Heat treatment clearly affects certain (but not all) foods, most commonly rendering them less likely to provoke an allergic reaction in a subject who is allergic. Occasionally, the reverse occurs, as in the celebrated case of Professor Heinz Kiistner, who was allergic to cooked and not raw fish.
In cow's milk, whey proteins are easily denatured by heat but casein is highly resistant. This observation led to the suggestion that the heat treatment of whey proteins may be a simple and logical strategy for producing a hypoallergenic infant milk formula. However, double-blind, placebo-controlled oral challenges gave rise to immediate hypersensitivity reactions to heat-treated whey protein in four of five children with cow's milk protein intolerance. The reason for these reactions is not known, but one possibility is a reaction to residual casein, which is often present in trace amounts in commercial whey preparations. The small proportion of patients with cow's milk protein intolerance likely to tolerate heat-treated cow's milk, such as evaporated milk, means that heat-treated milk is unlikely to be suitable as a substitute for a cow's milk infant formula.
Cooking reduces the allergenicity of eggs by 70%. However, one of the major allergens in eggs, ovomucoid, a heat-resistant glycoprotein that contributes to the gel-like structure of thick white, is resistant to heating. Heat appears to render a large number of fruits and vegetables less likely to provoke adverse reactions in subjects who are intolerant. Thus, for example, it is not uncommon to see children who are allergic to raw potatoes or fresh pineapple, but almost all such children can tolerate cooked potatoes or tinned pineapples. In some situations, it appears that heat can accelerate a process of denaturation that can in time occur on its own. For example, there have been studies of patients who reacted to fresh melon, pear, peach, pineapple, grape, and banana.
In each case, stewed or tinned fruit caused no reaction. Studies of fresh extracts of these fruits showed that when stored in a refrigerator, the extracts lost their ability to provoke a positive skin test after approximately 3 days.
Heating can increase the allergenicity of certain proteins through the induction of covalent modifications that lead to new antigens or increased stability. In peanuts, for example, the roasting process produces end products with greater resistance to digestion and heightened allergenicity compared with those produced by frying or boiling. This finding may partly account for the low prevalence of peanut allergy in China, where peanut is widely consumed but not roasted.
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