Vacuum frying was explored to cook donuts and compared to the conventional atmospheric frying. A temperature of 190°C was used for atmospheric frying. Three vacuum levels (3, 6, and 9 kPa vacuum) with three temperature levels (150, 165, and 180°C) were used for vacuum frying. The effects of initial moisture content (IMC), vacuum level and frying temperature on physicochemical properties, such as moisture loss, oil absorption, and quality were investigated. The properties of fried donuts were significantly affected by IMC. Under vacuum frying, volume and total color changes were affected by frying temperature; and oil uptake was affected by vacuum and frying temperature. Frying temperature and vacuum were not directly related to the final moisture content (MC) of donuts. There was no relationship between MC and fat content of donuts. Donut texture was directly related to the vacuum and frying temperature.
INTRODUCTION
Deep-fat fried donut is popular in North America, served as breakfast, convenience food or even snacks. The donut market alone is a $3–4 billion business in the U.S.[Citation1] Donuts are characterized by a golden-brown exterior color, a crisp crust, and an inner core that resembles a baked product more than a fried food.[Citation2] The fat content of the finished donuts varied in the range of 10–26%, depended on the formulation and frying conditions.[Citation2,Citation3] However, deep-fat fried foods have been causing concern due to their high fat content.[Citation4] Fried donut is not the exception. Atmospheric frying is usually employed for traditional fried foods. Recently, vacuum frying has been used as an alternative technique for fried foods with desired characteristics. Vacuum frying was used[Citation5] to prepare apple chips in place of common heat treatment to prevent enzymatic browning of the chips. Using break-force as a product quality indicator, at 2.66 kPa vacuum, the optimum frying temperature was 100–110°C with frying time of 20–25 minutes.
The effect of vacuum frying was reported[Citation6] on the drying rate and oil absorption of potato chips, as well as the product quality attributes such as shrinkage, color, and texture. Three levels of vacuum (16.7, 9.9, and 3.1 kPa vacuum) and oil temperature (118, 132, and 144°C) were considered. Furthermore, the characteristics of potato chips fried at 3.1 kPa vacuum and 144°C were compared to the chips fried under atmospheric pressure at165°C. During vacuum frying, oil temperature and vacuum had a significant effect on the moisture loss and oil absorption of chips. The chips fried at lower vacuum and higher temperature had less volume shrinkage; color was not affected by oil temperature and vacuum; hardness increased with increasing oil temperature and decreasing vacuum; and final oil content was higher for chips fried at atmospheric pressure than for those fried under vacuum. Chips fried under vacuum had higher volume shrinkage, were lighter, and softer than fried at atmospheric pressure. Hence, the objective of this article was to investigate the effects of initial moisture content (IMC) of dough, vacuum, and frying temperature on the time required for frying, and the properties of donuts such as volume and color, moisture (MC) and fat contents (FC), and texture.
MATERIALS AND METHODS
Experimental Design
First, the effects of IMC, with low and high MC formulations, on the donut properties were compared under atmospheric frying at 190°C. The desired IMC formulation was then selected for the subsequent vacuum frying study. Three levels of vacuum (3, 6, and 9 kPa vacuum) and oil temperature (150, 165, and 180°C) were considered. Samples were taken every minute during the frying for MC and FC determinations to calculate moisture loss and oil absorption. For the cooked donuts, MC, FC, volume, color, and texture were determined. Three replications were performed for each treatment.
Dough Preparation
Quick biscuit mix (President’s Choice, Sunfresh Limited, Toronto, Ontario, Canada) was used as a donut mix. The donut formulation with low IMC was 50 g mix with 15 g water, whereas the high IMC formulation was 50 g mix with 20 g water. The mix and water were placed into a mixer (Mixmaster stand mixer, model 2367–33, Sunbeam Corporation Limited, Mississauga, Ontario, Canada) and mixed for about 10 minutes until the dough base was evenly formed. Then the dough was cut into pieces, weighing about 25 g each, on a cutting board. Each piece of dough was moulded into cylindrical shape using a plastic mould, measuring approximately 24 mm (height) × 39 mm (diameter). The dough pieces were stored in polyethylene bags (Ziploc brand, Johnson and Son Limited, Brantford, Ontario, Canada) to prevent moisture loss. The IMC values were determined in triplicate.
Atmospheric Frying
A commercial cooker (Type F04–C1M, Supercontrol, Tefal, Mexico) with 8 L of capacity was used for atmospheric frying. The cooker was equipped with a thermostatic control for desired temperature in the range of 120–190°C. A steel basket with a wire mesh and a lid was specially designed in which donuts were placed. During the frying, the basket was fully immersed into the oil to ensure good contact between donuts and oil. A thermocouple probe (Type J-K-T thermocouple, model HH21, Omega Engineering, INC. USA) was plugged into the geometric center of a donut so that the core temperature was obtained during the frying. Another thermocouple probe was used to monitor the oil temperature. The fryer was filled by half of its height (7 cm) with about 4 L of oil (Creamy liquid frying shortening, CanAmera Foods, Oakville, Ontario, Canada), and heated at 190°C for 20 min before using, to ensure the oil temperature was stable and constant. Then the basket along with donuts was immersed into the oil for frying. According to the experimental design, some donuts were removed after 1st, 2nd, 3rd … minute, until the core temperature of donuts reached 99°C that indicated the end of the frying.[Citation1,Citation2] The fried donuts were removed from the oil, drained for a few seconds and turned over onto an absorbent paper, and then turned over to other side with a new absorbent paper. The samples were subsequently stored in Ziploc bags for further analysis.
Vacuum Frying
A pressure cooker (Mirro, 92D12, Sao Paulo, Brazil) of 11.4 L volume was modified to serve as a vacuum vessel. Two original safety valves were removed, and one was used for setting a vacuum gauge and other for thermocouple probes connections. The cooker was filled by half of its height (9 cm) with about 5.5 L of liquid shortening, heated to the designated temperature, and kept constant for 20 minutes before frying. The basket used for atmospheric frying was also used for vacuum frying. A dual-seal vacuum pump (model G180GDX, GE Motors & Industrial Systems, New York, USA) was used to provide required vacuum (). A heater (Standard Appliance, 1500 W, MFG Co. Ltd. Toronto, Canada) was used for heating the frying oil. A condenser, consisted of a dry-ice trap, was used to absorb the water vapor coming from the vacuum fryer and to prevent vapor entering the vacuum pump.[Citation6]
Figure 1. Schematic diagram of vacuum frying system.
The frying basket rod was held by a shaft seal (VRC shaft seal, Vacuum Research Corporation, Pittsburgh, PA, USA) attached on the center of the vessel lid. The specific design of the shaft seal allowed motion of the shaft along with the basket up and down, so that the donuts could be moved either into the oil while frying or above the oil after frying. During the experiment, the vacuum levels were controlled to 3 ± 0.5, 6 ± 0.5, and 9 ± 0.5 kPa vacuum by a valve between the pump and the condenser. The frying was terminated when the donut core temperature reached 99°C as darkening of the donut surface was observed above this temperature.
Analytical Methods
Moisture (MC) and fat (FC) contents
The fried donuts were cut into small pieces for MC analysis. MC was measured in duplicate using a convection oven (model 7094A, Toastmaster Inc., Boonville, MO, USA) at 105°C for about 24 hours by spreading about 5 g of sample on an aluminum dish, until the weight in mg (model PJ3000, Mettler Instrumente AG, Switzerland) was constant for 2 min.[Citation7] The dry samples from MC test were sealed in the aluminum dishes with plastic film (Cling Wrap, Clorox Company of Canada, Toronto, Canada) and stored in a freezer at −8°C. FC was determined in duplicate by Soxhlet extraction method using petroleum ether.[Citation7]
Volume change
The volumes of raw and fried donuts were calculated with the following equation as the donuts were cylindrical in shape with variation in diameter due to frying.
Where D is the larger diameter (mm), d is the smaller diameter (mm), and H is the height (mm).[Citation6] A pair of callipers with least count of 0.1 mm was used to measure the dimensions. The volume change (ΔV) was obtained by:[Citation3]
Where Vo is the volume of uncooked donut and Vt is the volume of fried donut at any time t. The volume was measured in 6 replicates.
Color
The Color of donut crust was measured by reflectance on a colorimeter (Chroma meter CR-300 series, Minolta, Tokyo, Japan), with L∗ (lightness), a∗ (redness), and b∗ (yellowness) parameters. Total color difference, ΔE, was calculated by[Citation8]
This difference was calculated with respect to the raw donut, as the source of ,
, and
parameters. The calibration was conducted using a standard white plate.
Texture
Puncture test was used as an objective method to measure the textural properties of fried donuts. It measures the force required to push a probe into the donut to a depth that causes irreversible crushing of the sample.[Citation9] The test was carried out on a Universal Testing Machine (Model 4202, Instron Corporation, Canton, MA, USA) equipped with a 1 kN load cell. The cylindrical probe used in the test was made of stainless steel with 2 mm diameter and 30 mm height. The machine was calibrated with a 1 kg weight. The force-displacement data were collected at the rate of 10 points per second. The probe was moved at a speed of 100 mm/min and allowed to penetrate 20 mm into the donut in a 2-cycle test.[Citation9] The measurement was conducted in triplicate.
Data Analysis
All data except those obtained in the puncture test were analyzed using the Statistical Analysis System (SAS).[Citation10] Analyses of variance were performed by ANOVA procedure of the SAS. Duncan’s multiple range test was used to determine differences among treatments. Statistical significance was expressed at P ≤ 0.05 level.
RESULTS AND DISCUSSION
Properties Changes Under Atmospheric Frying
Frying time, volume, and color
shows the effects of IMC on frying time, volume, color, MC, and FC of donuts fried under atmospheric pressure. Donuts with both IMC formulations needed the same frying time (5 minutes) to achieve a core temperature of 99 °C. Compared to donuts with higher IMC, donuts with lower IMC had bigger size, looked darker, and had lower MC and higher FC (P ≤ 0.05). A major difference at the surface between high and low IMC donuts was observed during their preparation, i.e. donuts with low IMC appeared much drier with a few cracks on their surfaces; in contrast, the counterpart looked much smoother and shinier. During the frying, oil easily penetrated into the donuts through the existing cracks and splits. Thus, donuts with low IMC absorbed more oil during frying and this might have resulted in more expansion and darkening.
Table 1 Effects of IMC on frying time, volume and color changes, MC and FC of donuts fried under atmospheric pressure
Moisture loss and oil uptake
shows the drying curve of dunuts during atmospheric frying. Loss of moisture during frying exhibited a typical drying profile. Moisture was rapidly lost in the 1st minute of frying, which could be considered as the first falling rate period (constant drying rate might be absent due to high initial solid content), then followed by a second falling rate period (due to change in the slope). This observation was in agreement with the published work[Citation11] where a similar drying curve for potato chips was obtained. The MC loss rate was slightly faster (based on the slope) for high IMC formulation than for low IMC formulation. During this period, the migration of moisture to the surface was more rapid than the evaporation taking place at the surface. The oil absorption curves during the frying are shown in . Within the first 2 minutes for high IMC formulation and the first 3 minutes for low IMC formulation, the oil uptakes were rapid, leading to the replacement of evaporated moisture. This phenomenon was also observed in the potato chip frying.[Citation11] The samples with low IMC formulation had a slightly higher oil uptake rate than the counterpart.
Figure 2. Moisture content versus frying time under atmospheric frying (50 + 20 means 50 g mix and 20 g water).
Figure 3. Oil uptake versus frying time under atmospheric frying (50 + 20 means 50 g mix and 20 g water).
Donuts with high IMC had higher MC and lower FC (), whereas the counterpart had lower MC and higher FC (P ≤ 0.05). The results confirmed the following findings: (i) Variation in the amount of water added to the mix would materially affect oil absorption.[Citation12] This study confirmed that higher IMC resulted in lower oil uptake. (ii) Under normal frying conditions, the amount of oil uptake reported to be directly proportional to the amount of moisture lost. The oil uptake to moisture loss ratio might vary with different frying products and conditions.[Citation2,Citation6,Citation11,Citation13,Citation14] Donuts with high IMC lost about 46% db (dry basis) moisture during frying, consequently led to an amount of 13.1% db oil uptake which was lower than 17.9% db oil uptake by lower IMC donuts corresponding to about 51% moisture loss.
Instrumental texture
The force-displacement curves (not shown) of puncture test indicated initial rapid rise in force as the probe moved onto the donut surface. The highest force values (brittleness) were 4.67 and 5.85 N for high and low IMC donuts respectively. During this stage, the crust was deformed under the load but no puncturing of the interior donut. This stage ended abruptly when the probe began to penetrate beyond the crust, which was represented by a sudden change in slope called the “yield point”.[Citation9] In the third phase of the puncture test, the force decreased greatly for both cases after the yield points, indicating the hardness (hardness 1) in the donut interior section much lower than in the crust. During this, the force fluctuated in the range of 0.5–1.5 N for high IMC donuts, and 1.6–3.0 N for the counterpart. Similarly, the force corresponding to the yield point for the second penetration (hardness 2) was 1.1 N for high IMC donuts, and 2.45 N for the low IMC. These values demonstrated that the interior sections of donuts with low IMC were much harder than those with high IMC. Overall, based on the results and visual observations, IMC apparently plays an important role in the characteristics of fried donuts, particularly in the fat-moisture ratio. Higher IMC resulted in a lower oil uptake. Similar conclusions were reported earlier.[Citation2,Citation15] The high IMC formulation (50 g mix + 20 g water) was accordingly chosen for vacuum frying experiments.
Properties Changes Under Vacuum Frying
Frying time, volume and color
The frying times of 8, 7, and 6 minutes were needed to achieve a core temperature of 99°C () for frying temperatures of 150, 165, and 180°C respectively, with 3 levels of vacuum. Frying time was a function of frying temperature and independent of vacuum since large temperature gradient affects the heat transfer more compared to low level of vacuum. Atmospheric frying required the shortest frying time but at a higher frying temperature of 190°C. At 150°C, volume changes (ΔV) at 3 levels of vacuum were significantly different. However, at 180°C, ΔV was not affected by the vacuum level. At 165°C, there was a significant difference in ΔV for 3 and 9 kPa vacuum, whereas there was no significant difference in ΔV for 3 and 6 kPa, and also for 6 and 9 kPa vacuum. Compared to vacuum frying, ΔV under atmospheric frying was significantly different and provided the smallest ΔV. The ΔV increased with increase in vacuum or decrease in frying temperature (). However, the influence of vacuum diminished as temperature increased. The ΔV reached the highest (208%) at 9 kPa vacuum and 150°C, and ΔV reached the smallest value (109%) for atmospheric frying at 190°C.
Table 2 Effects of vacuum and temperature on frying time, volume and color changes, MC and FC of donuts fried under vacuum frying
The ΔV during early stages of frying nearly equalled the volume of water loss,[Citation16] and in the final stages of frying ΔV was smaller. Thus, in the beginning of frying, moisture was lost rapidly and dough expanded quickly. For the same frying time, higher temperature resulted in a higher water loss, and consequently lower ΔV. In addition, higher temperature provided harder donut surface rapidly, thus providing increased resistance to ΔV. On the other hand, higher vacuum likely provided less compact donut structure than at lower vacuum, thus resulted in lower resistance to ΔV. The volume expansion data indicate the possibility of producing donuts with larger volume by using vacuum frying at appropriate temperature.
The color change in fried donuts might be caused by non-enzymatic browning during heating due to Maillard reactions as a result of the interaction of an amine group with reducing sugars.[Citation17] The ΔE is a function of frying temperature and independent of vacuum (). The ΔE increased progressively with the increase in frying temperature. The highest ΔE (21.3) was provided at the highest temperature of 190°C in atmospheric frying, and the lowest ΔE (13.8) was at the lowest temperature of 150°C and 9 kPa vacuum. Visual observation also confirmed the results, since the donuts fried under atmospheric pressure were darker, more reddish and yellowish than donuts fried under vacuum.
Moisture loss and oil uptake
shows that: (i) at a constant temperature, vacuum level did not affect (P ≤ 0.05) donut MC, except at 150°C where the MC values were different between 3 and 9 kPa vacuum; and (ii) MC values were independent of frying temperature and vacuum, except at 150°C and 3 kPa. Thus, at higher frying temperatures, MC in the fried donut is not directly related to frying temperature and vacuum levels. Generally, donut MC dropped rapidly during the first minute of frying ( a–c). During this period, the drying rate (based on the slopes of the curves) slightly increased with the increase in frying temperature. The highest drying rate was observed () at 9 kPa vacuum and 180°C, where the donut lost about 23% moisture (db) in the first minute; in contrast, at the lowest drying rate, about 17% moisture (db) loss was observed () at 3 kPa vacuum and 150°C. Only at 150°C, the initial drying rate increased as the vacuum level increased. At other temperatures (150°C), there was no effect of vacuum on the initial drying rate as the slopes of the lines are constant. At low temperatures, this was due to the lowering of the boiling point of water with reduced pressure. Thus, more water was removed with increased vacuum. This phenomenon was also observed in vacuum frying of potato chips.[Citation6] However, at higher temperatures, more energy is provided to facilitate water evaporating from the dough. This was why at higher temperatures, the effect of vacuum is negligible on water loss. During vacuum frying (), moisture history curves fluctuated up and down rather than a progressive drop as in atmospheric frying. Thus, shows that: (i) the MC fluctuated more frequently and in a slightly bigger range as vacuum level increased, and (ii) the changes of MC tended to be similar as the temperature increased (, ).
Figure 4. Moisture content versus frying time during vacuum frying.
The final MC in the donuts varied between 32 and 35% db (24 and 26% wb) (). These are typical MCs for such products[Citation18] as lower MC will provide a dry mouth feeling. Storage stability of such products at these MCs is good due to lower water activity, which is achieved by sugar and fat contents. Lower MC also creates the problem of fat oxidation. FC values, at a constant frying temperature except at 180°C, were significantly different at various vacuum levels (). At 180°C, no significant difference was observed between 14.5 and 17.6% db corresponding to 3 and 6 kPa vacuum respectively. Similarly, FC values, at a constant vacuum except at 3 kPa, were significantly different at various frying temperatures. At 3 kPa, no significant difference was observed between 24.7 and 21.6% db corresponding to 150 and 165°C respectively. Thus, FC was significantly affected by vacuum and frying temperature. Oil uptake increased as vacuum level increased or frying temperature decreased. and show that during the first minute of frying, oil uptake was rapid for all cases, due to the replacement of evaporated water. This coincided with the water evaporated from the donut at the fastest rate (). During this period, oil absorption rate increased as the vacuum level increased for a constant temperature () as mass transfer is strongly affected by pressure and temperature gradients. In contrast, oil uptake rate decreased as the temperature increased (). Accordingly, the oil uptake achieved a maximum value of 20.3% db at 9 kPa vacuum and 150°C after 1 minute ( and ).
Figure 5. Oil uptake versus frying time at different levels of temperature.
Figure 6. Oil uptake versus frying time at different levels of vacuum.
Interestingly, there was more than one oil desorption during the vacuum frying, compared to the atmospheric frying where only one oil desorption was observed. At 150°C, desorption occurred at the 4th minute for 3 kPa and at the 6th minute for 6 and 9 kPa (). As the temperature increased, the desorption occurred earlier, i.e. at the 2nd or 3rd minute (, ). The desorption level tended to decrease as the temperature increased (, , ). The desorption is defined as oil migrating from the product to the surroundings by capillary flow. The pressure difference in the capillaries causes a gradient in the capillaries resulting in oil desorption.[Citation19] As vacuum is employed, more pressure changes occur in the dough capillaries. Possibly, this is why more oil desorption was observed in vacuum frying than atmospheric frying. The MC was same for all treatments except for 3 kPa and 150°C where the MC was 35.3% db. Thus, FC was not directly associated to the MC. This is contrary to the findings of FC and MC correlations claiming that FC and MC were directly related.[Citation1,Citation11,Citation13] This finding was particularly pronounced at higher temperatures (, ).
Instrumental texture
Brittleness is the force required to break the food structure initially. In fried donuts, it was due to the presence of a crust. The brittleness decreased with the increase in vacuum or decrease in frying temperature (). The hardness-1 (H1) change was slower as the vacuum level increased. Thus, with the increase in vacuum level, the donut crust tended to be softer, and the internal structure less compact. At a constant vacuum, with the increase in frying temperature, donut crust became harder and internal structure more compact. Same observation was observed for hardness-2 (H2). The H1 and H2 were affected due to the presence of brittleness as some food structure was initially broken. Due to this, H1 and H2 decreased with the increase in vacuum or decrease in temperature up to 165°C. At 180°C, H1 and H2 reduced with the decrease in vacuum due to the formation of a thicker and harder crust at higher temperatures that was indicated by the higher brittleness. There was no consistent behavior in cohesiveness indicating uncertain effect of frying parameters. At 150°C, elasticity or springiness decreased with the increase of vacuum. However, at higher temperatures, there was no definite trend. Moreover, the largest elasticity was observed at the highest temperature of 180°C. Gumminess (a combination of hardness and cohesiveness) decreased with the increase in vacuum at 165°C. However, there was no definite trend at lower and higher temperatures. Similar behavior was observed for chewiness. Thus, the vacuum and frying temperature are directly related to the donut texture, as higher vacuum and lower temperature provided a less compact and firm product. The visual observations also confirmed these.
Table 3 Textural profile analysis parameters of vacuum fried donuts using 2 cycles penetration test
Three kPa vacuum and 180°C were considered as the desirable frying conditions based on: (i) lower frying temperature up to 10–20°C was achieved compared to atmospheric frying; (ii) maintained the same FC and MC levels, and similar textural properties compared to atmospheric frying; and (iii) product size increased, and crust color tended to be less darker and more yellowish.
CONCLUSIONS
The IMC plays an important role in the characteristics of fried donuts. The amount of oil uptake is inversely related to the MC of the finished product. Higher IMC provides the product with smaller size, less darker color, lower oil uptake, and higher MC in the final product. The frying behaviors are different for the donuts fried under vacuum and atmospheric conditions. The frying time is the function of frying temperature but independent of vacuum. Volume change of donuts is significantly affected by frying temperature and closely correlated to vacuum. The influence of vacuum decreased as temperature increased. Higher vacuum and lower temperature provide bigger donuts. The total color change is the function of frying temperature, and independent of vacuum; higher temperature causes more total color change. Frying temperature and vacuum are not directly related to the final MC of donuts. The final FC of donuts is significantly affected by vacuum and frying temperature. Oil uptake increases with increase in vacuum and decrease in frying temperature. The final MC has no direct relation with the final FC. Vacuum and frying temperature are directly related to the donut texture; and higher vacuum and lower temperature provide a less compact and firm product. Compared to atmospheric frying, vacuum frying can lower the frying temperature; achieve more desired volume and color, similar MC, more oil uptake, and less compact product. Three kPa vacuum and 180°C are the desired conditions for vacuum frying.
ACKNOWLEDGMENTS
Funding of this research by the Natural Sciences and Engineering Research Council of Canada is greatly appreciated.
Related Research Data
REFERENCES
- Shih , F.F. and Daigle , K.W. 2002 . Preparation and characterization of low oil uptake rice cake donuts . Cereal Chemistry , 79 ( 5 ) : 745 – 748 . [CSA]
- Shih , F.F. , Daigle , K.W. and Glawson , E.L. 2001 . Development of low oil-uptake donuts . Journal of Food Science , 66 : 141 – 144 . [CSA]
- Velez-Ruiz , J.F. and Sosa-Morales , M.E. 2003 . Evaluation of physical properties of dough of donuts during deep-fat frying at different temperatures . International Journal of Food Properties , 6 ( 2 ) : 341 – 353 . [CROSSREF] [CSA]
- Velez-Ruiz , J.F. , Vergara-Balderas , F.T. , Sosa-Morales , M.E. and Xique-Hernandez , J. 2002 . Effect of temperature on the physical properties of chicken strips during deep-fat frying . International Journal of Food Properties , 5 : 127 – 144 . [CROSSREF] [CSA]
- Shyu , S.L. , Hau , L.B. and Hwang , L.S. 1998 . Effect of vacuum frying on the oxidative stability of oils . Journal of American Oil Chemists’ Society , 75 : 1393 – 1398 . [CSA]
- Garayo , J. and Moreira , R. 2002 . Vacuum frying of potato chips . Journal of Food Engineering , 55 : 181 – 191 . [CROSSREF] [CSA]
- American Association of Cereal Chemists . 1983 . Approved Methods of the American Association of Cereal Chemists , St. Paul, Minnesota : AACC, Inc. .
- Clydesdale , F.M. and Francis , F.J. 1969 . Color measurements of foods: color scales . Food Product Development , 3 : 117 [CSA]
- Bourne , M.C. 1979 . “ Theory and application of the puncture test in food texture measurement ” . In Food Texture and Rheology , Edited by: Sherman , P. 95 – 142 . London, , UK : Academic Press .
- Statistical Analysis System . 1999 . PC-SAS User’s Guide , Cary, NC : SAS Institute, Inc. .
- Gamble , M.H. , Rice , P. and Selman , J.D. 1987 . Relationship between oil uptake and moisture loss during frying of potato slices from c.v. record U.K. tubers . International Journal of Food Science & Technology , 22 : 233 – 241 . [CSA]
- Wheeler , F.G. and Stingley , D.V. 1963 . Effect of moisture on fat absorption in cake doughnuts . Cereal Science Today , 8 : 102 – 108 . [CSA]
- Rice , P. and Gamble , M.H. 1989 . Modeling moisture loss during potato slice frying . International Journal of Food Science & Technology , 24 : 183 – 187 . [CSA]
- Tungsangprateep , S. and Jindal , V.K. 2004 . Sorption isotherms and moisture diffusivity in fried cassava-shrimp chips . International Journal of Food Properties , 7 : 215 – 227 . [CROSSREF] [CSA]
- Saguy , I.S. and Pinthus , E.J. 1995 . Oil uptake during deep-fat frying: factors and mechanism . Food Technology , 49 ( 4 ) : 142 – 145 . 152[CSA]
- Johnson , A.T. 1999 . Biological Process Engineering , New York : John Wiley & Sons, Inc. .
- Richardson , T. and Hyslop , D.B. 1985 . “ Enzymes ” . In Food Chemistry , Edited by: Fennema , O.R. 445 – 447 . New York : Marcel Dekker, Inc. .
- American Society of Heating, Refrigeration and Air conditioning Engineers . 1993 . ASHRAE Handbook of Fundamentals , Atlanta, GA : ASHRAE, Inc. .
- Ateba , P. and Mittal , G.S. 1994 . Modeling the deep-fat frying of beef meatballs . International Journal of Food Science & Technology , 29 : 429 – 440 . [CSA]