Food Lipids: Chemistry, Nutrition, and Biotechnology

7
Chemistry of Frying Oils
KATHLEEN WARNER
U.S. Department of Agriculture, Peoria, Illinois
I. INTRODUCTION
Deep-fat frying is a common method of food preparation that imparts desired sensory
characteristics of fried food flavor, golden brown color, and crisp texture. During
frying, at approximately 190�C, 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. 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.
II. PROCESS OF FRYING
A. Changes in Oils During Heating and Frying
1. Physical Changes
Deep-fat frying is a process of cooking and drying in hot oil with simultaneous heat
and mass transfer. As heat is transferred from the oil to the food, water is evaporated
from the food and oil is absorbed by the food [1] (Fig. 1). Many factors affect heat
and mass transfer, including thermal and physical properties of the food and oil,
shape and size of the food, and oil temperature [2]. Although this chapter primarily
presents information about the chemistry of frying, the physics of frying is of interest
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.