Introduction

Fried products are appreciated by all age groups and play an important role in consumer’s diet because of their unique flavor and texture. However, it is difficult to combine fried foods with the contemporary consumer trends toward healthier and low-fat products. There is an increased demand for healthy snack products with good taste, texture, and appearance [38]. This demand offers the opportunity to design novel fried products that have higher health properties such as fruit-based products. Increasing fruit consumption is promoted in all parts of the world to increase public health. Fruit implicitly has a strong health awareness based on the content of (micro) nutrients, fibers, and numerous bioactive phytochemicals [19, 51].

Vacuum frying is a frying process below atmospheric pressure (~ 100 kPa). At reduced pressure, the boiling point of oil and water is lower compared to atmospheric pressure [31]. Due to a lower frying temperature, vacuum frying better preserves the nutritional value, aroma, and color of the fried product compared to atmospheric frying [2].

Some anecdotic findings from existing studies highlighted several advantages vacuum frying might have over atmospheric frying:

  • Oil uptake in vacuum-fried apple chips is lower compared frying at atmospheric pressure [44];

  • 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];

  • Vacuum-fried mango was more uniform and crispier compared to soggy, burnt, and oily for atmospheric fried mango [50].

The multiple factors influencing the quality attributes of vacuum-fried fruit can be distinguished in vacuum-frying equipment (type and specifications), properties of the raw fruit (fruit matrix), pre- and post-treatments, and processing conditions. Time, temperature, and vacuum pressure influenced color, texture, nutrients, and oil content of fried fruits [2, 14, 73].

Another relevant aspect is the fruit matrix such as the fruit type and ripening stage that are affecting the vacuum-fried product quality attributes [20, 30, 84]. Pre-treatments such as blanching, drying, freezing, antioxidant, and coating applications have been used to preserve color, improve texture, and reduce oil absorption [9, 22, 24]. The use of post-frying steps such as centrifugation has a major effect on the oil content of fried product [47].

Some recent papers dealt with different aspects of vacuum-frying technology. The strategies to reduce oil absorption of vacuum-fried products have been studied intensively by Moreira [46], including optimizing temperature, pressure, pre-treatment, pressurization speed, and de-oiling time. The recent review by Diamante et al. [26] discussed the product and process optimization, oil uptake, oil quality, as well as packaging and storage of vacuum-fried fruits without mentioning matrix factors. Dueik and Bouchon [28] and Ayustaningwarno and Ananingsih [11] compared the quality changes comparing atmospheric and vacuum frying as well as the oil quality and packaging of fried products. Dueik and Bouchon [28] put emphasis on the microstructure, methods to reduce oil uptake, oil quality, bioactive compound degradation, and toxic compound generation. Andres-Bello et al. [8] reviewed the vacuum-frying processing for producing high-quality fried products, focusing on equipment types, pre-treatments, and vacuum-frying conditions.

Based on this existing background information, this review will consider the effects of vacuum frying on changes in quality attributes of tropical fruits with a focus on the role of the fruit matrix, since this is a very relevant but underexposed factor.

Vacuum Frying Versus Atmospheric Frying

The main difference between vacuum frying and atmospheric frying is the lower boiling point of water at lower pressures that enables to fry at lower temperatures. For that reason, vacuum frying has many advantages over atmospheric frying in relation to product quality attributes. Several comparative studies between vacuum and atmospheric frying were done on apple, plantain, banana, and mango.

Oil and Nutrient Content

The mechanism of oil uptake in atmospheric frying and vacuum frying is different. Oil uptake occurs mainly after frying: by the lower pressure in the pores, the oil present on the surface of the products is sucked into the pores. During atmospheric frying, this lower pressure in the pores is created by the evaporative cooling after frying [12]. On the other hand, at the end of vacuum frying, the vacuum breaking period produces a higher outside pressure then the pore pressure.

The oil content of vacuum-fried apples was lower compared to atmospheric fried one. Apple absorbed 1.2–2.0 times more oil by atmospheric frying compared to vacuum frying [29, 44]. This difference was explained by the lower temperatures during vacuum frying due to the lower vapor pressure of water. This low temperature will reduce temperature-induced tissue matrix degradation that increases the oil absorption. Dueik et al. [30] found that atmospheric fried apple had a larger portion of small pores and absorbed more oil by capillary suction compared to vacuum-fried apple. Larger pore formation was related to the higher specific volume of water vapor at lower pressure. These studies provided a convincing explanation about the mechanisms behind the reduced oil absorption of vacuum-fried products.

Shyu and Hwang [61] showed that oil absorption was highly correlated with moisture loss in vacuum-fried apple slices. At the beginning of the frying procedure, the outer surface of the product is dried, the moisture inside the product is converted into steam, and a pressure gradient is created. By prolonging the frying, the dried surface becomes more hydrophobic which facilitates the absorption of oil. This can explain the observed oil content that was increased from 33.64% in first 5 min of vacuum frying to 39.38% after 30 min of vacuum frying. This oil absorption mechanism is different compared with atmospheric frying in which most of the oil is absorbed after frying during the cooling period [12].

The situation found in apple is different as found in plantain and mango: as in plantain [2] and mango [17], vacuum frying resulted in a higher oil content compared to atmospheric frying. This difference could be attributed by matrix differences 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 product when the vacuum was broken to restore the system into atmospheric pressure. Additionally, vacuum-fried plantain had less gelatinized starch due to the lower temperature, thereby having more pores and absorbed more oil compared to atmospheric frying [2].

In general, a higher nutrient retention is expected with a lower temperature of vacuum frying. Ascorbic acid content of apple was found 1.7–1.9 times higher after vacuum frying compared to atmospheric frying [29]. Additionally, carotenoid retention in vacuum-fried mango was two times higher compared to atmospheric frying. High retention of carotenoid was attributed by the absence of oxygen, which induce oxidation in atmospheric frying [50]. In addition, a lower temperature of vacuum frying compared to atmospheric frying will have an effect on the nutrition degradation. A less pronounced effect was observed by Da Silva and Moreira [17] who found that vacuum-fried mango had 20–50% higher carotenoids compared to atmospheric fried mango.

Color, Texture, and Sensory Attributes

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 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 preserved the lightness and redness of apple chips compared to atmospheric frying [29, 44]. Similar results for lightness and redness were found in plantain [2] and mango [17, 50].

Vacuum frying of plantain produced a crispier product compared with plantain fried in atmospheric pressure, indicated 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 atmospheric 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/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

The vacuum-frying process consists of several steps as summarized 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 materials as fresh fruits. However, fruit paste also can be used by preparing a dough made up with fruit pulp and starch or flour [73]. Utilization of fresh fruit has some advantages as well as disadvantages. The product could be recognized by the consumer as the original fruit, but fresh fruits usually have a variety of shapes and irregularities resulting in uneven heat distribution during frying and a subsequent inhomogeneity in color and texture [36]. On the other side, using fruit paste a homogenous product in size and shape can be obtained, but the characteristic of the original fruit is lost [73].

Fig. 1
figure 1

Flow chart of vacuum-frying process

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Slicing of the fruit has a large influence on the final product characteristics. Fruit could be sliced into thin pieces from 1.5- to 7.5-mm thickness that need a relative short frying time. Fruit with thicker slices needs longer frying times to lower the water content, to get the desired crispiness and shelf life, leading to an elevated degradation of nutrients and bioactive compounds [17, 23].

Pre-treatments can be used to further improve quality attributes of the fried product, such as oil content, appearance, texture, taste, and retention of nutrients and phytochemicals. In this section, common pre-treatments used for vacuum-frying processing will be mentioned briefly and discussed further in separated sections. The pre-treatments that are reported in literature are blanching, pre-drying, impregnation, and freezing [20, 22, 23, 35, 43, 44, 50, 61, 64]. Blanching is used to minimize enzymatic browning [44, 61] and also to pre-gelatinize starch. Pre-drying is used to reduce the initial water content before frying and thus reduce frying time [44]. Osmotic dehydration is used to introduce salt or sugar to reduce initial water content [17, 22, 23, 50, 61, 64]. Application of anti-browning agents prevents browning reactions [44]. Freezing can be used to create a porous and spongy matrix in vacuum-fried fruit [61].

After the pre-treatment, fruits are ready to be fried. In a small-scale fryer, the process will start by placing the fruits inside a basket and placed in the vacuum chamber after which the vacuum pump is started. After the oil has reached the desired temperature and the chamber has the desired pressure, the basket is submerged in the oil to start the frying process. At the end of the frying time, the basket is lifted from the oil and shaken or spun to drain the surface oil. The pressure is gradually increased, and the product is centrifuged to eliminate part of the surface oil. Different setups could be found in larger scale and industrial scale vacuum fryer.

Vacuum-Frying Equipment

Vacuum frying is carried out in a closed system below atmospheric pressures. Schematic of a batch vacuum fryer can be observed in Fig. 2. Conceptually, different devices in batch and semi-continuous mode were used in the experimental studies. The batch vacuum frying is suitable for small production sizes [63], as well as for a larger capacity. Vacuum fryers with a low capacity (2–10 L) are also often used for research [14, 30, 50], while Diamante et al. [21] used a large capacity fryer (460 L) for their research.

Fig. 2
figure 2

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

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On the other hand, vacuum frying is also possible using a semi-continuous method, which is a batchwise process with 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 usually not includes a centrifuge inside the vacuum chamber like larger vacuum fryer do. A centrifugation before breaking the vacuum is desired to remove the surface oil that will otherwise get sucked into the pores. A centrifugation after breaking the vacuum could lead to higher oil content then when the centrifugation is done before. A high-capacity industrial fryer also usually includes several heat exchangers to maintain a constant and equally distributed oil temperature and an oil filter to maintain oil quality.

Vacuum-Frying Pre-treatments

Vacuum frying is an integral process which consists of pre-treatment, frying, and post-treatment. There are a few studies that described vacuum frying without a pre-treatment, but it cannot exclude the post-treatment. The discussion on effects of vacuum-frying parameters includes studies which applied pre-treatment in their method. The main parameters for the frying process are temperature, time, and pressure. However, the pre-treatments play a crucial role in the improvement of quality attributes as well. Therefore, the discussion of effects of pre-treatment and vacuum-frying parameters was separated into two sections.

Producing a high-quality vacuum-fried fruit which has desirable product quality attributes is a challenge in vacuum-fried fruit production, especially because of the high initial water content of fruits that requires long frying times. High oil absorptions, burnt product, and low crispness are the possible product quality attributes that are consequences of this high water content. Pre-treatments such as blanching, hot air pre-drying, immersion drying, freezing, anti-browning agent, and hydrocolloid application can limit these problems (Table 1).

Table 1 Pre-treatment effect on vacuum-fried product quality attributes
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Blanching

Blanching was used to minimize enzymatic browning in vacuum-fried apple chips [44, 61]. Enzymatic browning in fruits is the result of oxidation reactions of polyphenols with catalytic action of polyphenol oxidase (PPO) enzyme [58]. During blanching, PPO in mango can be inactivated by a 5-min treatment at 94 °C. However, blanching for more than 5 min resulted in color loss [49], even before frying. Blanching of jackfruit produced a negative effect on oil content and texture; a higher porosity matrix was formed during the vacuum frying causing a higher oil absorption compared to non-blanched jackfruit [20]; however, the mechanism behind the porosity formation is not clear. Nevertheless, Hasimah et al. [35] describe that blanched vacuum-fried pineapple at 100 °C for 3 min has shrunken cell due to air lost by blanching, and consequently produce a hard product.

On the other hand, blanching was found to limit oil uptake since the gelatinization leads to starch swelling and prevent oil to enter the product, as found in atmospheric fried tortilla chip [37], vacuum-fried sweet potato chips [57], and atmospheric-fried potato slices [3].

Pre-drying

Several strategies have been applied to reduce the initial water content of fruit such as pre-drying with hot air and osmotic dehydration. Hot air-drying as a pre-treatment at 80 °C, which produced final moisture content of 64% (wb), preserved apple slice color, which remains similar to that of raw apple [44]. This color preservation corresponds to lower water activity after hot air drying, which further inhibits non-enzymatic browning. Additionally, at 80 °C, hot air drying could decrease enzymatic activity which might reduce enzymatic browning. Hot air drying reduces moisture and form a crust which produce a high resistance to oil absorption during vacuum frying.

Osmotic Dehydration

Osmotic dehydration can be applied for reducing the initial water content by applying sugars like fructose, maltodextrin, 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 immersion in an osmotic solution (OS). An activity gradient between the fruit and OS causes a flow of water across fruit cell membranes which act as semi-permeable films [68]. The process results in modification of the fruit tissue which can be tailored toward compositional, textural, and sensorial quality of vacuum-fried fruit.

Osmotic dehydration reduced the initial water content by 10–70% depending on the process condition and fruit properties [45]. After osmotic dehydration with 40–65% maltodextrin, mango chip will have lower initial moisture content, and 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 increased from 30 to 40% [61]. Additionally, Nunes and Moreira [50] explained that the oil content reduction was affected by the water loss during the osmotic dehydration of mango by 40–65% maltodextrin in 5 h.

Osmotic dehydration by 30–40% fructose resulted in crispy texture of apple chips measured as low maximum breaking force [61]. Additionally, Diamante et al. [22] observed immersion with dextrose 55% increases crunchy texture of gold kiwifruit. The osmotic dehydration in fructose solution also produced chips with uniform porosity and reduced surface shrinkage of apple chips resulting in a smoother surface [61].

The negative effect of the osmotic dehydration with fructose on vacuum-fried fruits is the impact on color. Fructose 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 application of 55% maltodextrin increased the browning index of gold kiwifruit. Surprisingly, at higher maltodextrin concentration, the browning index decreased; the mechanism behind this is still unclear.

Freezing

Freezing is an alternative pre-treatment strategy to achieve a crispy fruit chips matrix in vacuum-frying processing [23, 25, 61]. Shyu and Hwang [61] found that freezing at − 30 °C overnight formed a porous sponge-like matrix in vacuum-fried apples. In fact, due to fast heat transfer to frozen tissue, ice crystal inside the frozen cells sublimed under vacuum condition leaving pores in the food matrix accelerated the moisture loss and sequentially decrease the final moisture content. Albertos et al. [4] found that moisture content in vacuum-fried carrot was lower in sample with – 20 °C blast freezing followed by overnight freezing pre-treatment compared to not frozen sample. To obtain the desired benefit of freezing, water in the fruit matrix should be in frozen condition, without thawed, to enable it for sublimed and left the matrix.

Freezing rate could affect vacuum-fried fruit. Slow freezing produces big size crystal, which damages the cell [7, 16]. Then it could increase oil penetration, since oil could penetrate into damaged cell during the frying [72]. Thus, fast freezing is preferred to minimize oil uptake.

Freezing is also used to preserve the raw material prior the frying process. During slow freezing processes, large ice crystal forms that damages the cell membranes and is causing water to leach upon thawing [18]. However, fruits have a different susceptibility to freezing injury. This difference is caused by the ability of cell membrane to adapt or resist the phase change during freezing which is different for each fruit [60]. Apricots, banana, and peaches are very susceptible, while apple, grapes, and pears are moderately susceptible and dates are least susceptible for freezing damage [77].

Anti-browning Agent

The application of an anti-browning agent could prevent further browning reaction in susceptible fruits. Pre-treatment by tartaric acid, cysteine, and calcium chloride have been used to prevent non-enzymatic browning in banana. Synergistic effect was observed by combining tartaric acid-ascorbic acid, calcium chloride-ascorbic acid, and cysteine-citric acid. However, using 1% cysteine-citric acid resulted in the highest overall preference evaluation in vacuum-fried banana [9]. Citric acid at 5.8% can also be applied to prevent non-enzymatic browning in vacuum-fried apple [44], and it was also able to reduce the rate of quinone formation and color development [5].

Hydrocolloids

Dipping the fruits in a solution of hydrocolloids such as guar gum and xanthan gum, pectin, carboxymethyl cellulose (CMC), gum arabic, and sodium alginate is a common fruit pre-treatment before vacuum frying to improve product quality attributes. Sothornvit [64] described that 1.5% of guar gum is able to reduce oil absorption by 25% and 1.5% of xanthan gum by 17% in banana chips. The application of hydrocolloids was not significantly improving the color of vacuum-fried banana chips. In the same paper, it was reported that hydrocolloid application increases the maximum breaking force. However, the differences were not observed during sensory study. This is explained because the hydrocolloids created a rigid, resistant film, protecting the inner matrix. Similar observation was made by Maity et al. [43] in jackfruit, showing that arabic gum was effective to reduce oil absorption up to 35.3%; on the other hand, it increased chip toughness, thus decreased crispness was observed.

Different hydrocolloids produce different effects when applied to the vacuum-fried fruits. CMC and other cellulose coatings produce a protective layer which induced gelatinization at 60 °C and subsequently prevent moisture loss and oil absorption. Meanwhile, guar gum reduces the formation of pores and cracks in the fried food, thereby reduce oil penetration [39].

Vacuum-Frying Parameters

Vacuum-frying process is mainly characterized by time-temperature and vacuum pressure as the main parameters, which should be adjusted to the fruits characteristics to produce high-quality vacuum-fried fruit. Vacuum-frying temperature for fruits ranged in a wide interval from 72 to 136 °C, as well as frying time (from 0.5 to 90 min), and the vacuum pressure (from 1.3 to 98.7 kPa).

Clearly, increasing temperature from 70 to 90 °C and time from 35 to 65 min results in an increased oil content for gold kiwi fruit [23]. On the other hand, increasing temperature from 112 to 136 °C and time from 3 to 9 min results insignificant increase of oil content in plantain [2]. Mariscal and Bouchon [44] found that increasing temperature from 95 to 115 °C induces structural changes such as tissue degradation that enhanced the oil absorption in apple chips. Additionally, Shyu and Hwang [61] explained that the increase of oil content when temperature increase from 90 to 110 °C was caused by a higher speed of water escaping from the matrix of apple. When the water is removed from the matrix, the process will damage the cells and make the surface hydrophobic, and thus oil can absorb into the damaged sites.

The maximum breaking force of the vacuum-fried apricot [25] increased as the temperature and time were increased from 70 to 90 °C and 35 to 65 min; similar effect was observed in plantain [2]. Accordingly, Shyu and Hwang [61] found that increasing of frying time (from 5 to 30 min) leads to a higher crispness of apple chips. However, Yamsaengsung et al. [84] found that increasing temperature from 100 to 120 °C did not affect the crispness of banana chips. At the beginning of the frying, fruit tissue becomes soft due to cell rupture and solubilization of the middle lamellae and leads to rubbery and soggy products. Continuing the frying, the rapid loss of moisture from the surface leads to crust formation and an increase of the maximum breaking force. In the final stages of the process, the crust thickened until the end of the process [2, 25, 84].

Vitamin C content of the vacuum-fried gold kiwifruit [23] and apple [29] was decreased as the temperature increased from 70 to 90 °C (gold kiwifruit) and 160 to 180 °C (apple) because of heat sensitivity of vitamin C. However, an increasing frying time from 35 to 55 min of vacuum-fried gold kiwifruit was found to have only a slight effect on vitamin C [23]. Diamante et al. [24] found that in apricot, the β-carotene content increased upon frying temperature increase from 70 to 90 °C; they attributed this to the higher accessibility of the β-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 yellowness values decreased, and redness increased as found in plantain, gold kiwifruit (from 70 to 90 °C and from 35 to 65 min), apple, and mango (from 100 to 120 °C, and from 30 to 90 s) [2, 22, 61, 73]. No significant color change was found by Dueik and Bouchon [29], Mariscal and Bouchon [44], and Diamante et al. [25], who found that there was no difference in color when the frying temperature was increased for apple (from 160 to 180 °C), mango, and apricot. Moreover, Mariscal and Bouchon [44] and Diamante et al. [25] found that frying time does not influence the color of the vacuum-fried apple (between 2 and 15 min) and apricot. The a* and L* values as indicators of the browning reaction were similar to the value of raw product. As the frying time increased for plantain and apple (from 5 to 30 min), the Maillard reaction was more pronounced; and as the moisture removed, the lightness was decreased while redness and yellowness were increased [2, 61].

Another vital processing parameter is the pressure: decreasing the frying pressure which decreases the oil content. A lower pressure (from 13.14 to 26.54 kPa) produces a faster moisture removal, reducing the rate of oil diffusion into the pores of vacuum-fried plantain [2]. On the other hand, a lower pressure (from 40 to 60 Pa) leads to decrease of the texture quality and darker color in vacuum-fried plantain and mango [2, 73].

Vacuum-Frying Post-treatment

Centrifugation for removing the surface oil is an important part of the post-frying process and can be part of the frying equipment. Centrifugation done while the pressure is still low will significantly decrease the amount of surface oil that can penetrate the porous products when breaking the vacuum. Tarmizi and Niranjan [66] found that centrifugation under high vacuum following moderate vacuum frying has potency to reduce oil uptake in potato slices. Furthermore, Tarmizi and Niranjan [67] also found that potato chip, centrifuged under vacuum, has a significantly lower oil content than atmospheric centrifuged chip (56.85-g oil/100 g and 35.01-g oil/100 g defatted dry matter, respectively).

On the other hand, atmospheric centrifugation is also promising. Sothornvit [64] compared two atmospheric centrifugation speeds 140 and 280 rpm to remove oil after vacuum frying of banana. They found centrifugation at 280 rpm reduced oil content, 17.3% higher than at 140 rpm. Similar findings were reported by Dueik et al. [30] who found centrifugation of vacuum-fried apple at 400 rpm for 3 min reduced the oil content by 24% compared to without centrifugation. In general, data show that increasing centrifugation speed decreased the oil uptake. However, the centrifugation speed has to be limited according to the product hardness to prevent product breakage.

Effect of Matrix to Vacuum-Fried Fruit Quality

The matrix of food products is defined as “the whole of the chemical components of food and their molecular relationships, the chemical composition of food, and the way those components are structurally organized at micro-, meso-, and macroscopic scales” [15]. Tropical fruits have diverse matrix characteristics that could have different effects on vacuum-fried fruit quality. Those characteristics include cell size, cell wall, flesh thickness, firmness, intracellular spaces, sugar content, fiber content, and fiber type. Some matrix characteristics of the fruits that are usually quantified and processed by vacuum frying are described in Table 2. The effect of different matrix characteristic will be discussed in this chapter.

Table 2 Fresh tropical fruit matrix characteristic
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Fruits can have two possible types of ripening. The first are called climacteric fruits, whose respiration and ethylene biosynthesis rates increase during ripening. The second are non-climacteric fruits, whose respiration and ethylene biosynthesis rates do not increase during ripening [32]. This characteristic is important to select which fruit is suitable for frying. A characteristic of climacteric fruits will change substantially over time during storage. The characteristics of non-climacteric fruits will stay more constant after harvest.

Ripening stage has an important role on the vacuum-fried fruit quality attributes: as a general rule, the riper the fruit, the higher the oil content in the vacuum-fried chips [20]. Yashoda et al. [86] explained that during the early ripening stage of mango, the cell wall is compact and rigid, and as the ripening continues, the cell become more loose and expanded. This expansion is due to the movement of water into the voids that form after pectin solubilization. Pectin is important because of its role in gluing the adjacent cell which results in tissue rigidity and firmness. Moreover, pectin is essential to maintain the matrix cohesiveness during frying [1].

The effect of differences in ripening stages on the texture of vacuum-fried banana has been described by Yamsaengsung et al. [84]. They found that at the first stage of ripening, sugar to starch ratio was 2.95 and the vacuum-fried banana chips have the highest maximum breaking value as an indicator of compactness and hardness of the chips. This high maximum breaking value was caused by the high content of starch which helps forming a crust [87]. At the second stage of ripening, the maximum breaking value is lower than early ripening stage as an indicator of crispy and porous matrix. At this stage, sugar to starch ratio was 8.75, which is the most optimum value to produce crispy vacuum-fried banana. However, at the third stage of ripening, the maximum breaking value is increased again, and the product is becoming hard and compact. At this ripening stage, the sugar to starch ratio was decreasing again to 4.05. The sugar to starch ratio should be increasing during the ripening process; this reverse effect could be because of the high biological variance in the banana. The high sugar content slows down the gelatinization process, thus produces a shrunk banana chip [84].

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 mango has 18% starch, 1.9% pectin, and 1% total soluble sugar. However, after ripening, mango has 0.1% starch, 0.5% pectin, and 15% of total soluble sugar [86]. This composition changes during ripening could have effect on texture of vacuum-fried mango.

Starch content could play a major role during vacuum frying of fruit and determine the final quality of fried products. Banana and plantain are examples of high-content starchy fruit, which are commonly used for vacuum frying. Starch in the fruit will be gelatinized, swollen, and prevents moisture and oil transport. Giraldo Toro et al. [33] found that 35–25-mm vacuum-packed plantain slices were gelatinized for 80% at 85 °C and the degree of gelatinization increased even more at higher temperatures.

Also, the fiber content could play a significant role in the final quality of vacuum-fried fruits. Fruits with high fiber content could influence the fat and water transfer to and from the product; fiber could get gelatinized, swollen, and inhibit fat entering the product [39].

After vacuum frying, fruit at an early ripening stage produced a low-color-intensity product; at later ripening stage, the color of the product will be more intense, which also contributed by Maillard reaction. The color of the vacuum-fried fruit may be affected by the sugar content that increased during ripening. Yashoda et al. [86] described that the alcohol-soluble sugar in unripe mango is mostly oligosaccharides; on the other hand, in ripe mango, it is mainly glucose and fructose. The increasing content of glucose and fructose will increase the Maillard reaction that produces brown color. A similar finding was found by Li et al. [40] in banana in which the sugar content was increasing as the starch content was decreasing.

Conclusions

Vacuum frying is a processing method that is suitable to produce high-quality fried fruit products. Several factors have been reviewed for their influence on product quality attributes like oil content, texture, color, and nutrient content. Although some contradictory results have been reported for the different fruits, there are several indications for a higher quality of vacuum-frying products compared to atmospheric frying of fruit. Different equipment used in vacuum-frying processing have different characteristics, which leads to different processing conditions and different product quality attributes. Pre-treatments could improve most of the product quality attributes; however, the treatment should be tailored on the characteristics of the raw material and on the desired final properties. We can conclude that information about the role of the fruit matrix is a very important factor in vacuum processing, but is described very limited, fragmentary, and anecdotal in the literature. During the ripening process, the fruit matrix and chemical composition will change, which will have an effect on the texture, oil content, and color of vacuum-fried fruits. Especially, tropical fruits have quite different ripening properties, firmness, texture, and porosity that will influence the quality attributes of vacuum-fried tropical fruits. More systematic research into the effects of the fruit matrix on the vacuum-frying process and the quality attributes of the fried fruits is needed. By such research, the mechanistic understanding can be used to optimize the frying process to produce high-quality vacuum-fried fruits.