43 MICROBIOLOGY OF SPECIAL INDUSTRIES. 



or completely spoil the wine. Its beneficial effects are due primarily to 

 its action on microorganisms, on enzymes and on the color of the wine. 



In the small quantities properly used in wine-making, it is antiseptic in a degree 

 varying with the amount. All microorganisms are susceptible to its action in varying 

 degrees. Bacteria are particularly sensitive, molds and pseudo-yeasts less so, while wine 

 yeast is the most resistant of the ordinary forms found in must and wine. 



The result of the use of the proper amount of sulphurous acid in crushed grapes 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 suppression 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 produced during the burn- 

 ing 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 

 and progressive wine-makers are adopting more reliable sources. 



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

 can be obtained in the requisite purity in commerce containing 50 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. 



