a. Results of Enzyme Action. Deterioration of foods resulting from the catalytic action of enzymes is easy to observe.
(1) An example is the ripening process of the banana wherein it becomes sweeter, softer, less astringent in taste, and more odorous. One of the reactions in this process is the hydrolysis (degradation) of the starch (essentially tasteless and insoluble in the water of the banana) to simple water-soluble sugars.
(2) Another example is the overripe tomato. Here the softening is due largely to the hydrolysis of the pectins to their simpler carbohydrate building blocks. Pectin is a water-soluble carbohydrate found in ripe fruits and has strong gelling properties which are used in cooking.
b. Changes in Proteins and Fats. Proteins in such foods as cheese, meat, and fish may be hydrolyzed to simpler compounds by the enzymes naturally present. Fats in some foods may also be hydrolyzed by naturally present enzymes. Such chemical changes are often manifested as changes in taste, odor, texture, and so forth.
c. Oxidative Enzymes. When freshly harvested products are processed for eating, the normal cellular organization of the tissues may be disrupted, with the result that residual enzymes may initiate degradative changes at a very rapid rate. One of the most common examples of such changes is the rapid darkening of freshly peeled potatoes, apples, peaches, and pears. Here the oxidative enzymes of the freshly exposed tissues use oxygen from the air to change many naturally occurring colorless compounds (phenols) to colored compounds (quinones). Other oxidizing enzymes induce the common and sometimes intense "hay" flavor of vegetables such as lima beans, corn, and broccoli if they are not cooked soon enough after harvesting.
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°-80°C (140°-176°F), although some enzymes are destroyed below 60°C (140°F) and some require heating to temperature above 80°C (176°F) before they lose their catalytic properties.
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 100°C (212°F) before they are frozen to prevent deterioration in frozen storage.
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.
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