K.— BOTANY 20 1 



inscriptions and the manuscripts of Greek mythology. The yeasts 

 bringing about the fermentation of the grape sugar when the juice (must) 

 is pressed out are present on the skins of the fruit : they winter in the 

 soil. Different forms of yeast occur in different vineyards, though they 

 are usually of the Saccharomyces ellipsoideus type. The character of the 

 wine depends upon the kind of grape and the manner and period of 

 fermentation : red wines are formed when the colour from the skins 

 is extracted by the fermented liquor. Brandy or cognac is the alcoholic 

 distillate from wine. 



In fermentation processes it is common to find that a practice handed 

 down from antiquity was carried on in essentially the same way until 

 recent times, and then there has been some method of control. Naturally 

 in- so important an industry as wine-making — e.g. France devotes four 

 million acres to vineyards — -scientific methods have been widely adopted. 

 It is a little too haphazard to depend upon the naturally occurring yeasts 

 on the grape skin. Consequently the skins are sterilised either by Pasteur- 

 isation, or more commonly by the addition of a small amount of a dilute 

 solution of sulphurous acid or one of its salts, generally potassium meta- 

 bisulphite. Pure-culture yeast is added to the must as a ' starter.' 



As fermentation is carried out in the open it is obvious that other 

 yeasts enter the fermenting liquor. Mycoderma vini, 10 ' la fleur du vin,' is 

 active in bringing about the ageing of sherries kept on ullage, by inducing 

 oxidation changes and esterification. In some districts of France, 

 Botrytis cinerea is allowed to infect grapes which are to be used in 

 making wine of relatively high alcohol content (e.g. Sauterne), which 

 usually contains some unfermentable sugar. This ' noble mould ' 

 produces no objectionable odour or flavour ; its growth merely results 

 in considerable loss of water from the grape. 



The preparation of cider and perry is similar to that of wine. Formerly 

 the juice of apples or pears when pressed out of the pulp was allowed to 

 ferment with the yeasts occurring naturally on the surface of the fruits. 

 Modern manufacturies, however, use pure-cultures of appropriate yeasts, 

 which enable them to standardise their products in a manner not possible 

 if reliance is placed on the mixed natural population. 



Mead is sometimes regarded as the oldest beverage of the human 

 race, for it was probably brewed from the washings of emptied honey- 

 combs before crops were cultivated. It is still made in English farm- 

 houses, and sold to a small extent on the Continent. Water is added to 

 the honey and well mixed and sterilised by boiling. As the liquid cools 

 flavouring is added, and it is then fermented with brewers' yeast. 



The general routine of beer-brewing is well known. Brewers' yeast is 

 Saccharomyces cerevisice. Many strains of this species are known ; they 

 are generally classified as top or high yeasts, and bottom or low yeasts. 

 Brewery yeast must generate certain substances possessing a characteristic 

 aromatic taste or odour. It must also readily separate from the fluid, 

 leaving a clear, bright liquid. This species has been studied more 

 thoroughly than any other fungus. Many breweries have long had their 



10 Not to be confused with Mycoderma aceti = A cetobacter aceti, the vinegar 

 plant. 



H 2 



