Heat and Mass Transfer in Foods During Deep-Fat Frying

Chapter 11

Heat and Mass Transfer in Foods During Deep-Fat Frying R. Paul Singh Department of Biological and Agricultural Engineering University of California Davis, California, U.S.A. Sukumar Debnath Department of Lipid Science and Traditional Foods Central Food Technological Research Institute Mysore, India

Frying is a popular method of cooking and processing foods. Fried fish and chicken are ubiquitous in diets around the world. Frying is also a method of choice in manufacturing numerous types of snack foods. The high rates of heat transfer obtained in frying are desirable in processing french-fried potatoes, potato chips (also called crisps), extruded noodles, doughnuts, expanded snacks, and roasted nuts. Typically, frying involves immersing a food in heated oil (150–190°C); the process is called immersion-oil frying or deep-fat frying. Other methods of frying include shallow-pan frying, in which the food is kept partially immersed in oil, and stir-frying, in which a yet smaller quantity of oil is used. A variety of edible oils of plant and animal origin are used in frying. Palm oil is commonly used in Southeast Asia, coconut and groundnut oil on the Indian subcontinent, lard in Central and South America, and olive oil in the Mediterranean region. In this chapter, the term frying is used to describe a process in which a food is cooked by total immersion in heated oil.

Heat Transfer During Frying In immersion-oil frying, heat is transferred from heated oil to the surface of a product. The high-temperature, short-time cooking process results in a variety of physical and chemical changes in the food such as starch gelatinization, protein denaturation, water vaporization, and crust formation, accompanied by the development of color and texture. Simultaneously, mass transfer occurs, as characterized by the intrusion of oil into the food and removal of water, in the form of vapor, out of the food. The complexity of the frying process is evident when one considers the simultaneous transport of heat, oil, and water vapors in a food and the structural changes in food, with the development of unique textural properties (Singh 1995). This chapter first examines the role of oil as a heating medium in frying, with specific reference to heat and mass transfer. Frying may be considered analogous to drying. While, in drying, ambient or heated air is used to reduce the moisture content of a food, in frying, oil is 185

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used as the heat transfer medium. Oil, heated to a temperature in the range of 150–190°C, provides a high temperature gradient and a better contact between oil and the surface of a food, resulting in higher rates of heat transfer than is possible when food is put in contact with heated air. However, several significant changes take place in oil during the frying process, including • oxidative changes due to atmospheric oxygen entering the oil from the surface of the oil, which is exposed to the surrounding atmosphere, • hydrolytic changes due to water vapors coming from the product being fried, and • thermal changes due to oil being maintained at high temperature. These changes have a major impact on how oil behaves as a heat-transfer agent. In drying, air provides a large sink for moisture removed from the food, and freshly heated air often replaces moisture-laden air. In frying, oil becomes contaminated by food components leaching into the oil. Additionally oil is contaminated by water vapor exiting the food, by oxygen absorbed at the oilair interface, and by thermal breakdown of the oil itself. Some of the contaminants accumulating in oil act as surfactants and reduce the surface tension of oil. As the surfactant levels increase, the wetting of the food surface by the hot oil increases, which influences the heat and mass transfer processes. With an increase in surfactant level, there is also increased wetting of heater surfaces located in oil, causing a potential increase in the rate of oil breakdown. Surfactants also enter the food with the oil and may influence moisture pickup by the food during subsequent storage, hence reducing its shelf life. The violent heat and mass transfer occurring on the surface of a food after it is immersed in hot oil is an example of the nucleate boiling of liquid, which is governed by the balance of surface, inertial, and viscous forces. The surface tension of the oil, and the contact angle between the food surface and the oil, control the formation of water vapor nuclei. Furthermore, the surface tension at the oil–water-vapor interface influences the dynamics of growth and detachment of water vapor bubbles following nucleation.

Interfacial Properties The interfacial properties such as surface tension and contact angle between the oil and water, oil and food surface, and oil and air are topics that are not very well understood. Only limited information is available regarding the interfacial properties for food-grade oils at the high temperatures used in frying. According to Gomes Da Silva and Singh (1995), oil viscosity increases and surface tension decreases with thermal degradation. They found that viscosity increased from 2.55 to 3.28 mm2/s for corn oil. Miller (1992) and Miller et al (1994) determined the viscosity of canola, palm, corn, and soybean oils at 170, 180, and 190°C and found the values to range between 2.5 and 3.95 mm2/s. Vijayan et al (1996) reported that the specific heat of corn oil did not show any significant change with oil degradation over a period of 7 h. The specific heat of the oil was measured using a differential scanning calorimeter, and values at 50°C were used for comparison purposes. The specific heat of the corn oil varied between 1.952 and 2.134 J/g°C. Oil degradation also results in changes in the optical properties of oil; these changes have been used in developing optical sensors to monitor the quality of oil (Sebben et al 1998).

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The mechanism of oil absorption by tortilla chips during cooling was studied by Moreira and Barrufet (1998). They reported that the major portion (~80%) of the total oil content in tortilla chips was absorbed during the cooling period immediately after frying. They also found that, as the product temperature decreases, the interfacial tension between gas and oil increases, which raises the capillary pressure and sucks the surface oil into the porous medium, thus increasing the oil content. Interfacial tension was also reported by Pinthus and Saguy (1994) to have a significant influence on oil uptake after deep-fat frying. They developed an equation, based on contact angle, for estimating interfacial tension between food material and frying oil. Goel et al (1999) investigated the oil content of fried noodlelike products prepared from blends of corn starch with casein, soy protein, or their hydrolysates in the ratios of 80:20 to 20:80 (protein/hydrolysate to corn starch). They found that oil content increased for all blends with an increase in protein or protein hydrolysate as a result of hydrophobicity and the surface tension of proteins and protein hydrolysates in the presence of corn starch.

Deep-Fat Frying of Selected Foods Deep-fat frying is a process of simultaneous heat and mass transfer. In recent years, much research has been aimed toward the development of fried food products with reduced fat and cholesterol levels in response to increasing consumer awareness of nutrition (Krokida et al 2001). Studies on deep-fat frying of selected foods are discussed in the following sections. DOUGHNUTS The effect of the addition of curdlan (0–0.5%) to decrease oil uptake and moisture loss in doughnuts (or donuts) during deep-fat frying was studied by Funami et al (1999). They determined that curdlan is more effective than other cellulose derivatives. After curdlan addition, a linear reduction in each parameter within the above range was seen. They also attributed the effect of curdlan to its thermal gelling property during frying, which exerts a plasticizing effect and acts as an oil and moisture barrier. Vélez-Ruiz and Sosa-Morales (2003) evaluated the physical and thermal properties of the dough used for doughnuts during deep-fat frying in sunflower oil at 180–200°C. They also reported that doughnuts should be fried for 75– 120 s in inverse relation to oil temperature to get an acceptable product. CHICKPEA FLOUR-BASED SNACK FOOD Debnath et al (2003) investigated mass transfer kinetics of moisture outflow and oil uptake during deep-fat frying and prefry drying of a ribbonshaped (15 × 1 mm) snack food prepared from a blend of chickpea flour and modified starch. They found an increase in the mass transfer coefficient for moisture (range: 0.045–0.096 s–1) as well as for oil (0.056–0.082 s–1) with an increase in the frying temperature (range: 150–200°C). They also reported a decrease in the kinetic coefficient for moisture transfer (0.056–0.039 s–1) as well as oil transfer (0.063–0.035 s–1) for the same snack with an increase of prefry drying time (0–90 min) before frying at 175°C. The optimum prefry

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drying and frying times were 40 min and 45 s, respectively, which resulted in a 54% reduction in the oil content of the fried product, due to the compactness of the material matrix. POTATO DOUGH The convective heat transfer coefficient (h) of a potato-dough ball during deep-fat frying was measured by Budžaki and Šeruga (2005) at elevated temperatures (160–190°C). Initially, during frying, the value of h varied from 197.25 ± 1.39 to 774.88 ± 3.90 W/m2 K, and it varied from 94.22 ± 0.33 to 774.88 ± 3.89 W/m2 K during the boiling phase. They also demonstrated that the addition of potato flour to the dough decreased the value of h even with an increase in oil temperature. CASSAVA Vitrac et al (2002) investigated heat and mass transfer phenomena in thin slices (1–2 mm) of cassava during deep-fat frying. They found that these transfer phenomena occurred in two stages due to vaporization of 1) available water that was controlled by diffusion and thermal resistance and 2) bound water depending upon the final core temperature. TOFU The convective heat transfer coefficient during deep-fat frying at 147– 172°C at different locations on the surface of flat discs made of tofu was first studied by Baik and Mittal (2002). They found that h values are higher at the top surface and vary from 722 to 827 W/m2 K due to bubbling and 644 to 724 W/m2 K at the bottom surface. KROŠTULE DOUGH The convective heat transfer coefficient of dough for “Kroštule” (a type of sweet fritter popular in Coratia) during deep-fat frying at 160–190 ± 1°C was studied by Šeruga and Budžaki (2005). The value of h varied initially from 579.12 ± 2.46 to 657.91 ± 0.95 W/m2 K and, during the boiling phase of frying, from 26.53 ± 0.63 to 657.91 ± 0.95 W/m2 K at the higher temperatures. TAPIOCA Nair et al (1996) characterized the interactions among different frying parameters such as time, temperature, moisture content, oil uptake, and expansion (due to vaporization) during deep-fat frying of tapioca starch chips. They found an optimum condition for frying parameters; the maximum expansion was determined at 200°C for a frying time of 40 s at an initial moisture content of 15%. PORK MEAT Sosa-Morales et al (2006) studied heat and mass transfer and physical properties of pork meat during deep-fat frying using sunflower oil at 90– 110°C and shortening at 100°C. They observed that the moisture diffusivity coefficient (1.5–30.2 × 10–9 m2/s), convective heat transfer coefficient (187.7– 226.1 W/m2 °C), density, crust color, and texture were affected by frying temperature, while thermophysical properties, except thermal diffusivity, and quality were affected by frying time as well as temperature.

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Heat Transfer Coefficients Water escaping from a food causes bubbles to form around the surface of the product being fried. This phenomenon has a marked influence on the convective heat transfer coefficient (Levine 1990a–c). Miller et al (1994) determined convective heat transfer coefficients for the initial phase (lasting only a few seconds) of frying when there are no bubbles present around a food. For canola, palm, and soybean oil at 170, 180, and 190°C, h varied between 250 and 280 W/m2 °C. Mittelman et al (1984) reported that h was not affected by the oil temperature (135–175°C) and presented an average value of 1,800 W/m2 °C. Califano and Calvelo (1991) reported h values of 150–165 W/m2 °C for oil temperatures ranging from 50 to 100°C. Hubbard and Farkas (1999) developed a method to determine the convective heat transfer coefficient during frying. They found that, during frying of a potato cylinder, the highest value of h (~1,100 W/m2 K) occurred after about 180 s of frying. The influence of oil temperature (120, 150, and 180°C) on the convective heat transfer coefficient and on heat flux was reported by Hubbard and Farkas (2000). They observed that an increase in oil temperature causes an increase in maximum convective heat transfer coefficient, heat flux, and drying rate. The maxima for each of these values occurred early in the process as oil temperature was increased. Heat transfer from oil into a food is a complex process. Farkas (1994) suggested that four stages are observable in frying any food. When a food is immersed in hot oil, the process begins with an initial heating stage that lasts only a few seconds. During this short time, the surface temperature of the food rises to the boiling point of water. The mode of heat transfer from the hot oil to the surface of the food is by natural convection. Soon after, the second stage begins with surface boiling, during which the surface moisture starts to boil and vaporize. Because of the turbulence caused by the bubbling action, the mode of heat transfer changes from natural to forced convection. During this stage, crust begins to develop within the food and moves into the interior of the food mass. The third stage is the falling rate stage, during which additional internal moisture starts to leave the food. This stage is similar to the falling rate period in a typical drying process. During this stage, the core temperature increases to 100°C. The crust continues to grow and, after sufficient time, the rate of vapor removal at the surface reduces. Surface vapor transfer ceases when the food is fried for a long duration. The final stage is called the bubbleend point. This stage is not common because food begins to burn or char, giving an undesirable color and flavor to the product. Similar steps to describe the frying process were presented by Hällstrom (1980).

Heat Transfer into Foods During Frying Using the above description of the frying process, we may consider a food (such as a potato strip) to consist of three characteristic zones at some elapsed time after the frying process has begun. They are 1. the interface between food material and hot oil, 2. the crust area that is porous and contains most of the oil, and 3. the inner core region that is moist and largely devoid of oil.

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Heat transfer at the interface of food and oil is a convective process involving the surface heat transfer coefficient. Heat transfer through the crust region may involve convective and conductive heating depending on the structure of the crust and its composition. Within the core region, the heat diffusion equation describes unsteady-state conduction heating. During frying, the interface between the crust and core does not remain stationary; rather, it moves from the surface toward the central plane of the food. Thus, the crustcore interface, referred to as the “moving boundary,” becomes an important part of the physical mechanism governing heat transfer (Singh 2000). Liquid water at the crust-core interface converts into a vapor state, absorbing the latent heat of vaporization. The vapor formed at this interface travels through the porous crust region and leaves from the surface of the object being fried. As frying proceeds, the crust region develops, thus advancing the position of the crust-core interface or moving boundary; meanwhile, the internal core region recedes toward the product center. In the case of french-fried potato, the crust thickness in the final product may be only 1–2 mm, but its importance in texture development is profound (Lima and Singh 2001).

Mathematical Models Describing Heat Transfer Mathematical models describing heat and mass transfer mimic the actual process. These models can be run on computers to obtain results for conditions similar to actual experimental settings. Mathematical models are valuable tools used by food engineers because predictive modeling saves both cost and time. Researchers have made several attempts to model the heat and mass transfer appropriate to the frying process. The following section considers some of the proposed mathematical models for this purpose. The role of the crust zone on heat transfer during frying is evident from the study reported by Mittelman et al (1984). They proposed two different mathematical models, one for an analogue system (urea formaldehyde foam) that remains crustless during frying and another for a crust-forming (potato strip) product. Both these mathematical models were developed for an infiniteslab geometry. The following model was proposed for the crustless capillary porous product. 2kA2  m2  (T0  Tw )(t  t0 )  where A = area of heat and mass transfer (m2), k = thermal conductivity of the dry layer (W/m °C), m = cumulative quantity of water evaporated (kg), t = time (s), t0 = time elapsed until evaporation begins (s), T0 = temperature of the oil (°C), Tw = boiling temperature of water (°C),  = latent heat of vaporization, and  = specific gravity of the wet material (kg/m3). For the crust-forming product (potato strip), they proposed the following model. m2 

K ' m2 D (t  t0 ) a2

where a = half-thickness of the slab (m), D = diffusivity of water in tissue (m2/s), K = proportionality constant, and m = quantity of water evaporated after infinite time (kg).

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From the experimental data with both crustless and crust-forming products, they obtained a linear relationship with a high correlation coefficient (r = 0.998) when the square of water evaporated was plotted against frying time. The effect of oil temperature on the rate of evaporation rate was given by m 2  Ce(0.0113T0 ) (t  t0 )

where C = a proportionality constant. Moisture loss during frying of potato slices was modeled by Rice and Gamble (1989). They noted that the driving force for mass transfer was from vaporization of water, and the main resistance to mass transfer was provided by the internal resistance. Using experimental data and Newman’s solution for Fick’s first law, they back-calculated the apparent diffusion coefficients. They found that the diffusion coefficients were approximately constant between 60 and 240 s for each temperature (145, 165, and 185°C) of frying. They observed a sudden change in the diffusion coefficient after 300 s of frying and attributed this change to an alteration in the moisture loss. The apparent diffusion coefficients were correlated with the oil temperature by the following Arrhenius-type relationship:  Ea    RT 

Da  D0e

where Da diffusion coefficient (m2/s), D0 = a constant (m2/s), Ea = activation energy (J/mol), R = universal gas constant (8.314 J/mol K), and T = absolute temperature (K). By fitting their data in Arrhenius plots, they calculated the activation energy as Ea = 24.2 ± 4 kJ/mol for the temperature range of 145–185°C. In a study on immersion frying of meat balls, Ateba and Mittal (1994a) developed a model of heat, moisture, and fat transfer. They postulated that foods containing a significant amount of fat undergo two fat transfer periods: 1) a fat absorption period, when fat diffuses into the product from the surrounding oil, and 2) a fat desorption period, when fat migrates from the product to the surrounding oil. They assumed that the movement of fat and moisture is by diffusion. They did not consider formation of crust or shrinkage effects. They reported good agreement between the mathematical models and experimental data. In a follow-up study, Ateba and Mittal (1994b) described crust formation and the kinetics of crust color and firmness changes during deep-fat frying of meat balls. Using regression analysis, they presented the following equation to determine the crust thickness as a function of frying time: Y  3.9024  0.0105t

where t = time (s) and Y = thickness of the crust (mm). Moreira et al (1995) studied the frying of tortilla chips. They used onedimensional heat and mass transfer in tortilla chips, assuming the chip to be an infinite slab. Shrinkage during frying was neglected, and thermal and moisture diffusivities were kept constant during the whole process. They proposed the following decoupled models:

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 2MW  M W  D 2  t  x    2    2  t  x 

where D = mass diffusivity (m2/s), Mw = moisture content, dry basis, t = time (s), x = variable thickness across the chip (m),  = thermal diffusivity (m2/s), and  = tortilla chip temperature (°C). The above partial differential equations were solved with three boundary and one initial condition using an explicit finite-difference method. Average oil content and frying time data were correlated using the following equation: Mf(t) = Mfe [1 – exp(–kt)] where Mf = oil content (%), Mfe = equilibrium oil content (%), k = constant (1/s), and t = time (s). The proposed model was validated with experimental data. Another model was developed specifically for frying of chicken by Rao and Delaney (1995). They used dimensional analysis to model moisture loss for a given time-temperature history from the exponential decline in moisture content. The moisture content at any time of frying was modeled as MC t  MC e  e  kmt MC i  MC e

where k m  k1e  Em /( RT ) ,

MCe = equilibrium moisture content at the mass average temperature of the food, MCi = initial moisture content, and MCt = moisture content at any time. Em and k1 were Arrhenius terms obtained from the regression of moisture content versus temperature for a range of degrees of frying and mass average temperatures at the end of the frying. Using this modeling approach, the authors could successfully optimize the product juiciness. Yamsaengsung and Moreira (2002a,b) presented a two-dimensional model to predict the heat and mass transfer during frying and cooling of tortilla chips. They used semiempirical correlations to account for structural changes, such as shrinkage and expansion due to puffing. The gas movement inside a chip during frying was considered to be a result of convective flow due to the total gas pressure gradient and Knudsen diffusion due to the concentration gradient. During cooling, the only mass transport resulted from oil absorption, which was assumed to be a function of the capillary pressure. Other attempts to model the frying process recognize a moving boundary in the food undergoing frying. In the engineering literature, a heat transfer problem involving moving boundaries is also referred to as the “Stefan problem.” These types of problems are named after a 19th century scientist, J. Stefan, who defined this problem when he was studying the melting of polar caps (Voller and Cross 1981). The solution to this type of problem is intrinsically difficult because the interface between solid and liquid phases moves as the latent heat is absorbed or released at the interface (Özisik 1993). This makes the location of the interface an unknown parameter, which also must become a

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part of the solution. Moving boundary problems are time-dependent, and the position of the interface must be determined as a function of space and time (Crank 1984). These kinds of problems are nonlinear in nature, and analytical solutions are difficult to obtain for all the required conditions (Carslaw and Jaeger 1959). When exact analytical solutions are not available, approximate, semianalytic, and numerical methods are used to solve the phase-change problems (Özisik 1993). A predictive model for heat transfer during frying based on the fundamental principles of heat and mass transfer and accounting for the moving boundary was presented by Farkas et al (1996a). They considered the frying process to be a moving boundary problem and modeled a one-dimensional, semiinfiniteslab geometry assuming the material to be homogenous and isothermal (Fig. 11.1). The following partial differential equations were developed to describe heat and mass transfer in the crust and core regions: Core region heat transfer  k eff

 2T T T  N  x C p  (    C p     C p ) x 2 x t

Core region mass transfer N  x   D

c x

Fig. 11.1. Schematic of a slab of food undergoing frying, assuming one-dimensional heat and mass transfer. X is the space variable; L is the thickness of the slab; x is the location along the x-axis. See text for more definitions.

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Crust region heat transfer (   C p     C p )

T  2T T   keff  N  xC p t x 2 x

Crust region mass transfer N x  

 K P  x

where Cpi = heat capacity of species i (kJ/kg K); c = concentration of  =  (kg/m3); D = diffusivity (m2/s); K = permeability (m2); keff = effective thermal conductivity of region i (W/m K); Ni = flux of species i (kg/m2s); P = pressure (N/m2); qx = heat flow in x direction (W); T = temperature (K); t = time (s); x = variable location; i = volume fraction of species i; γ = viscosity of the  species (Ns/m2); i = density of species i (kg/m3); and , , ,  = represent solid, liquid, vapor, and oil phases, respectively. The procedure given by Landau (1950) was used to transform the above partial differential equations into a different coordinate system for immobilizing the moving boundary. They used the Crank-Nicolson method (Smith 1985), an implicit finite-difference scheme, to convert the partial differential

Fig. 11.2. Predicted and experimentally determined temperature profiles in a 2.54cm-thick patty made of restructured potato during frying. Frying oil temperature: 180°C; T1 = center point, T2 = 1/3 out, T3 = 2/3 out, T4 = crust. (Reprinted, with permission, from Farkas 1994)

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equations into nonlinear algebraic equations. These equations were then solved by Gauss-Seidel iteration along with a set of eight boundary conditions and four initial conditions. The solution of these equations provides a prediction of temperatures at the center and at any other desired location. The authors fried a patty of restructured potato 2.5 cm thick for experimental validation. The experimental results were in good agreement with the predicted values of temperature from the model (Fig. 11.2). From the predicted and experimental data, they observed that the crust-core interface temperature remained around the boiling point of water. The crust layer got thicker with frying time, and the crust temperature went above 100°C (Fig. 11.3). The temperature within the core increased with time but remained below 100°C. The predicted decrease in moisture content in the core region compared well with the experimental results (Fig. 11.4). Farkas et al (1996b) conducted a sensitivity analysis using their model and found that the crust temperature profile was a function of the oil temperature and had a linear temperature profile. The core temperature was unaffected by the oil temperature because of the presence of a crust-core boundary at around 100°C. The thermal conductivity and specific heat of the core region governed its heating rate. The moisture content of the fried sample depended on the moisture diffusivity of the material, and finally, the crust thickness was governed by the thermal conductivity of the crust, oil temperature, moisture content of the food, and thermal conductivity of the core region. It is a common practice in the foodservice industry to fry many frozen foods by directly immersing them in heated oil (e.g., french fries are often processed by immersing frozen potato strips in hot oil). The formulation and solution of moving-boundary problems becomes more complicated when addi ional phase changes take place. When a frozen food (such as a frozen potato

Fig. 11.3. Predicted and experimentally determined crust thickness in a 2.54-cmthick patty made of restructured potato during frying. Frying oil temperature: 160°C. (Reprinted, with permission, from Farkas 1994)

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Fig. 11.4. Predicted and experimentally determined moisture content in a 2.54-cmthick patty made of restructured potato during frying. Frying oil temperature: 160°C. ddb = decimal dry basis. (Reprinted, with permission, from Farkas 1994)

Fig. 11.5. Predicted and experimentally determined temperature profiles at the center of a frozen patty made of restructured potato during frying. Initial patty temperature: –19.3°C, frying oil temperature: 182°C, slab thickness: 2.54 cm. (Reprinted, with permission, from Vijayan 1986)

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strip) is directly submerged in hot oil, there arise two moving boundaries, the crust-core interface and the unfrozen-frozen interface. These interfaces separate different regions in the food material with uniquely different physical and thermal properties. Vijayan and Singh (1997) developed a predictive model to simulate frying of frozen products. They used the enthalpy method as described by Mannapperuma and Singh (1988) to predict temperatures in the core region as it undergoes phase change. Good agreement was obtained between the predicted temperatures and the experimental data (Fig. 11.5). The predictive model was useful in developing temperature profiles within a product at different frying times (Fig. 11.6). The predictive heat transfer models used to simulate the frying process require reliable data on physical and thermal properties of foods. The key properties include thermal conductivity, thermal diffusivity, and specific heat of the food. The data on thermal properties of fried foods are extremely limited. Califano and Calvelo (1991) obtained thermal conductivity values of cylindrical potato samples between 50 and 100°C. The results ranged between 0.545 and 0.957 W/m °C for potatoes with a specific gravity of 1,070 kg/m3 and moisture content of 80%. Arifin and Singh (1993) measured the thermal properties of various materials at temperatures below 100°C using a differential scanning calorimeter. They obtained thermal conductivity of 0.552 W/m K at 74.9% (wb) moisture content for the potato substrate and 0.1191 W/m K at 4.2% (wb) moisture content for the crust (obtained from fried restructured potato). Yıldız et al (2007) demonstrated a more realistic method to measure the effective heat transfer coefficient during deep-fat frying of a potato slice.

Fig. 11.6. Simulated temperature profiles within a frozen patty of restructured potato during frying. Initial product temperature: –20°C, frying oil temperature: 180°C, thickness of patty: 2.57 cm. (Reprinted, with permission, from Vijayan 1986)

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According to them, heat transfer coefficients were found to decrease from 286.7 ± 15.5 to 181.3 ± 6.5 with increasing temperature of the frying oil from 150 to 190°C. Vitrac et al (2003) investigated the direct and continuous method for the measurement of convective heat flux during deep-frying of cassava slices using the inverse modeling method. According to them, frying was related to unstable superficial vaporization, regular internal vaporization, and decreasing vaporization rate. This method may be useful during on-line process control of batch or continuous frying processes.

Mass Transfer During Frying Mass transfer during frying is characterized by the flux of two immiscible species, oil and water vapor. Water vapor is generated at the crust-core interface, the moving boundary, at a rate dependent on the heat flux to the interface. Mass transfer occurs by several modes, such as diffusion, capillary action, and bulk flow. Generally, the internal moisture movement in a food material undergoing frying has been considered to be a diffusional process. Studies with frying of tortilla chips have shown an increase in porosity and oil uptake with increasing frying time. The oil migration appears to occur largely during the time the product is immersed in oil, although it continues even after the food is removed. Different techniques have been used to visualize how far the oil migrates into a food during frying. Keller et al (1986) and Lamberg et al (1990) used a fat-staining technique to study the oil uptake by foods during frying. These studies involved addition of a thermally stable oil-soluble dye, Sudan Red, into the frying oil. After frying, sectioning of the food showed a red region corresponding to the depth of oil penetration. Farkas et al (1992) used magnetic resonance imaging for visualizing oil and water concentration gradients within a food material. They reported that oil penetrates only up to the crust-core interface in a potato strip during frying. McDonough et al (1993) used an environmental scanning electron microscope for studying the oil uptake in corn tortilla chips during frying. They observed a spongelike structure being created as a result of steam leaving the product, which was later filled with oil. They also noted that, as soon as a tortilla chip is immersed in frying oil, the oil coats the chip’s surface and starts to diffuse into the product immediately. Scanning electron microscopy (SEM) was used by Farkas et al (1992) to study the crust of fried products. Using infrared microspectroscopy to monitor the oil distribution in fried potato cylinders, Bouchon et al (2001) presented quantitative data on oil distribution within a fried product. The anisotropic nature of porous crust region influenced the oil distribution. Bouchon and Aguilera (2001) studied the microstructure of frying potatoes. Rao and Delaney (1995) used SEM for studying the structure of the crust of fried, breaded chicken. The effective mass transfer coefficient and moisture diffusivity were determined by Yıldız et al (2007) during frying of potato slices in sunflower oil at 150–190°C. They observed an increased mass transfer coefficient (1.12 ± 0.22 to 2.07 ± 0.24) and increased moisture diffusivity (9.2 ± 1.1 to 18.2 ± 0.7) with an increase in the temperature of the frying medium.

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Factors Affecting Oil Uptake and Migration Many factors affect oil migration into a food during immersion frying. These include oil quality and composition, frying temperature and duration, product shape, moisture content of the food, composition of the food, porosity of the food, prefrying and surface treatments, gel strength, and initial interfacial tension (Gamble and Rice 1987, Saguy and Pinthus 1995). Bouchon et al (2003) examined the oil-absorption process in potato cylinders fried at 155, 170, and 185°C. They noted three oil fractions, namely, structural oil (absorbed during frying), penetrated surface oil (suctioned during cooling), and surface oil. Their results indicate that only a small amount of oil penetrates during frying; most of the oil is picked up at the end of the process. Upon examining samples after cooling, they determined that oil was located either on the surface of the chip or suctioned into the porous crust microstructure. This study suggests that oil uptake and water removal do not occur in a synchronous manner. Oil intrusion into a food is known to increase with frying time (Du Pont et al 1992). As oil degrades with frying time, the amount of surfactant increases, which in turn affects the food’s oil uptake (Blumenthal 1991). The amount of moisture in a food influences oil uptake. Weaver et al (1975) reported that the oil uptake increases with higher initial moisture content of the food. Lamberg et al (1990) concluded that an increase in surface moisture content can cause increased oil uptake. Both blanching and lowering the moisture content of a food before frying reduced oil absorption by the food (Saguy and Pinthus 1995). A linear relation between surface area of the product and amount of oil uptake during frying of potato slices 1.04–2.11 mm thick was reported by Gamble et al (1987) and Gamble and Rice (1988). Bouchon and Pyle (2004) found that oil absorption significantly decreased when the thickness of restructured potato chips was increased. They concluded that this decrease in oil absorption was mainly due to the higher final density of the thick fried chip. Several studies have shown that moisture loss is proportional to the square root of frying time (Mittleman et al 1984, Rice and Gamble 1989). Pinthus et al (1997) determined the effective diffusivity of water in a deep-fatfried restructured potato product. Surface roughness was mentioned to cause increased oil uptake during frying (Saguy and Pinthus 1995). Lulai and Orr (1979) reported that the oil content of potato chips decreased linearly when the specific gravity of the tubers increased from 1.06 to 1.11. Pinthus and Saguy (1994) developed a coefficient, UR, to describe oil uptake. UR was defined as the ratio of the amount of oil uptake to the amount of water removed during frying. They found a linear relationship between UR and the initial porosity of the product (Pinthus et al 1992). They introduced a new term “net porosity” to ignore changes in porosity due to oil uptake; this property showed a significant linear correlation with oil uptake. The process of wetting a food surface with oil and treating it with hot water vapor several times before frying was reported to be an effective method to reduce oil uptake (Henderson 1988). According to Weaver et al (1975), freezing the food material before frying could reduce oil uptake. A lower oil uptake by food products when they were cooked under pressure was noted by Saguy and Pinthus (1995).

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Addition of powdered cellulose (Ang 1989), soy protein (Finley and Simpson 1976), and methylcellulose (MC) (Pinthus et al 1993) have been shown to reduce oil uptake. Increase in the gel strength of a food was reported to decrease oil uptake during frying (Pinthus et al 1992). The mechanisms governing the intrusion of oil into a food material are not clearly understood. Saguy and Pinthus (1995) listed some of these mechanisms: replacement of moisture, crust formation and preferential adsorption, interfacial tension, capillary rise, and product porosity. During frying, as water evaporates from the crust-core interface, it causes a pressure difference. Rice and Gamble (1989) stated that this pressure drop within the food causes a vacuum effect, which causes increased oil uptake. Steam escapes through the porous crust, forming capillaries. Oil is then absorbed into the food through these capillaries. Pinthus and Saguy (1994) developed an equation to calculate the interfacial tension between a product and the frying medium. This equation was then related to the equilibrium contact angle and the surface tension of the medium, which are relatively easy to measure. From the study, they proposed the following relationship:

 

Oil uptake (dry basis) = a  b

where  = interfacial tension between the food and frying oil (N/s) and a and b are constants. According to the above relationship, additives such as Tween 80 that increase interfacial tension may be used to reduce oil uptake. The oil viscosity may affect the heat transfer rate, oil uptake by the food, and the degree of foaming during frying (Roth and Rock 1972). Kress-Rogers et al (1990) developed a sensor to assess frying oil quality based on its viscosity. A review by White (1982) provides more information on different methods that may be used for measuring frying oil quality. Various coating materials may be used on food products during frying to reduce oil intrusion and increase moisture retention. The use of water-binding additives like alginates or cellulose helps to reduce oil uptake and moisture loss by the food (Duxbury 1989). Results from many of these types of studies are in patent literature (Prosise 1990, El-Noklay and Hiler 1992, Feeney et al 1993, Kazlas et al 1994).

Physicochemical Changes During Frying Physicochemical changes occurring in starch during frying of a corn-starch patty were reported by Fan et al (1997). Pinthus et al (1998) found that resistant starch forms during deep-fat frying of patties made of corn, rice, wheat, or potato starch. They studied the role of resistant starch in modifying the mechanical properties of fried products. Lima and Singh (2001) examined the role of oil temperature on the hardness of a crust layer in a restructured fried potato. Using a solid mechanical analyzer, they determined quantitative data regarding the increase of hardness with increasing frying time and oil temperature. During the postfrying period, crust hardness decreases as water migrates into the crust region (Rovedo et al 1999a,b). The mechanical properties of a fried starch-gluten gel during the postfrying period were reported by Normén et al (1998).

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Effect of Edible Coatings on Frying In the snack-food processing industry, a major share of the production is in manufacturing deep-fat-fried products. Several researchers have investigated methods to reduce oil uptake in deep-fat-fried foods by using edible coatings or films. Balasubramaniam et al (1995) and Mellema (2003) studied the application of hydrocolloid coatings such as gellan and cellulose derivatives (e.g., MC and hydroxypropyl MC [HPMC]) on reduction of oil content. They found that these hydrocolloids reduce the oil content of deep-fat-fried products by means of their lipid barrier properties or film formation when added in dry form (Meyers 1990) or by their gelation properties during heating (Grover 1993). Deep-fat frying of coated foods in a model system was studied by Mallikarjunan et al (1997) and Huse et al (1998). They explained the effectiveness of various edible coatings for moisture retention and oil uptake reduction in fat-free starchy products. After developing a mathematical model for frying of cereal foods coated with gellan gum, MC, and hydroxypropylcellulose, Williams and Mittal (1999a,b) determined the effectiveness of edible films for reduced oil uptake. Khalil (1999) demonstrated that coating of potato tissue with hydrocolloid gel-forming compounds and a calcium cross-linking agent, followed by additional hydrocolloid coating, was effective in protecting potato tissue from the thermal effects of deep-fat frying, reducing the oil content and preventing water evaporation. The effect of external addition of wheat starch, modified cornstarch, dextrin, dried egg, and gluten to the batter mixture on pickup, flow properties, color, and texture characteristics of fried coatings for squid rings was examined by Salvador et al (2005). They reported that the highest absorption values were found in gluten-added batters; the highest contribution to crispness was found in dextrin-added batters; and gluten caused lower oil absorption. Mittal and Zhang (2000) developed a mathematical model using an artificial neural network (ANN) to predict heat and mass transfer during deep-fat frying of infinite-slab-shaped foods coated with edible films. They showed that the process of deep-fat frying food coated with an edible film can be predicted from the variables used in processing by using ANN from validated mathematical models. Baixauli et al (2003) demonstrated the effect of added dextrin or dried whole egg on the rheological properties of raw coating batters and the batter’s textural properties after frying. They observed that addition of dextrin to a coating batter produces a crispier texture than dried egg and that the texture is retained for longer periods of time after frying. García et al (2002) studied the effect of MC, HPMC, and sorbitol (as plasticizer) in coating formulations to reduce oil uptake in the deep-fat frying of potato strips and dough discs. They observed that MC coatings were more effective in reducing oil uptake than HPMC coatings, whereas sorbitol maintained coating integrity and improved barrier properties. Holownia et al (2000) studied the effect of edible films such as MC and HPMC on marinated whole chicken strips. The films were added either before or after breading or incorporated into the breading. They reported that HPMC

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reduces oil degradation by 25–50% when applied to marinated chicken strips before breading. The effects of the edible coatings MC and whey protein isolate (WPI) on the quality parameters of breaded fried chicken nuggets were evaluated under nitrogen atmosphere and steam pressure by Ballard and Mallikarjunan (2006). They determined that pressure frying under nitrogen atmosphere produced fried products crispier and more tender than samples fried using steam and that MC-coated products were crispier than WPI-coated products. However, Dogan et al (2005) evaluated the addition of soy and rice flour to the batter formulation on the quality of deep-fat-fried chicken nuggets. They reported that batters with added soy and rice flour showed shear thinning and thioxotropic flow behavior and were better than the original formulation (batter based on wheat and corn flours) with respect to low oil uptake. Maskat and Kerr (2004) examined the effect of surfactant (0–75 ppm) and batter-mix–solvent ratio (1:1.5–1:3) on various coating yield and frying parameters during deep-fat frying of chicken breasts at 160°C for 240 s. They observed that higher surfactant levels lowered the interfacial tension between the frying oil and coating, which, in turn, increased fat absorption by the coating and reduced coating adhesion.

Summary In summary, heat transfer and mass transfer in foods during frying are complex processes. Quantitative information regarding the rates of these processes is often necessary to design and improve the industrial systems used in frying foods. Mathematical simulations are valuable tools with which to study the effects of the many operating conditions encountered in frying.

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