi, D. L., & Salunkhe, D. K. (1998). Tomatoes. In D. K. Salunkhe, & S. S. Kadam., (Eds). Handbook of Vegetable Science and Technology: Production, Composition, Storage, and Processing, (pp. 171- 202). New York: CRC Press.
Martı́nez-Valverde, I., Periago, M. J., Provan, G., & Chesson, A. (February 01, 2002). Phenolic compounds, lycopene and antioxidant activity in commercial varieties of
71 tomato (Lycopersicum esculentum). Journal of the Science of Food and Agriculture, 82(3), 323-330. DOI: 10.1002/jsfa.1035
Mazur, P. (1984). Freezing of living cells: mechanisms and implications. American Journal of Physiology-Cell Physiology, 247(3), C125-C142.Retrieved from http://ajpcell.physiology.org/content/ajpcell/247/3/local/ed-board.pdf
Mertz, C., Gancel, A.-L., Gunata, Z., Alter, P., Dhuique-Mayer, C., Vaillant, F., Perez, A. M., ... Brat, P. (August 01, 2009). Phenolic compounds, carotenoids and antioxidant activity of three tropical fruits. Journal of Food Composition and Analysis, 22, 5, 381-387. doi:10.1016/j.jfca.2008.06.008
Mohr, W. P., and M. Stein. "Effect of different freeze-thaw regimes on ice formation and ultrastructural changes in tomato fruit parenchyma tissue."Cryobiology 6.1 (1969): 15-31. doi:10.1016/S0011-2240(69)80004-0
Nakagawa, K., Hottot, A., Vessot, S., & Andrieu, J. (2006). Influence of controlled nucleation by ultrasounds on ice morphology of frozen formulations for pharmaceutical proteins freeze-drying. Chemical Engineering and Processing: Process Intensification, 45(9), 783-791. doi:10.1016/j.cep.2006.03.007
Nesvadba, P. (2008). Thermal properties and ice crystal development in frozen food. In: J. A. Evans. Frozen Food Science and Technology (pp 1-25). Oxford, UK: Blackwell Publishing Ltd.
Ngapo, T. M., Babare, I. H., Reynolds, J., & Mawson, R. F. (1999). Freezing rate and frozen storage effects on the ultrastructure of samples of pork. Meat science, 53(3), 159-168. doi:10.1016/S0309-1740(99)00051-0
Oey, I., Lille, M., Van, L. A., & Hendrickx, M. (June 01, 2008). Effect of high-pressure processing on colour, texture and flavour of fruit- and vegetable-based food products: a review. Trends in Food Science & Technology, 19, 6, 320-328. doi:10.1016/j.tifs.2008.04.001
Otero, L., Martino, M., Zaritzky, N., Solas, M., & Sanz, P. D. (2000). Preservation of Microstructure in Peach and Mango during High‐pressure‐shift Freezing. Journal of Food Science, 65(3), 466-470. DOI: 10.1111/j.1365-2621.2000.tb16029.x
Parkin, K. L. (n.d.). Enzymes. In S. Damodaran, K. L. Parkin, & O. R. Fennema (Eds.), Fennema's food chemistry (4th ed., pp. 332-429). Boca Raton: CRC Press/Taylor & Francis.
Paliyath, G., Tiwari, K., Sitbon, C., & Whitaker, B. D. (January 01, 2012). Fruits, Vegetables, and Cereals. In: Simpson, Leo M.L. Nollet, Fidel Toldra, Soottawat Benjakul, Gopinadhan Paliyath and Y.H. Hui. Food Biochemistry and Food Processing (2nd ed) (pp 531-553). Oxford, UK: John Wiley & Sons, Inc.
72 Peng, Y., Zhang, Y., & Ye, J. (January 01, 2008). Determination of phenolic compounds and ascorbic acid in different fractions of tomato by capillary electrophoresis with electrochemical detection. Journal of Agricultural and Food Chemistry, 56(6), 1838-1844. DOI: 10.1021/jf0727544
Pernice, R., Parisi, M., Giordano, I., Pentangelo, A., Graziani, G., Gallo, M., Fogliano, V., Ritieni, A. (September 13, 2010). Antioxidants profile of small tomato fruits: Effect of irrigation and industrial process. Scientia Horticulturae, 126, 2, 156- 163. doi:10.1016/j.scienta.2010.06.021
Pham, Q. T. (1996). Prediction of calorimetric properties and freezing time of foods from composition data. Journal of Food Engineering, 30(1), 95-107. doi:10.1016/S0260-8774(96)00036-2
Pinheiro, S. C. F., & Almeida, D. P. F. (January 01, 2008). Modulation of tomato pericarp firmness through pH and calcium: Implications for the texture of fresh- cut fruit. Postharvest Biology and Technology, 47 (1), 119-125.doi: 10.1016/j.postharvbio.2007.06.002
Produce for Better Health Foundation. State of the Plate, 2015 Study on America’s Consumption of Fruit and Vegetables, Produce for Better Health Foundation, 2015. Web.
Quintero-Ramos, A., Bourne, M., Barnard, J., Gonzalez-Laredo, R., Anzaldua-Morales, A., Pensaben-Esquivel, M., & Marquez-Melendez, R. (2002). Low temperature blanching of frozen carrots with calcium chloride solutions at different holding times on texture of frozen carrots. Journal of food processing and preservation, 26, 361-374.
Rao, S. N., & Barringer, S. A. (2005). Timing of calcium treatment on resistance of raw and canned diced tomatoes to mechanical abuse. Journal of food processing and preservation, 29(1), 1-7. DOI: 10.1111/j.1745-4549.2005.00008.x
Rao, S. N., & Barringer, S. A. (2006). Calcification of diced tomatoes by liquid dipping versus electrostatic powder coating. Journal of food processing and preservation, 30(1), 71-78. DOI: 10.1111/j.1745-4549.2005.00048.x
Reid, D.S. (1997). Overview of Physical/Chemical Aspect of Frezzing. In: M.C. Erickson & Y. C. Huang. Quality in Frozen Food (pp 10-28). Boston, MA: Springer US.
Rickman, J. C., Bruhn, C. M., & Barrett, D. M. (2007). Nutritional comparison of fresh, frozen, and canned fruits and vegetables II. Vitamin A and carotenoids, vitamin E, minerals and fiber. Journal of the Science of Food and Agriculture, 87(7), 1185- 1196. DOI: 10.1002/jsfa.2824
73 Ridley, B. L., O'Neill, M. A., & Mohnen, D. (2001). Pectins: structure, biosynthesis, and oligogalacturonide-related signaling. Phytochemistry, 57 (6), 929-967. Doi: doi: 10.1016/S0031-9422(01)00113-3*
Seynave, E. N. (1972). Factors Affecting the Quality of Frozen Fresh Tomatoes. University of Georgia.
Silva, C. L. M., Gonçalves, E. M. and Brandão, T. R. S. (2008) Freezing of Fruits and Vegetables. In: J. A. Evans (eds) Frozen Food Science and Technology (pp 165- 183). Oxford, UK: Blackwell Publishing Ltd. doi: 10.1002/9781444302325.ch8
Singh, R. P., & Heldman, D. R. (2014). Introduction to food engineering (pp521-563). Amsterdam: Elsevier/Academic Press.
Slade, L., Levine, H., & Reid, D. S. (1991). Beyond water activity: recent advances based on an alternative approach to the assessment of food quality and safety. Critical Reviews in Food Science & Nutrition, 30(2-3), 115-360
Shi, J., & Maguer, M. L. (2000). Lycopene in tomatoes: chemical and physical properties affected by food processing. Critical reviews in food science and nutrition, 40(1), 1-42. DOI: 10.1080/10408690091189275
Shivhare, U. S., Gupta, M., Basu, S., & Raghavan, G. S. V. (2009). Optimization of blanching process for carrots. Journal of food process engineering, 32(4), 587- 605. DOI: 10.1111/j.1745-4530.2007.00234.x
Sirijariyawat, A., & Charoenrein, S. (June 01, 2014). Texture and Pectin Content of Four Frozen Fruits Treated with Calcium. Journal of Food Processing and Preservation, 38(3), 1346-1355. Doi:10.1111/jfpp.12096
Souci, S. W., Fachmann, W., Kraut, H., Kirchhoff, E., Germany., & Deutsche Forschungsanstalt fü r Lebensmittelchemie. (2008). Food composition and nutrition tables: On behalf of the Bundesministerium fü r Ernä hrung, Landwirtschaft und Verbraucherschutz. (pp 905-907). Stuttgart: MedPharm Scientific Publishers.
Spiazzi, E., & Mascheroni, R. (1997). Mass transfer model for osmotic dehydration of fruits and vegetables—I. Development of the simulation model. Journal of Food Engineering, 34(4), 387-410. doi:10.1016/S0260-8774(97)00102-7
Stewart, A. J., Bozonnet, S., Mullen, W., Jenkins, G. I., Lean, M. E., & Crozier, A. (January 01, 2000). Occurrence of flavonols in tomatoes and tomato-based products. Journal of Agricultural and Food Chemistry, 48, 7, 2663-9. DOI: 10.1021/jf000070p
Stinco, C. M., Rodríguez-Pulido, F. J., Escudero-Gilete, M. L., Gordillo, B., Vicario, I. M., & Meléndez-Martínez, A. J. (2013). Lycopene isomers in fresh and processed 74 tomato products: Correlations with instrumental color measurements by digital image analysis and spectroradiometry. Food research international, 50(1), 111- 120. doi:10.1016/j.foodres.2012.10.011
Sun, D. W., & Li, B. (2003). Microstructural change of potato tissues frozen by ultrasound-assisted immersion freezing. Journal of food engineering, 57(4), 337- 345. doi:10.1016/S0260-8774(02)00354-0
Sutton, R. L., Lips, A., Piccirillo, G., & Sztehlo, A. (1996). Kinetics of ice recrystallization in aqueous fructose solutions. Journal of food science, 61(4), 741-745. DOI:10.1111/j.1365-2621.1996.tb12194.x
Thakur, B. R., Singh, R. K., & Nelson, P. E. (1996). Quality attributes of processed tomato products: a review. Food Reviews International, 12(3), 375-401. Doi:10.1080/87559129609541085
Torreggiani, D. (1993). Osmotic dehydration in fruit and vegetable processing. Food Research International, 26(1), 59-68. doi:10.1016/0963-9969(93)90106-S
Tucker, G. A., Robertson, N. G., & Grierson, D. (1980). Changes in polygalacturonase isoenzymes during the ‘ripening’of normal and mutant tomato fruit. European Journal of Biochemistry, 112(1), 119-124. DOI: 10.1111/j.1432- 1033.1980.tb04993.x
United States Department of Agriculture, Economic Research Service. (2016). Tomatoes. Retrieved from http://www.ers.usda.gov/topics/crops/vegetables- pulses/tomatoes.aspx.
US Department of Agriculture, Agricultural Research Service, Nutrient Data Laboratory. (2015) USDA National Nutrient Database for Standard Reference, Release 28. Version Current: September 2015. Retrieved from: http://www.ars.usda.gov/nea/bhnrc/ndl
Vail, G. E., Jeffery, M., Forney, H., & Wiley, O. (July 01, 1943). Effect of method of thawing upon losses, shear, and press fluid of frozen beefsteaks and pork roasts. Journal of Food Science, 8(4), 337-342. Retrieved from http://onlinelibrary.wiley.com.proxy.lib.ohio-state.edu/doi/10.1111/j.1365- 2621.1943.tb18010.x/epdf
Verdini, R. A., Zorrilla, S. E., & Rubiolo, A. C. (September 01, 2008). Calcium Uptake during Immersion of Strawberries in CaCl₂ Solutions. Journal of Food Science, 73 (7), C533-539.doi: 10.1111/j.1750-3841.2008.00868.x
Ware, G. W., & McCollum, J. P. (1980). Producing vegetable crops. Danville, Ill: Interstate Printers & Publishers.
75 Willats, W. G., McCartney, L., Mackie, W., & Knox, J. P. (2001). Pectin: cell biology and prospects for functional analysis. In Plant Cell Walls (pp. 9-27). Netherlands: Springer.
Wu, M. A., & Burrell, R. C. (1958). Flavonoid pigments of the tomato (Lycopersicum esculentum Mill.). Archives of biochemistry and biophysics, 74(1), 114-118. Doi: 10.1016/0003-9861(58)90205-4
Xiong, Y.L. (1997). Protein Denaturation and Functionality loss. In: M.C. Erickson & Y. C. Huang. Quality in Frozen Food (pp 111-140). Boston, MA: Springer US.
Zaritzky, N.E. (2008). Frozen Storage. In: J. A. Evans. Frozen Food Science and Technology (pp 224-247). Oxford, UK: Blackwell Publishing Ltd.
Zheng, L., & Sun, D. W. (2006). Innovative applications of power ultrasound during food freezing processes—a review. Trends in Food Science & Technology, 17(1), 16- 23. doi:10.1016/j.tifs.2005.08.010
76 Appendix A: Temperature history at the center of the tomato slice when frozen at a range of “Time to freeze” without calcium treatment 30 25 105 64 57 50 46 20
） 15 C
10 5 0 -5
Temperature Temperature ( -10 -15 -20 -25 0 20 40 60 80 100 120 Time (s) Figure A.1 Temperature history at the center of the tomato slice when frozen at a range of “Time to freeze” without calcium treatment.
77