General

The amount and type of endogenous enzyme present in the fruit of a plant, whether a cereal grain seed or a fruit such as an apple or orange, depends, to a large extent, on the maturation stage of the plant. As a seed or fruit ripens, its enzyme content changes. As ripening proceeds, more hy-drolytic enzymes are produced. In the case of a cereal grain, these enzymes begin the process of breaking down carbohydrate and protein stored in the seed in order to provide energy and amino acids necessary for the synthesis of new plant parts. While necessary for plant growth, these enzymes can have quite detrimental effects when flour from germinated wheat kernels is used in the production of foods, particularly in baked goods.

In ripening fruits, analogous enzymes begin a similar process of cellular breakdown that leads to characteristic loss of firmness normally associated with overripe, less desirable produce. The freshness of fruits and vegetables is often judged by the color and texture of the produce such as, for example, the bright green color and firm texture of fresh peas, beans, or peppers. When many products such as these ripen, their color changes to red, orange, yellow, brown, or even black. This is due, to a large degree, to endogenous enzymes, some of which degrade the green chlorophyll, thus changing the color of the product. Other enzymes such as lipoxygenase attack triglycerides forming free radicals and hydroperoxides. These free radicals can also lead to loss of green color due to oxidative degradation of chlorophyll and also changes in reds and orange colors due to reactions with carotenoid compounds. Oxidative reactions from these compounds can also lead to nutritional damage in the food product through detrimental action on vitamins as well as some proteins. Other color changes are also found to result from enzymatic browning reactions involving polyphenol oxidase (see "Browning Reactions— Color and Flavor") (1).

Texture is also affected by endogenous enzymes that hy-drolyze cellular material, most of which is composed of various forms of carbohydrate material. Most commonly found in plants, these include cellulose, hemicellulose, starch, pectic material, and lignin. These carbohydrates are involved in cell structure of most plants and are responsible for the firm texture in young, preripe fruits and vegetables.

Meat color is also influenced by endogenous enzymes involved in oxidation-reduction reactions that alter the oxidation state of several forms of myoglobin as well as the amount of available oxygen. Changes in the oxidation state of oxymyoglobin and deoxymyoglobin can lead to production of metmyoglobin, which is responsible for the brown coloration in meat (1).

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