Frying oil polymerization

Fig. 34. 3. Structures of selected sterols. Sources animal - lanosterol, cholesterol and ergosterol (also microbial) plant - all others. (From Warner, K., Su, C, and White, P.J. "Role of Antioxidants and Polymerization Inhibitors in Protecting Frying Oils" in Frying Technology and Practices, M.K. Gupta, K. Warner, and P.J. White (Eds.), pp. 37-49, AOCS Press, Champaign, IL 2004. With permission.)...

On the other hand, oxidative polymerization involves formation of C —O —C bonds. Polymers with ether and peroxide linkages are formed in the presence of oxygen. They may also contain hydroxy, oxo, or epoxy groups. Such compounds are undesirable in deep-fried oil or fat because they permanently diminish the flavoring characteristics of the oil or fat and also cause a foaming problem. 

Saturated FA, such as palmitic and stearic acids, are stable toward oxidation and polymerization. Because they have a high melting point, they add structure to certain products. However, due to this characteristic the appearance of a fried food may be adversely affected (e.g., waxy mouth-feel, dry surface of stored fried food). Monoenoic FA, primarily oleic acid, are considered to be beneficial from a health standpoint. Frying oils rich in such FA do not add to the structure they are stable against oxidation and provide a light taste. Polyenoic FA (PEFA) deteriorate more rapidly that monoenoic FA and the shelf life of products fried in oils rich in these acids is shorter. Oxidation products formed from PEFA vary widely, depending on the structure of the FA and the relative concentration of linoleic and linolenic acids. The percentage of linolenic acid in heated oils should be very low. 

Teflon (polytetrafluoroethylene) is a chemically inert polymer used to create nonstick frying pans. Polytetrafluoroethylene can be modified to form a coating, Gore-Tex, which allows the passage of water vapor, but not liquid water, and is used in many articles of clothing. Polymeric perfluorinated ethers are widely used as high performance oils and lubricants. 

It has been mentioned earlier that several chemical reactions take place in the oil during frying (10, 11). These include hydrolysis, autoxidation, oxidative polymerization, and thermal polymerization, as explained below. 

Polymerization of oil occurs under heat with or without the presence of oxygen. Heat can cleave the oil molecule or fatty acid. These cleaved compounds can then react with each other, forming large molecules. These polymers are referred to as thermal polymers. In the frying process, excessive fryer heat and excessive fryer down time can produce high levels of thermal polymers. Thermal polymers can be detected in the fresh product by expert panelists because they generally impart a bitter aftertaste to the fried food. 

Referring to Figure 3, one can see that a multitude of reactions occur in the fryer oil simultaneously. This includes hydrolysis, autoxidation, polymerization, and many others. Therefore, the fryer oil should also be analyzed for the state of oxidative and polymeric degradation aside from free fatty acids. These analyses wUl not be the same for all types of fried food. The specific analysis needs to be established through shelf life study and consumer acceptabihty tests on the product. 

During frying, the oil is continuously exposed to high temperatures in the presence of air and moisture. Under such conditions, many complex reactions occur these reactions can be classified as oxidation, polymerization, and hydrolysis. Some of these reactions result in the desirable flavor, color, and texture of the fried food, while others are undesirable from the perspectives of quality, nutrition, and toxicology. 

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