556 RESPIRATION AND FERMENTATION 



results show that cane-sugar cannot be inverted by the protoplasm without 

 the intervention of an enzyme (Sect. 91). Hence it may be concluded that 

 reserve and plastic carbohydrate materials are utilized for fermentation only 

 in so far as they can be converted into monosaccharides. 



The fcrmentability of different hexoses varies widely : thus yeasts 

 ferment dextrose and laevulose, but not their optical counterparts, although 

 it is uncertain whether or not the latter may be used as food by the aid 

 of oxygen-respiration. It is, however, well known that two stereo-isomeric 

 substances may have a widely different nutritive value (Sect. 66), and of 

 two structurally different compounds, such as dextro-rotatory glucose (an 

 aldose) and dextro-rotatory fructose (a ketose), certain yeasts preferably 

 ferment the former, others the latter. Bacillus bittylicus is, however, able to 

 carry on alcoholic fermentation when supplied with glycerine 1 , and there is 

 no reason to suppose that the glycerine is previously converted into sugar. 



Nageli and other authors 2 have shown that the fermentative activity 

 of normally cultivated yeast is not suppressed even in the presence of an 

 abundance of oxygen, but on the other hand the same amount of alcohol is 

 produced as before. In Giltay and Aberson's experiments, however, about 

 21 per cent, of the sugar was consumed in oxygen-respiration, and hence 

 relatively less alcohol was produced in relation to the amount of sugar 

 consumed, while since aeration caused more rapid multiplication it is evident 

 that the fermentative activity of the individual yeast-cells must have been 

 correspondingly reduced. Hence in the absence of oxygen the fermentation 

 of sugar is more perfect and economical, while in its presence the aid of 

 oxygen-respiration enables the plant to employ the sugar consumed to 

 better purpose in growth and reproduction. In the absence of oxygen, 

 yeast remains living and capable of exciting fermentation for a long time, 

 but it is impossible to say whether under prolonged periods of experimenta- 

 tion the conjoint maximum of growth and of fermentative activity would 

 occur with a continuous deficiency of oxygen or when air was only 

 intermittently supplied. 



Chudiakow found that in the presence of peptone or malt-extract aeration 

 does not diminish the fermentative activity of yeast, whereas if ammonium salts 

 provide the sole source of nitrogen the evolution of carbon dioxide gradually 



E. Fischer, Ber. d. Chem. Gcs., 1895, pp. 984, 3024, 3034, and Beyerinck, Ceutralbl. f. Bact., 1895, 

 Abth. ii, Bd. I, p. 224. 



1 Buchner, Zeitschr. f. physiol. Chem., 1885, Bd. ix, p. 393. 



2 Nageli, Theorie d. Gahrung, 1879, p. 17; Giltay u. Aberson, Jahrb. f. wiss. Bot., 1894, 

 Bd. xxvi, p. 543; Chudiakow, Landw. Jahrb., 1894, Bd. XXIir, p. 428; Iwanowsky, Jahresb. d. 

 Gahrungsorg., 1894, p. 116. In these works the other researches are given. It is not necessary to 

 discuss the correct usage of the term ' fermentative power.' Cf. Giltay u. Aberson, 1. c., and Jahrb. 

 f. wiss. Bot, 1896, Bd. xxx, p. 71 ; A. Brown, Centralbl. f. Bact., 1897, Bd. in, p. 33. None of the 

 above designations are clearly defined constants. 



