ABSTRACT
I. Introduction.......................................................................................................................... 189 II. Process of Frying................................................................................................................. 190
A. Changes in Oils during Heating and Frying................................................................ 190 1. Physical Changes................................................................................................... 190 2. Chemical Changes................................................................................................. 191
B. Factors Affecting Oil Decomposition .......................................................................... 194 III. Decomposition Products ...................................................................................................... 195
A. Volatile Decomposition Compounds........................................................................... 195 B. Nonvolatile Decomposition Compounds ..................................................................... 195
IV. Measurement of Decomposition Products: Significance, Advantages, Limitations............ 196 A. Nonvolatile Decomposition Products .......................................................................... 196 B. Volatile Decomposition Products ................................................................................ 198
1. Peroxides ............................................................................................................... 198 2. Volatile Compounds.............................................................................................. 198 3. Sensory .................................................................................................................. 199
References ..................................................................................................................................... 200
Deep-fat frying imparts desired sensory characteristics of fried food flavor, golden brown color, and crisp texture in foods. During frying, at ~1908C, as oils thermally and oxidatively decompose, volatile and nonvolatile products are formed that alter functional, sensory, and nutritional qualities of oils. During the past 30 years, scientists have reported extensively on the physical and chemical changes that occur during frying and on the wide variety of decomposition products formed in frying oils. A small amount of oxidation in frying oils is important to develop the delicious deepfried flavor characteristic of fried foods. However, as oils breakdown further because of the processes of oxidation, hydrolysis, and polymerization, compounds are formed that can cause offflavors and may even be toxic if formed in high concentrations. Hydrogenated oils have been commonly used for commercial deep-fat frying in the United States since the 1950s, but concerns about trans fatty acids in hydrogenated oils have encouraged the use of alternative oils. Sometimes, these alternatives are less oxidatively stable oils and subsequent problems developed because of the use of unstable oils for frying. The chemistry of frying is especially important to understand, as less stable alternative oils are investigated as potential substitutes for hydrogenated oil. This chapter will review the physical and chemical changes in oils during frying, including reactions that occur in the frying process. In addition to discussing the degradation products formed, their effects on oil stability and quality of fried food will be included. Methods to measure oil deterioration will be discussed in terms of their significance, advantages, and limitations.
