Food Lipids: Chemistry, Nutrition, and Biotechnology

or intermittent frying, oil filtration, and turnover of oil. Frying protocols of intermittent
or continuous frying affect fry life. Perkins and Van Akkeren [20] found that
cottonseed oil intermittently heated for 62 hours had as much polar material as oil
heated continuously for 166 hours. They suggested that this may be caused by increased
amounts of fatty acyl peroxides which decompose upon repeated heating
and cooling, causing further damage to the oil. Replenishing the fryer with fresh oil
is commonly done in most frying operations; however, in the snack food industry
where more makeup oil is added than in restaurant-style frying, a complete turnover
of the oil in the first 8–12 hours of the frying cycle can be achieved [2]. Levels of
the reaction products in frying oil can also be affected by absorption into the fried
food and evaporation [21]. However, accumulation of degradation products in the
frying medium and their eventual incorporation in fried foods is of primary concern
when commercial or industrial frying operations are carried out under abusive conditions
[9]. Fritsch [1] pointed out that combinations of these factors (Table 2) determine
the rate at which the individual reaction takes place. For example, in one
operation, the rate of hydrolysis may be twice that of the rate of oxidation, whereas
in another operation with different conditions, the reverse may occur.
III. DECOMPOSITION PRODUCTS
During frying, oils degrade to form volatile and nonvolatile decomposition products.
Foods fried in deteriorated oils may contain a significant amount of decomposition
products to cause potential adverse effects to safety, flavor, flavor stability, color, and
texture of the fried food. Although volatile compounds are primarily responsible for
flavor—both positive and negative—thermal polymers do not affect flavor directly.
Therefore, thermal polymers may exist in an edible product, but the conditions leading
to their formation are not usually encountered in commercial practice. Chang
and coworkers [22] isolated the nonvolatile fraction as a brownish, transparent viscous
liquid, which is indicative of a considerable amount of decomposition products
in the oil.
A. Volatile Decomposition Compounds
In deep-fat frying, as oil is continuously or intermittently heated in the presence of
air, both thermal and oxidative decomposition of the oil occurs producing both volatile
and nonvolatile decomposition products (Table 3). Selke et al. [23] identified
volatile odor constituents and their precursors from heated soybean oil, using model
triglycerides [pure triolein, mixture of triolein (25%)-tristearin, and a randomly esterified
triglyceride of stearic and 25% oleic acids] heated at 192�C in air for 10
minutes. Each model system produced the same major compounds, identified as
heptane, octane, heptanal, octanal, nonanal, 2-decenal, and 2-undecenal. These seven
compounds were unique to the oxidation of the oleate fatty acid in each triglyceride
sample. Later, Selke and coworkers [24] analyzed pure trilinolein and mixtures of
trilinolein-tristearin, trilinolein-triolein, and trilinolein-triolein-tristearin heated to
192�C in air. Major volatiles included pentane, acrolein, pentanal, 1-pentanal, hexanal,
2- and/or 3 hexanal, 2-heptenal, 2-octenal, 2,4-decadienal, and 4,5-epoxide-2enal.
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