Frying Life
The unavoidable exposure of frying shortening to the adverse conditions in
varying degrees dictates that frying stability is a major economic and quality factor
that frying operations must consider when choosing a frying shortening. The frying
stability of a shortening is dependent upon the fat composition, physical characteristics,
and how it is used. Exposure time, as determined by turnover rate, is one the
most important factors for identification of the shortening characteristics required
to maintain the required fried product quality for the maximum time. High-turnover
situations will allow the use of frying shortenings with a lower stability, which
usually equates to higher liquid oil levels. Low-turnover situations demand highstability
shortenings, which normally equates to a higher degree of selective hydrogenation.
The factors that adversely affect frying stability or frying shortening life
are oxidation, polymerization, and hydrolysis.
Oxidation is the combining of oxygen with the unsaturated fatty acids, which
causes off-flavor and odor development along with color darkening. Oxidation is
accelerated at the higher temperatures necessary for frying foods. The oxidation rate
is roughly proportional to the degree of unsaturation; linolenic fatty acid, with three
double bonds, is much more susceptible than oleic fatty acid, which has only one.
Therefore, shortenings with high polyunsaturated fatty-acid levels have the least
resistance to oxidation, which precedes polymerization. The frying shortening factors
that affect the oxidation rate are (1) the degree of hydrogenation or isomerization
or the unsaturation level, (2) the addition of antifoamers, (3) the addition of antioxidants,
(4) the tocopherol or natural antioxidant level, and (5) the use of chelating
agents.