Journal of Food Engineering 55 (2002) 181–191 www.elsevier.com/locate/jfoodeng Vacuum frying of potato chips Jagoba Garayo, Rosana Moreira * Department of Biological and Agricultural Engineering, Texas A & M University, College Station, TX 77843-2117, USA Received 31 October 2001; accepted 5 February 2002 Abstract Vacuum frying was tested as an alternative technique to develop low oil content potato chips. The effect of oil temperature (118, 132, 144 °C) and vacuum pressure (16.661, 9.888, and 3.115 kPa) on the drying rate and oil absorption of potato chips and on the product quality attributes such as shrinkage, color, and texture was investigated. Furthermore, the characteristics of the vacuumfried potato chips (3.115 kPa and 144 °C) were compared to potato chips fried under atmospheric conditions (165 °C). During vacuum frying, oil temperature and vacuum pressure had a significant effect on the drying rate and oil absorption rate of potato chips. Potato chips fried at lower vacuum pressure and higher temperature had less volume shrinkage. Color was not significantly affected by the oil temperature and vacuum pressure. Hardness values increased with increasing oil temperature and decreasing vacuum levels. Potato chips fried under vacuum (3.115 kPa and 144 °C) had more volume shrinkage, were slightly softer, and lighter in color than the potato chips fried under atmospheric conditions (165 °C). It was concluded that vacuum frying is a process that could be a feasible alternative to produce potato chips with lower oil content and desirable color and texture. Ó 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction Potato chips represent 33% of total sales of snack in the US Market. They have an oil content that ranges from 35.3% to 44.5% w.b. that gives the product the unique texture-flavor combination that makes them so desirable. In recent years, consumer’s preference for low-fat and fat-free products has been the driving force of the snack food industry to produce lower oil content products that still retain the desirable texture and flavor. Vacuum frying may be an option for production of fruits and vegetables with low oil content and the desired texture and flavor characteristics. It is defined as the frying process that is carried out under pressures well below atmospheric levels, preferably below 50 Torr (6.65 kPa). Due to the pressure lowering, the boiling points both of the oil and the moisture in the foods are lowered. Vacuum frying poses some advantages that include: (1) can reduce oil content in the fried product, * Corresponding author. Tel.: +1-979-847-8794; fax: +1-979-8453932. E-mail address: [email protected] (R. Moreira). (2) can preserve natural color and flavors of the product due to the low temperature and oxygen content during the process, and (3) has less adverse effects on oil quality (Shyu, Hau, & Hwang, 1998). Shyu and Hwang (2001) studied the effects of processing conditions on the quality of vacuum fried apple chips. They used a single vacuum pressure condition, 3.115 kPa, and three levels of temperature, 90, 100, and 110 °C to fry the chips. After frying, the chips were centrifuged for 30 min at 350 rpm to remove the surface frying oil and then packed in polyethylene bags and stored at 30 °C. Using texture (hardness) as an indicator of product quality, the optimum conditions were: vacuum frying temperature of 100–110 °C, vacuum time of 20–25 min, and a concentration of immersing fructose solution of 30–40%. Even though frying is an old process of manufacturing a food product worldwide, most of the research found in the literature is related to atmospheric frying. No studies are available on the effect of vacuum on the oil content, crispness, color, and nutritional value of the final product. The main objective of this research was to develop a vacuum frying system to produce high quality potato chips in terms of reduced oil content, good texture, and color. 0260-8774/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 2 ) 0 0 0 6 2 - 6 182 J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191 The sealing mechanism consists of two Buna-N washers, the inner edges of which, under compression, seal against the shaft outer diameter. 2. Materials and methods 2.1. Vacuum frying setup An electric pressure cooker (Wisconsin Aluminum Foundry Co., Manitowoc, Wisconsin), which has a capacity of 24 l, was modified to serve as the vacuum vessel. The inner diameter of the vessel is 28 cm and the wall thickness is 0.8 cm. It is constructed of cast aluminum and can withstand an internal pressure of 2 atm. The maximum temperature that the heater can generate is 145 °C for an oil capacity of 6.5 l. A dual seal vacuum pump (model 1402 Welch Scientific Co., Skokie, Illinois) that can generate a vacuum up to 10 Torr (1.333 kPa) is used to provide vacuum to the vessel. The vacuum frying system is illustrated in Fig. 1. The condenser consists of a dry-ice trap. The objective of the condenser is to prevent the water vapor from the product to mix with the pump’s oil, thus prone to damage the pump. The condenser has a removable three-quart center well designed for dry-ice/alcohol slurry. A 50–50 mix of dry ice and 95% ethanol was used in this study. The fryer’s basket rod is held by a motion feedthrough-type shaft-seal attached at an NTP-threaded port (Kurt J. Lesker, Clairton, PA). This shaft-seal allows rotary motion of the shaft running from the atmosphere side to the vacuum side of the chamber wall. 2.1.1. Experimental conditions Three levels of vacuum pressure (16.661, 9.888, and 3.115 kPa) and three levels of oil temperature (118, 132, and 144 °C) were considered in this study. The vacuum vessel was set to the target temperature and allowed to operate for one hour before frying started. The oil volume in the vessel was 6.5 l. For this oil volume, there was a temperature differential between the bottom layer and the top layer of oil in the vessel of about 3 °C. Fresh hydrogenated soybean oil was used in all experiments. The potato chips were fried until the equilibrium moisture content was reached. 2.1.2. Atmospheric deep-fat frying experiments For the atmospheric frying experiments, an electricfired type fryer (Hobart model HK3-2, Hobart Corp, Troy, Ohio) was used. It is a bench top type fryer with a frying oil capacity of 7.5 l. About 50–60 g of potato slices was fried in each batch. To compare the atmospheric frying to the vacuum frying on potato chip quality and frying rate, an oil temperature of 165 °C was considered. The atmospheric fryer was set to the required frying temperature and left for 30 min to ensure the oil temperature was equili- Fig. 1. Schematic of the vacuum frying system. J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191 Table 1 Solids content and color for the potato variety used in this study Characteristics Values Solids content (%) Color––lightness (L) Color––green/red chromaticity (a) 21.67 67.04 1.495 brated. The samples were fried to the equilibrium moisture content. Potato samples: A food company provided the potatoes used in this study. The potato breed is special for frying and was stored in a cooling chamber at a temperature range of 10–12 °C and a relative humidity of 55%. Table 1 shows the solid content and color specifications determined by the scientists of the food company for this potato variety. Fig. 2 presents a block diagram of the vacuum frying process for producing potato chips. The potatoes were taken out of storage at least 12 h before frying to let them reach room temperature and for the reducing sugar contents to decrease. Once the potatoes were peeled and sliced, they were soaked in water for a few seconds, and then dried in paper towels prior to frying. About 5–6 slices of potatoes (20–25 g) were fried each time. Once the oil temperature reached the target value, the potatoes were then placed into the basket, the lid closed, the vessel evacuated. At this moment, the basket was lowered to the oil and frying began for the desired frying time. Once the potato slices were fried, the basket was lifted from the oil and the vessel pressurized. Then, the lid of the vessel was opened and the potato chips were removed from the basket. The potato chips were then allowed to cool to room temperature, dried with paper towel, and later stored in polyethylene bags for further analysis. 2.2. Analytical methods Moisture content of raw potatoes: Potato tubers have considerable variability in initial moisture content from potato to potato. To account for this variability, the 183 initial moisture content of each potato was measured before the slices of the potato were fried. The initial moisture content was determined by drying samples of 5 g of the potato (slices) to a constant mass for 72 h at 105 °C (AACC, 1986). The tests were done in triplicate. Moisture content of potato chips: Potato chips were ground in a coffee grinder after frying. Moisture content of potato chips was determined by weight loss after drying 5 g samples of ground chips in a forced air oven at 105 °C for 24 h (AACC, 1986). The test was performed in duplicate. Oil Content: Total oil content of potato chips was determined by using the Soxtec System HT (Pertorp, Inc., Silver Spring, Maryland) extraction unit with petroleum ether (AACC, 1986). The test was performed in duplicate. 2.3. Product characteristics in frying Shrinkage: Degree of shrinkage in volume (Sv ) was evaluated by: Sv ¼ Vo V ðtÞ 100 Vo ð1Þ where Vo is the original volume of the sample (m3 ) and V ðtÞ is the volume (m3 ) of the sample at time t. The volume of the sample (elliptical shaped) at any given time can be calculated by V ¼ ðpDd=4ÞL where D is the larger diameter of the sample (m) and d is the smaller diameter [m] of sample, and L the sample’s thickness (m). Ten samples were taken to determine shrinkage for each frying condition at the equilibrium time. Color: The color of the potato chips was measured using a Hunter Lab Colorimeter Labscan XE (Hunter Associates Laboratory, Reston, VA). Color was measured in potato chips obtained at the equilibrium moisture content for each condition. Measurements were taken for ten chips of each condition, and two readings were taken for each potato chip by rotating the chip with an 180° angle. The colorimeter was standardized utilizing a white calibration plate, and a glass plate was used as the standard. The samples were placed always Fig. 2. Flow diagram of the vacuum frying process. 184 J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191 against the same background to obtain the color measurements. Texture: A rupture test was performed on the fried potato chips obtained at different frying times using a TA-XT2 Texture Analyzer (Texture Technologies Corporation, Scardale, New York). The test involves applying a direct force to a sample, which is placed over the end of a 0.018 m hollow cylinder. A 0.00635 m ball probe moving at a constant rate was used to break the chips. The data for each sample was saved for further manipulation. The data collected included the force (N) at each time (s) and corresponding distance (mm). Texture was evaluated on samples fried at several times corresponding to different levels of moisture content for each condition. The tests were performed using ten chips per condition. For good experimental practice, all tests were run on the same day the chips were processed. The property used to describe the texture of the samples was hardness, defined as the force at maximum compression (Steffe, 1996). It was measured by recording the peak force value of each sample rupture test. 2.4. Data analysis 2.4.1. Vacuum frying experiment The effect of temperature and vacuum pressure on the drying curve, the oil content, shrinkage, color, and texture of the chips was evaluated using a factorial de- sign with three levels for temperature and three levels for vacuum pressure. Analysis of variance, Duncan’s New Multiple range test (a ¼ 0:05) and Tukey’s tests were used to determine differences among treatments. The statistical analysis of the data was conducted using SAS statistical software package (version 6.10, 1996). Statistical significance was expressed at the p < 0:05 level. The experiments were performed at least in duplicate. Calculations of non-linear regression parameters were done using the Levenberg-Marquard iteration method with the graphics software package PlotIt (version 3.2, 1999). 3. Results 3.1. Moisture loss Figs. 3 and 4 represent the drying curves for vacuum frying at various experimental conditions (temperature and vacuum pressure). The loss of moisture during frying exhibited a classical drying profile. The drying process of foods is generally characterized by three distinct periods. The first is an initial heat-up period during which the wet solid material absorbs heat from the surrounding media. The product is heated up from its initial temperature to a temperature where the moisture begins to evaporate from the food. In vacuum frying, this initial heat-up period is very short and therefore difficult to quantify. Fig. 3. Effect of oil temperature on the moisture loss of potato chips versus frying time for different vacuum pressures: (a) Pvac ¼ 16:661 kPa; (b) Pvac ¼ 9:888 kPa; and (c) Pvac ¼ 3:115 kPa. J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191 185 Fig. 4. Effect of vacuum pressure on the moisture loss of potato chips versus frying time for different oil temperatures: (a) Toil ¼ 118 °C; (b) Toil ¼ 132 °C; and (c) Toil ¼ 144 °C. At a vacuum pressure of 3.115 kPa, for example, the boiling point of water is around 25 °C. The temperature in the potato slice is slightly higher than the boiling point of water due to the presence of some solutes. Since the temperature of the potato slices prior to frying was at room temperature (23–24 °C), the slices only had to warm up a few degrees for the water to start boiling. For this reason, the heat-up period in vacuum frying is very short. The second drying period is known as the constant rate period. Here, the rate of drying is limited by the rate at which heat is transferred from the drying medium to the product. The constant drying conditions continue as long as the food surface remains wetted with water. In the case of potato chips fried under vacuum frying, we could not observe any constant rate period (see Fig. 3). When the moisture level in the product is so low that its surface is no longer wetted, the drying rate decreases entering the falling rate period. During this period, drying rate is controlled by moisture diffusion mechanisms. The water during this period is held in the material by multi-molecular adsorption and capillary condensation (Toledo, 1991). Table 3 shows that potato chips fried at an oil temperature 144 °C and 3.115 kPa took the shortest time to fry (360 s). Potato chips fried at the same pressure and at 132 and 118 °C, took 480 and 600 s to fry, respectively. Decreasing the oil temperature increased the frying time of potato chips fried under vacuum as expected. The same behavior was observed for the other vacuum pressures. Gamble, Rice, and Selman (1987) reported that potato chips fried at atmospheric conditions at higher temperatures resulted in an initial rapid fall followed by a continuous drying period. Fig. 3a, b and c confirm this point, where the drying rate in the initial 150 s of frying was higher for 144 °C than for 132 °C. The difference was even more pronounced when comparing the drying rate at 132 and 118 °C at all pressures. Fig. 4a, b and c show the effect of vacuum pressure on moisture content history for potato chips fried at a temperature of 118, 132 and 144 °C, respectively. The rate of moisture loss was affected in a significant manner (p < 0:05) by the vacuum pressure treatment. Fig. 4a–c show that there is a negative correlation between vacuum pressure and moisture loss rate for the same oil temperature. This is due to the fact that the more the pressure is lowered; the further the boiling point of water is reduced. As a consequence, the water in the potato chips will begin to vaporize faster at a higher vacuum level. Table 2 shows that by decreasing the vacuum pressure, for a fixed value of temperature, it took less time for the chips to reach the same final moisture content (1:90 0:05% w.b.). For example, at 132 °C, the frying time decreased from 480 s (Pvac ¼ 16:661 kPa) to 360 s (Pvac ¼ 9:888 kPa) and then to 300 s (Pvac ¼ 3:115 kPa). The equilibrium moisture content was significantly 186 J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191 Table 2 Initial moisture content, moisture content after frying, and oil content for potato chips fried under different vacuum pressures and oil temperatures Toil (°C) Pvac (kPa) FT (s) IMC (% w.b.) EMC (% w.b.) FOC (% w.b.) 118 132 144 118 132 144 118 132 144 16.661 16.661 16.661 9.888 9.888 9.888 3.115 3.115 3.115 600 480 360 600 480 360 600 480 360 78:96 1:53a 77:92 0:52a 73:55 0:63a 77:98 1:01a 75:65 0:33a 73:45 0:35a 79:97 0:70a 80:11 0:68a 75:04 0:56a 1:84 0:19a;a (1:84 0:19) 1:90 0:21a;a (1:90 0:21) 1:93 0:02a;a (1:93 0:01) 1:31 0:10a;b (1:80 0:01) 1:37 0:04a;b (1:89 0:19) 0:94 0:02b;c (1:95 0:05) 1:06 0:19a;c (1:94 0:08) 1:12 0:01a;c (1:85 0:17) 0:75 0:00b;d (1:90 0:06) 25:74 0:36a 26:22 0:34a 26:66 0:26a 26:07 0:45a 26:16 0:20a 27:22 0:32a 26:66 0:31a 26:16 0:03a 26:63 0:02a (600) (480) (360) (480) (360) (200) (360) (300) (190) (35.54) (36.48) (37.33) (35.90) (36.09) (37.88) (36.89) (35.89) (36.67) Tests were performed in duplicate. Means with the same letter are not significantly different (p < 0:05). Toil ¼ oil temperature, FT ¼ frying time (values in parenthesis are the frying times to obtain chips with final moisture content around 1.90% w.b.), IMC ¼ initial moisture content, FOC ¼ final oil content (values in parenthesis is oil content in dry basis); EMC ¼ equilibrium moisture content (number in parenthesis is the final moisture content related to FT values in parenthesis––column 3). affected (p < 0:05) by the vacuum pressure as indicated in Table 2. 3.2. Oil absorption Oil absorption is a complex phenomenon that happens mostly when the product is removed from the fryer during the cooling stage (Sun & Moreira, 1994). Potato slices where fried for different times (and different operating conditions), until the equilibrium moisture content was reached, to study the effect of frying time on the chip’s total (surface plus internal) oil content. Figs. 5 and 6 present the oil absorption curves for vacuum frying at various experimental conditions (temperature and vacuum pressure). It can be observed that within the first 150 s, the chips’ oil content increased as frying time increased for all temperatures and vacuum pressures (Fig. 5a–c). This coincided with the period of time at which water evaporated from the chips at the fastest rate (Fig. 3a–c). The chips absorbed more oil when fried at Pvac ¼ 3:115 kPa and Toil ¼ 144 °C when the chips’ oil content reached a maximum value of about 56% d.b (33% w.b.) after 150 s of frying and then reduced to about 37% d.b. (27% w.b.) as frying reached 360 s. Baumann and Escher (1995) found that the varying frying oil temperature under atmospheric conditions caused a slight increase in final oil content of the chips. Higher oil temperatures lead to a faster development of a solid crust and consequently surface properties that are favorable for oil absorption. In the case of frying under vacuum, the final oil content of the potato chips was not significantly affected (p < 0:05) by the oil temperature (Table 2). This suggests that the final oil content of the potato chips is not a function of oil temperature, but it depends on the frying time (thus remaining moisture), which increases with decreasing oil temperature. Fig. 5a shows that for a Pvac of 16.661 kPa the product absorbed more oil, initially, when fried at 144 °C than at the other two lower temperatures. This behavior was similar for vacuum pressures of 9.888 and 3.115 kPa as illustrated in Fig. 5b and c, respectively. So, the higher the oil temperature, the higher the oil pickup by the potato chips during the first 150 s of frying. Note that the vacuum pressure level did not affect the final oil content of potato chips fried at any temperature (Table 2). Higher vacuum levels lead to faster development of a crust and thus to faster oil absorption rate (due to loss of the characteristic hidrophilicity of raw potatoes) during the process. Therefore, the oil absorption rate was faster as the vacuum level increased (Fig. 6a–c). Vacuum pressure affects the speed at which moisture leaves the potato chips (by lowering the Tsat of water). Fig. 6c shows that the initial oil pickup (around 150 s) by the potato chips was higher at 3.115 kPa than at 9.888 and 16.661 kPa, respectively. However, the effect of vacuum pressure on the chips’ oil content at different frying time was not as large as the effect of temperature for the conditions analyzed in this study. This can be observed by comparing Fig. 5a–c with Fig. 6a–c. It is interesting to notice that the oil absorption versus frying time curves for the chips fried at Pvac ¼ 3:115 and 9.888 kPa at Toil ¼ 144 °C (Fig. 6c) behaved differently than those chips fried at the other Toil and Pvac conditions, showing a maximum value for oil absorption around 150 s of frying. These results indicate that probably more oil adheres to the chip’s surface as Pvac decreases (Tsat of water decreases, moisture loss rate increases, thus the crust develops fast). As the chip is removed from the oil bath, the adhered oil intends to flow into the pore spaces during the pressurization period. The amount of free water in the product seems to be related to oil absorption, which is higher when more free spaces are available (when the amount of free water is small). That is, an increase in the surrounding pressure at constant temperature, during the pressurization period, causes the vapor inside the pore to condense (at constant T and P). As the water vapor condenses the pressure difference J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191 187 Fig. 5. Effect of oil temperature on the oil absorption of potato chips versus frying time for different vacuum pressures: (a) Pvac ¼ 16:661 kPa; (b) Pvac ¼ 9:888 kPa; and (c) Pvac ¼ 3:115 kPa. Fig. 6. Effect of vacuum pressure on the oil absorption of potato chips versus frying time for different oil temperatures: (a) Toil ¼ 118 °C; (b) Toil ¼ 132 °C; and (c) Toil ¼ 144 °C. (Psurroundings Ppore ) causes the oil to be absorbed into the pore space. Under these operating conditions, most of the oil will be absorbed during the pressurization pro- cess. For the chips fried at a higher Pvac (16.661 kPa), the higher Tsat slowed down the crust formation so less oil adhered to the chips’ surface. In addition, due to the low 188 J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191 moisture loss rate, the higher free water content hindered the oil to enter the pore spaces during pressurization. Once the free water is reduced to a critical level (at the 150 s in this case), oil absorption during the pressurization process decreases (since (Psurroundings Ppore ) is negligible). Air then diffuses faster than the oil into the pore spaces thus blocking the oil to flow into the product. In this case, most of the oil is absorbed during cooling. These observations indicate that the pressurization process plays an important role in the oil absorption mechanism. It can increase or decrease oil absorption in the product depending on the amount of surface oil and free water presented in the product. The lower the vacuum pressure and the higher the temperature (i.e. Pvac ¼ 3:115 kPa and Toil ¼ 144 °C), the highest is the oil content of the chips during the initial period of frying when free water is still available in the product. Therefore, for these conditions, the lowest oil content is obtained when the chips are fried as close as to the equilibrium moisture content. It seems that oil temperature has a major effect on the oil absorption rate by increasing the moisture loss rate (see Fig. 6a–c). Pressure affects the Tsat of water and thus has a minor effect on the oil absorption phenomenon during vacuum frying (Fig. 5a–c). Since the best frying time and quality results were obtained at 3.115 kPa and 144 °C with the same final oil content and quality results, only the products fried under this vacuum condition will be compared with those products fried under the atmospheric condition. Fig. 7a shows that the drying rate of potato chips was similar for vacuum-fried potato chips and for potato chips fried under atmospheric conditions (at 165 °C). Table 3 shows that potato chips fried faster at atmospheric conditions (300 s) compared to the chips fried at vacuum (360 s). Also, the equilibrium moisture content for these two conditions shows significant difference (p < 0:05) between the treatments, with the chips fried at the atmospheric fryer showing lower value for EMC. Table 3 also shows that the final oil content values were significantly different (p < 0:05) for potato chips fried under vacuum (37% d.b. (27% w.b.)) and atmospheric pressure (66% d.b. (40% w.b.)). Fig. 7b shows the difference between oil absorption rate for potato Fig. 7. Comparison between potato chips fried under vacuum pressure and atmospheric pressure (a) moisture loss vs. frying time, (b) oil absorption vs. frying time. chips fried at atmospheric conditions (at 165 °C) and potato chips fried under vacuum at Pvac ¼ 3:115 kPa and Toil ¼ 144 °C. The frying behavior of chips is clearly different from each treatment as indicated by the oil absorption curves in Fig. 7b. One major difference between potato slices fried under vacuum and atmospheric conditions is the surface structure of the potato chips formed during the process. Visual observations indicated that the surface of a vacuum fried potato chip had less expansion and numerous small bubbles, as opposed to a potato chip fried under atmospheric pressure, which showed more expansion and lesser but larger bubbles. The bubble formation at the product’s surface is the results of gas expansion inside the pores. For the vacuum fried chips (Pvac ¼ 3:115 kPa, Tsat ¼ 24:6 °C), once Table 3 Comparison of initial moisture content, moisture content after frying, and oil content for potato chips fried under vacuum and atmospheric conditions Toil (°C), P (kPa) 144, 3.115 165, 101.35 FT (s) 360 300 IMC (% w.b.) a 75:04 0:56 78:50 0:00a EMC (% w.b.) a 0:75 0:01 0:45 0:01a FOC (% w.b.) 26:63 0:03a (36.67) 39:65 0:16b (66.20) Tests were performed in duplicate. Means with the same letter are not significantly different (p < 0:05). Toil ¼ oil temperature, Pac ¼ vacuum pressure, FT ¼ frying time, IMC ¼ initial moisture content, EMC ¼ equilibrium moisture content, FOC ¼ final oil content (values in parenthesis is for oil content in dry basis). J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191 the fryer is evacuated, the water vapor in the pores expands with little resistance (since the crust has not been formed and/or starch gelatinized) even before the product is fried. During frying, little expansion may be produced by the superheated vapor trying to escape the pore space. For the chips fried under atmospheric conditions, the expansion happens when the product is immersed in the oil. Water is heated first and then the vapor expanded. As it is heated starch will gelatinize producing a barrier for the saturated vapor to escape (Kawas & Moreira, 2001). As a result, large, but few, bubbles will be formed at the chips surface. Mass transfer mechanisms in atmospheric frying can be divided into two periods: the frying period and the cooling period. The frying period is characterized by a heating-up stage in which the product’s temperature is raised from room temperature (23–24 °C) to the boiling point of water (100 °C). This process occurs at an extremely fast pace, since liquid water has a high specific heat. Once the water inside the potato chip reaches the boiling point, it vaporizes. Once the pores are filled out with gas (water vapor and air), the temperature, and then the pressure, increase at a fast rate (due to high temperature differential with the oil). During this period, the capillary pressure is negligible (Moreira, CastellPerez, & Barrufet, 1999), so there is no driving force for the oil to flow into the chip’s pores. During the cooling period the surface oil that adhered to the surface of the potato chips during frying penetrates into the pores of the chips. As the potato chip cools down, the pressure inside the pores changes as a consequence of the so-called capillary pressure rise (Moreira & Barrufet, 1995). This pressure difference between the surface and the pores of the potato chips creates a driving force for the oil and air to penetrate the pores. The mass transfer mechanisms in vacuum frying can be understood by dividing the process into three periods: the frying, the pressurization, and the cooling period. At the beginning of the frying period, the boiling temperature of the water has been lowered to a level lower than 100 °C (56.0 °C for Pvac ¼ 16:661 kPa, 45.4 °C for Pvac ¼ 9:888 kPa, and 24.6 °C for Pvac ¼ 3:115 kPa). So, the water in the potato chips evaporates quicker under vacuum conditions and it will be less vigorous. Capillary pressure (between oil and gas) during this period is negligible. Thus, no oil is absorbed at this stage. The crust in vacuum frying begins to form as soon as the potato slices are immersed in the oil. The second period in vacuum frying is the pressurization from vacuum to atmospheric conditions. This stage begins when the potato chips are removed from the frying oil and are kept under vacuum and temperature conditions (the fryer vessel is still closed). At this stage, as the vessel is vented, the pressure in the pores of the potato chips rapidly increases to atmospheric levels, 189 as air and surface oil are carried into the empty pores spaces until the pressure reaches atmospheric levels. However, because of the low pressure, gas diffuses much faster into the pore space thus obstructing the oil’s passage to enter the product’s pores. The third period starts when the potato chips are removed from the fryer and it is known as the cooling period, when part of the adhered oil continues to penetrate the pore spaces. Since less oil adheres to the chips surface during vacuum frying, less oil is absorbed during cooling. 3.3. Product characteristics 3.3.1. Shrinkage Fig. 8a presents the effect of oil temperature on the degree of volume shrinkage of potato chips fried under vacuum frying as a function of temperature and vacuum pressure. Volume shrinkage during early stages of frying very nearly equals the volume of water loss; however, in the Fig. 8. Shrinkage in volume of potato chips as function of frying time. (a) Effect of oil temperature and vacuum pressure; (b) comparison between vacuum-fried chips and atmospheric-fried chips. 190 J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191 final stages of drying the volume shrinkage is smaller (Johnson, 1999). Therefore, volume shrinkage depends on water transfer within the product. For the same frying time, higher temperatures result in a higher mass diffusivity, higher water loss, and consequently lower volume shrinkage. The final shrinkage in volume of potato slices fried under different vacuum pressures was shown to decrease with oil temperature (Fig. 8a). In addition, higher oil temperature caused the potato chip surface to become rigid more rapidly, thus producing increased resistance to volume change. Lamberg, Hallstrom, and Olsson (1990) suggested that the formation of a rigid surface in atmospheric frying at higher drying rates. Fig. 8a also shows that by decreasing the vacuum pressure, the chips shrunk more. Probably, the structured formed in the chips during low vacuum pressure was less rigid than that formed under high pressures thus resulting in less resistance to volume change. Fig. 8b represents a comparison of the degree of shrinkage in volume for potato chips fried under vacuum and atmospheric conditions. Potato chips showed a higher degree of volume change when fried under vacuum at Pvac ¼ 3:115 kPa and Toil ¼ 144 °C than when fried at atmospheric conditions at Toil ¼ 165 °C. This is probably because the product becomes more rigid when fried under atmospheric frying than when fried under vacuum frying. 3.3.2. Color The statistical analysis showed that there were no significant differences (p < 0:05) for lightness (L ), green– red chromaticity (a ), and blue-yellow chromaticity (b ) of potato chips as a function of vacuum temperature and pressure (not shown). A comparison of the color attributes of potato chips fried under vacuum and atmospheric conditions is shown in Fig. 9. The potato chips fried at Pvac ¼ 3:115 kPa and Toil ¼ 144 °C had L values that were significantly higher (p < 0:05) than the values corresponding to the potato chips fried under the atmospheric condition (Toil ¼ 165 °C). A higher L value indicates a lighter color, which is desirable in potato chips. The a values were significantly higher (p < 0:05) for potato chips fried at atmospheric pressure than for potato chips fried at the vacuum conditions, indicating more Maillard reaction occurred at the atmospheric frying conditions. The blue–yellow chromatically (b ) values were also significantly higher (p < 0:05) for the potato chips fried at atmospheric pressure and Toil ¼ 165 °C than for potato chips fried at vacuum conditions. Visual observation confirmed the results obtained with the colorimeter, since the potato chips fried under atmospheric conditions were darker, more red, and yellowish than potato chips fried under vacuum. The change Fig. 9. Comparison of color of potato chips fried under vacuum pressure and atmospheric pressure. in color in fried potato chips is due to the interaction of an amine group with a reducing sugar, which is a nonoxidative browning also known as Maillard reaction. 3.3.3. Texture During frying, most of the water is removed from the potato slices resulting in textural changes. Table 4 shows the effect of pressure and temperature on the texture characteristic of potato chips at the end of frying. The force required to break the chips (hardness) increased for oil temperature range used in this study. However, the oil temperature did not affect significantly (p < 0:05) the chip’s texture. Higher vacuum levels (lower vacuum pressure) produced chips with lower hardness values. However, the texture was not significantly (p < 0:05) affected by vacuum pressure. For the potato chips fried under atmospheric conditions (Table 5), during the first 50 of frying, their structure was very weak and collapsed easily when applying the rupture test. For the potato chips fried under vacuum conditions, the texture in the initial stages of Table 4 Effect of frying time on the texture (hardness) of potato chips fried under different operating conditons Toil (°C) Pvac (kPa) FT (s) Hardness (N) 118 118 118 16.661 9.888 3.115 600 600 600 2:73 0:81a 2:47 0:44a 2:23 0:51a 132 132 132 16.661 9.888 3.115 480 480 480 3:13 0:64a 2:58 0:42a 2:05 0:40a 144 144 144 16.661 9.888 3.115 360 360 360 3:65 0:42a 2:86 0:86a 2:71 0:79a Means with the same letter are not significantly different (p < 0:05). Toil ¼ oil temperature, Pvac ¼ vacuum pressure, FT ¼ frying time. J. Garayo, R. Moreira / Journal of Food Engineering 55 (2002) 181–191 Table 5 Comparison of textures values of potato chips fried under atmospheric and vacuum conditions (Toil ¼ 144 °C, Pvac ¼ 3:115 kPa) Toil (°C) Pvac (kPa) FT (s) Hardness (N) 165 165 165 165 165 101.3 101.3 101.3 101.3 101.3 30 150 180 240 300 0:65 0:11a 2:65 0:59b 2:89 0:91b 3:45 0:88c 2:88 0:54b 144 144 144 144 3.115 3.115 3.115 3.115 30 150 240 360 3:78 1:83a 3:31 0:55b 3:17 0:49b 2:71 0:79b 191 Comparing atmospheric frying to vacuum frying the following conclusions were obtained. The drying rate for potato chips fried under atmospheric and were the same for the two treatments compared in this study. The final oil content was higher for potato chips fried under atmospheric conditions than for potato chips fried under vacuum conditions. The frying behavior was different for the chips fried under vacuum and atmospheric conditions. Potato chips fried under vacuum pressure had higher volume shrinkage, were lighter, and softer than potato chips fried under atmospheric conditions. Means with the same letter are not significantly different ðp < 0:05Þ. Toil ¼ oil temperature, Pvac ¼ vacuum pressure, FT ¼ frying time. References frying was also leathery, but much more flexible and the chips became brittle very early during the frying process. However, the final hardness values were not significantly different (p < 0:05) for the potato chips fried at vacuum and atmospheric conditions. 4. Conclusions The main purpose of using vacuum frying in this study was to evaluate its feasibility for production of low oil content potato chips. Oil absorption rate during vacuum frying of potato chips was related to the moisture loss rate. The highest drying loss rate (thus the highest the oil absorption rate) was obtained with the highest oil temperature (Toil ¼ 144 °C) and lowest vacuum pressure (Pvac ¼ 3:115 kPa). The fryer operating conditions did not affect the final oil content of the vacuum-fried chips. However, results showed that the faster the water loss rate, the higher the oil adhesion at the chips surface and then the higher oil absorption. In addition, as the percentage of free water is depleted in the product, less oil is absorbed. The pressurization step plays an important role in reducing the oil absorption during vacuum frying. The vacuum frying operating conditions tested in this study did not affect significantly (p < 0:05) the color or the texture characteristics of the chips at the end of frying. The percentage of volume shrinkage of the chips decreased when Toil increases and increased when Pvac decreases. AACC. (1986). Approved methods of the American Association of Cereal Chemists. Minneapolis MN: AACC. Baumann, B., & Escher, F. (1995). Mass and heat transfer during deep-fat frying of potato slices, rate of drying and oil uptake. Lebensmittel Wissenschaft und Technologie, 28, 395–403. Gamble, M. H., Rice, P., & Selman, J. D. (1987). Relationship between oil uptake and moisture loss during frying of potato slices from the UK tubers. International Journal of Food Science and Technology, 22, 233–241. Johnson, A. T. (1999). Biological process engineering. New York: John Wiley & Sons, Inc. Kawas, M. L., & Moreira, R. G. (2001). Effect of degree of starch gelatinization on product quality attributes of tortilla chips during frying. Journal of Food Science, 66(2), 195–210. Lamberg, I., Hallstrom, B., & Olsson, H. (1990). Fat uptake in a potato drying/frying process. Lebensmittel Wissenschaft und Technologie, 23, 295–300. Moreira, R. G., & Barrufet, M. A. (1995). Spatial distribution of oil after deep-fat frying from a stochastic model. Journal of Food Engineering, 27(2), 205–220. Moreira, R. G., Castell-Perez, M. E., & Barrufet, M. A. (1999). Deepfat frying: Fundamentals and applications. Gaithersburg, MD: Aspen Publishers. Shyu, S., & Hwang, L. S. (2001). Effects of processing conditions on the quality of vacuum fried apple chips. Food Research International, 34, 133–142. Shyu, S., Hau, L., & Hwang, L. S. (1998). Effect of vacuum frying on the oxidative stability of oils. Journal of American Oil Chemical Society, 75, 1393–1398. Steffe, J. F. (1996). Rheological methods in food process engineering (2nd ed.). East Lansing, MI: Freeman Press Inc. Sun, X., Moreira, R.G. (1994). Interfacial tension on oil uptake during deep fat frying of tortilla chips. Poster Presentation––IFT Annual Meeting, Atlanta, GA. Toledo, R. R. (1991). Fundamentals of food process engineering (2nd ed.). New York: Van Nostrand Reinhold Company, Inc.
© Copyright 2024