Food Lipids: Chemistry, Nutrition, and Biotechnology

Table 1 Effects of Physical and Chemical Reactions During Deep-Fat Frying
Physical Changes
Increased viscosity, color, and foaming
Decreased smoke point
Chemical Changes
Increased free fatty acids, carbonyl compounds, high molecular weight products
Decreased unsaturation, flavor quality, nutritive value (essential fatty acids)
sively, or when the oil color darkens. Other characteristics of abused oil include
increased viscosity, off-odors such as acrid and burnt, and development of off-flavors
in fried food. Of course, instrumental or chemical analyses are preferable to visual
inspection in measuring frying oil deterioration, especially in research applications.
The relationship of physical and chemical changes in oil is usually predictable since
the many variables in the frying process affect the onset and magnitude of these
changes, as will be discussed later in this chapter. Nonvolatile decomposition products
eventually produce physical changes in frying oil, such as increases in viscosity,
color, and foaming [8]. Chemical changes during frying increase the concentration
of free fatty acids as well as carbonyl and polymeric compounds, and decrease fatty
acid unsaturation. The effect of chemical changes on flavor quality of fried food and
on the stability of the oil will be discussed in this chapter; however, potential toxic
effects of degradation products on health will not be included because this topic is
well covered in the literature [4,9].
2. Chemical Changes
Frying oils not only transfer heat to cook foods but also help to produce distinctive
fried-food flavor and, unfortunately, undesirable off-flavors if deteriorated oil is used.
During deep-fat frying various deteriorative chemical processes (e.g., hydrolysis,
oxidation, and polymerization) take place, and oils decompose to form volatile products
and nonvolatile monomeric and polymeric compounds (Fig. 1). With continued
heating and frying, these compounds decompose further until breakdown products
accumulate to levels that produce off-flavors and potentially toxic effects, rendering
the oil unsuitable for frying. The amounts of these compounds that are formed and
their chemical structures depend on many factors, including oil and food types, frying
conditions, and oxygen availability [10]. In addition, these chemical reactions—
hydrolysis, oxidation, and polymerization—are interrelated producing a complex
mixture of products. The individual processes of hydrolysis, oxidation, and polymerization
and their degradation products are described below.
a. Hydrolysis. As food is placed in oil at frying temperatures, air and water
initiate a series of interrelated reactions. Water and steam hydrolyze triglycerides,
which produces mono- and diglycerides, and eventually free fatty acids and glycerol
(Fig. 2). Glycerol partially evaporates because it volatilizes above 150�C, and the
reaction equilibrium is shifted in favor of other hydrolysis products [5]. The extent
of hydrolysis is a function of various factors, such as oil temperature, interface area
between the oil and the aqueous phases, and amount of water and steam because
water hydrolyzes oil more quickly than steam [5]. Free fatty acids and low molec-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.