Physical Changes in Oils during Deep Fat Frying

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Deep fat frying is the most commonly used thermal processing method within the food industry. Despite its popularity, the theoretical aspects of the frying process are highly complex and can be difficult to grasp. It is essential to understand the series of phenomena that occur during the deep fat frying of foods in order to continuously produce high quality fried products.

Frying Process

The actual technology of frying is claimed to have originated and been developed near the Mediterranean area due to the influence of olive oil (Moreira et al., 1999). Fried food products generate billions of dollars. In the United States alone, more than 500,000 commercial restaurants use one million metric tons (MMT) or 2.0 x 109 lb. of frying fats and oils annually (Moreira et al., 1999). Frying is such a popular process due to the characteristics of the foods that are produced. Deep fat frying produces foods that are not only flavorful, but have tremendous aesthetic appeal due to the golden brown color.
Frying is a highly complex process where a series of phenomena occur simultaneously throughout the entire process. More specifically, there is simultaneous heat, moisture and oil transfer taking place between the product and the heating medium (frying oil). There is also the formation of a crust layer. To complicate the issue even further, the composition of the oil is steadily changing throughout the process. It is extremely important to have an understanding of what is happening during the frying process such that optimization of the process can be achieved.
There have been several simplified models developed to simulate the frying process, but all are specific to the food material being fried. Not all food material behaves in the same manner upon being fried due to differences in geometry and chemical and physical makeup.

Frying Mechanism

Deep fat frying is a process in which a food material is cooked through continued contact with hot oil that usually ranges between 180 and 205°C. The frying process involves simultaneous heat and mass transfer. The heat is transferred from the oil into the food material while moisture is transferred from within the food material to the oil. Once moisture evaporates from the interior of the food material, void spaces are created. Oil is then able to occupy the space. An understanding of these phenomena is critical to ensuring that high quality fried products are produced with every fry cycle.

Heat Transfer

There are two basic modes of heat transfer involved in the process of deep-fat frying, and those are convection and conduction. The oil serves as the heating medium. Heat is transferred from the oil to the surface of the product by way of convection. It is then transferred from the surface to the center by conduction. Thermal properties of the food material such as specific heat, thermal conductivity and density affect the rate at which heat is conducted. The magnitudes of these properties change throughout the frying process. The literature that is currently available is nearly devoid of reliable thermal properties data relevant to the frying process (Singh, 1995).
Water is an important factor in convective heat transfer. Water migrates from the central portion of the food material radially outward to the walls and edges to replace that which is lost by dehydration at the surface. As the phase change from liquid water to steam occurs, thermal energy from the frying oil is carried off, which prevents burning caused by excessive dehydration (Blumenthal, 1991). Due to the ability of the water to remove thermal energy from the oil, the temperature of the food material only reaches approximately 100°C even though the oil temperature may be around 180°C. As water escapes from the inner portion of the product and comes into contact with the hot oil, bubbles form and move vigorously throughout the oil, therefore causing turbulence (Innawong, 2001). In general, turbulent conditions promote more rapid heat transfer. The amount of water vapor bubbles decrease with increased frying time due to the decreased amount of remaining moisture within the product.

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Moisture Transfer

There is typically a mass loss experienced by the product during the frying process due to constant diffusion of water from within the material core. Moisture is evaporated at the product surface as a result of the partial vapor pressure difference between the product and the frying oil. The rate of moisture transfer is directly related to frying time and oil temperature. According to Mittelman et al. (1984), moisture diffusion during the frying of French fries is proportional to the square root of the frying time. The rate of moisture loss significantly decreases with increased frying time. During a study on the drying rate of potato chips, it was shown that a reduction in drying time of 60 seconds occurred when the potatoes were fried at 180°C as opposed to 150°C (Moreira et al., 1999).
Rapid drying is critical for ensuring desirable texture of the final product. However, it is undesirable to have excessive moisture loss as it may result in greater absorption of oil by the product. Sustaining higher moisture content in the final product normally results in products having a low final fat content. As water retention is strongly affected by some food additives, incorporating alginates or cellulose could play a major role in changing the amount of moisture loss and oil uptake (Saguy and Pinthus, 1995).

Oil Transfer

Oil absorption into the product is influenced by oil temperature, frying time and surface moisture content, product surface area and pressure (Innawong, 2001). A linear relationship exists between the surface area and the amount of fat uptake. More specifically, the ratios of product weight to frying oil volume and product surface area to volume are extremely important because they determine the extent to which oil is able to penetrate the food material (Moreira et al., 1999). An increase in the surface-to-mass ratio of the product will yield an increase in the amount of oil absorption by the product (Saguy and Pinthus, 1995). Blumenthal (1991) postulates that there are basically three surface-to-volume ratios for foods being fried: (1) foods having an all interior volume with a crispy external surface and no possibility of crust differentiation; example: a food product underneath a breading and battering such as chicken; (2) foods having a significant interior volume with a significant external surface and good crust differentiation; example: French fry; (3) foods having no significant interior volume but a high exterior surface area approximating an all crust and no center product; example: potato chip.
In a study of the modeling of deep fat frying of meatballs, Ateba and Mittal (1994), suggest that foods containing fat undergo two fat transfer periods during the frying process: fat absorption and fat desorption. During the fat absorption period, oil diffuses into the product. The fat desorption period is marked by the migration of fat from the product to the surroundings due to capillary forces in the pores. Foods lacking an initial fat content do not experience the fat desorption period. It has been postulated that fat is able to be absorbed into the product due to the moisture migration from within the product. When the moisture is evaporated from the surface, void spaces are left behind in the product. Fat is then absorbed and fills those void spaces. Another interpretation for the mechanism of oil absorption in fried products proposed that most of the oil enters the product from the adhering oil being pulled into the product when it is removed from the fryer, due to the condensation of steam in the product pores, which produces a vacuum (Moreira et al., 1999). This oil absorption mainly occurs during the post-frying (cooling) period. In a study conducted by Moreira and Barrufet (1998) on tortilla chips, it was found that most of the oil (80%) was absorbed during the post-frying period.

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2.1 Frying Process
2.2 Frying Mechanism
2.2.1 Heat Transfer
2.2.2 Moisture Transfer
2.2.3 Oil Transfer
2.2.4 Crust Formation
2.3 Mathematical Modeling of Deep Fat Frying
2.4 Frying Oil.
2.4.1 Frying Oil and Food Quality
2.4.2 Changes in Oil during Deep Fat Frying Physical Changes in Oils during Deep Fat Frying Chemical Changes in Oils during Deep Fat Frying
2.5 Pressure Frying
2.6 Batter and Breading in Deep Fat Fried Foods
2.7 Edible Coatings in Frying Process.
2.8 Quantifying and Evaluating Crispness.
2.8.1 Instrumental Evaluation of Texture
2.8.2 Sensory Evaluation of Crispness
2.8.3 Ultrasonic Evaluation of Crispness.
3. The Effect of Pressure Conditions and Coating Type on Quality of Chicken Nuggets.
4. The Effect of Edible Coatings and Pressure Conditions on the Crispness of Chicken Nuggets.
5. Consumer Assessment of Crispness of Pressure Fried Chicken Nuggets Using Nitrogen Gas

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