Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 5 Analytical schemes for volatile and nonvolatile compounds in frying oils.
Smith et al. [35] evaluated 65 samples of partially hydrogenated soybean oil
used for frying battered chicken and french fried potatoes in fast-service restaurants.
Frying times correlated highly with increases in dielectric constant, polar materials,
and free fatty acids. Oleic and linoleic acids increased in the shortenings with hours
of use, whereas stearic acid decreased because of contamination with chicken fat.
Collected samples that had been discarded before 100 hours of frying time had values
of 4.0 for the Foodoil Sensor, 1% free fatty acids and 27% polar materials which
have been suggested as end points for discarding frying oil. Perkins [8] measured
nonvolatile decomposition products in cottonseed oil and tallow to show that polymers
increased with increasing heating time, and that cottonseed oil was deteriorated
more by intermittent heating and/or added water than by continuous heating and no
water addition. Cuesta et al. [36] measured polar components by HPSEC to investigate
the thermo-oxidative and hydrolytic changes in frying oil. These researchers
were able to quantitate triacylglycerol polymers and dimers, oxidized triacylglycerols,
diacylglycerols, and free fatty acids.
Christopoulou and Perkins [25] recommended model systems such as pure fatty
acids and triglycerides oxidized under simulated deep-fat frying conditions to control
the various factors (Table 2) affecting the thermal-oxidative reactions and to facilitate
the structure elucidation of the decomposition products such as thermal and oxidative
dimers. Arroyo et al. [14] reported a linear correlation of r = 0.99 between number
of fryings and amount of decomposition products, including total polar compounds,
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.