Abstract
The surface wax (bloom) of the grape berry occurs in amounts of about 0.1 mg/cm2 and is chemically a mixture of long-chain alcohols, esters of such alcohols and fatty acids, free fatty acids, long-chain aldehydes, and hydrocarbons. The chain length of these compounds is 12 to 35, with a maximum in the range of 24 to 28. The main constituent of grape wax is oleanolic acid, which is absent from the cuticular wax of grapevine leaves. The concentration of surface wax on leaves is about 1/10th that on grape berries.
The cuticular wax is the main barrier to the evaporation of water, and careful removal of the surface wax, by vapor of solvents, is almost as effective in promoting evaporation as peeling the berry. The effect of wax and its components in hindering evaporation can be simulated in model systems, i.e., wax-coated plastic membranes. In this system, aldehydes, alcohols, and paraffins were found to be equally effective in inhibiting evaporation. Fatty acids show some activity, whereas oleanolic acid is ineffective when applied in amounts similar to those on the cuticle.
The drying rate of grapes can be increased by chemical treatments. Dipping of grapes in hot caustic soda solution removes part of the wax and probably destroys the cuticular structure irreversibly. However, in the cold-dipping process suitable "oils" are reversibly added to the grape surface, whereby the drying rate is increased 2-3 times. The cold-dipping process also has a secondary effect. Grapes that are naturally dried usually darken in color, whereas dipped sultanas stay yellow-green. This is due to inhibition of the action of the polyphenol oxidases by quick drying.
In addition to the naturally occurring lipids, during the processing of dried grapes, some lipids are added to prevent stickiness. These added lipids can be detected and determined by chromatographic methods.
- Accepted April 1971.
- Published online January 1971
- Copyright 1971 by the American Society for Enology and Viticulture
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