The effect of pretreatment and subsequent drying on the activity of grape oxidase

Abstract does not appear. First page follows.

Thompson Seedless grapes usually darken during drying in the preparation of raisins. The intensity of this darkening depends to a considerable extent on the treatment of the fresh fruit prior to drying. Raisins prepared by drying grapes in the sunlight without other treatment are characteristically of a dark color similar to the clove-brown of (Ridgway (1912)).5 Raisins produced by the cold-dip, mixed-dip, soda-dip (hot lye), sulfur-bleach, or golden-bleach procedures, on the other hand, usually have a light color ranging in hue from cinnamon-buff to sepia as judged by the color standards of Ridgway. (Hussein and Cruess (1940)) investigated the properties of grape oxidase and suggested that oxidizing enzymes are involved to a considerable extent in the darkening of grapes during the preparation of raisins, and to some extent in the darkening of wines. These authors, however, limited their investigations to the enzyme preparation obtained from fresh, untreated grapes. There is no available published information concerning the effects of the various treatments used in the production of light-colored raisins on the oxidizing enzymes occurring in grapes.

A series of experiments was conducted during the 1939 season in order to determine the effects of the mixed-, soda-, and cold-dip pretreatments, and of sulfuring and drying, on the oxidase activity of raisins made from Thompson Seedless grapes.

Experimental Procedure

Materials Used.—Thompson Seedless grapes were used in all experiments unless otherwise indicated.

The sulfur-bleach and cold-, mixed-, and soda-dip procedures employed were similar to those in commercial use as described by (Mrak and Long (1941)). The golden-bleach procedure was similar to that used in the preparation of sulfur-bleach raisins, except that the fruit was dried in a dehydrater having a relative humidity of 25 per cent at the hot end, and a dry-bulb temperature of 71.1° C (160° F).