Food Lipids: Chemistry, Nutrition, and Biotechnology

for the ultimate criteria to evaluate frying stability of oils and fried-food flavor quality
and stability. White [27] reviewed existing analyses to measure formation of volatile
and nonvolatile components in order to detect deterioration in frying oils including
the standard methods of polar components, conjugated dienes, and fatty acids, as
well as rapid analyses such as dielectric constant. Croon et al. [28] compared methods
to evaluate 100 frying oil samples using four quick test methods (Foodoil Sensor–
dielectric constant, RAU Test, Fritest, and spot test) and two laboratory methods
(free fatty acids and chromatographic analysis of triglyceride dimers) with a standard
column chromatographic determination of polar compounds. Foodoil Sensor (FOS)
Model NI-20 (Northern Instruments Corp., Lino Lakes, MN, USA) measures dielectric
constant in frying fat relative to fresh oil; RAU Test is a colorimetric test kit
that contains redox indicators reacting with the total amount of oxidized compounds.
Fritest (E Merck, Darmstadt) is a calorimetric test kit sensitive to carbonyl compounds
and the spot test assays free fatty acids to indicate hydrolytic degradation
and free fatty acid. The Foodoil Sensor correlated with polar compounds more than
did the RAU Test, Fritest, and spot test. The amount of free fatty acids was found
to be an unreliable indication of deteriorated frying fat.
From a practical point of view, Fritsch [1] noted that commercial and industrial
frying oil operators want to know the answer to one primary question: When should
frying oil be discarded? Since there are many variables that affect oil degradation
(Table 2), a specific method may be ideal for one operation but completely useless
in another. Fritsch stated that determination of the end point of a frying oil is dependent
on good judgment and knowledge of the particular frying operation, as well
as on the type of frying oil and the analytical measurements used [1]. Some of the
methods used to measure degradation products in frying oil are listed in Table 4 and
are discussed in the following section. Schemes for the chromatographic separation
of volatile and nonvolatile products in frying oil appear in Figure 5. After column
chromatography, both polar and nonpolar compounds can be detected by high-performance
size exclusion chromatography (HPSEC). Volatile compounds can be collected
by several techniques, including direct injection, static head space, dynamic
or purge-and-trap head space, and solid phase microextraction and analyzed by capillary
gas chromatography.
A. Nonvolatile Decomposition Products
Paradis and Nawar [29] reported that nonvolatile higher molecular weight compounds
are reliable indicators of fat deterioration because their accumulation is steady
and they are not volatile. As mentioned earlier, the formation and accumulation of
nonvolatile compounds are responsible for physical changes in frying oil (Table 1)
[8]. Most methods for assessing deterioration of frying fats are then based on these
changes. Nonspecific methods for measuring nonvolatile compounds in deteriorated
frying oil include free fatty acids [1], iodine value [30], non–urea adduct–forming
esters [26], viscosity [32], and petroleum ether–insoluble oxidized fatty acids [33].
White acknowledged that none of these methods has proved to be a good measure
of heat abuse [27]. Melton et al. [34] noted that nonvolatile decomposition products
are a better measure of degradation of a frying oil than volatile products are and
concluded that more research is needed to determine the total polar components
levels at which different frying oils should be discarded and to relate those levels to
fried-food quality for each oil type.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.