FERMENTATION OF SUGARS 131 



fungi, have all proved to be erroneous (106). As to the mode of propaga- 

 tion by budding, there is no reason why it should not have appeared 

 independently in several groups, and there are therefore no adequate 

 grounds for refusing the Saccharomycetes the position of a separate self- 

 contained phylum. 



The Saccharomycetes are mctatrophic and need the same food-stuffs 

 as many bacteria. As a source of nitrogen, peptone is by far the best, and 

 after it asparagine ; but ammonia salts suffice if nothing better is to be 

 had. Carbon is obtained from the fermentable material, sugar, which may 

 be present in quantities up to 35 per cent, (optimum, 2-4 per cent., or 

 20-25 P cr cent.). Where the object is not fermentation of the substratum, 

 but culture of the organisms, the sugar may be replaced by other sub- 

 stances, such as glycerine or mannite. In direct contrast to most bacteria, 

 the yeasts are not affected by an acid reaction of the culture medium, 

 but are arrested in their growth if free alkali be present. This peculiarity 

 is of great advantage in brewing and distillery, for by maintaining the 

 acidity of the liquid (wort) in the vats the development of many injurious 

 bacteria is prevented, whilst the yeast cells are unhindered in their fer- 

 mentative power (see p. 120). 



Of the carbohydrates only the monosaccharides with the formula 

 C c H li; O c (e.g. glucose, fructose, galactose) are directly fermentable (107). 

 The disaccharides (C 12 H 22 O U ), cane sugar, maltose and lactose must be in- 

 verted before they can be fermented ; that is to say, they must be changed 

 into monosaccharides (107) *. This is effected, as we have already seen, 

 by the hydrolytic action of enzymes excreted by the yeast cells themselves. 

 By an enzyme called invertase the beer and wine yeasts change cane sugar 

 into the so-called invert sugar, and by another, known as maltose^ they 

 change maltose into glucose. Neither of these yeasts can hydrolyze milk- 

 sugar, and therefore cannot ferment it. This is effected, however, by the 

 Saccharomycetes found in Kephir grains (p. 120) which secrete an enzyme 

 (lactase) that inverts the lactose in milk. Every species of yeast has its 

 own peculiar zymolytic properties. 



The polysaccharides other than sugars (cellulose, starch, dextrine, 

 gums, &c.) arc not attacked by the yeasts until they have been converted 

 into sugars by enzymes of other origin. In the preparation of beer, for 

 instance, the starch of the barley grains is converted into maltose by an 

 enzyme (diastase) contained in the grains themselves. 



Every fermentation gives rise, apart from the chief products, to 

 numerous other substances. The following table shows the quantities of 

 these by-products formed in the fermentation of i,coo gr. of grape-sugar. 



* Probably the various new synthetically-produced sugars will also be nrrangeable in accordance 

 with their structure and ability to be fermented. 



K 2 



