6l6 MICROBIOLOGY OF ALCOHOLIC FERMENTATION 



and must before fermentation is the almost complete suppression of bacterial action, 

 the discouragement of molds and pseudo-yeasts and the promotion of the growth 

 of wine yeast which is given a clear field unhindered by the deleterious excretions 

 of competitors. 



Its action as regards enzymes is hardly less important. It would be impossible to 

 make the finest wines of Sauternes and the Rheingau without its use on account of 

 the oxidase produced by the Botrytis cinerea which is abundant and necessary on 

 the best grapes of these regions. In other regions where this mold and others 

 occasionally occur its use is also necessary. In hot climates it is especially useful, 

 not only because bacterial action is more intense in such regions but because of its 

 action in preserving the natural fixed acids of the grape, which are, there, nearly 

 always deficient. This preservation, according to Wortmann, is due to the sup- 

 pression of acid-consuming bacteria, but experiments of Astruc tend to show that 

 the prevention of the action of unknown acid-destroying enzymes is in part the 

 cause. 



Its action on the color of wines is also of importance. By the action of oxygen, 

 the color of red wine is gradually made insoluble and precipitated, and the greenish 

 or golden color of white wine is turned to brown. Both these actions are prevented 

 or much diminished by the use of minute quantities of sulphurous acid. 



The most commonly used source of sulphurous acid is the fumes of burning 

 sulphur. Sulphur is burned in a cask and the must caused to take up the fumes by 

 being pumped into the cask through the upper bung hole. It is almost impracticable 

 to apply sulphurous acid from this source to crushed grapes for red wine. 



The method is defective in many ways. It is impossible to tell within very wide 

 limits how much sulphur dioxide has been absorbed by the wine. Moreover, the 

 sulphur burns incompletely and the volatilized sulphur acted upon by the yeast 

 may produce sulphuretted hydrogen. Other sulphur compounds are also pro- 

 duced during the burning, to some of which the so-called sulphur taste of wine is 

 said to be due. Several devices have been invented to decrease these defects but 

 none remove them completely; accordingly progressive wine-makers are adopting 

 more reliable sources. 



An improvement is the use of potassium metabisulphite (K 2 S 2 O5) a salt which 

 can be obtained in the requisite purity in commerce containing 50 to 55 per cent by 

 weight of sulphur dioxide. The amount of potash added by this salt in the doses 

 used, is very small, and far within the limits of variation between different wines. 

 By the use of this salt, exact amounts of sulphur dioxide can be applied both to 

 white and red wines. Other sulphites are not permissible. 



The best source of the acid, recently brought into limited use, is the liquefied gas, 

 which can be manufactured comparatively cheaply in great purity. By its use all 

 the^benefits of sulphurous acid are obtained and the defects eliminated. 



Some grapes, owing to their composition, especially their high 

 acidity, are very resistant to the attacks of injurious bacteria. Others, 

 owing to their low acidity or highly nitrogenous nature, are very 

 susceptible. The addition of tartaric or citric acid to the latter has 



