CHANGES IN PHYSICAL AND CHEMICAL-PROPERTIES OF SHORTENINGS USED FOR COMMERCIAL DEEP-FAT FRYING

This study evaluated some of the changes that occur in shortenings used for commercial deep-fat frying in fast-service restaurants. Foods cooked in partially hydrogenated soybean oil were battered chicken parts and french fries. Sixty-five samples of fresh and used shortenings were collected from nine restaurants on three occasions over a three-month period. Frying periods varied from 0 to 300 hr, and most samples were taken just before the used fat was discarded. For fresh shortenings, percentages of polar materials, free fatty acids (FFA), materials not eluted by gas chromatography, and fatty acid profiles differed only slightly. For used samples, there were marked variations in these analyses and in increases of dielectric constant measurements. Frying times were highly correlated with increases in dielectric constant, polar materials and FFA. The greatest change in fatty acid profiles occurred in trans-C18 monoenes which decreased from over 40% to as low as 13%. Due to lipid exchange with chicken fat, both oleic and linoleic acids increased in the shortenings with hours of use, whereas stearic acid decreased. There were high correlations, among increases in dielectric constant, percentages of polar materials and FFA, demonstrating that each of these methods could predict degradation of the shortening. However, the increase in dielectric constant, as measured by a Foodoil Sensor (FOS), was the most convenient for quality control in restaurant situations. In most cases, used shortening was discarded before 100 hr of frying time; and only a few of these samples had FOS readings near 4.0, FFA over 1.00%, or percentages of polar materials over 27%. These values have been suggested as discard criteria. However, a number of samples used between 100 and 300 hr exceeded these limits. There is a need to specify suitable limits, related to quality and health factors, to determine at what point a cooking fat should be discarded. © 1986 The American Oil Chemists' Society.