EVALUATING THE EFFECT OF SELECTED SOAKING PRETREATMENTS ON
THE COLOR QUALITY AND PHENOLIC CONTENT OF PURPLE POTATO CHIPS
THESIS
Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in
the Graduate School of The Ohio State University
By
KAI ZHANG
Graduate Program in Food Science and Technology
The Ohio State University
2017
Master’s Examination Committee:
Dr. M. Monica Giusti, Advisor
Dr. Christopher T. Simons
Dr. Lynn C. Knipe
Copyrighted by
Kai Zhang 2017
Abstract
Purple potato chips made from natural potato varieties with purple-bluish flesh are a novel alternative product to traditional potato chips. However, color loss and anthocyanin degradation occur during the processing of purple chips due to high frying temperature and Maillard browning. The objective of this study was to evaluate the influence of pretreatment soaking solutions on the color formation and phenolic content of purple potato chips.
Purple potatoes (Purple majesty) were obtained from a local market in Columbus (OH,
USA), and cut into slices of 2±0.2 mm. After washing in running water, slices were soaked in solutions of citric acid, acetic acid, sodium chloride (NaCl) or calcium chloride
(CaCl2) using four different concentrations (0.1%, 0.2%, 1%, 2%) for 10 minutes, superficially dried and fried for 3 minutes in vegetable oil at 170 °C. Color (CIELab), monomeric anthocyanins, and total phenolics were monitored on the final product.
Potato slices pretreated with 1% and 2% citric acid solutions had brighter purple-bluish color (L*=39.1±0.4, h*=317.9°±3.6°) with less browning compared to samples pretreated with other solutions (L*=22-33, h*=306˚-329˚). Potato chips pretreated with 1% and 2% citric acid solutions had higher monomeric anthocyanin content (50.3-63.7 mg Cyanidin-
3-glucoside equivalents /100g fries) and less polymeric anthocyanin color
ii (25.4-35.7%) as compared to other samples (monomeric: 30.6-44.6 mg/100g Cyanidin-3- glucoside equivalents, polymeric: 51.4-69.4%). Soaking treatments had less impact on total phenolics. The phenolic content of potato chips with 1% and 2% citric acid ranged from 538.5-554.7 mg gallic acid equivalents (GAE)/100g chips, against 400.7-519.1
GAE mg/100g in other soaked samples.
The major anthocyanin in raw purple potatoes was identified by HPLC as Petunidin-3-(p- coumaroyl)-rutinoside-5-glucoside which remained unchanged during the processing, along with Petunidin-3-(caffeoyl)-rutinoside-5-glucoside and Malvidin-3-(p-coumaroyl)- rutinoside-5-glucoside as minors.
Potato chips were vacuum packed and stored at room temperature in the dark. Longer storage period resulted in the accumulation of polymerized anthocyanins in the purple potato chips. Changes in the phenolic content of chips remained constant around 520 mg gallic acid equivalents (GAE) /100g dry weight.
Reducing sugar content in potato slices decreased from ~3.4 to 2.6 g/100g DW with water rinsing. Most soaking treatments helped decrease sugars levels even further, as low as ~1.0g/100g DW with citric acid. Citric and acetic acid soaking solutions had pH levels below 4, especially in 1% and 2% citric acid solutions (pH 2). This may have helped retain color and monomeric anthocyanin content, particularly on potato chips pretreated with 1% and 2% citric acid, since anthocyanins are inherently more stable under acidic conditions. Citric acid’s ability to function as an acidulant resulted in greater benefit as compared to other pretreatment chemicals tested. In conclusion, soaking colored potato slices in citric acid could be used as pretreatment to enhance the color and phenolic quality of fried purple potato chips with potential application on industrial scale.
iii Dedication
This document is dedicated to my family.
iv Acknowledgements
I would like to thank my advisor, Dr. M. Monica Giusti, for her generous support with encouragement, knowledge and time during the entire master program. I greatly appreciate her guidance of great patience through my experiment and assistance in writing my thesis.
I would like to thank Dr. Christopher T. Simons and Dr. Lynn C. Knipe for kindly being my committee and giving pertinent advice on my thesis.
I would like to thank Mr. Steven Simons and the Food Industries Center of The Ohio
State University for his help on the material preparation of this research.
Finally, thanks my parents and my sister for their enduring love and metal support on my study.
v Vita
September 22nd, 1989……………………………….Born – Zhengzhou, Henan China
2015…………………………………………………B.S. Food Science and Technology,
The Ohio State University
2015-2017…………………………………………...M.S. Food Science and Technology,
The Ohio State University
Fields of Study
Major Field: Food Science and Technology
vi Table of Contents
Abstract ...... ii
Dedication………………………………………………………………………………………………………iv
Acknowledgements ...... v
Vita ...... vi
List of Figures ...... x
List of Tables ...... xi
Chapter 1 Introduction ...... 1
Chapter 2 Literature Review ...... 3
2.1 Potato Chips ...... 3
2.2 Potato Chips Production ...... 4
2.3 Potato Chips Color ...... 5
2.3.1 Color Formation – Maillard Reaction ...... 5
2.3.2 Color Parameters ...... 6
2.4 Process Variables Affecting Chips Color ...... 7
2.4.1 Frying Temperature ...... 7
2.4.2 Frying Time ...... 8
2.4.3 Storage Conditions ...... 8
vii 2.4.4 Slice Thickness ...... 9
2.5 Cooking Methods Affecting Chip Color and Phenolic Contents ...... 10
2.5.1 Vacuum Frying ...... 10
2.5.2 Baking ...... 11
2.5.3 Microwaving ...... 12
2.6 Pre-treatments Prior to Frying Affecting Chips Color and Phenolic Contents ...... 12
2.6.1 Blanching ...... 12
2.6.2 Pre-drying ...... 14
2.6.3 Soaking ...... 15
2.7 Colored-flesh Potatoes ...... 17
2.7.1 Potato Varieties and Characteristics ...... 17
2.7.1.1 Potato History and Composition ...... 17
2.7.1.2 Potato Varieties and Pigmented Cultivars ...... 19
2.7.2 Anthocyanins ...... 20
2.7.2.1 Anthocyanins Structure ...... 20
2.7.2.2 Anthocyanins in Pigmented Potatoes ...... 22
2.7.2.3 Potential Health Benefits of Anthocyanin-rich Potatoes ...... 24
2.7.3 Purple-fleshed Potatoes Cultivars and Anthocyanins ...... 26
2.8 Purple Potato Chips ...... 27
2.8.1 Quality Degradation during Purple Potato Chips Production ...... 27
2.8.2 Factors Leading to Color Development of Chips ...... 28
Chapter 3 Materials and Methods ...... 31
3.1 Raw Materials ...... 31
3.2 Purple Potato Chips Production ...... 31
3.3 CIELab Analysis of Chips Color ...... 32
viii 3.4 Anthocyanin Extraction and Purification from Purple Potatoes and Chips ...... 32
3.5 Determination of Total Monomeric Anthocyanin Content ...... 33
3.6 Determination of Percent Polymeric Color ...... 34
3.7 Determination of Total Phenolic Content ...... 34
3.8 Determination of Reducing Sugar Content by HPLC ...... 35
3.9 pH Value of Pretreatment Solutions and Raw Potatoes ...... 35
3.10 Anthocyanin Analysis by HPLC ...... 36
3.11 Storage Stability Analysis ...... 36
3.12 Statistical Analysis ...... 37
Chapter 4 Results and Discussion ...... 38
4.1 Colorimetric Evaluation ...... 38
4.2 Analytical Testing ...... 41
4.3 Anthocyanins in Raw Purple Potatoes and Potato Chips ...... 49
Chapter 5 Conclusion ...... 51
References ...... 53
ix List of Figures
Figure 1. Flow chart of the fried potato chips process …………………………………...4
Figure 2. Formation of advanced Maillard reaction products (MRPs)….………………..6
Figure 3. Basic chemical structure of aglycone (anthocyanidin)………………………..21
Figure 4. Structures of major anthocyanins isolated from blue-fleshed potato varieties…...... 22
Figure 5. Main anthocyanin aglycones of potatoes……………………………………...24
Figure 6. Predominant structural forms of anthocyanins present at different pH levels..30
Figure 7. Lightness (L*) and hue angle (h*) of purple potato chips pretreated with different soaking solutions……………………………………………………………….40
Figure 8. Color of purple potato extract at pH 3 to 10……………………………...... 40
Figure 9. Content of reducing sugar in potato slices pretreated with soaking solutions...42
Figure 10. Changes in monomeric anthocyanin content and percent polymeric color of purple potato chips during storage…..……………………………………...... 48
Figure 11. Anthocyanins from purple potato and purple potato chips extracts…………49
x List of Tables
Table 1. Differences in chemical structure and color of most common anthocyanidins..22
Table 2. Color characteristics of raw purple-fleshed potato and purple potato chips with
1% and 2% citric acid pretreatments using CIE Lab color ……………………………...39
Table 3. Changes in pH of pretreatment solutions after soaking purple potato slices…..44
Table 4. Average of monomeric, polymeric anthocyanins and total phenolics content in dried raw purple potatoes and chips…………………………...…………………………47
xi Chapter 1 Introduction
Color is one of the most important attributes of a product that is perceived by consumers and affects consumers’ willingness to purchase. Traditionally, chips made from yellow- fleshed potatoes are required to exhibit a well-accepted golden-yellow color on the final products. Color of chips is attributed to the result of a non-enzymatic browning due to
Maillard reaction, a reaction of amino acids with reducing sugars in potatoes. High reducing sugar contained in potatoes would develop darker color and off-flavors, which are not acceptable for chips production (Sahin 2000). Some process variables, like frying time and temperature, are reasonably to affect the color of fried chips.
Some pre-treatments of the sliced potatoes are often required to produce paler brown- yellow chips with acceptable browning color (Krokida et al. 2001). It has been reported that soaking process would lead to a high leaching of reducing sugars like glucose that finally results in lower Maillard reaction (Pedreschi et al. 2004). Using organic acid solutions (citric acid, acetic acid) and salt solutions (Na+, Ca2+) at certain concentration as pre-treatments would produce natural potato chips with lower browning color (Mestdagh et al. 2008a). Jung et al. (2003) stated that soaking in lower pH solutions such as citric acid would suppress Maillard reaction by reducing sugars from the surface layer of potato slices. It has also been found that soaking potato slices in NaCl or CaCl2 solutions before
1 frying minimizes the browning, leading to paler color of traditional potato chips
(Pedreschi et al. 2007a; Ou et al. 2008). Purple potato chips are a novel alternative produced from natural potato varieties with purple-bluish flesh.
Compared to traditional potato chips, purple potato chips are more expensive because of their potential health benefits and antioxidant power by some natural polyphenols, like anthocyanins. Anthocyanins are water-soluble natural pigments that are accredited to the red, pink, and blue color of many plants in nature. This natural pigment is sensitive and may be affected by different processing methods. During the production of purple potato chips, high frying temperature is a major factor leading to the degradation of anthocyanin, a heat-sensitive compound in purple potatoes that not only provides appealing purple color to chips but also boosts nutritional value due to its anti-oxidative property (Kita et al. 2015). In addition, superficial reducing sugar content on the potato slices would cause Maillard, a non-enzymatic browning reaction that would bring undesirable dark color to the purple-fleshed chips and loss of anthocyanin in the chips
(Pedreschi et al. 2007b).
In this study, fresh purple-fleshed potatoes (Purple Majesty) grown in California, USA were selected and used. We hypothesized that soaking as a purple potato chips processing method prior to frying may help enhance color quality and inhibit anthocyanin degradation. The purpose of this study was to evaluate the influence of pretreatment soaking solutions prior to frying on the color formation and changes in phenolic content of purple potato chips.
2 Chapter 2 Literature Review
2.1 Potato Chips
Potato chips, as a common crunchy snack food, are traditionally made from thin slices of yellow-fleshed potatoes through deep frying methods, which were originally produced in
England two hundred years ago (Berry & Norman 2014). As reported by Fox News
(2016), chips were accidentally invented by Mr. George Crum, an African American working for a restaurant in New York State. The story of chips creation started with complaints about Mr. Crum’s too thick fried potatoes from a customer in 1853. George wanted to teach him a lesson by slicing a new batch of potatoes as thin as he could and frying the slices until they became crunchy. Fried slices were then salted and thought highly of by the customer, which deeply surprised Mr. Crum. He did not patent his invention, fried potato chips; however, this new fried potato snack food was eventually mass-produced and sold in bag without giving any credits to George. With centuries of innovation in potato chips industry, many other processing methods, such as baking, kettle-cooking, and vacuum frying, have been successfully applied to potato chips production. Besides, chips with assorted flavors have been developed and manufactured using various ingredients. As a predominant snack food in the United States, potato chips has become the top salty snack seller with a sales of $7.5 billion in 2015, the enduring
3 market need allows potato chips to serve as a strong driver for the $22 billion U.S. salty snack sales till 2020 (IFT 2016). According to a report by IMARC Group (2016), worldwide potato chips market has increased at an annual rate of 4% from 2009-2016, reaching a market value of $26 billion.
2.2 Potato Chips Production
Traditionally, potato chips can go from freshly harvested raw potatoes to crispy chips in several processing steps in manufacturer (Figure 1). Quality potatoes are harvested and selected for chips making. Potato tubers ware gently peeled after thoroughly cleaned in cold water. Then peeled potatoes are sliced into thin round pieces, followed by rinsing in cold water to remove excessive starch on their surface. Potato slices are superficially dried and deeply fried into crispy crunch in vegetable oil. Fried chips are seasoned with salt and other flavoring additives. Final products are cooled, bagged and packed in cases for sale in markets. Processing variables like frying time and temperature should be strictly controlled to make chips of desirable golden-yellow color.
Figure 1. Flow chart of the fried potato chips process
4 2.3 Potato Chips Color
2.3.1 Color Formation – Maillard Reaction
Color, as well as crispness, is a parameter of importance to indicate chips quality that should be strictly controlled during production. Golden-yellow color of chips is intimately associated with a high-grade product in consumers’ minds. The desirable color of potato chips is attributed to the chemical compounds produced by Maillard reaction, a non-enzymatic browning caused by the carbonyl groups of reducing sugar and the amino groups of amino acids at higher temperature during frying period (Márquez & Añón
1986). This reaction is a form of non-enzymatic browning named after a French chemist
Louis-Camille Maillard who first described the reaction between amino acids and sugars at elevated temperature (Maillard 1912). In 1953, an American chemist John E. Hodge explained the mechanism of the Maillard reaction (Hodge 1953; Everts 2012).
A complex mix of various compounds is produced by Maillard reaction to contribute to flavor and color of the finished products, including flavoring compounds like mono-/di- carbonyl and coloring agents like melanoidins (Mookherjee et al. 1965; Wang et al.
2011). Melanoidins are brown nitrogenous polymers generated in the late stages of the
Maillard reaction in potato chips making, which brings golden-yellow color to chips. In the food product with brown color, melanoidins in the Maillard reaction products (MRPs) are predominantly high molecular weight (HMW) compounds (Hofmann 1998).
Mechanisms of melanoidins production are illustrated in Figure 2. At the beginning, the carbonyl group reacts with the amino group, generating the N-substituted glycosylamine that is further stabilized to form ketoamine via Amadori rearrangement (Wang et al.
5 2011). Intermediate products are generated through two pathways: furaldehydes, furanones, dicarbonyls and aldehydes from ketoamines; furaldehydes and furanones due to the degradation of reducing sugars (Wang et al. 2011). In the final stages, HMW advanced MRPs containing brown melanoidins are produced by polymerization of low molecular weight (LMW) compounds from the reaction of intermediate products and amino acids, and by the modification of proteins with intermediate products.
Figure 2. Formation of advanced Maillard reaction products (MRPs) (Wang et al. 2011)
2.3.2 Color Parameters
Color of fried potato chips is an important attribute that has to be well controlled during processing. To clearly monitor any changes in chips color that occurred during frying, color measurements of potato chips are typically completed in CIELab using a
6 colorimeter. CIELab is an international standard for color data and has been utilized for decades of years by the Commission Internationale d’Eclairage (CIE) since 1976. L* is the lightness parameter, ranging from 0 to 100 as the lightness increased; a* (greenness to redness) and b* (blueness to yellowness) are the two chromatic parameter of the
CIELab color scale, ranging from -120 to 120 (Papadakis et al. 2000). For traditional potato chips, a golden-yellow color is considered desirable for customers. To make chips with acceptable color, process variables during the frying process are strictly controlled by the industry.
2.4 Process Variables Affecting Chips Color
2.4.1 Frying Temperature
Traditional potato chips are produced by deep-fat frying, a process method to cook and dehydrate food that immersed in heated oil at 165-190ºC (Moreira 2004). Temperature of frying is a crucial variable to influence the chips color because potato chips are fried at an elevated temperature that rapidly proceeds the Maillard reaction that is highly dependent on frying temperature (Márquez & Añón 1986). L* (lightness) of fried potato chips is negatively influenced by the frying temperature, and lightness decreases as the frying temperature increases at the same frying time condition (Krokida et al. 2001). a* value increases significantly as the frying temperature goes up within same time, indicating that fried chips tend to get darker and more red, which is undesirable and unacceptable
(Pedreschi et al. 2005; Krokida et al. 2001). Similarly as the manner that frying temperature influence a*, b* slightly increases with temperature, generating more
7 desirable yellowness to the chips (Krokida et al. 2001). Lowering the frying temperature is acceptable to produce lighter colored fried potato chips with less darkening browning during frying.
2.4.2 Frying Time
Traditional potato chips are fried in heated oil for 3-5 minutes until a crispy texture is achieved (Kita et al. 2009). Frying time is another crucial parameter to be monitored that deeply influences the reaction of Maillard browning. The lightness parameter of potato chips is progressively increased as the frying time proceeds: L* value remarkably increases during the early stages of frying but remains constant afterwards (Krokida et al.
2001). The fried chips continuously turn darker during frying as indicating by the increasing a* value (Pedreschi et al. 2005; Krokida et al. 2001). Compared with the obvious increase in parameter a*, b* value undergoes a sluggish increase during frying at the same temperature (Krokida et al. 2001).
2.4.3 Storage Conditions
Color of potato chips has been associated with the reducing sugar content of the raw potatoes, and higher reducing sugar leads to excessive browning of potato chips
(Márquez & Añón 1986). During the storage, reducing sugar in raw potatoes accumulates with the storage time (Watada & Kunkel, 1954; Hyde & Morrison, 1964). Storage temperature also influences the amount of the reducing sugar of the raw potatoes. Raw potatoes are usually stored at lower temperature around 4 ℃ to inhibit sprouting;
8 however, tubers stored at temperatures of less than 5 ℃ would produce undesirable darker fried products due to the increased concentration of reducing sugars at lower storage temperature (Talburt & Smith 1967; Mackay et al. 1990). During low- temperature storage around 4 ℃, sufficient starch is more actively degraded and converted to reducing sugar by alpha- and beta-amylase compared to those stored at temperatures in excess of 5 ℃ (Sowokinos et al. 1985; Cottrell et al. 1993). Potato tubers stored at lower temperature are usually reconditioned at around 21 ℃ for 1-3 weeks to decrease the accumulated reducing sugar prior to frying (Heinze et al. 1955; Kirkpatrick et al. 1956).
2.4.4 Slice Thickness
Some research showed that lightness value would be lower for smaller slice thickness and higher sample thickness could lead to lighter colored fried potato chips under the same frying time and temperature (Krokida et al. 2001). Potato slices of 1/8 of an inch thick are highly recommended to fry perfect potato chips (FoxNews 2013). Smaller slice thickness would result in higher values both in a* and b* of fried chips as the frying temperature increases (Krokida et al. 2001). However, Abong et al. (2011) reported that lightness and yellowness values of potato crisps significantly decreased as the slice thickness increased while redness (a*) changes varied among potato crisps of different cultivars as the slice thickness increased.
9 2.5 Cooking Methods Affecting Chips Color and Phenolic Contents
2.5.1 Vacuum Frying
Vacuum frying is an efficient alternative method for the traditional deep fry process of potato chips. Under vacuum, the frying process is carried out at lower pressures below atmospheric levels, leading to lower boiling points of the frying oil and the moisture in the potatoes. Under this circumstances, ideal oil temperature for the chips and oxygen content are decreased, yielding fried chips with more natural color and flavor preservation together with less oil absorption (Shyu et al. 1998). Some investigation has shown that potato chips fried under vacuum were lighter, less red and yellowish than potato chips fried under atmospheric conditions like traditional deep fry (Garayo &
Moreira 2002). In terms of the color-fleshed potatoes, blue potato chips by vacuum frying showed more blue (higher b*) and less red (lower a*) compared with the blue potato chips by traditional frying method at atmospheric pressure (Da Silva & Moreira 2008).
Frying variables, such as frying method and time length, are crucial factors that influence the stability and content of phytochemicals (Shirsat & Thomas 1998). Some study reported that the presence of oxygen would result in anthocyanin degradation and color loss, while vacuum frying method generates products with higher retention of some phytochemicals content due to the lower temperature and the absence of oxygen in the vacuum fryer that alleviate the degradation of compounds like carotenoids and anthocyanins (Da Silva & Moreira 2008; Caieta et al. 2001; Garcia-Viguera & Briddle
1999). In addition, the total monomeric anthocyanins in vacuum-fried blue potato chips
10 were 60% higher than those samples fried at atmospheric pressure (Da Silva & Moreira
2008).
2.5.2 Baking
Baking process is considered as a healthy alternative to traditional frying method because of its potential to provide a similar product with no added fat (Palazoglu et al. 2010).
Effect of baking as a cooking method on the color of potato chips has been a great interest to researchers. Compared to the color parameters of fried chips, values of lightness (L*) and yellowness (b*) were lower while redness (a*) was higher, indicating darker colors of baked potato chips (Tuta & Palazoglu 2017).
Some researchers stated that since frying medium had no oxygen and chips color only formed by Maillard reaction, redness (a*) had a significant change while L* and b* parameters did not significantly change (Khraishes et al. 2003). While during baking process, potato slices are heated by air convection and darker color is generated resulting from the ascorbic acid oxidation in addition to Maillard reaction due to the existence of oxygen (Abu-Ali & Barringer 2007; Demiray & Tulek 2015; Haase & Weber 2003).
Compared with other cooking methods like frying or microwave, oven baking had the highest amount of phenolic compounds remained after processing, with more than 80% retention of chlorogenic acid and its isomer that constituted the majority of total phenolics in potatoes (Im et al. 2008).
11 2.5.3 Microwaving
Microwaving method has a slower rate of color development than frying because microwave heating generates the heat transfer through air, which is slower than that via liquids during frying method (Yost, Abu-Ali & Barringer 2006). When potato chips are under microwaves, nearly no browning produced while both baking and frying could develop a satisfactory brown color in the fries, which is because microwave heating generates a heat transfer throughout fries and the escaped moisture keeps the fry surface temperature below 100 °C to inhibit the Maillard browning (Yost, Abu-Ali & Barringer
2006). Microwave is significant at the loss of phenolic compounds in potato chips. ~70% chlorogenic acid remained after microwave heating, which is lower than that by oven heating (Im et al. 2008). The change of phenolic compounds in potato slices might vary according to the power level of microwave. One study reported that potato slices microwaved at 500 W for 120 s had the least loss of phenolic compounds (3.99%) compared to samples treated at other energy levels like 300 W and 700 W (Barba et al.
2008).
2.6 Pre-treatments Prior to Frying Affecting Chips Color and Phenolic
Contents
2.6.1 Blanching
Blanching process is a method that is traditionally carried out within 20 s to 15 min under the temperature between 80 – 100 ℃, consisting of the immersion of food materials in hot water or steam (Pedreschi et al 2009; Reis et al. 2008). Blanching helps destroy some
12 enzymes in the potato tissues that lead to enzymatic browning and degrade quality of potato products (Moreno-Perez et al. 1996). Blanching previous to frying is also widely used in potato chips production for the leaching of precursors reducing sugars and asparagine on the slice surface to inhibit the non-enzymatic Maillard browning reaction occurred during frying, which is able to improve the product quality with desirable color and texture of potato chips (González-Martínez et al. 2004; Pedreschi et al. 2004;
Califano and Calvelo 1988). Temperature and time are significant factors affecting the loss of reducing sugar content during blanching, and long time or high temperature blanching helps increase the content of reducing sugars leaching (Agblor and Scanlon
2000). Loss rate of reducing sugar increases as the temperature goes up, while the amount of reducing sugar extracted from potato slices accumulates as the blanching time increases with decreasing rate (Pedreschi et al 2009). Blanching generates lighter
(increased L*) and less red (decreased a* ) fried chips than those unblanched potatoes due to more reducing sugars and asparagine diffusing into the blanching water
(Leeratanarak et al. 2006; Pedreschi et al. 2005). Change in the yellowness (b*) of blanched potato chips is also reduced after frying compared to those unblanched chips, resulting in relatively stable yellow color (Leeratanarak et al. 2006). Low temperature and long time blanching outweighs the high temperature and short time, with lighter desirable color of fried potato chips (Pedreschi et al. 2004). When frying the blanched potato slices at the same temperature, L* diminish while a* and b* increase with frying time (Pedreschi et al 2007). Therefore, blanching as a pretreatment is used by some potato processing plants to produce fried chips with desirable lighter color by leaching out reducing sugars from the potato tissues (Andersson 1994).
13 2.6.2 Pre-drying
The pre-drying process before frying is commonly used in the food industry due to its advantages like uniform color and much less fat easily obtained after the frying (Lisińska
& Leszczyński, 1989). Pre-drying process includes various methods, such as hot air drying and microwave drying. Among all the methods, microwave drying is able to rapidly and uniformly pre-dry a large quantity of materials, and maintain desirable product quality with less loss of nutrients and color compared with conventional hot air drying (Drouzas & Schubert 1996; Jones 1992; Maskan 2000). Vacuum microwave has been widely utilized as a pre-drying processing method in the production of fried food products (Song et al. 2007). Quality of French fries were significantly improved with proper color by microwave pre-drying methods, and some study reported that over 4.5 points were assigned with French fries samples pre-dried with vacuum microwave method based on the results of sensory evaluations using a 5-point scale (Tajner-Czopek et al 2007). Song et al. (2007) reported that vacuum-microwave pre-drying resulted in a negative effect on potato chips color with decreased L* value but increased a* and b* values. However, in a study of drying pretreatment by hot air drier, browning reactions that takes place during drying decreased lightness (L*) and parameter b* of initial potato strips but increased parameter a* of potato strips as drying time increased, which contributes to a negative effect on French fries color of darker (L*: 40-50), more redness
(a*: 0-5) and less yellowness (Krokida et al. 2001). Time and temperature of pre-drying determines the color change of product during drying because thermal drying leads to enzymatic and non-enzymatic browning reactions, which would significantly influence color parameters of final products after frying (Tan et al. 2001). Long-time and high-
14 temperature drying would cause color degradation to a product, and it is recommended to cause minimum damage to potato products by choosing appropriate drying time and temperature (Drouzas & Schubert 1996; Bouraout et al. 1994).
2.6.3 Soaking
Soaking of raw potato slices prior to frying in a method used in the potato industry in order to minimize browning and improve color quality of chips by lowering precursors of non-enzymatic browning reactions. It has been reported that soaking process would lead to a high leaching of water-soluble reducing sugars like glucose from raw potato slices that finally results in lower Maillard reaction (Grob et al. 2003; Haase et al. 2003).
Pedreschi et al. (2004) found that 32% decrease in glucose content of potato slices soaked for 90 min and 25% for 40 min soaking in distilled water. Some study stated that French fries soaked in distilled water had lighter color due to a reduction in browning when fried compared to those samples without pre-treatment (Burch et al. 2008).
Distilled water is often combined with chemical browning inhibitors like salts or organic acids at appropriate concentration as pre-treatments to produce desirable potato products with lower browning color (Mestdagh et al. 2008a). Fried potato products are usually salted with sodium chloride after frying and it is reasonable and acceptable to process raw potato materials with NaCl soaking solution prior to frying to maintain sensory quality of product (Blahovec et al. 1999). NaCl has been found as an inhibitor to Maillard reaction, significantly reducing 30% color density of Maillard browning products as 10% NaCl was added to the reaction model (Kwak & Lim 2004). It has been reported that soaking potato strips in a NaCl solution previous to frying for proper time would cause osmotic
15 dehydration to help solutes (organic acids, reducing sugars etc.) present in potato tissues leach into the soaking solution, which removes the Maillard reactants on the surface of potato strips and improves the product color (Bunger et al. 2003; Califano & Calvelo
1988). NaCl soaking solution as a pre-treatment is able to result in lighter fried potato chips at 120, 140, 160 and 180 ℃ than those non-soaked chips (Pedreschi et al. 2007a).
Color of potato chips was improved by NaCl soaking treatment with an increase of 7 L* units compared with water-soaked chip samples, indicating a paler color of final chips; however, the concentration of NaCl solution was not significantly associated with the extent of browning by the parameter L* during frying process (Santis et al. 2007). This finding is in agreement with the sensory evaluation, in which the color scores remained constant values regardless of NaCl concentrations and soaking times (Bunger et al. 2003).
Like Na+, Ca2+ was also used as chemical additive in potato products due to its inhibition of enzymatic browning and reduction of acrylamide by reacting with asparagine to prevent the formation of Schiff base, a key intermediate in the Maillard reaction
(Gökmen & Senyuva 2007; Lindsay et al. 2005; Delgado-Andrade et al. 2004). As the
Ca2+ added crosslinks the pectin in potato tissues, hydrogen chloride is produced, which induces a pH drop and prejudices the subsequent Maillard reaction during frying
(Andersson et al. 1994). On the contrary, some other study showed that no significantly reduced browning was found after frying upon the addition of CaCl2 to potato strips
(Franke et al. 2005). pH value is an important factor in Maillard browning, and lowering the pH would suppress the reaction by blocking the addition of asparagine with a carbonyl compound and preventing the formation of a key intermediate in the Maillard reaction, the Schiff
16 base (Kita et al. 2004; Rydberg et al. 2003). Using organic acids, like citric acid and acetic acid, is able to lower the non-enzymatic browning due to the reduction of pH
(Mestdagh et al. 2008b). Besides that, dipping peeled potatoes in citric acid solution would inactivate the polyphenoloxidase, preventing discoloration caused by enzymatic browning (Sapers & Miller 1995). Jung et al. (2003) stated that soaking in lower pH solutions such as citric acid would suppress Maillard reaction by lowering pH in potatoes and leaching out reducing sugars and asparagine from the surface layer of potato slices.
Mestdagh et al. (2008a) found that blanched potato crisps with citric or acetic acid obtained brighter color compared to those samples blanched in distilled water. However, the addition of acidulants in the soaking solution is likely to generate a sour product taste, depending on the type and concentration of the acid used (Franke et al. 2005). There was a slight sourness detected in the French fries dipping with the 2% citric acid while 1% citric acid solution as pre-treatment was an ideal level for French fries (Jung et al. 2003).
Comparing citric acid with acetic acid, it was recommended that acetic acid resulted in less sourness and could be used as a better acidulant to pre-treat the potato crisps (Kita et al. 2004).
2.7 Colored-fleshed Potatoes
2.7.1 Potato Varieties and Characteristics
2.7.1.1 Potato History and Composition
Potato (Solanum tuberosum, belonging to the nightshade family) is a starchy, tuberous crop with cultivars in assorted colors, shapes and sizes. It was originally domesticated in
17 South America, and a large variety of wild species found in the Andes of Peru and
Bolivia was brought into cultivation as a staple food several thousand years ago (Hawkes
1992; Salaman et al. 1985). Potato was primarily introduced outside the Andes region nearly 400 years ago, in the 1700s to Europe and later to North America (Ochoa 1990).
Now it has been spread and planted in more than 100 countries, and ranks as the world’s fourth most important crops following maize, wheat and rice (FAO 2009; Hendley 2006).
The global production of potatoes in 2014 was over 477 million tonnes, and more than two thirds of the world production was consumed by humans (FAOSTAT 2015; FAO
2009).
Potato has been playing an important role in the food supply, fueling human body with high levels of starch as important carbohydrate source. According to USDA database
(2016), a raw potato tuber contains around 75% water, and over 15% starch, followed by other trace amount of vitamins and minerals. Contents of total sugars determine the quality of raw potatoes and potato products, with a reported average amount of 6.8 g/kg fructose, followed by 5.3 g/kg and 4.2 g/kg in sixteen commercial potato varieties from eight countries (Zhu et al. 2010). While the composition varies by geographic origins and cultivars, Burlingame et al. (2009) found that sugar contents in some varieties from Italy differentiated with 0.12 g/100g glucose, 0.07 g/100g fructose, and 0.40 g/100g sucrose in average. In addition, antioxidants, such as carotenoids and phenolic compounds, are also found in potatoes. Brown (2005) reported that carotenoids presented in the flesh of all potatoes (white-fleshed: 50–100µg/100g fresh weight, deeply yellow to orange-fleshed:
50–2000 µg/100g fresh weight). The amount of total phenolics in diverse potato
18 genotypes ranged from 1.8 to 11 mg/g dry weight, in which the chlorogenic acid dominated with a content of 22 – 473 mg/100g dry weight (Navarre et al. 2011).
2.7.1.2 Potato Varieties and Pigmented Cultivars
About 5000 potato varieties are recorded worldwide, with a vast genetic diversity in cultivars of white-, yellow-, orange-, red-, purple-, and blue-flesh (Burlingame et al. 2009;
Kaspar et al. 2013). The flesh may be partially or solidly pigmented while the maybe only the potato skin is pigmented. White- and yellow-fleshed potatoes are the most well- known to people as rich sources of carotenoids, and yellow-flesh cultivars contain 10 times higher concentration of total carotenoids than white (Brown et al. 2005). Pigmented varieties, such as red-, purple-, and blue-fleshed have been interested by consumers due to their higher concentration of anthocyanins that are potentially beneficial to human health (Brown 2005; Lachman and Hamouz 2005). Higher contents of polyphenols are contained in red and purple flesh cultivars than those with white flesh (Hamouz et al.
2011). Significant differences in total polyphenols content found in purple and yellow- fleshed potatoes: 4.68 g gallic acid equivalents /kg dry weight in purple-fleshed cultivars,
2.96 g gallic acid equivalents /kg dry weight in yellow-fleshed cultivars (Lachman et al.
2008). Total anthocyanin content in some cultivars of red and purple-fleshed potatoes ranged from 0.7 to 74.3 mg/100g FW expressed as cyaniding-3-glucoside (Lachman et al.
2009).
Red and purple flesh potatoes contain higher amounts of total anthocyanin, with 6.9 - 35 mg/100 g fresh weight and 5.5 – 17.1 mg/100 g fresh weight, respectively (Lachman and
Hamouz 2005). Hamouz et al. (2011) reported that approximately 135.3 – 573.5 mg cyanidin /kg fresh weight was found in some purple and red varieties. Anthocyanin
19 content of purple-fleshed potatoes was higher than that of red-fleshed potatoes (Ezekiel et al. 2013). Lewis et al. (1998) found that potato cultivars of purple flesh and red flesh, anthocyanin content was determined as 368 mg/100g FW and 22 mg/100g FW, respectively.
2.7.2 Anthocyanins
Anthocyanins have been known as the largest group of water-soluble natural plant pigments (Takeoka & Dao 2002; Mateus & Freitas 2009). Up to date, more than 700 anthocyanins have been identified in nature, which contributes to the red, blue, and purple color of many plants (Anderson 2012). Potato varieties that are red, purple, or blue pigmented are accredited to the presence of anthocyanins.
2.7.2.1 Anthocyanins Structure
Anthocyanins are from a large family of phytochemical compounds named flavonoids, which share a three-ring carbon backbone structure consisting of C-6 (A ring), C-3 (C ring), and C-6 (B ring) (Cavalcanti et al. 2011; Harborne 1998). Under acidic conditions, anthocyanin pigments are resulted from the eight conjugated double bonds and the resonant structure of the flavylium cation, which act as chromophores and absorb light around 500 nm (Cavalcanti et al. 2011; Castaneda-Ovando et al. 2009).
Anthocyanins mainly differentiate in three aspects: aglycone (anthocyanidin), glycosylation, and acylation. Anthocyanidins, the aglycone form of anthocyanins, differ in the positions of hydroxylation and methoxylation on the rings, as shown in Figure 3
(Andersen & Jordheim 2006). There are 27 different anthocyanidins isolated from plants,
20 of which only six are the most abundant and constitute approximately 90% of anthocyanins found in nature: cyanidin (Cy), delphinidin (Dp), pelargonidin (Pg), peonidin (Pn), malvidin (Mv), and petunidin (Pt) in Table 1 (Andersen & Jordheim 2014;
Kong et al. 2003). Anthocyanins rarely exist in aglycon forms in nature, and sugar moieties may be glycosylated through glycosidic bonds at the 3 and/or 5 positions of A and C rings, which is known as anthocyanin (Wrolstad 2004). Glycosides most commonly found include glucose (glu), rhamnose (rha), galactose (gal), arabinose (ara), xylose (xyl), and rutinose (rut) (Delgado-Vargas et al. 2000; Kähkönen et al. 2003; He &
Giusti 2010). In addition to glycosylation, many glycosylated anthocyanins are acylated with organic acids at the glucoside via ester bonds, including cinnamic acids (p-coumaric, caffeic, ferulic and gallic acid) and aliphatic acids (acetic, malic, malonic, oxalic and succinic acid) (Francis, 1989; Delgado-Vargas et al., 2000; Robbins, 2003). Both glycosylation and acylation (Figure 4) enhance the stability of anthocyanins (Borkowski et al. 2005; Stintzing & Carle 2004).
R1
OH
B HO O R2 A C
OH
OH
Figure 3. Basic chemical structure of aglycone (anthocyanidin)
21 Table 1. Differences in chemical structure and color of most common anthocyanidins (Delgado-Vargas et al. 2000; Mateus & Freitas 2009)
Name R1 substitution R2 substitution Color Cyanidin (Cy) OH H Magenta Delphinidin (Dp) OH OH Purple, Blue Pelargonidin (Pg) H H Red, Orange Peonidin (Pn) OCH3 H Magenta Malvidin (Mv) OCH3 OCH3 Purple, Blue Petunidin (Pt) OCH3 OH Purple
Figure 4. Structures of major anthocyanins isolated from blue-fleshed potato varieties: Pt-3- (p-coumaroyl-rut)-5-glu (R1 = OCH3, R2 = OH), Pn-3- (p-coumaroyl-rut)-5-glu (R1 = OCH3, R2 = H), Mv-3- (p-coumaroyl-rut)-5-glu (R1 = OCH3, R2 = OCH3) (Hillebrand et al. 2009)
2.7.2.2 Anthocyanins in Pigmented Potatoes
Pigmented potato cultivars derive their color from anthocyanins, and type of
anthocyanins varies by the pigment of the potatoes, including skins and the flesh of
tubers. Anthocyanins contained in the potatoes with pigmented flesh have been
investigated by many researchers due to their antioxidant activity and as alternatives to
synthetic colorants. It was found that approximately over 98% of the total anthocyanins
in potatoes were acylated (Lachman & Hamouz 2005). Acylated glucosides of
pelargonidin dominated in the red-fleshed potato comprising about 80% of the total and
22 those compounds as well as acylated petunidin glycosides in a 2:1 ratio were contained in blue-fleshed potatoes, while acylated glucosides of predominant petunidin and peonidin with smaller amounts of delphinidin and malvidin were found in the purple-fleshed potato (Brown 2004; Brown et al. 2005). In some blue-fleshed potato varieties, it was found that p-coumaric acid acylated derivatives of petunidin, malvidin, and peonidin were predominantly shown (Hillebrand et al. 2009; Eichhorn & Winterhalter 2005).
Major anthocyanins isolated from potatoes with different colored-flesh were characterized by many researchers and main anthocyanin aglycones in potatoes are presented in Figure 5. Rodriguez-Saona et al. (1998) reported that the major pigments in red-fleshed potatoes were identified to be acylated glucosides of pelargonidin, which was in agreement with the finding by Naito et al. (1998). In addition, the major anthocyanin in red-fleshed potatoes was identified as Pelargonidin-3-rutinoside-5-glucoside acylated with p-coumaric acid, which took up about 70% from the total anthocyanin content
(Rodriguez-Saona et al. 1998). Lewis et al. (1998) found that the major anthocyanins in the potatoes with red skins and flesh was Pelargonidin-3-(p-coumaroyl)-rutinoside-5- glucoside, followed by lesser amounts of Peonidin-3-(p-coumaroyl)-rutinoside-5- glucoside; however, in purple tubers tested, Malvidin-3-(p-coumaroyl)-rutinoside-5- glucoside was predominant, which was much higher than the amount of Petunidin-3-(p- coumaroyl)-rutinoside-5-glucoside. In some red and purple Andean potatoes, the most dominant anthocyanins were identified as Pelargonidin-3-(p-coumaroyl)-rutinoside-5- glucoside and Petunidin-3-(p-coumaroyl)-rutinoside-5-glucoside, respectively (Giusti et al. 2014).
23 R1
OH
B HO O
R2 A C
OH
OH
R =R =H Pelargonidin 1 2 R =OCH , R =H Peonidin 1 3 2 R =OCH , R =OH Petunidin 1 3 2 R =R =OCH Malvidin 1 2 3
Figure 5. Main anthocyanin aglycones of potatoes
2.7.2.3 Potential Health Benefits of Anthocyanin-rich Potatoes
Pigmented potatoes like purple- and red-fleshed tubers are common low-cost crops and rich in anthocyanin pigments, which could serve as a potential source of natural colorants by the food or non-food industry (Jansen & Flamme 2006; Reyes & Cisneros-Zevallos,
2007; Vögel et al. 2004). Phenolic acids like chlorogenic, caffeic and p-coumaric acid have been detected in purple- and red-fleshed potatoes, among which chlorogenic acid dominates. (Lewis et al. 1998; Rodriguez-Saona et al. 1998; Navarre et al. 2011). Red- and purple-fleshed potatoes provide higher amount of phenolic compounds that has been found to be associated with potential benefits to health due to their antioxidant activity
(Espin et al. 2000). In red- and purple-fleshed tubers, the concentration of phenolic acids is 3-4 times of that in white-fleshed cultivars (Ezekiel et al. 2013). Flavonoid concentration in red- and purple-fleshed potatoes is as twice as that in white-fleshed cultivars (Lewis et al. 1998). Total antioxidant activity of purple-fleshed potatoes is
24 determined by the content of anthocyanins and phenolics acids like isomers of chlorogenic acids (Hamouz et al. 1999, Lachman et al. 2000). Due to the significantly higher amount of anthocyanins and phenolic acids, antioxidant potential of pigmented potatoes like red- or purple-fleshed is 2-3 times higher than that of white-fleshed potatoes
(Brown 2004). The number of free hydroxyl groups (-OH) in anthocyanin structure determines the level of antioxidant activity of anthocyanins, and it is said that the antioxidant ability of anthocyanins are stronger than some well-known antioxidant vitamins (Lachman & Hamouz 2005; Mitcheva et al. 1993; Harasym & Oledzki 2014).
In epidemiological studies, the correlation between phenolics ingestion and health improvement has been reported (Dragsted et al. 1997; Knekt et al. 2002; Giusti et al.
2014). Diets rich in antioxidant compounds maintain a higher level of antioxidant in blood serum, and the effects of anthocyanins consumption has been investigated by many researchers (Mazza et al. 2002; Prior & Cao 2000). Medical researches have shown that consumption of anthocyanin-enriched food has a largely association with a reduction in incidence and severity of certain cancers and heart disease (Hertog et al. 1993).
Anthocyanins have been known to lower the risks of diseases like cardiovascular diseases, cancer and diabetes (Reddivari et al. 2007; Konczak & Zhang 2004). It was found that colored potato extracts provide protection against the proliferation of prostate cancer cell
(Reddivari et al. 2010). In the study of anthocyanin on rats, purple potato flakes could prevent adverse effects of oxidative damage and reduce the risk of liver injury caused by carbon tetrachloride while red potato flakes could enhance the hepatic superoxide dismutase mRNA and improve the antioxidant system (Han et al. 2007a; Mitcheva et al.
1993; Han et al. 2007b). Jiang et al. (2016) stated that the supplementation of purple
25 potato anthocyanins was able to fight against the development of fatty liver induced by chronic intake of alcohol.
2.7.3 Purple-fleshed Potatoes Cultivars and Anthocyanins
Purple-fleshed cultivars have a large collection of potato tubers with entirely or partially purple flesh, such as Purple Peruvian, All Blue, Shetland Black, Vitelotte, Purple Majesty and Purple Mackintosh (Reyes et al. 2004; Lachman & Hamouz 2005; Li et al. 2012).
Type of anthocyanins differs in assorted cultivars of purple-fleshed potatoes. Six common anthocyanidins except delphinidin were present in the Andean purple potato extracts, in which petunidin and peonidin were the most predominant, and three acylating groups (caffeic, p-coumaric and ferulic) were also identified; acylated anthocyanins were
3-coumaroylrutinoside-5-glucoside derivatives of cyanidin, petunidin, pelargonidin, peonidin and malvidin, and 3-feruloylrutinoside-5- glucoside derivatives of peonidin and petunidin as well as petunidin-3- (caffeoyl)-rutinoside-5-glucoside, of which Petunidin-3-
(p-coumaroyl)-rutinoside-5-glucoside accounted for about 63.2% of the total anthocyanin content as the highest portion (Giusti et al. 2014). Major anthocyanidins in the cultivar
Shetland Black were petunidin (52%) and peonidin (38%), including Petunidin-3-(p- coumaroyl)-rutinoside-5-glucoside, Peonidin-3-rutinoside-5-glucoside and Peonidin-3-(p- coumaroyl)-rutinoside-5-glucoside; however, in potato cultivar Vitelotte, Malvidin-3- rutinoside-5-glucoside was identified as the major pigment with minor amount of
Malvidin-3-(p-coumaroyl)-rutinoside-5-glucoside and Petunidin-3-(p-coumaroyl)- rutinoside-5-glucoside (Singh & Kaur 2016; Eichhorn & Winterhalter 2005). Purple cultivars of Mackintosh and Majesty contained Petunidin-3-(p-coumaroyl)-rutinoside-5-
26 glucoside as the major anthocyanin, followed by Petunidin-3-(caffeoyl)-rutinoside-5- glucoside and Malvidin-3-(p-coumaroyl)-rutinoside-5-glucoside (Li et al. 2012). Identity of anthocyanins in Purple majesty potatoes was also reported by Stushnoff et al. (2008) with five petunidin glucosides and one single glucoside of malvidin, peonidin, and delphinidin.
2.8 Purple Potato Chips
2.8.1 Quality Degradation during Purple Potato Chips Production
Purple potato chips, as an alternative to traditional potato chips, are made with purple potato tubers due to the anthocyanins naturally present in potatoes that not only have health-promoting properties but also bring appealing purple color to the chips (Kita et al.
2015). However, as a processing method, frying would influence the color formation and phenolic content. Total phenolic content in purple potato chips is also reduced compared with raw potato tubers, with a loss range of 0 – 64% found in some chips samples from purple varieties (Kita et al. 2015). Frying method, as a thermal treatment, would cause degradation and transformation of different groups in polyphenols, which explained the diversity of phenolic compounds remaining in purple potato chips (Bąkowska et al. 2003).
Brown et al. (2008) found 70% loss of anthocyanins in purple-fleshed potatoes due to its greater sensitivity during frying than that in red-fleshed potatoes.
Frying leads to anthocyanins degradation and color loss with more browning in purple chips, which is primarily caused by high frying temperature and Maillard reaction (Kita et al. 2015; Jiménez et al. 2012). Up to 99% loss of anthocyanins were observed by Kita
27 et al. (2015) in purple potato chips made from some purple varieties, while Kita et al.
(2013) reported that 50-80% loss of anthocyanin content occurred in purple potato cultivars during frying. Kita et al. (2013) stated that the original color of raw potato flesh was different with the color of the resulting chips, with a* of 5.33 to 8.56 and b* of -3.23 to -0.22 in purple potato chips. Chips made with purple potatoes were characterized with
L* (below 29), a* (2.15-6.61), and b* (-2.62 – 4.42). Typical color of raw purple potatoes differed from the color of chips, which was caused by the masking effect of melanoidins, colored products produced during Maillard reaction (Kita et al. 2015; Hofmann 1998). Da
Silva & Moreira (2008) vacuum frying could be used as an efficient alternative to traditional frying to maintain about 60% higher anthocyanin content in some chips made with Blue Congo variety.
2.8.2 Factors Leading to Color Development of Chips
High temperature during frying is critical to potato chips processing (Kita et al. 2009).
Discoloration of purple-fleshed potato chips occurs during the frying of chips production due to Maillard browning reaction, anthocyanin degradation and enzymatic browning reaction.
Maillard reaction is a non-enzymatic browning caused by the carbonyl groups of reducing sugars and the amino groups of amino acids at higher temperature during frying, during which melanoidins are produced to mask the chips with brown color (Márquez &
Añón 1986; Hofmann 1998). High reducing sugar content contained in potatoes would cause brown and darker color as a result of Maillard reaction during frying (Kita et al.
28 2013). In the production of purple potato chips, purple color in the final product is obtained with brown color spots caused by Maillard browning.
High temperature is a lethal factor affecting color loss of purple potato chips during the frying process due to the thermal sensitivity of anthocyanins (Wrolstad 2000). Color retention of purple-flesh potato extracts at pH 3 stored at 80 and 98℃ in the dark was significantly decreased, indicating a lower thermal stability and degradation during the storage (Reyes & Cisneros-Zevallos 2007). Potato chips are fried at over 160 ℃, and thermal degradation of anthocyanins results in color loss and brown-colored appearance due to the severe polymerization and the formation of brown pigment in purple potatoes
(Starr & Francis 1968; Reyes & Cisneros-Zevallos 2007).
Anthocyanins are sensitive to the changes in pH values that determine the color and stability (Figure 6). Anthocyanins present as the flavylium cation form at low pH, showing bright red color with the most stability (Jackman et al., 1987; Giusti & Wrolstad
2001). As the pH level increases, the flavylium cation transforms into colorless carbinol pseudobases and chalcone forms, followed by blue quinonoidal base at pH 7 (Brouillard
1982; Giusti & Wrolstad 2001). In the study by Reyes & Cisneros-Zevallos (2007), purple-fleshed potato extract showed red color at low pH (1-3), and anthocyanins stability decreased at room temperature as the pH increased.
29
Figure 6. Predominant structural forms of anthocyanins present at different pH levels (Brouillard & Delaporte 1977)
Enzymatic browning is also detrimental to anthocyanins, and some enzymes naturally contained in purple potato tissues, such as glycosidases, polyphenoloxidases (PPO) and peroxidases, can degrade the pigment (Francis 1989; Giusti et al. 2014). Although those enzymes could be inactivated by the high frying temperature, discoloration occurs during the preparation of purple potato slices. Glycosidases are able to turn the pigments into anthocyanidins that are easily degraded and become colorless (Shiroma-Kian et al. 2008;
Schwartz et al. 2008). Anthocyanins can also be oxidized by the reaction with o- benzoquinone, a product of o-diphenols oxidation by PPO (Schwartz et al. 2008).
30 Chapter 3 Materials and Methods
3.1 Raw Materials
Fresh purple-fleshed potatoes, cultivar Purple Majesty grown in California, were purchased from a local grocery store in Columbus, OH, USA.
Raw purple potato slices (2±0.2 mm) were dried after soaking pretreatments using a
VirTis Ultra freeze dryer (SP Scientific, Stone Ridge, NY) and vacuum packed for further analysis of reducing sugar.
3.2 Purple Potato Chips Production
Purple-fleshed potatoes were cut into slices of 2±0.2 mm using an electric slicer after washing. Approximately 60 g potato slices were washed in cold running tap water and soaked in one of four different pre-treatment solutions for 10 minutes: citric acid (0.1%,
0.2%, 1%, 2%), acetic acid (0.1%, 0.2%, 1%, 2%), NaCl (0.1%, 0.2%, 1%, 2%), CaCl2
(0.1%, 0.2%, 1%, 2%), as well as no soaking (control1) and soaking in distilled water
(control 2). After soaking, potato slices were superficially dried using paper towels. Dried slices were fried in a 2-liter deep fryer (Hamilton Beach, Glen Allen, VA) for 3 minutes in vegetable oil heated to 170 °C, followed by discharging of the oil and cooling the chips.
Chips samples (100 g) were then vacuum packed in plastic bag and covered with aluminum foil for laboratory analysis.
31 3.3 CIELab Analysis of Chips Color
CIELab Color of Solid Sample by Reflection. A Hunter ColorQuest XE (Hunter
Laboratories, Reston, VA, USA) was used to measure CIELab color. Solid purple potato samples were crushed, loaded in a small plastic bag and read for CIElab, chroma, and hue angle, with the equipment set for total reflectance, illuminant D65, and a 10° observer angle.
3.4 Anthocyanin Extraction and Purification from Purple Potatoes and
Chips
(a) Extraction of anthocyanins from purple-fleshed potato and chips samples followed procedures by Rodriguez-Saona and Wrolstad (2001). Samples of fresh purple potato, dried potato slices, and chips were powdered with liquid nitrogen and treated with 1.5% trifluoroacetic acid acidified 70% aqueous acetone to reduce pigment degradation due to high polyphenol oxidase activity before filtration. The filtrate was transferred and mixed with 2 volumes of chloroform after a discolored solution was obtained. A clear phase partition was obtained after overnight storage at 4 °C. The upper aqueous layer containing anthocyanin pigments was transferred and dried in a rotary evaporator at
40 °C under vacuum while the chloroform layer containing lipophilic compounds was discarded.
(b) Prior to extraction of anthocyanins from purple-fleshed potato chips, chip samples were weighed, ground by hand, and soaked in chloroform (w/v: 1:2) for one hour at room temperature to remove oil from the chips. The mixture was then filtered by vacuum funnel. An aqueous extract was produced with 1.5% trifluoroacetic acid acidified 70%
32 aqueous acetone to reduce pigment degradation. The chip-chloroform slurry was filtered using vacuum filtration. The residue was re-extracted with acidified acetone water until a colorless solution was obtained. The filtrate was transferred and mixed with 2 volumes of chloroform after a discolored solution was obtained. A clear phase partition was obtained after overnight storage at 4 °C. The upper aqueous layer containing anthocyanin pigments was transferred into a 50-ml centrifuge tube. Each aqueous extract was centrifuged at a rate of 5,000 rpm for 5 min. Centrifuged sample extract was dried in a rotary evaporator at 40 °C under vacuum and collected in a 100 ml volumetric flask with 0.01% acidified distilled water.
(c) Anthocyanin extracts from samples of purple-fleshed potatoes and chips were purified using the method by Rodriguez-Saona and Wrolstad (2001). Crude extracts were purified by filtering the sample through a C18 cartridge and washed with 0.01% HCl acidified water to remove sugars and acids not absorbed, followed by ethyl acetate loaded to remove polyphenolic compounds. Anthocyanin pigments were eluted with 0.01% HCl acidified methanol and collected to remove the methanol in a rotary evaporator at 40 °C under vacuum. Purified anthocyanin extract was stored in acidified water at 4 °C for further analysis.
3.5 Determination of Total Monomeric Anthocyanin Content
Total monomeric anthocyanin was quantitated using the pH differential method by Giusti and Wrolstad (2001). The concentration of the monomeric anthocyanin pigment was determined based on the UV-Visible difference of each sample at λvis-max and 700 nm at pH 1.0 and pH 4.5, using the formula: ACN (mg/L) = (A×MW×DF×1000)/(ε×1). A was the absorbance, calculated by (Aλmax – A700nm)pH1 – (Aλmax – A700nm)pH4.5, MW was
33 molecular weight of ACN (449.2), DF was dilution factor, and ε was molar absorptivity of ACN (26900).
3.6 Determination of Percent Polymeric Color
Polymeric color percentage was also quantitated using the method by Giusti and
Wrolstad (2001). Using the dilution factor determined previously, control sample were made with distilled water and bleached sample were made with bisulfite solution, followed by 15-minute equilibration. Absorbance of both samples with different pre- treatments at 420 nm, λvis-max, and 700 nm were measured. Polymeric anthocyanin content was determined using the equations given by Giusti and Wrolstad (2001).
3.7 Determination of Total Phenolic Content
Total phenolics were determined using the adapted microscale protocol for Folin-
Ciocalteau colorimetry by Waterhouse (2002). Each 20 µl purple-fleshed chips extract, a gallic acid calibration standard and distilled water were put into a 1-cm path length plastic cuvette (2 ml), respectively. 1.58 ml water with 100 µl FC reagent were added to each cuvette and mixed thoroughly, followed by 5 min incubation. Then 300 µl sodium carbonate solution was added, mixed and incubated for 2 hours at room temperature.
Absorbances of all samples were measured at 765 nm. A calibration curve from the standards was created after subtracting the absorbance of the blank. Total phenolics were determined using the curve and expressed as gallic acid equivalents (GAE).
34 3.8 Determination of Reducing Sugar Content by HPLC
In this experiment, glucose and fructose levels will be determined using high pressure liquid chromatography (HPLC, Shimadzu Scientific Instruments, Inc., Columbia, MD) equipped with LC-6AD pumps, SIL-20A HT autosampler, CTO-20A column oven, and
Refractive Index Detector. Separations of sugars were carried out using a RezexTM RCM-
Monosaccharide Ca+ column (300 × 7.8 mm; Phenomenex, Torrance, CA) column.
Purple potato extracts (2 mL) were filtered into the HPLC vials through 0.2 µm, 15 mm membrane syringe filter (Phenomenex®, Torrance, CA, USA). Filtered extract (10 µL) was injected and isocratic separation of the sugars was achieved at 80 °C using HPLC grade water with a flow rate of 1 mL/min during the 20 min runs. Chromatograms were collected and analyzed using LC Solutions software (Shimadzu, Columbia, MD). Pure glucose and fructose (Fisher Scientific, Fair Lawn, NJ) with concentrations from 0.625-
40 mg/mL were run to obtain a standard calibration curve and quantify the concentration of each sugar in the sample extracts. Contents of reducing sugars were calculated by adding values of these two sugars.
3.9 pH Value of Pretreatment Solutions and Raw Potatoes
Using a pH meter (Mettler Toledo, Columbus, OH), pH values of the selected pretreatment solutions were measured before and after soaking potato slices. pH of raw purple potatoes was measured by inserting the pH probe into the hole made by a spatula in the potato tubers.
35 3.10 Anthocyanin Analysis by HPLC
Anthocyanins of purple potatoes and chips samples were analyzed using an HPLC
(Shimadzu, Columbia, MD) system equipped with LC-20AD pumps, a SIL-20AC autosampler and an LCMS Shimadzu 2010 Liquid mass spectrometer with an SPD-
M20A Photodiode Array Detector. LCMS Solution Software (Shimadzu, Columbia, MD) was used to collect the results. A reverse-phase symmetry C18 column with 5 µm particle size, 100Å pore size and 150 x 4.6 mm column size (Phenomenex®, Torrance, CA, USA), was used to separate anthocyanins, with a C18 column guard (Phenomenex®, Torrance,
CA, USA). Samples were purified using C18 column, filtered through 0.25 µm, 15 mm membrane syringe filter (Phenomenex®, Torrance, CA, USA), and a 75μL sample extract was injected . Separation of the anthocyanins was achieved at 25 °C using two solvents. The solvents were phase A, 4.5% formic acid in H2O, and phase B, acetonitrile.
A binary gradient was used for solvent B with a flow rate of 0.8 mL/min: 0-2 min for 7%,
2-30 min for 7-20%, 30-36 min for 20-60% and 36-50 min for 60-7%; spectral data were obtained from 200-700 nm, and the measurement of anthocyanin eluted was at 520 nm.
3.11 Storage Stability Analysis
20 grams of purple potato chips pretreated with 1% and 2% citric acid solutions were vacuum packed in plastic bags to prevent oxidation by air, covered with aluminum foil to avoid light exposure, and stored for stability analysis. CIELab color of chips were measured every two weeks as well as total monomeric anthocyanin content, percent polymeric color and total phenolics following extraction and purification.
36 3.12 Statistical Analysis
Microsoft Excel 2013 was used to process and analyze data of colorimetric measurements, reducing sugar amount, monomeric anthocyanins, percent polymeric color, and total phenolics. Analysis of variance (ANOVA) was completed by using SPSS 24.0, and significance of difference was identified by Tukey’s post hoc test (α=0.05).
37 Chapter 4 Results and Discussion
4.1 Colorimetric Evaluation
Color is an important attribute affecting product quality and customers’ willingness to purchase. CIELab color parameters of all samples, purple potato flesh as well as freshly- made chips and stored chips were measured using a HunterLab benchtop spectrophotometer.
Color of fresh purple potato flesh and chips are shown in Table 2 and Figure 7, respectively. Purple potato flesh had values of lightness (L*) of 36.2 ± 2.9, hue angle (h*) of 330.7 ± 2.0, as well as a*= 7.7 ± 1.1 and b* = -4.3 ± 0.6, indicating a bright purple reddish pigment of the flesh. By contrast, all purple potato chips pretreated with 0.1% and 0.2% citric acid solution, as well as those pre-treated with the different concentrations of acetic acid, NaCl, CaCl2 soaking solutions had darker colors with L* ranging from 21.6 to 32.9 and h* from 306° to 329°. Potato slices pre-soaked in 1% and
2% citric acid solutions had lighter color with average values of 39.1 in lightness and
317.9°in hue angle. Color of purple potato extract at pH 3 to 10 are presented in Figure 8 to help perceive the color based on CIELab data. Significant differences between chips soaked in 1%, 2% citric acid and other solutions were identified in lightness (p ≤ 0.003) while there was no significant differences in hue angles (p ≥ 0.772). Chips colors of most
38 of all samples were darkened mainly as a result of melanoidins, brown-colored compounds produced by Maillard reaction occurred under high temperature (Kita et al.
2015; Hofmann 1998). Besides that, discoloration due to enzymatic browning could be prevented by dipping peeled potatoes in citric acid solution with low pH to inactivate the polyphenoloxidase (Sapers & Miller 1995).
Table 2. Color characteristics of raw purple-fleshed potato and purple potato chips with 1% and 2% citric acid pretreatments using CIELab color
Time Sample L* a* b* L* C* h*(°) ∆E* (week) Raw purple 36.2 7.7 -4.3 36.2 8.8 330.7 0 --- potato (2.9) (1.1) (0.6) (2.9) (1.2) (2.0) 38.8 3.3 -3.0 38.8 4.5 317.9 0 --- (0.4) (0.3) (0.1) (0.4) (0.2) (3.6) 39.3 3.7 -4.5 39.3 5.9 309.7 1.8 2 (0.7) (0.1) (0.3) (0.7) (0.2) (1.2) (0.5) Chips 39.3 3.7 -3.8 39.3 5.3 314.4 1.4 4 soaked in (0.8) (0.3) (0.3) (0.8) (0.4) (1.8) (0.2) 1% citric 39.3 3.7 -4.0 39.3 5.5 312.7 1.3 6 acid (0.3) (0.2) (0.3) (0.3) (0.3) (2.0) (0.4) 40.5 4.1 -4.2 40.5 5.8 314.1 2.3 8 (0.8) (0.4) (0.4) (0.8) (0.5) (1.4) (0.6) 40.3 4.1 -4.9 40.3 6.4 310.1 2.5 10 (0.2) (0.3) (0.2) (0.2) (0.3) (1.6) (0.3) 39.3 4.2 -4.3 39.3 6.0 314.3 0 --- (1.5) (0.3) (0.5) (1.5) (0.5) (1.8) 39.6 4.7 -4.5 39.6 6.5 316.3 1.4 2 (0.3) (0.2) (0.3) (0.3) (0.3) (1.5) (0.5) Chips 39.3 4.3 -4.4 39.3 6.1 314.8 1.4 4 soaked in (0.3) (0.2) (0.5) (0.3) (0.5) (2.6) (0.3) 2% citric 39.4 4.3 -3.5 39.4 5.6 321.3 1.7 6 acid (0.5) (0.3) (0.2) (0.5) (0.3) (1.0) (0.5) 39.9 4.4 -4.1 39.9 6.0 317.4 1.4 8 (0.3) (0.2) (0.3) (0.3) (0.4) (1.4) (0.9) 39.1 4.3 -4.0 39.1 5.9 317.3 1.4 10 (0.2) (0.3) (0.5) (0.2) (0.5) (2.2) (0.4)
39
Figure 7. Lightness (L*) and hue angle (h*) of purple potato chips pretreated with different soaking solutions. Columns with the same letters are not significantly different from each other at p > 0.05; columns with different letters are significantly different from each other at p < 0.05.
pH 3 4 5 6 7 8 9 10
L* 75.3 79.6 86.2 52.6 37.2 37.3 39.2 46.2 h* 347.4 349.3 353.4 327.7 310.1 281.6 266.2 252.1
Figure 8. Color of purple potato extract at pH 3 to 10
Color stability of chips pretreated with 1% and 2% citric acid during the storage was completed in the 10-week study, and CIELab color characteristics were measured every two weeks and presented in Table 2. Color data from week 0 (one day after frying) were
40 considered as the control for 1% and 2% citric acid-treated chips separately. In 1% citric acid-treated chips, average color values were maintained almost unchanged, with lightness (L*) ~ 40, hue angle (h*) ~ 313°, a* ~ 3.8 and b* ~ -4.0. As a comparison, color data from 2% citric acid-treated chips were measured as of L* 39.4, h* 316.9º, a* 4.4 and b* -4.1 in average. As the study progressed, significant differences of blueness (-b*) and color intensity (C*) were found in chips made with 1% citric acid (p ≤ 0.004) while lightness (L*) and redness (a*) started to show significance of difference since week 8 (p
≤0.006). In chips soaked in 2% citric acid solutions, there was no significant change in all color parameters (L*, a*, b*, C*, h*) during the 10-week storage (p ≥ 0.109). Total color difference (△E) was included in the color scale to quantify the overall difference of one sample and one standard on the three dimensional color space (△L*, △a*, △b*) of
CIELab. Nearly all △E values in color of both pre-treated purple potato chips were below 2, indicating that there were very small difference identified; however, color difference from week 8 and 10 of 1% citric acid-treated chips were above 2 but below 3.5, indicating a medium difference in total color. △E value less than 2 is expected to be only noticed by trained eyes, and the color changes of purple potato chips during storage are not obvious and not able to be detected by consumers, especially the chips treated with 2% citric acid.
4.2 Analytical Testing
Reducing sugars, together with amino acids, are the major reactants of the Maillard browning reaction, and the reduction in the amount of reducing sugar has been used as a means to inhibit or limit the extent of this reaction (Márquez & Añón 1986). Sugars in
41 potatoes consist of glucose and fructose as free reducing sugars, as well as sucrose (Zhu et al 2010). Reducing sugar content of raw purple potatoes was 3.38±0.03 g/100g in dry weight (DW); reducing sugar in potato slices washed but without soaking (Control 1) and soaked in distilled water (Control 2) decreased, with values of 2.63±0.03 g and 2.06±0.02 g per 100 g (DW), respectively. Compared to Control 1 and Control 2, the amount of reducing sugar of purple potato slices soaked in citric acid, acetic acid and NaCl, were all significantly reduced (p ≤ 0.005), while no significant changes were identified in reducing sugar content of samples treated with CaCl2 solutions (p = 0.641). Citric acid solutions decreased the reducing sugar content of the potato slices the most, followed by
NaCl and acetic acid solutions (Figure 9).
Figure 9. Content of reducing sugar in potato slices pretreated with soaking solutions. Columns with the same letters are not significantly different from each other at p > 0.05; columns with different letters are significantly different from each other at p < 0.05.
42 Soaking in lower pH solutions such as citric acid would not only help lower the pH of potatoes but also leach out reducing sugars and asparagine from the surface layer of potato slices (Jung et al. 2003). In this study, soaking in distilled water did not induce pH reduction in potato slices; however, soaking solutions of citric acid and acetic acid possessed higher acidity values, with pH between 2 and 3, which helped with the removal of reducing sugar content and explained the larger reduction of reducing sugar content during soaking.
Once potato slices were soaked in NaCl solutions, concentration differences formed between cell membranes of potato tissues and hypertonic NaCl solutions. As previously reported (Bunger et al. 2003; Califano & Calvelo 1988), soaking potato strips into NaCl solution prior to frying could favor osmotic dehydration to help with the removal of solutes (organic acids, reducing sugars etc.) present in potato tissues by using water as the carrier, which removed the precursors of the Maillard reaction in potatoes.
By contrast with other soaking pretreatments, CaCl2 solutions did not significantly lower the reducing sugar amounts in the soaked potato slices. This could be explained because adjacent chains of pectin polymers can be bridged by Ca2+ and the formation of cross links between pectin polymers in potato tissues could have trapped the sugars in (Ralet et al. 2001; Thakur et al. 2009; Andersson 1994).
The pH values of pretreatment solutions along with purple potatoes were measured before and after soaking process (Table 3). The pH of purple potatoes was around 6.4, and a pH value of 5.8 – 6.0 was found in distilled water. Citric acid and acetic acid made soaking solutions below pH 4, especially approximately pH 2 in 1% and 2% citric acid
43 solutions. Soaking solutions of NaCl and CaCl2 had pH values around 6, closer to the pH level of purple potatoes. The soaking process slightly increased the pH of each solution.
Table 3. Changes in pH of pretreatment solutions after soaking purple potato slices
Soaking pH of soaking solutions with different concentrations solutions Control 1 No soaking 6.3-6.5 before 5.8-6.0 Control 2 after 6.3-6.4 0.1% 0.2% 1% 2% before 2.7-2.9 2.5-2.7 2.1-2.3 1.9-2.1 Citric acid after 2.9-3.0 2.7-2.8 2.2-2.3 2.0-2.1 before 3.2-3.3 3.0-3.2 2.7-2.8 2.5-2.7 Acetic acid after 3.7-3.8 3.4-3.6 2.9-3.0 2.8-2.9 before 5.7-5.9 5.6-5.9 5.6-5.7 5.8-6.1 NaCl after 6.0-6.1 6.0-6.1 5.8-6.0 5.8-5.9 before 5.7-5.9 5.8-5.9 5.9-6.0 6.1-6.2 CaCl 2 after 5.9-6.0 5.8-5.9 5.7-5.8 5.5-5.6
Average of monomeric, polymeric anthocyanins and total phenolics content in raw purple potatoes and chips are presented in Table 4. Content of monomeric anthocyanins and total phenolics was expressed as mg of Cyanidin-3-glucoside equivalents/100g (mg Cy-3- gluc eq./100g) and mg of gallic acid equivalents/100g (mg GAE/100g), respectively.
Freeze-dried purple potato powder was determined to contain 150.6±1.8 mg Cy-3-glu eq./100g monomeric anthocyanins and 634.2±3.9 mg GAE/100g total phenolics with
11.6% polymeric color. Anthocyanins concentration in some purple-fleshed tubers was estimated and reported as 5.5 – 17.1 mg/100g FW by Brown et al. (2003), while Kita et al.
(2015) reported a range from 37.20-57.18 mg/100g DW in some purple flesh potato tubers. Nemś et al. (2015) stated that the amount of total anthocyanins in different raw purple potato varieties ranged from 38.7-93.7 mg/100g DW. Total polyphenols content in
44 some purple-fleshed potatoes was quantified as 455-481 mg GAE /100g dry weight
(Lachman et al. 2008). The content of mono- and polymeric anthocyanins differed among publications due to different cultivars and harvest time of purple potatoes.
Compared to raw potatoes, the contents of monomeric anthocyanins and total phenolics in all chips samples decreased significantly (p ~ 0.000) while the percentage of polymeric color increased to a great extent. A loss of 57.7-79.7% monomeric anthocyanins and
12.5-36.8% loss of polymeric anthocyanins were found in all purple potato chips samples.
Loss of total phenolics was found up to 64% in potato chips made with some purple- fleshed cultivars (Kita et al. 2015).
High temperature is critical in the chips production to achieve dehydration and final product with appealing color and crunchy texture; however, high temperatures can also be detrimental to pigments in purple potatoes during the chips frying because of the thermal sensitivity of anthocyanins, constituting a primary caused of degradation
(Wrolstad 2000; Kita et al. 2015). Similarly, high frying temperature could also degrade polyphenols and transform polyphenols of different groups (Bąkowska et al. 2003). In addition, enzymes like polyphenoloxidase (PPO) contained in potato slices soaked in non-acidic solutions (NaCl, CaCl2) were not inactivated during the sample preparation, which would also contribute to the loss of phenolics.
However, purple potato chips samples pretreated with 1% and 2% citric acid showed better color quality and retention of phenolics after frying. Higher monomeric anthocyanin (50.3-63.7 vs 30.6-44.6 mg Cy-3-glu eq/100g) and less polymeric color
(25.4-35.7% vs 51.4-69.4%) were also found in 1% and 2% citric acid samples compared with other chips, as well as lightly more total phenolics (538.5-554.7 vs 400.7-519.1 mg
45 GAE/100g). Significant differences in mono-, polymeric anthocyanins, and total phenolics were identified with p ≤ 0.015. The 1% and 2% citric acid soaking solutions had pH values around 2, in which anthocyanins are mostly present as the flavylium cation form, showing bright red color with the most stability (Jackman et al. 1987; Giusti &
Wrolstad 2001). pH value is an important factor in Maillard browning, and pH reduction would inhibit the reaction by blocking the addition of asparagine with a carbonyl compound and preventing the formation of a key intermediate in the Maillard reaction, the Schiff base (Kita et al. 2004; Rydberg et al. 2003). Soaking potato slices in acidic solutions like citric acid and acetic acid, is able to not only lower the non-enzymatic browning by reducing pH of potatoes and leaching out reducing sugars, but also inhibit the enzymatic browning by inactivating polyphenoloxidases (Mestdagh et al. 2008b;
Sapers & Miller 1995; Jung et al. 2003). Polyphenols in purple potatoes were protected from enzymatic reaction due to the inactivation by low pH of 1% and 2% citric acid soaking solutions. Thus, the acidity of 1% and 2% citric acid soaking stabilized anthocyanin in potatoes, resulting in brighter purple-reddish chips with (Table 3, Figure
8).
46 Table 4. Average of monomeric, polymeric anthocyanins and total phenolics content in dried raw purple potatoes and chips. Monomeric anthocyanin expressed as mg of cy-3- glu equivalents / 100 grams, total phenolics expressed as mg of gallic acid equivalents / 100 grams.
Monomeric Polymeric Total phenolics anthocyanins color (%) (mg/100g) (mg/100g) Raw purple potato (dry basis) 150.6 (1.8) a 11.6 (2.9) a 634.2 (3.9) a Control 1 No soaking 36.9 (0.3) b 64.6 (2.8) b 400.7 (4.1) b Control 2 Distilled water 36.8 (1.5) b 51.4 (0.6) c 432.5 (4.5) c 0.1% 40.8 (0.7) c 53.8 (4.4) c 489.8 (3.8) d 0.2% 38.3 (0.8) c 51.9 (5.1) c 519.1 (5.0) e Citric acid 1% 50.3 (1.7) d 35.7 (2.6) d 538.5 (4.8) f 2% 63.7 (1.0) e 25.4 (1.3) e 554.7 (4.2) g 0.1% 37.2 (0.4) b 60.8 (2.9) b 443.6 (3.0) h 0.2% 40.8 (0.5) c 63.8 (4.4) b 415.7 (3.3) i Acetic acid 1% 39.6 (0.6) c 66.5 (0.8) b 449.4 (4.3) h 2% 40.3 (0.7) c 63.2 (4.3) b 469.5 (3.9) j 0.1% 42.7 (1.8) f 67.1 (2.4) b 423.4 (5.1) c 0.2% 40.3 (0.9) c 62.1 (3.5) b 463.7 (3.5) j NaCl 1% 41.1 (0.5) c 65.1 (2.6) b 435.7 (2.9) c 2% 44.6 (1.5) f 62.1 (3.7) b 510.1 (4.7) e 0.1% 30.6 (0.4) g 69.4 (4.6) b 420.6 (3.9) i 0.2% 37.4 (0.9) b 64.0 (1.8) b 439.1 (3.7) c CaCl 2 1% 36.4 (1.4) b 66.1 (3.1) b 417.6 (4.1) i 2% 37.8 (1.8) b 68.1 (1.7) b 445.1 (4.9) h Within each column, data with the same letters are not significantly different from each other at p > 0.05 while data with different letters are significantly different from each other at p < 0.05.
Stability tests were carried out over 10 weeks of storage, and measurements were completed every two weeks. In chips samples with 1% citric acid pretreatment, monomeric anthocyanin content decreased from 50.3 to 29.0 Cy-3-glu eq. mg/100g, and polymeric anthocyanins percentage increased from 22.5% to 36.3% during the first 8 weeks of storage (Figure 10), likely due to the polymerization of monomeric anthocyanins. After week 8, the amount of monomeric anthocyanins increased to 38.6
47 Cy-3-glu eq. mg/100g while percent polymeric color fell down to 22.4% at the same time as the polymers started to precipitate due to lower solubility. Compared with the fresh chips, stored samples did not show significant difference in the monomeric anthocyanins change until week 6 (p ~ 0.000). Similar trends presented on 2% citric acid-treated chips
(monomeric anthocyanins: from 63.7 to 36.7 to 42.9 Cy-3-glu eq. mg/100g; percent polymeric color: from 18.2% to 33.4% to 22.4%).
Figure 10. Changes in monomeric anthocyanin content and percent polymeric color of purple potato chips during storage. Columns with the same letters are not significantly different from each other at p > 0.05; columns with different letters are significantly different from each other at p < 0.05.
48 Total phenolics of purple potato chips were also monitored over the 10-week storage period. Changes in the phenolic content of chips were not largely affected as mono- and polymeric anthocyanins, and it remained around 520 GAE mg/100g DW.
4.3 Anthocyanins in Raw Purple Potatoes and Potato Chips
The major anthocyanin (peak 2) in both, extracts of purple raw potatoes and purple chips, eluted in approximately 15.2 minutes and were identified as the same compound,
Petunidin-3-(p-coumaroyl)-rutinoside-5-glucoside (Figure 11).
Figure 11. Anthocyanins from purple potato and purple potato chips extracts
49 This indicated that the type of major anthocyanin in raw purple potatoes did not change during the production of chips. Other anthocyanins were also identified, including
Petunidin-3-(caffeoyl)-rutinoside-5-glucoside (peak 1) and Malvidin-3-(p-coumaroyl)- rutinoside-5-glucoside (peak 3). Petunidin-3-(p-coumaroyl)-rutinoside-5-glucoside, as the dominant anthocyanin found in the Purple Majesty, were also reported by other researchers (Stushnoff et al. 2008; Nayak et al. 2011). In addition to Petunidin-3-(p- coumaroyl)-rutinoside-5-glucoside, minor anthocyanins like Petunidin-3-(caffeoyl)- rutinoside-5-glucoside and Malvidin-3-(p-coumaroyl)-rutinoside-5-glucoside were tentatively identified, which was in agreement with the identification of predominant pigment in Purple Majesty cultivar by Li et al. (2012).
50 Chapter 5 Conclusion
Soaking purple potato slices in selected solutions as pretreatments before frying had an effect on the color quality and phenolic content of purple potato chips. However, the results obtained differed depending on the type and concentration of the chemicals used for soaking. Compared to other solutions (acetic acid, NaCl, CaCl2), soaking purple potato slices in citric acid solutions of 1% and 2% resulted in chips of appealing brighter purple-reddish color and higher retention of monomeric anthocyanins and total phenolics.
Citric acid solutions of 1% and 2% had acidity around pH 2, which not only helped lower the pH level of soaked purple potato slices but also suppress the Maillard reaction and enzymatic browning, resulting in less brown color development and brighter purple- reddish chip color. In the meantime, soaking potato slices in 1% and 2% citric acid also made chips of higher monomeric anthocyanins and total phenolics as well as less polymeric color due to the inhibition of both enzymatic and non-enzymatic reaction. The frying process influenced the color appearance and phenolics content of purple potato chips, but the type of major anthocyanin in purple potatoes did not change, which was identified as Petunidin-3-(p-coumaroyl)-rutinoside-5-glucoside. Chips made with 2% citric acid solutions showed better color stability with no significance (p ≥ 0.109) of color changes during the storage test. Long storage period of chips would accumulate the
51 polymerization of anthocyanins from fresh purple potato chips. Total phenolic content in chips were not largely changed as much as anthocyanins.
Citric acid’s ability to function as an acidulant resulted in greater benefit as compared to other pretreatment chemicals tested. Soaking potato slices in citric acid at certain concentrations could be used as pretreatment methods to enhance the color and phenolic quality of fried purple potato chips, which could bring potential benefits to the potato chips industry.
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