2 BULLETIN* 335, U. S- DEPARTMENT OF AGEICULTURE. 



sition of grape products, and it would appear that the data secured 

 should have practical value to growers who wish to secure the highest 

 quality of fruit; therefore the analytical results are presented quite 

 fully. 



CHANGES IN THE CHEMICAL COMPOSITION OF GRAPES DURING THE 



RIPENING PERIOD. 



The ripeness of the grape should not be judged merely by its color, 

 for many varieties color long before they are fully ripe. Ripeness can 

 be properly determined only when both chemical composition and 

 physiological condition are considered. Many of the varieties of 

 grapes derived from native American species at present commercially 

 grown show a rather high acidity. In such grapes the qualities de- 

 sired are the highest content of sugar and the lowest content of acid 

 which can be secured without deterioration of the marketable condi- 

 tion of the crop. Some exceptions to this statement may be noted, 

 as, for instance, the Delaware, which, when fully ripe, shows an acid- 

 sugar ratio too wide for dry wine manufacture. 



In the ripening of grapes the increase of sugar and the elimination 

 of acid are dependent upon many factors, such as climatic and soil 

 conditions, the vigor of the plant, the site, pruning and training, and 

 species and variety characteristics. With certain reservations, sugar 

 should increase and acid diminish as long as the leaves function 

 properly. This, however, is not always the case, for as soon as the 

 pedicels — the small stems which carry the berries — begin to wither, 

 the fruit is gradually cut off from further influence of the growth 

 processes taking place in the plant, and its sugar content may remain 

 fairly constant for some time. It may appear to increase by reason 

 of evaporation of water from the berries, or in certain cases may 

 seem to be reduced by changes induced by the respiratory processes 

 of the fruit. The acid content is notably affected by respiration, 

 especially if malic acid is present in any quantity. 1 



The total sugar and acid content are not the sole factors of impor- 

 tance in determining the acceptability of the fruit for manufacturing 

 purposes, because the total acid-reacting substances are composed of 

 several substances, among which the malic acid is at times quite as 

 important as the tartaric acid. The content of malic acid is of 

 importance, because this acid is to a considerable extent eliminated 

 by oxidation through respiratory processes during the development 

 of the fruit. 2 As far as this process occurs it causes a positive lower- 

 ing of the acid content of the juice. In wine making whatever por- 

 tion of the malic acid remains free and goes into the young wine 



i Babo und Mach, Weinbau, 3d ed., 1909, ;.• 117, 157-159; Ibid, Kellerwirtschaft, 4th ed., 1910, 8: 12, 13, 

 31, 162. 

 2 Ibid. 



