Enzymes control many of the complex chemical processes of plant metabolism. They accelerate reactions which would otherwise take place very slowly, and though they may initiate the reaction, do not form part of its final product. Reactions of several enzymes have been mentioned. A large number of the enzymes are hydrolytic, including the proteinases, lipases, amylases, and others; but there are also oxidizing and reducing and other groups of enzymes. Some of the plant enzymes that digest proteins are of slight interest in cookery. An enzyme of the pitcher-plant hydrolyzes fibrin. Bromelin of pineapple acts on native proteins, its effect being more often noticed in cookery when uncooked pineapple is added to gelatin, which is liquefied. Papain of the papaw leaf acts on native proteins. Some experiments were tried at the Office of Home Economics to utilize the papaw leaf, the dried powdered leaf, or the extracted enzyme to render tough meat tender. These did not indicate that papain could be successfully used in this way, as the enzyme acted on the surface, powdering or pulverizing it in only a thin layer even after being in contact with the meat for several hours, the interior of the meat not being affected.

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 oxygenase, (2) an aromatic substance containing an ortho-dihydroxy grouping such as that in catechol, and (3) a peroxidase.

Catechol

Catechol

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 contain 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 components of the oxidase system, and the following contain only a peroxidase: 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 destroys 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 (pineapple juice was less acid and more efficient than lemon juice), was not known until the work of Balls and Hale was published.

Cruess, Mark, and Quinn state that the oxidase of peaches is not all destroyed by blanching at 120° to 160°F., thus causing internal browning with the formation of crescent shaped areas often seen in sliced peaches. To destroy the action of the oxidase of large peach halves requires heating for 10 minutes at 180° to 200°F. These investigators state that fruit acids such as citric and tartaric are not so effective as hydrochloric acid in retarding browning. Oxidation was completely held in check by 0.25 per cent of hydrochloric acid, so that this concentration on the surface of the peach would prevent browning. Salt and other chlorides will check browning temporarily; but, after standing in a 2-per cent salt brine, the fruit must be rinsed to remove the flavor of salt.

Prevention of browning by reducing substances. Joslyn and Marsh state that the primary oxidation step may be prevented by removal of oxygen or the addition of reducing substances. And without oxidation browning does not occur. Sulfites and stannous salts are good reducing substances. Sulfur dioxide has been used for many years to prevent discoloration. They state that orange juice does not brown at high or low temperatures when stored in tin cans. The reason given is absence of oxygen and the reducing action of stannous salts. The addition of ferrous salts in concentrations of 25 parts per million to the orange juice increased browning. Ferrous salts were more effective than ferric salts. Nickel, copper, or stannic salts were without effect, but stannous salts and sulfites protected against browning. However, after these salts were oxidized, then browning of the juice could occur. The addition of analine or tryptophane caused immediate browning.

Balls and Hale found that sulfhydryl compounds, some of which occur in common foods, prevent browning of apples. Gluthathione and cystein prevent darkening of the apples even when applied in very dilute concentration. Pineapple juice contains a sulfhydryl compound which is the cause for its inhibition of development of brown color.

Oxidation and color loss. A type of oxidation that causes fading of red fruits in canning is explained by Kohman and Sandborn. All fruits and vegetables use oxygen in respiration, and some oxygen is found within the fruit or vegetable. The color of strawberries is deadened if they are heated very rapidly after refrigeration, during which the respiratory processes have been inhibited. However, if they are heated slowly so that the interior oxygen is used in respiration the color remains bright.