Chemical Changes of Oil during Frying

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The most popular foods in the industry are produced through frying. This is a complicated method for cooking involving many different simultaneous processes.Although popular, they have a perception of being unhealthy. This review will provide information about the frying process, issues related to frying, and the current research on frying.

Frying Process

During the frying process, water evaporates from inside the product being fried, creating voids that are penetrated by the oil. As long as there is moisture to evaporate, the product will stay at a temperature of roughly 100 oC (Blumenthal, 1996). As the water evaporates, the crust indicative of fried foods is formed on the exterior of the product. There are three simultaneous processes occurring: heat transfer from the hot oil to the product, mass transfer of the water from the interior of the product to the exterior and then into the oil, and mass transfer of the oil into the product. Because of the high temperatures and the presence of water vapor (steam), the oil is also undergoing chemical changes.

Heat Transfer

There is a rapid rise in the surface temperature of a food placed in hot oil. As the product’s internal moisture is vaporized into steam, the surface begins to dry. A crunchy crust forms on the outside as the plane of evaporation moves inside the food (Fellows,2000) and hot fat begins to penetrate the food. The crust prevents large amounts of fat from passing through to the inside of the product (Moreiras-Varelo et al., 1988). A film of oil surrounds the product upon removal from the fryer. The thickness of this film controls the rate of heat and mass transfer. The viscosity and velocity of oil determine the thickness of this boundary layer (Fellows, 2000). The initial mechanism considered for oil uptake was the condensation mechanism.Although there is no direct proof of this mechanism, it’s been composed using common sense. According to the condensation mechanism for oil uptake, as vapor escapes foods frying in hot oil, an overpressure is created inside the food’s pores. This prevents the oil from penetrating the food. This “barrier of escaping steam” continues until just after the food is removed from the oil (Mellema, 2003). As the food cools, the vapor inside the crust condenses, creating an underpressure in the pores. The film of oil surrounding the food product is driven into the pores by the underpressure (Mellema, 2003). Because there is no evidence to support this theory, it has largely been replaced by the capillary mechanism of oil uptake, which is based on fluid flow through porous media. After frying, according to the capillary mechanism, the pores of a food are filled with vapor. During cooling, the surface tension between the thin film of frying oil surrounding the product and the vapor inside the pores increases as the temperature decreases. The capillary pressure, or pressure difference between two immiscible fluids (in this case, water vapor and oil), subsequently increases, causing the surface oil to enter the pores (Moreira et al., 1999). The hot oil fills the larger capillaries first, because water vapor is removed from them first (Fellows, 2000). This mechanism has experimental evidence to support it (Saguy and Pinthus, 1995; Moreira and Barrufet, 1998). There is both convective and conductive heat transfer during frying. The convection occurs between the hot oil and the food, while conduction occurs within the product being fried (Singh, 1995). The rate of convective heat transfer depends on the temperature difference between the oil and the food and by the product surface heat transfer coefficient (Fellows, 2000). Water vapor bubbles escaping from the food also affect the transfer of heat from the oil to the food (Singh, 1995). The rate of conductive heat transfer depends on the thermal properties of the food, such as specific heat, thermal conductivity, and thermal diffusivity. To complicate matters, these properties will change with temperature during the frying process (Singh, 1995). Farkas et al. (1996) have suggested that the frying process can be divided into four stages: initial heating, surface boiling, falling rate, and bubble end-point. During the initial heating stage, which usually only lasts a few seconds, the food surface is heated to the elevated boiling point of water (approximately 103oC) through natural heat convection. There is no vaporization of water from the surface of the food during this short time. When vaporization begins, so does the surface boiling stage. The mode of heat transfer changes to forced convection, because of the turbulence in the surrounding oil caused by escaping water vapor. The crust begins to form at the surface of the food during this stage. The falling rate stage is similar to the falling rate period that takes place during food dehydration processes. This is when more internal moisture leaves the food and the internal core temperature rises to the boiling point. The internal core region is undergoing several physicochemical changes, such as cooking and starch gelatinization, and the crust layer is increasing in thickness. After sufficient time, the vapor transfer to the surface decreases. If the food is fried for a considerably long period of time, the bubble end point will be reached. At this final stage, the rate of moisture removal diminishes and no more bubbles are seen escaping the food surface. The crust layer continues to increase in thickness as the frying process proceeds.

Reduction of Fat Uptake

There are three basic routes for the reduction of fat uptake – modification of frying techniques, edible coatings and batters, and modification of frying medium

Table of Contents
List of Tables
List of Figures
1. Introduction
2. Literature Review
2.1 Frying Process
2.1.1 Heat Transfer
2.1.2 Reduction of Fat Uptake Modification of Frying Techniques Coatings and Batters Modification of Frying Medium
2.2 Frying Oil
2.2.1 Chemical Changes of Oil during Frying
2.2.2 Effect of Oil Quality on Fried Foods
2.3 Frying Methods
2.3.1 Pressure Frying
2.3.2 Vacuum Frying
2.4 Fried Product Analyses
2.4.1 Breading Adhesion
2.4.2 Color
2.4.3 Juiciness
2.4.4 Moisture Content
2.4.5 Oil Content
2.4.6 Texture
2.5 Health Issues
3. The Effect of External Pressurizing Media on Quality of Breaded Fried Foods
4. The Effect of Vacuum Frying on Quality of Breaded Fried Foods
5. Summary and Conclusions


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