Extensive studies have been carried out to control enzymatic browning ever since Lindet (1) recognized in 1895 that the change in color occurring in freshly pressed cider was enzymatic. The practical control of enzymatic browning in foods has been carried out by several methods. The method of choice is dependent upon the food product and the intended use. The enzymatic browning in fruits and vegetables can be controlled at the beginning by selecting cultivars that are least susceptible to discoloration either because of the absence of the certain phenolic substrate or because the substrate or enzyme is present at low concentration. It is also possible to select fruits and vegetables at stages of maturity where discoloration is at minimum. Other methods include the removal of oxygen from fruit and vegetable tissues as well as from the atmosphere surrounding the food; the addition of acids to reduce the pH and thus reduce PPO activity; the addition of antioxidants or reducing substances; the addition or treatment with permissible inhibitors; and heat inactivation of the enzyme. The use of antioxidants during processing and heat inactivation of the enzyme are widely used practices that have been moderately successful.
All fruits susceptible to browning should be processed as quickly as possible. Heating destroys the enzymes responsible for the reaction; thus when fruit is canned or made into jams or jellies, browning stops as soon as the fruit is heated sufficiently to denature the enzyme. The exact temperature necessary varies with enzyme, rate of heating, pH, and other factors such as size of the fruits. Unfortunately some undesirable effects on quality often result from an adequate heat process. In general, cherry, peach, and apricot take 2 to 3 minutes in boiling water and apple, pear, and plum take 4 to 5 minutes to inactivate enzymes responsible for browning on surface. Vegetables such as beans and peas also take 3 to 5 minutes. The optimum blanching conditions for strawberry, black currant, sour cherry, and prune range from 1.5 to 3 minutes at 85°C.
In the preparation of fruit for freezing, sugars and sugar solution have been used to sweeten and to exclude direct contact of the fruit tissue with molecular oxygen. The sugar solutions inhibit discoloration by reducing the concentration of dissolved oxygen and by retarding the rate of diffusion of the oxygen from the air into the fruit tissues. Concentrated sugar solutions also exert an inhibiting effect on fruit PPO. The pH also affects the rate of the browning reaction: acid dips are sometimes used to lower the pH and by this method delay or retard browning. The effect of many salts and compounds such as sulfur dioxide, hydrogen sulfide, hydrocyanic acid, and thiourea on PPO has been studied extensively.
In the home preparation of fruits, pineapple juice and lemon juice have long been used to prevent browning. Pineapple juice has a relatively high concentration of sulfhydryl compounds, which are active antioxidants, whereas lemon juice contains relatively high amounts of both citric acid and ascorbic acid.
Heat treatments and the application of sulfur dioxide or sulfites are common commercial methods for inactivating PPO. PPO activity can be inhibited by the addition of
Tyrosinase aCH2 +0
' Slow NH2
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