Experimental cookery

Oxidizing enzymes. The oxidizing enzymes are concerned with the processes of oxidation and reduction in the plant cells. They are the cause of some of the brown color changes in fruits and vegetables when they are bruised or pared. The principal enzymes that produce the color changes are the peroxidases, the oxidases (classed as laccases or phenolases by some authors), and tyrosinase. The internal browning of fruits when injured involves oxidation as a primary step. But the oxidation requires peroxide oxygen. Peroxidases will decompose hydrogen peroxide giving "active" (atomic) oxygen. They are practically always present in the cells of the higher plants. Onslow states that fruits and vegetables containing only peroxidases do not brown when injured. She reports the following of the oxidases. The oxidases are present in about 63 per cent of the higher plants. A plant oxidase is made up of three components : ( 1 ) An enzyme, termed oxy- genase, (2) an aromatic substance containing an ortho-dihydroxy grouping such as that in catechol, and (3) a peroxidase. \ Catechol OH OH There may be several 'substances with the catechol grouping, i.e., two hydroxyl groups in the ortho position, found in plants. If the substances with the catechol grouping are present in plant tissue, but enzymes are not present, browning of the tissue in injury takes place slowly, but with oxygenase and peroxidase oxidation occurs rapidly and the material turns brown on injury. Onslow has reported that apples, apricots, cherries, grapes, figs, mulberries, pears, plums, peaches, potatoes, and strawberries all con- tain oxidases. Bananas sometimes contain substances with the catechol grouping, and sometimes they are absent from the flesh but are found in the skin. Oranges, lemons, limes, and raspberries do not contain all three PREVENTION OF BROWNING 125 components of the oxidase system, and the following contain only a peroxi- dase: blackberries, pineapple, melon, and tomatoes. Zerban found that the polyphenols in cane juice may be oxidized by enzymes to a brown color, and to a less extent tyrosin may be oxidized by tyrosinase, giving a dark color. Discoloration in pared foods. Pared potatoes, apples and some other foods will turn dark unless cooked or put under water. The cooking de- stroys the enzymes, and putting under water prevents the oxygen of the air from coming in contact with the food. This darkening may be due to tannic substances which contain a catechol group or oxidases or both. It may also be due to the flavone and anthocyan pigments, as many of them show the black reaction typical of tannins since they have the same chemical linkage. Fresh peaches, pears, apricots, apples, bananas, etc., that are to be used for a salad and would lose sugar if kept in water may be pared six to eight hours before serving and kept from turning brown by being dipped in lemon or pineapple juice and put away in a covered fruit jar. Although the writer has used lemon and pineapple juice for years to prevent discoloration of freshly pared fruits, the reason for this, other than increased acidity, which did not entirely explain the results (pine- apple juice was less acid and more efficient than lemon juice), was not known until the work of Balls and Hale was published.