Grape juice composition

The complexity of wine composition is a central reason for the vast variety of wines in the marketplace. In addition to water and ethanol, the major components, a variety of organic acids as well as metal ions from minerals in the skin of the grape are present. Initially, all of these substances remain dissolved in the bottled grape juice. As the fermentation process occurs, the increasing alcohol concentration in the wine alters the solubility of particular combinations of acid and metal ions. Unable to remain in solution, the insoluble substances settle as crystals. Since the process of red-wine making involves extended contact of the grape juice with the skins of the grapes (where the minerals are concentrated), wine crystals are more common in red wines than in white wines. 

Spanos, G.A. and Wrolstad, R.E., Influence of processing and storage on the phenolic composition of Thompson seedless grape juice, J. Agric. Food Chem., 38, 1565, 1990. 

Early studies of Concord and other native grape juices (7, 8) found soluble solids content of these juices in the 12°-20° Brix range and found them to be variable with location and season. The chemical composition of New York State grapes and juices was studied in 1938-1939 ( 9). Brix of Concord juice from ripe fruit from different areas within the state ranged... 

All amino acids, except proline, may be used by Saccharomyces cerevisiae in grape juice fermentation. Amino acids can be directly used to synthesize proteins. However, the amino acid composition of the grape juice is not necessarily similar to the needs of the cell. For that reason, yeasts must use the remaining amino acids to synthesize those which it lacks (Hensche and Jiranek 1993 Ribereau-Gayon et al. 2000b). In this case, ammonia is incorporated into other amino acids whereas the carbon skeleton is metabolized by the cell. 

Finally, the relationship between the amino acid composition of grapes and the final aromatic composition of wine has been recently described (Hernandez-Orte et al. 2002, 2006). Therefore, it is possible that in the near future grape juice will be complemented with specific mixtures of amino acids in order to improve the aromatic quality of wine. 

Houtman, A. C., Morais, J., Du Plessis, C. S. (1980) Factors affecting the reproducibflity of fermentation of grape juice and of the aroma composition of wines. I Grape maturity, sugar, inoculation concentration, aeration, juice turbidity and ergosterol. Vitis, 19, 37-54. 

TABLE 2 The phenolic composition of grape juice (Amerine et al., 1980)... 

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