2IO METABOLISM 



A large number of yeasts are able to hydrolyse not only cane sugar but 

 maltose also, and to induce fermentation in the resulting products {= two 

 molecules of glucose). Others (5. marxianus, ludwigii, exiguus) attack cane 

 sugar only, others still (5. apiculatus, Schizosaccharomyces octosporus) maltose 

 only. Maltase must, therefore, be looked on as a distinct enzyme from 

 saccharase. Its recognition was rendered all the more difficult because it was 

 at first to be obtained only from the dead cells by drying, since it was obviously 

 unable to penetrate the living protoplasm. A third disaccharide, lactose (milk- 

 sugar), is broken up by yet other yeasts, which together can also hydrolyse 

 maltose and saccharose. Similarly a fourth natural disaccharide, trehalose, is 

 acted upon by certain other yeasts. 



We need not discuss the artificial disaccharides nor the fermentable tri- 

 saccharide, raffinose, since they add nothing essentially new to the facts already 

 known. Finally, as to the polysaccharides, e. g. starch, we already know that 

 they also are transformed into sugar by an enzyme, diastase ; in general, however, 

 organisms which can produce alcoholic fermentation are unable to use starch, 

 although there are certain Fungi, members of the Mucorineae — though certainly 

 not Saccharomycetes— such as, for example, Mucor alter nans, Amylomyces 

 rouxii, which can do so, and these during the last few years have been 

 employed technically for the purpose. 



There is another question which we need only glance at in passing, and 

 that is whether the possession of a definite enzyme and the power of producing 

 fermentation are constant characters of the organism or whether they may be 

 induced to appear in it by culture methods and change of habit. The latter 

 view is held by French investigators, and although, according to the results of 

 the work carried out in the Carlsberg Laboratories, these results do not appear 

 to be entirely reliable (Klocker, 1901), still it must be said that critical evidence 

 against such a possibility does not exist, since experience has taught us that 

 micro-organisms may be easily affected by culture methods, acquiring or losing 

 certain characteristics according to circumstances. 



As already noted, sugar is broken down into carbon-dioxide and alcohol 

 during alcoholic fermentation, both of which appear in about equal amounts ; 

 thus 100 parts of cane sugar in one of Pasteur's experiments, giving by hydro- 

 lysis 105-26 g. of invert sugar, gave rise to 51-0 g. of alcohol and 49-1 g. of 

 carbon-dioxide ; the remainder, about 5 g., went to the nourishment of the yeast 

 and the formation of certain secondary products. The proportional relations 

 of carbon-dioxide and sugar is about what one would expect if a molecule of 

 cane sugar, on taking up water, broke down into four molecules of carbon- 

 dioxide and four of alcohol : — 



CijHjAi+HjO = 4C,H60 + 4C03 

 100 g. + 5.3 g. = 53-8 g. + 51-5 &• 



It is quite possible that the process of fermentation consists in a decom- 

 position of sugar of this nature. As a matter of fact, however, there are always 

 secondary products formed which, in the older analyses at least, must have 

 been due to the presence of foreign organisms in the fermenting fluid ; still some 

 of these products, especially glycerine and succinic acid, appear during the action 

 of perfectly pure yeast. The amounts of these substances vary, in the first place, 

 according to the species of yeast employed, and in the second, according to 

 the nature of the food, not only of the fermentable carbohydrate but also of 

 the nitrogenous constituent. Laborde (1899) found that 2-5-775 g. of gly- 

 cerine were formed from 100 g. of fermenting sugar as the result of the action 

 of different yeasts, and Wortmann (1892) also showed that very variable 

 amounts of glycerine were produced by different wine yeasts. Succinic acid 

 comes next to glycerine in amount and reaches about 0-5 g. per 100 g. of sugar ; 

 the amount is also liable to considerable variation. Since both these secondary 

 products are formed by yeast under conditions which exclude the formation of 



