Effect of Vacuum Frying on Quality Attributes of Fruits

Food Engineering Reviews https://doi.org/10.1007/s12393-018-9178-x

REVIEW ARTICLE

Fitriyono Ayustaningwarno 1,2,3 & Matthijs Dekker1 & Vincenzo Fogliano1 & Ruud Verkerk1

Received: 6 December 2017 /Accepted: 12 April 2018 # The Author(s) 2018

Abstract Vacuum frying of fruits enables frying at lower temperatures compared to atmospheric frying, thereby improving quality attributes of the fried product, such as oil content, texture, retention of nutrients, and color. Producing high-quality vacuum-fried fruit is a challenge, especially because of the high initial water content of fruits that requires long frying times. Factors influencing vacuum-fried fruit quality attributes are the type of equipment, pre-treatments, processing conditions, fruit type, and fruit matrix. Pre-treatments such as hot air, osmotic drying, blanching, freezing, impregnation, anti-browning agents, and hydrocolloid application strongly influence the final quality attributes of the products. The vacuum-frying processing parameters, namely frying time, temperature, and vacuum pressure, have to be adjusted to the fruit characteristics. Tropical fruits have different matrix properties, including physical and chemical, which changed during ripening and influenced vacuum-fried tropical fruit quality. This paper reviews the state of the art of vacuum frying of fruit with a specific focus on the effect of fruit type and matrix on the quality attributes of the fried product.

Keywords Vacuum frying . Fruit . Quality attributes . Matrix . Phytochemicals

Introduction Vacuum frying is a frying process below atmospheric pressure (~ 100 kPa). At reduced pressure, the boiling Fried products are appreciated by all age groups and play an point of oil and water is lower compared to atmospheric important role in consumer’s diet because of their unique fla- pressure [31]. Due to a lower frying temperature, vacu- vor and texture. However, it is difficult to combine fried foods um frying better preserves the nutritional value, aroma, with the contemporary consumer trends toward healthier and and color of the fried product compared to atmospheric low-fat products. There is an increased demand for healthy frying [2]. snack products with good taste, texture, and appearance Some anecdotic findings from existing studies highlighted [38]. This demand offers the opportunity to design novel fried several advantages vacuum frying might have over atmo- products that have higher health properties such as fruit-based spheric frying: products. Increasing fruit consumption is promoted in all parts of the world to increase public health. Fruit implicitly has a & Oil uptake in vacuum-fried apple chips is lower compared strong health awareness based on the content of (micro) nutri- frying at atmospheric pressure [44]; ents, fibers, and numerous bioactive phytochemicals [19, 51]. & Color of vacuum-fried mango was lighter compared to atmospheric frying [17]; & Carotenoid retention was higher in vacuum-fried mango compared to atmospheric frying [50]; * Ruud Verkerk & Vacuum-fried mango was more uniform and crispier [email protected] compared to soggy, burnt, and oily for atmospheric 1 Food Quality and Design, Wageningen University & Research, fried mango [50]. Bornse Weilanden 9, 6708 WG Wageningen, Netherlands 2 Department of Nutrition Science, Faculty of Medicine, Diponegoro The multiple factors influencing the quality attributes of University, Semarang, Indonesia vacuum-fried fruit can be distinguished in vacuum-frying equip- 3 Center of Nutrition Research, Diponegoro University, Jalan Prof. ment (type and specifications), properties of the raw fruit (fruit Soedarto, SH Tembalang, Semarang, Jawa Tengah 1269, Indonesia matrix), pre- and post-treatments, and processing conditions. Food Eng Rev

Time, temperature, and vacuum pressure influenced color, tex- the end of vacuum frying, the vacuum breaking period pro- ture, nutrients, and oil content of fried fruits [2, 14, 73]. duces a higher outside pressure then the pore pressure. Another relevant aspect is the fruit matrix such as the fruit The oil content of vacuum-fried apples was lower com- type and ripening stage that are affecting the vacuum-fried pared to atmospheric fried one. Apple absorbed 1.2–2.0 times product quality attributes [20, 30, 84]. Pre-treatments such as more oil by atmospheric frying compared to vacuum frying blanching, drying, freezing, antioxidant, and coating applica- [29, 44]. This difference was explained by the lower temper- tions have been used to preserve color, improve texture, and atures during vacuum frying due to the lower vapor pressure reduce oil absorption [9, 22, 24]. The use of post-frying steps of water. This low temperature will reduce temperature- such as centrifugation has a major effect on the oil content of induced tissue matrix degradation that increases the oil ab- fried product [47]. sorption. Dueik et al. [30] found that atmospheric fried apple Some recent papers dealt with different aspects of vacuum- had a larger portion of small pores and absorbed more oil by frying technology. The strategies to reduce oil absorption of capillary suction compared to vacuum-fried apple. Larger vacuum-fried products have been studied intensively by pore formation was related to the higher specific volume of Moreira [46], including optimizing temperature, pressure, water vapor at lower pressure. These studies provided a con- pre-treatment, pressurization speed, and de-oiling time. The vincing explanation about the mechanisms behind the reduced recent review by Diamante et al. [26] discussed the product oil absorption of vacuum-fried products. and process optimization, oil uptake, oil quality, as well as Shyu and Hwang [61] showed that oil absorption was high- packaging and storage of vacuum-fried fruits without men- ly correlated with moisture loss in vacuum-fried apple slices. tioning matrix factors. Dueik and Bouchon [28]and At the beginning of the frying procedure, the outer surface of Ayustaningwarno and Ananingsih [11] compared the quality the product is dried, the moisture inside the product is con- changes comparing atmospheric and vacuum frying as well as verted into steam, and a pressure gradient is created. By the oil quality and packaging of fried products. Dueik and prolonging the frying, the dried surface becomes more hydro- Bouchon [28] put emphasis on the microstructure, methods phobic which facilitates the absorption of oil. This can explain to reduce oil uptake, oil quality, bioactive compound degrada- the observed oil content that was increased from 33.64% in tion, and toxic compound generation. Andres-Bello et al. [8] first 5 min of vacuum frying to 39.38% after 30 min of vacu- reviewed the vacuum-frying processing for producing high- um frying. This oil absorption mechanism is different com- quality fried products, focusing on equipment types, pre-treat- pared with atmospheric frying in which most of the oil is ments, and vacuum-frying conditions. absorbed after frying during the cooling period [12]. Based on this existing background information, this The situation found in apple is different as found in plantain review will consider the effects of vacuum frying on andmango:asinplantain[2] and mango [17], vacuum frying changes in quality attributes of tropical fruits with a focus resulted in a higher oil content compared to atmospheric fry- on the role of the fruit matrix, since this is a very relevant ing. This difference could be attributed by matrix differences but underexposed factor. of apple with plantain and mango. Wexler et al. [80] explained that at the end of vacuum frying of papaya, capillary absorption of surface oil was favored to be absorbed inside the prod- Vacuum Frying Versus Atmospheric Frying uct when the vacuum was broken to restore the system into atmospheric pressure. Additionally, vacuum-fried plantain The main difference between vacuum frying and atmospheric had less gelatinized starch due to the lower temperature, there- frying is the lower boiling point of water at lower pressures by having more pores and absorbed more oil compared to that enables to fry at lower temperatures. For that reason, atmospheric frying [2]. vacuum frying has many advantages over atmospheric frying In general, a higher nutrient retention is expected with a in relation to product quality attributes. Several comparative lower temperature of vacuum frying. Ascorbic acid content of studies between vacuum and atmospheric frying were done on apple was found 1.7–1.9 times higher after vacuum frying apple, plantain, banana, and mango. compared to atmospheric frying [29]. Additionally, carotenoid retention in vacuum-fried mango was two times higher com- Oil and Nutrient Content pared to atmospheric frying. High retention of carotenoid was attributed by the absence of oxygen, which induce oxidation The mechanism of oil uptake in atmospheric frying and vac- in atmospheric frying [50]. In addition, a lower temperature of uum frying is different. Oil uptake occurs mainly after frying: vacuum frying compared to atmospheric frying will have an by the lower pressure in the pores, the oil present on the sur- effect on the nutrition degradation. A less pronounced effect face of the products is sucked into the pores. During atmo- was observed by Da Silva and Moreira [17] who found that spheric frying, this lower pressure in the pores is created by vacuum-fried mango had 20–50% higher carotenoids com- the evaporative cooling after frying [12]. On the other hand, at pared to atmospheric fried mango. Food Eng Rev

Color, Texture, and Sensory Attributes Fresh Fruits Natural fruit color preservation is an important product quality attribute for vacuum-fried fruit [46]. This color preservation can be attributed to the low pressure and temperature of the Peeling and slicing vacuum-frying process. A low pressure means a low oxygen level, thereby reducing oxidation processes, which could lead to darkening of the color. In addition, low temperature slows down non-oxidative browning reaction. Vacuum frying better Pre-drying Blanching Pulping preserved the lightness and redness of apple chips compared and to atmospheric frying [29, 44]. Similar results for lightness dough and redness were found in plantain [2] and mango [17, 50]. Immersing Freezing making Vacuum frying of plantain produced a crispier product compared with plantain fried in atmospheric pressure, in- dicated by a lower maximum breaking force value [2]. Less effect was observed in mango which had no maximum breaking force value difference between vacuum and atmo- Vacuum Frying spheric fried mango [17]. Based on sensory analysis, vacuum-fried plantain chips have significantly higher scores on sensory attributes as taste, aroma, overall appearance (color), and texture (crispiness/ Centrifugation crunchiness) [2]. Similar observations were done by [17], showing that vacuum-fried mango has significantly higher sensory score in color, odor, texture, flavor, and a higher overall quality than perceived for atmospheric fried mango.

Vacuum-Frying Process Vacuum Fried Fruits The vacuum-frying process consists of several steps as sum- Fig. 1 Flow chart of vacuum-frying process marized in Fig. 1. These steps include fruit preparation, peeling and slicing, pre-treatment, vacuum-frying process, and removal of excess oil. Vacuum frying usually uses raw mate- In this section, common pre-treatments used for vacuum- rials as fresh fruits. However, fruit paste also can be used by frying processing will be mentioned briefly and discussed preparing a dough made up with fruit pulp and starch or flour further in separated sections. The pre-treatments that are re- [73]. Utilization of fresh fruit has some advantages as well as ported in literature are blanching, pre-drying, impregnation, disadvantages. The product could be recognized by the con- and freezing [20, 22, 23, 35, 43, 44, 50, 61, 64]. Blanching sumer as the original fruit, but fresh fruits usually have a is used to minimize enzymatic browning [44, 61] and also to variety of shapes and irregularities resulting in uneven heat pre-gelatinize starch. Pre-drying is used to reduce the initial distribution during frying and a subsequent inhomogeneity water content before frying and thus reduce frying time [44]. in color and texture [36]. On the other side, using fruit paste Osmotic dehydration is used to introduce salt or sugar to re- a homogenous product in size and shape can be obtained, but duce initial water content [17, 22, 23, 50, 61, 64]. Application the characteristic of the original fruit is lost [73]. of anti-browning agents prevents browning reactions [44]. Slicing of the fruit has a large influence on the final product Freezing can be used to create a porous and spongy matrix characteristics. Fruit could be sliced into thin pieces from 1.5- in vacuum-fried fruit [61]. to 7.5-mm thickness that need a relative short frying time. After the pre-treatment, fruits are ready to be fried. In a Fruit with thicker slices needs longer frying times to lower small-scale fryer, the process will start by placing the fruits the water content, to get the desired crispiness and shelf life, inside a basket and placed in the vacuum chamber after which leading to an elevated degradation of nutrients and bioactive the vacuum pump is started. After the oil has reached the compounds [17, 23]. desired temperature and the chamber has the desired pressure, Pre-treatments can be used to further improve quality attri- the basket is submerged in the oil to start the frying process. At butes of the fried product, such as oil content, appearance, the end of the frying time, the basket is lifted from the oil and texture, taste, and retention of nutrients and phytochemicals. shaken or spun to drain the surface oil. The pressure is Food Eng Rev gradually increased, and the product is centrifuged to elimi- of vacuum-frying parameters includes studies which applied nate part of the surface oil. Different setups could be found in pre-treatment in their method. The main parameters for the larger scale and industrial scale vacuum fryer. frying process are temperature, time, and pressure. However, the pre-treatments play a crucial role in the improvement of Vacuum-Frying Equipment quality attributes as well. Therefore, the discussion of effects of pre-treatment and vacuum-frying parameters was separated Vacuum frying is carried out in a closed system below atmo- into two sections. spheric pressures. Schematic of a batch vacuum fryer can be Producing a high-quality vacuum-fried fruit which has observed in Fig. 2. Conceptually, different devices in batch desirable product quality attributes is a challenge in and semi-continuous mode were used in the experimental vacuum-fried fruit production, especially because of the studies. The batch vacuum frying is suitable for small produc- high initial water content of fruits that requires long frying tion sizes [63], as well as for a larger capacity. Vacuum fryers times. High oil absorptions, burnt product, and low crisp- with a low capacity (2–10 L) are also often used for research ness are the possible product quality attributes that are [14, 30, 50], while Diamante et al. [21]usedalargecapacity consequences of this high water content. Pre-treatments fryer (460 L) for their research. such as blanching, hot air pre-drying, immersion drying, On the other hand, vacuum frying is also possible using a freezing, anti-browning agent, and hydrocolloid applica- semi-continuous method, which is a batchwise process with tion can limit these problems (Table 1). aspects of continuous processing [63]. This process was adopted by Perez-Tinoco et al. [54], who used a conveyor belt frying system inside a vacuum chamber. A small vacuum fryer Blanching usually not includes a centrifuge inside the vacuum chamber like larger vacuum fryer do. A centrifugation before breaking Blanching was used to minimize enzymatic browning in the vacuum is desired to remove the surface oil that will oth- vacuum-fried apple chips [44, 61]. Enzymatic browning in erwise get sucked into the pores. A centrifugation after break- fruits is the result of oxidation reactions of polyphenols with ing the vacuum could lead to higher oil content then when the catalytic action of polyphenol oxidase (PPO) enzyme [58]. centrifugation is done before. A high-capacity industrial fryer During blanching, PPO in mango can be inactivated by a 5- also usually includes several heat exchangers to maintain a min treatment at 94 °C. However, blanching for more than constant and equally distributed oil temperature and an oil 5 min resulted in color loss [49], even before frying. filter to maintain oil quality. Blanching of jackfruit produced a negative effect on oil content and texture; a higher porosity matrix was formed during Vacuum-Frying Pre-treatments the vacuum frying causing a higher oil absorption compared to non-blanched jackfruit [20]; however, the mechanism be- Vacuum frying is an integral process which consists of pre- hind the porosity formation is not clear. Nevertheless, treatment, frying, and post-treatment. There are a few studies Hasimah et al. [35] describe that blanched vacuum-fried pine- that described vacuum frying without a pre-treatment, but it apple at 100 °C for 3 min has shrunken cell due to air lost by cannot exclude the post-treatment. The discussion on effects blanching, and consequently produce a hard product.

Fig. 2 Schematic representation of a vacuum fryer. a Vacuum c chamber. b Frying basket. c Electric motor. d Oil filter. e Oil heater. f Oil cooler. g Condenser. h Vacuum pump. i Centrifuge

i h a d e f Food Eng Rev

Table 1 Pre-treatment effect on vacuum-fried product quality Pre-treatments Quality attributes References attributes Oil content Texture Nutrient Color

Blanching Negative – N.A. N.A. [20] – Negative –– [35] – Negative –– [61] Hot air drying Positive N.A N.A. Positive [44] Osmotic dehydration Positive Positive – Negative [61] Positive –––[50] Positive – Neutral – [23] – Positive – Negative [22] Freezing N.A. Positive N.A. N.A. [61] Anti-browning agent N.A. N.A N.A. Positive [44] Hydrocolloids Positive Negative N.A. Positive [43, 64]

N.A. data not available

On the other hand, blanching was found to limit oil uptake Osmotic dehydration reduced the initial water content by since the gelatinization leads to starch swelling and prevent oil 10–70% depending on the process condition and fruit proper- to enter the product, as found in atmospheric fried tortilla chip ties [45]. After osmotic dehydration with 40–65% maltodex- [37], vacuum-fried sweet potato chips [57], and atmospheric- trin, mango chip will have lower initial moisture content, and fried potato slices [3]. thus time needed to reach same final frying time will be shorter [50]. On the other hand, in vacuum-fried apple, oil content was decreased as the concentration of fructose was Pre-drying increased from 30 to 40% [61]. Additionally, Nunes and Moreira [50] explained that the oil content reduction was af- Several strategies have been applied to reduce the initial water fected by the water loss during the osmotic dehydration of content of fruit such as pre-drying with hot air and osmotic mango by 40–65% maltodextrin in 5 h. dehydration. Hot air-drying as a pre-treatment at 80 °C, which Osmotic dehydration by 30–40% fructose resulted in crispy produced final moisture content of 64% (wb), preserved apple texture of apple chips measured as low maximum breaking force slice color, which remains similar to that of raw apple [44]. [61]. Additionally, Diamante et al. [22] observed immersion with This color preservation corresponds to lower water activity dextrose 55% increases crunchy texture of gold kiwifruit. The after hot air drying, which further inhibits non-enzymatic osmotic dehydration in fructose solution also produced chips browning. Additionally, at 80 °C, hot air drying could de- with uniform porosity and reduced surface shrinkage of apple crease enzymatic activity which might reduce enzymatic chips resulting in a smoother surface [61]. browning. Hot air drying reduces moisture and form a crust The negative effect of the osmotic dehydration with fruc- which produce a high resistance to oil absorption during vac- tose on vacuum-fried fruits is the impact on color. Fructose uum frying. application decreased the lightness of products because of the Maillard reaction during vacuum frying of apple [61]. A similar result was also found by Diamante et al. [22] whose ap- Osmotic Dehydration plication of 55% maltodextrin increased the browning index of gold kiwifruit. Surprisingly, at higher maltodextrin concen- Osmotic dehydration can be applied for reducing the initial tration, the browning index decreased; the mechanism behind water content by applying sugars like fructose, maltodextrin, this is still unclear. and salts like NaCl [17, 22, 23, 50, 61]. Osmotic dehydration is a mass transfer process, which removes partially water and simultaneously increases the soluble solid content of fruit by Freezing immersion in an osmotic solution (OS). An activity gradient between the fruit and OS causes a flow of water across fruit Freezing is an alternative pre-treatment strategy to achieve a cell membranes which act as semi-permeable films [68]. The crispy fruit chips matrix in vacuum-frying processing [23, 25, process results in modification of the fruit tissue which can be 61]. Shyu and Hwang [61] found that freezing at − 30 °C tailored toward compositional, textural, and sensorial quality overnight formed a porous sponge-like matrix in vacuum- of vacuum-fried fruit. fried apples. In fact, due to fast heat transfer to frozen tissue, Food Eng Rev ice crystal inside the frozen cells sublimed under vacuum con- created a rigid, resistant film, protecting the inner matrix. dition leaving pores in the food matrix accelerated the mois- Similar observation was made by Maity et al. [43]injackfruit, ture loss and sequentially decrease the final moisture content. showing that arabic gum was effective to reduce oil absorption Albertos et al. [4] found that moisture content in vacuum-fried up to 35.3%; on the other hand, it increased chip toughness, carrot was lower in sample with – 20 °C blast freezing follow- thus decreased crispness was observed. ed by overnight freezing pre-treatment compared to not frozen Different hydrocolloids produce different effects when ap- sample. To obtain the desired benefit of freezing, water in the plied to the vacuum-fried fruits. CMC and other cellulose coat- fruit matrix should be in frozen condition, without thawed, to ings produce a protective layer which induced gelatinization at enable it for sublimed and left the matrix. 60 °C and subsequently prevent moisture loss and oil absorption. Freezing rate could affect vacuum-fried fruit. Slow freezing Meanwhile, guar gum reduces the formation of pores and cracks produces big size crystal, which damages the cell [7, 16]. Then in the fried food, thereby reduce oil penetration [39]. it could increase oil penetration, since oil could penetrate into damaged cell during the frying [72]. Thus, fast freezing is Vacuum-Frying Parameters preferred to minimize oil uptake. Freezing is also used to preserve the raw material prior the Vacuum-frying process is mainly characterized by time- frying process. During slow freezing processes, large ice crys- temperature and vacuum pressure as the main parameters, tal forms that damages the cell membranes and is causing which should be adjusted to the fruits characteristics to pro- water to leach upon thawing [18]. However, fruits have a duce high-quality vacuum-fried fruit. Vacuum-frying temper- different susceptibility to freezing injury. This difference is ature for fruits ranged in a wide interval from 72 to 136 °C, as caused by the ability of cell membrane to adapt or resist the well as frying time (from 0.5 to 90 min), and the vacuum phase change during freezing which is different for each fruit pressure (from 1.3 to 98.7 kPa). [60]. Apricots, banana, and peaches are very susceptible, Clearly, increasing temperature from 70 to 90 °C and time while apple, grapes, and pears are moderately susceptible from 35 to 65 min results in an increased oil content for gold and dates are least susceptible for freezing damage [77]. kiwi fruit [23]. On the other hand, increasing temperature from 112 to 136 °C and time from 3 to 9 min results insignificant Anti-browning Agent increase of oil content in plantain [2]. Mariscal and Bouchon [44] found that increasing temperature from 95 to 115 °C The application of an anti-browning agent could prevent fur- induces structural changes such as tissue degradation that en- ther browning reaction in susceptible fruits. Pre-treatment by hanced the oil absorption in apple chips. Additionally, Shyu tartaric acid, cysteine, and calcium chloride have been used to and Hwang [61] explained that the increase of oil content prevent non-enzymatic browning in banana. Synergistic effect when temperature increase from 90 to 110 °C was caused by was observed by combining tartaric acid-ascorbic acid, calci- a higher speed of water escaping from the matrix of apple. um chloride-ascorbic acid, and cysteine-citric acid. However, When the water is removed from the matrix, the process will using 1% cysteine-citric acid resulted in the highest overall damage the cells and make the surface hydrophobic, and thus preference evaluation in vacuum-fried banana [9]. Citric acid oil can absorb into the damaged sites. at 5.8% can also be applied to prevent non-enzymatic brow- The maximum breaking force of the vacuum-fried apricot ning in vacuum-fried apple [44], and it was also able to reduce [25] increased as the temperature and time were increased from the rate of quinone formation and color development [5]. 70 to 90 °C and 35 to 65 min; similar effect was observed in plantain [2]. Accordingly, Shyu and Hwang [61] found that Hydrocolloids increasing of frying time (from 5 to 30 min) leads to a higher crispness of apple chips. However, Yamsaengsung et al. [84] Dipping the fruits in a solution of hydrocolloids such as guar found that increasing temperature from 100 to 120 °C did not gum and xanthan gum, pectin, carboxymethyl cellulose affect the crispness of banana chips. At the beginning of the (CMC), gum arabic, and sodium alginate is a common fruit frying, fruit tissue becomes soft due to cell rupture and solubi- pre-treatment before vacuum frying to improve product qual- lization of the middle lamellae and leads to rubbery and soggy ity attributes. Sothornvit [64] described that 1.5% of guar gum products. Continuing the frying, the rapid loss of moisture from is able to reduce oil absorption by 25% and 1.5% of xanthan the surface leads to crust formation and an increase of the max- gum by 17% in banana chips. The application of hydrocol- imum breaking force. In the final stages of the process, the crust loids was not significantly improving the color of vacuum- thickened until the end of the process [2, 25, 84]. fried banana chips. In the same paper, it was reported that Vitamin C content of the vacuum-fried gold kiwifruit [23] hydrocolloid application increases the maximum breaking and apple [29] was decreased as the temperature increased force. However, the differences were not observed during sen- from 70 to 90 °C (gold kiwifruit) and 160 to 180 °C (apple) sory study. This is explained because the hydrocolloids because of heat sensitivity of vitamin C. However, an Food Eng Rev increasing frying time from 35 to 55 min of vacuum-fried gold the oil content by 24% compared to without centrifugation. In kiwifruit was found to have only a slight effect on vitamin C general, data show that increasing centrifugation speed de- [23]. Diamante et al. [24] found that in apricot, the β-carotene creased the oil uptake. However, the centrifugation speed content increased upon frying temperature increase from 70 to has to be limited according to the product hardness to prevent 90 °C; they attributed this to the higher accessibility of the β- product breakage. carotene by the oil which penetrates the fruit. The color of the fruit chips was affected as the temperature- time of the frying process is increased. Lightness and Effect of Matrix to Vacuum-Fried Fruit Quality yellowness values decreased, and redness increased as found in plantain, gold kiwifruit (from70to90°Candfrom35to The matrix of food products is defined as Bthe whole of the 65 min), apple, and mango (from 100 to 120 °C, and from 30 to chemical components of food and their molecular relation- 90 s) [2, 22, 61, 73]. No significant color change was found by ships, the chemical composition of food, and the way those Dueik and Bouchon [29], Mariscal and Bouchon [44], and components are structurally organized at micro-, meso-, and Diamante et al. [25], who found that there was no difference macroscopic scales^ [15]. Tropical fruits have diverse matrix in color when the frying temperature was increased for apple characteristics that could have different effects on vacuum- (from 160 to 180 °C), mango, and apricot. Moreover, Mariscal fried fruit quality. Those characteristics include cell size, cell and Bouchon [44] and Diamante et al. [25] found that frying wall, flesh thickness, firmness, intracellular spaces, sugar con- time does not influence the color of the vacuum-fried apple tent, fiber content, and fiber type. Some matrix characteristics (between 2 and 15 min) and apricot. The a* and L* values as of the fruits that are usually quantified and processed by vac- indicators of the browning reaction were similar to the value of uum frying are described in Table 2. The effect of different raw product. As the frying time increased for plantain and apple matrix characteristic will be discussed in this chapter. (from 5 to 30 min), the Maillard reaction was more pronounced; Fruits can have two possible types of ripening. The first are and as the moisture removed, the lightness was decreased while called climacteric fruits, whose respiration and ethylene bio- redness and yellowness were increased [2, 61]. synthesis rates increase during ripening. The second are non- Another vital processing parameter is the pressure: decreas- climacteric fruits, whose respiration and ethylene biosynthesis ing the frying pressure which decreases the oil content. A lower rates do not increase during ripening [32]. This characteristic pressure (from 13.14 to 26.54 kPa) produces a faster moisture is important to select which fruit is suitable for frying. A char- removal, reducing the rate of oil diffusion into the pores of acteristic of climacteric fruits will change substantially over vacuum-fried plantain [2]. On the other hand, a lower pressure time during storage. The characteristics of non-climacteric (from40to60Pa)leadstodecreaseofthetexturequalityand fruits will stay more constant after harvest. darkercolorinvacuum-friedplantainandmango[2, 73]. Ripening stage has an important role on the vacuum-fried fruit quality attributes: as a general rule, the riper the fruit, the Vacuum-Frying Post-treatment higher the oil content in the vacuum-fried chips [20]. Yashoda et al. [86] explained that during the early ripening stage of Centrifugation for removing the surface oil is an important mango, the cell wall is compact and rigid, and as the ripening part of the post-frying process and can be part of the frying continues, the cell become more loose and expanded. This equipment. Centrifugation done while the pressure is still low expansion is due to the movement of water into the voids that will significantly decrease the amount of surface oil that can form after pectin solubilization. Pectin is important because of penetrate the porous products when breaking the vacuum. its role in gluing the adjacent cell which results in tissue rigid- Tarmizi and Niranjan [66] found that centrifugation under ity and firmness. Moreover, pectin is essential to maintain the high vacuum following moderate vacuum frying has potency matrix cohesiveness during frying [1]. to reduce oil uptake in potato slices. Furthermore, Tarmizi and The effect of differences in ripening stages on the texture of Niranjan [67] also found that potato chip, centrifuged under vacuum-fried banana has been described by Yamsaengsung vacuum, has a significantly lower oil content than atmospheric et al. [84]. They found that at the first stage of ripening, sugar centrifuged chip (56.85-g oil/100 g and 35.01-g oil/100 g to starch ratio was 2.95 and the vacuum-fried banana chips defatted dry matter, respectively). have the highest maximum breaking value as an indicator of On the other hand, atmospheric centrifugation is also prom- compactness and hardness of the chips. This high maximum ising. Sothornvit [64] compared two atmospheric centrifuga- breaking value was caused by the high content of starch which tion speeds 140 and 280 rpm to remove oil after vacuum helps forming a crust [87]. At the second stage of ripening, the frying of banana. They found centrifugation at 280 rpm re- maximum breaking value is lower than early ripening stage as duced oil content, 17.3% higher than at 140 rpm. Similar an indicator of crispy and porous matrix. At this stage, sugar to findings were reported by Dueik et al. [30] who found centri- starch ratio was 8.75, which is the most optimum value to fugation of vacuum-fried apple at 400 rpm for 3 min reduced produce crispy vacuum-fried banana. However, at the third Food Eng Rev

Table 2 Fresh tropical fruit matrix characteristic

Fruits Fruit ripeninga Firmness Water contentb Porosity References

Apple Non-climacteric 4.0 N 85.5 0.15 [70, 78, 62] Avocado Climacteric 5.5 N/mm 73.2 0.16 [41, 27, 69] Banana Climacteric 12.0 N/mm 71.8 0.06 [13, 56, 9, 85] Dragon fruits Non-climacteric 7.0 N/mm 83.6 N.A. [79, 74, 42] Jackfruit Climacteric 14.0 N 73.5 N.A. [82, 59] Longan Non-climacteric 18.2 N/g 81.9 N.A. [88, 75] Mango Climacteric 22.2–35.6 N 83.0 0.05 [48, 83, 85] Pineapple Non-climacteric 11.2 N 85.7 0.11 [52, 53, 85] Rambutan Climacteric 1.5 N 80.0 N.A. [34, 76, 10] Snake fruit Non-climacteric 32.7 N 81.0 N.A. [65] Watermelon Non-climacteric 24.1 N 91.5 N.A. [6, 55]

N.A. data not available a Wongs-Aree et al.[81] b US Department of Agriculture. Agricultural Research Service. Nutrient Data Laboratory [71] stage of ripening, the maximum breaking value is increased fruit may be affected by the sugar content that increased during again, and the product is becoming hard and compact. At this ripening. Yashoda et al. [86] described that the alcohol-soluble ripening stage, the sugar to starch ratio was decreasing again sugar in unripe mango is mostly oligosaccharides; on the other to 4.05. The sugar to starch ratio should be increasing during hand, in ripe mango, it is mainly glucose and fructose. The the ripening process; this reverse effect could be because of increasing content of glucose and fructose will increase the the high biological variance in the banana. The high sugar Maillard reaction that produces brown color. A similar finding content slows down the gelatinization process, thus produces wasfoundbyLietal.[40] in banana in which the sugar content a shrunk banana chip [84]. was increasing as the starch content was decreasing. Similar results were found in mango. Starch and pectin concentrations in mango are decreasing during ripening. On the other hand, sugar is increasing during ripening. Unripe Conclusions mango has 18% starch, 1.9% pectin, and 1% total soluble sugar. However, after ripening, mango has 0.1% starch, Vacuum frying is a processing method that is suitable to pro- 0.5% pectin, and 15% of total soluble sugar [86]. This com- duce high-quality fried fruit products. Several factors have position changes during ripening could have effect on texture been reviewed for their influence on product quality attributes of vacuum-fried mango. like oil content, texture, color, and nutrient content. Although Starch content could play a major role during vacuum fry- some contradictory results have been reported for the different ing of fruit and determine the final quality of fried products. fruits, there are several indications for a higher quality of Banana and plantain are examples of high-content starchy vacuum-frying products compared to atmospheric frying of fruit, which are commonly used for vacuum frying. Starch in fruit. Different equipment used in vacuum-frying processing the fruit will be gelatinized, swollen, and prevents moisture have different characteristics, which leads to different process- and oil transport. Giraldo Toro et al. [33] found that 35–25- ing conditions and different product quality attributes. Pre- mm vacuum-packed plantain slices were gelatinized for 80% treatments could improve most of the product quality attri- at 85 °C and the degree of gelatinization increased even more butes; however, the treatment should be tailored on the char- at higher temperatures. acteristics of the raw material and on the desired final proper- Also, the fiber content could play a significant role in the ties. We can conclude that information about the role of the final quality of vacuum-fried fruits. Fruits with high fiber con- fruit matrix is a very important factor in vacuum processing, tent could influence the fat and water transfer to and from the but is described very limited, fragmentary, and anecdotal in product; fiber could get gelatinized, swollen, and inhibit fat the literature. During the ripening process, the fruit matrix and entering the product [39]. chemical composition will change, which will have an effect After vacuum frying, fruit at an early ripening stage pro- on the texture, oil content, and color of vacuum-fried fruits. duced a low-color-intensity product; at later ripening stage, Especially, tropical fruits have quite different ripening proper- the color of the product will be more intense, which also con- ties, firmness, texture, and porosity that will influence the tributed by Maillard reaction. The color of the vacuum-fried quality attributes of vacuum-fried tropical fruits. More Food Eng Rev systematic research into the effects of the fruit matrix on the 9. Apintanapong M, Cheachuminang K, Sulansawan P, Thongprasert vacuum-frying process and the quality attributes of the fried N (2007) Effect of antibrowning agents on banana slices and vacuum-fried slices. J Food Agric Environ 5:151–157 fruits is needed. By such research, the mechanistic under- 10. Arenas MGH, Angel DN, Damian MTM, Ortiz DT, Diaz CN, standing can be used to optimize the frying process to produce Martinez NB (2010) Characterization of Rambutan (Nephelium high-quality vacuum-fried fruits. lappaceum) Fruits from Outstanding Mexican Selections. Revista Brasileira De Fruticultura 32:1098–1104. https://doi.org/10.1590/ S0100-29452011005000004 Funding Information Financial support for this study was provided by the Indonesia Endowment Fund for Education (LPDP) within the 11. Ayustaningwarno F, Ananingsih VK (2007) Vacuum frying usage Ministry of Finance, Indonesia (grant number PRJ-201/LPDP/2015). on increasing food diversity. Paper presented at the International Agricultural Engineering Conference: Cutting Edge Technologies and Innovations on Sustainable Resources for World Food Compliance with Ethical Standards Sufficiency, Bangkok, Thailand 12. Bouchon PB, Aguilera JM, Pyle DL (2003) Structure oil-absorption Conflict of Interest The authors declare that they have no conflict of relationships during deep-fat frying. J Food Sci 68:2711–2716. interest. https://doi.org/10.1111/j.1365-2621.2003.tb05793.x 13. Boudhrioua N, Michon C, Cuvelier G, Bonazzi C (2002) Influence Open Access This article is distributed under the terms of the Creative of ripeness and air temperature on changes in banana texture during Commons Attribution 4.0 International License (http:// drying. J Food Eng 55:115–121. https://doi.org/10.1016/s0260- creativecommons.org/licenses/by/4.0/), which permits unrestricted use, 8774(02)00025-0 distribution, and reproduction in any medium, provided you give appro- 14. Bravo J, Sanjuan N, Clemente G, Mulet A (2011) Pressure effect on priate credit to the original author(s) and the source, provide a link to the deep fat frying of apple chips. Dry Technol 29:472–477. https://doi. 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