SUGAR AND ACID IN GRAPES DURING RIPENING. 3 



can be largely changed by after-fermentation into lactic acid and 

 carbon dioxid, 1 thus still further lowering the acid content of the 

 final product. This latter change in the malic acid can not, how- 

 ever, occur in juices that are used in unfermented condition, as 

 bottled grape juices. 



The occurrence of malic acid in the grape is very well known, yet 

 it is difficult to determine the acid as such by the methods available 

 in this laboratory; hence the acid substances here discussed as malic 

 acid were not estimated by direct determination. These substances 

 are in fact the fixed acids other than tartaric, but it is known that of 

 these only malic acid 2 occurs in important quantities. 



The total acidity of the fruit is also very sensibly lowered some 

 seasons through the disappearance of free tartaric acid during ripen- 

 ing. It appears from our results that free tartaric acid in many 

 cases entirely disappears in the ripe fruit, especially from those 

 varieties with dark-colored juice. In some varieties with light- 

 colored juice, like Catawba, the disappearance of free tartaric acid 

 does not seem so complete, especially when the fruit ripens late. 



The disappearance of free tartaric acid 3 as the fruit matures is 

 primarily due to the influx, into the developing fruit, of potassium, 

 which combines with the free tartaric acid to form cream of tartar, 

 the insoluble acid tartrate of potassium. This compound is present 

 in all grape juices and its titratable acidity is but one-half that of 

 the free tartaric acid before combination with the potassium, hence 

 those conditions which encourage the formation of this acid salt 

 sensibly reduce the acidity of the juice of the fruit. Other bases, such 

 as lime and magnesia, also enter into combination with tartaric acid 

 to form the alkaline-earth tartrates; but these salts occur only in 

 slight quantities, and their presence does not materially affect the 

 total acidity from the manufacturing standpoint. The formation of 

 crystals of cream of tartar in the fruit, and the fact that these crystals 

 occur in the pressed residue of freshly-crushed grapes, has been de- 

 termined by the authors, 4 but these, phenomena require further study 

 before their effect upon total acidity can be stated. The precipitation 

 of this salt of tartaric acid, in whatever manner it may occur, has a 

 marked effect upon the final acidity of food products made from 

 grape juice. Thus a freshly-pressed grape juice or young wine will 

 ordinarily show a decided precipitation of tartar or argol within a 

 few months. 



After the disappearance of all free tartaric acid any further influx 

 of potassium or of other bases will result in a union with the malic 



1 Kiilisch, Weinverbesserung, 3d ed., 1909, p. 19. 



2 Babo und Mach, Kellerwirtschaft, 4th ed., 2: 12, 162-163. 



3 Babo und Mach, Weinbau, 3d ed., 1909, 1: (1) 159-161, tables; Ibid, Kellerwirtschaft, 4th ed., 1910, 

 2: 13-14; Koenig, Nahrungs und Genussmittel, 4th ed., 1903, p. 1174. 



* J. Agr. Res., 1913, /.- 513. 



