10. Why do cooking oils go bad? How do you prevent them from going rancid?
1. Autoxidation. Of particular interest in the food field is the
process of oxidation induced by air at room temperature referred to as
"autoxidation." Ordinarily, this is a slow process which occurs only to a
limited degree. In autoxidation, oxygen reacts with unsaturated fatty acids. Initially,
peroxides are formed which in turn break down to hydrocarbons, ketones, aldehydes, and
smaller amounts of epoxides and alcohols. Heavy metals present at low levels in fats and
oils can promote autoxidation. Fats and oils often are treated with chelating agents such
as citric acid (Also see Tables IV and V to inactivate
heavy metals.)
The result of the autoxidation of fats and oils is the development of
objectionable flavors and odors characteristic of the condition known as "oxidative
rancidity." Some fats resist this change to a remarkable extent while others are more
susceptible depending on the degree of unsaturation, the presence of antioxidants, and
other factors. The presence of light, for example, increases the rate of oxidation. It is
a common practice in the industry to protect fats and oils from oxidation to preserve
their acceptable flavor and shelf life.
When rancidity has progressed significantly, it is apparent from the
flavor and odor of the oil. Expert tasters are able to detect the development of rancidity
in its early stages. The peroxide value determination, if used judiciously, may be helpful
in measuring the degree of oxidative rancidity in the fat.
It has been found that oxidatively abused fats can complicate
nutritional and biochemical studies in animals because they can affect food consumption
under ad libitum feeding conditions and reduce the vitamin content of the food. If
the diet has become unpalatable due to excessive oxidation of the fat component and is not
accepted by the animal, a lack of growth by the animal could be due to its unwillingness
to consume the diet. Thus, the experimental results might be attributed unwittingly to the
type of fat or other nutrient being studied rather than to the condition of the ration.
Knowing the oxidative condition of unsaturated fats is extremely important in biochemical
and nutritional studies with animals.
2. Oxidation at Higher Temperatures. Although the rate of
oxidation is greatly accelerated at higher temperatures, oxidative reactions which occur
at higher temperatures may not follow precisely the same routes and mechanisms as the
reactions at room temperatures. Thus, differences in the stability of fats and oils often
become more apparent when the fats are used for frying or slow baking. The more
unsaturated the fat or oil, the greater will be its susceptibility to oxidative rancidity.
Predominantly unsaturated oils such as soybean, cottonseed, or corn oil are less stable
than predominantly saturated oils such as coconut oil. Methylsilicone often is added to
institutional frying fats and oils to reduce oxidation tendency and foaming at elevated
temperatures. Frequently, partial hydrogenation is employed in the processing of liquid
vegetable oil to increase the stability of the oil. Also oxidative stability has been
increased in many of the oils developed through biotechnological engineering. The
stability of a fat or oil may be predicted to some degree by the oxidative stability index
(OSI).
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