VINTAGE WINES. FERMENTATION AND RESPIRATION 135 



of wine depends not only on the way it is made, but 

 on the kind of grape used, the climate and soil in which 

 the vines are grown, and the weather during the season 

 in which the grapes ripen. Thus not only are claret, 

 port, sherry and hock, for instance, totally different 

 wines, because they are made in different ways, but 

 particular vineyards in each wine-growing district 

 produce particular qualities of wine, and finally the 

 different years or " vintages " from the same vineyard 

 have different characteristics owing to the weather 

 during the year. Sometimes the characters so pro- 

 duced actually override the differences due to the 

 different vineyards, so that a connoisseur can recognise 

 a particular vintage. A fine wine is an immensely 

 complex solution, and the chemistry of many of the 

 organic substances it contains is still very imperfectly 

 understood. Slow chemical changes continue in the 

 wine while it is in cask and while it is in bottle. Up 

 to a certain point the wine constantly " improves," 

 and after that point deteriorates. 



In making sparkling wines the later stages of fer- 

 mentation are allowed to take place in the bottles, and 

 the carbon dioxide, which develops a considerable pres- 

 sure, is thus retained. Fermentation is, however, always 

 stopped before it is complete and the yeast removed. 



Relation of Fermentation to Respiration. The 

 alcoholic fermentation of sugar by the yeast plant, or 

 rather by the enzyme zyma.se which it produces, is, as 

 has been said, to be regarded as a modified kind of 

 respiration into which free oxygen does not enter 

 (anaerobic respiration). But yeast cannot go on fer- 

 menting sugar indefinitely. After a time it must have 

 free oxygen and respire aerobically. It is interesting 

 to note that ordinary (aerobic) respiration begins with 



