FRUIT Darkening Can be Prevented by New Process

The tendency of many fruits and vegetables to darken at freshly cut surfaces is well known. Slices of apple, for example, by the time they have been left in the air long enough to dry, are usually a deep brown. This is a serious loss to the fruit drier, because such dark-colored products are not received well on the market. It is impossible to prepare from them an article of food which even remotely resembles the original fruit in respect to color.

   Only one method has ever been applied which satisfactorily prevented this discoloration of the cut fruits while they were being dried.  This consists of treating the freshly-cut fruit with sulphur dioxide (the gas evolved from burning sulphur). The fruit dried after sulphuring has a good color, but retains considerable amounts of the gas. Its export to foreign countries is restricted, since the food laws of many European nations do not permit food to be sold which contains more than a very small amount of sulphur dioxide.

Research was begun recently by the Bureau of Chemistry and Soils with the object of finding a satisfactory method of preventing the discoloration of the cut fruit (while it was kept or being dried) which could replace the sulphuring process. The investigation started with a study of the enzyme reactions that caused the darkening of cut fruits. It was successful in producing these reactions in the test tube where they could be investigated very thoroughly. As a result it appeared that several classes of substances ought to possess the property of inhibiting the discoloration. Of these, the great majority were poisonous, but one class seems to be harmless, because it occurs in many foods. This is the class of sulphydryl-containing amino acids and peptides, typified by glutathione and cysteine.

Experiments with these substances were made on apples of a variety which darkens rapidly when cut (Paragon). The results showed that only very small amounts of these substances were necessary to completely inhibit the darkening of the apples.

The application was simple; the sliced apples were sprayed with a very dilute solution (0.1 to 0.25 percent) of the chemical and then placed in a drier where they were handled as in an apple-drying plant.

In technology these chemicals are as yet rare, although if there were any great demand for them they could probably be made cheaply enough. The investigators knew, however, that a substance related to those with which they had experimented is often found in pineapple juice. The next step was therefore to spray the fruit with pineapple juice which contained this substance. The effect of the pineapple juice was weaker than that of the chemicals, but the result was quite satisfactory.

Fruit dried after spraying with pineapple juice is, of course, covered with a thin film of dry residue from the juice. This does not seem objectionable but it may be avoided by first fermenting the juice, removing the yeast and alcohol, and using the greatly purified liquid in the spraying process.

Another application of this finding is that cut-up fruit, such as apples, apricots, bananas (if they are not too ripe), peaches and probably many others can be stored in the cold for as long as 24 hours without turning dark if they are immersed in pineapple juice or if a small amount of one of the chemicals mentioned is added to the juice which covers them. In the event that the pineapple juice is not already acid a little lemon juice should be added to it, since the darkening is more easily prevented in acid solutions.