13.6.2   Oxidative fat cleavage due to action of oxygen
Food and feedstuff components frequently react with atmospheric oxygen in spoilage reactions. Atmospheric oxygen may enter into an addition reaction with unsaturated fatty acids, which is known as oxidative rancidity, through the simultaneous assistance of light, heat and certain fat companion substances, and possibly also traces of heavy metals. This phenomenon is particularly noticeable in shelled shell fruit (e.g. walnut kernels, hazelnut kernels, shelled peanuts). Storage in the dark and protection from oxygen and steel parts are necessary in order to prevent consequent brown discoloration and a rancid odor and flavor.
The principal cause underlying self-heating caused by oxidative fat cleavage is the high oil content of, for example, nuts. An elevated unsaturated fatty acid content has a strong tendency to undergo autoxidation with atmospheric oxygen to form saturated fatty acids, a process which is associated with considerable evolution of heat.
Self-heating of the goods not only damages the goods themselves (rancid odor and flavor), but also has a qualitative and quantitative impact on oil yield: the color and bleachability of the oils are negatively affected and oil yield may be cut in half.

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