d. Control by Heating. Chemical reactions catalyzed by enzymes can be
stopped by destroying or removing the enzymes. The method is simply heating or
cooking food. All enzymes are proteins, and proteins are easily changed or denatured
by heating. The temperature required to inactivate most enzymes are in the range of
60-80C (140-176F), although some enzymes are destroyed below 60C (140F) and
some require heating to temperature above 80C (176F) before they lose their catalytic
e. Control by Freezing. Many foods are preserved by freezing. However,
freezing does not destroy most enzymes, and many frozen foods can deteriorate
enzymatically, even though the rates of the reactions may be slowed. Vegetables are
the worst offenders. So, in order to preserve peas, green beans, corn, and so forth, by freezing,
it is first necessary to heat them briefly to almost 100C (212F) before they are frozen
f. Control Techniques for Fruit. Generally, fruits do not require such heat
treatment (blanching), which is fortunate, since many of them are adversely altered in
flavor when heated. However, enzymatic darkening often occurs in frozen fruits such as
sliced peaches. To counteract this, they are often packed with sugar syrups containing
ascorbic acid or similar oxidation inhibitors (antioxidants). Along with proper packaging,
this diminishes greatly the amount of atmospheric oxygen reaching the fruit, which is
necessary for the darkening reactions.
1-18. NONENZYMATIC BROWNING
a. Caused by Sugars. Nonenzymatic browning is a process in which foods
darken without the catalytic effect of enzymes. It differs markedly from the darkening of
fresh fruits and vegetables due to enzymatic degradation. The food constituents
responsible are certain simple sugars which react with proteins, amino acids, or other
substances normally present in the foods.
b. Flavor Change in Some Foods. Although the darkening in color of the food
may be undesirable, the more notable changes are often those of flavor.
(1) The rate of browning reactions increases with temperature, and these
chemical changes may contribute to desirable flavor development on cooking.
(2) Maple syrup gets its characteristic flavor from the chemical process as
the maple sap is concentrated by boiling.
(3) A similar reaction causes the caramelized flavor in canned evaporated
milk, which many people find much less acceptable than the taste of fresh milk. But the
same flavors of evaporated milk are characteristic of the desirable flavors of caramel
candy and certain other foods.