234 CULTIVATION AND REPRODUCTION OF YEAST. 



Since the discovery that alcoholic fermentation by yeast is a 

 purely enzymatic action (see chapter Ixiii.), and not directly and 

 inseparably connected with the life of the cell, it has become 

 necessary to bear in mind the two-fold character of the sources 

 from which the liberation of carbon dioxide may proceed, and 

 therefore to consider respiration separately from fermentation. 

 Strictly speaking, the only way in which the influencing of 

 respiration can be reliably tested is by employing conditions in 

 which the nature of the yeast and medium precludes the possi- 

 bility of any alcoholic fermentation a stipulation which at once 

 disqualifies a whole series of investigations on the " carbon 

 balance" of the fermentation process, the stimulation of the 

 latter by aeration, &c. Little is known as to the intermediate 

 stages of the combustion of the substances undergoing respiration. 

 Saccharomyces Hansenii isolated by W. ZOPFF (XIII.) from 

 American cotton-seed meal a yeast incapable of inciting 

 alcoholic fermentation forms large quantities of oxalic acid as 

 the final oxidation product (instead of carbon dioxide) of the 

 sugars (glucose, galactose, saccharose, maltose, lactose, mannitol, 

 dulcitol, and glycerol) added to the nutrient medium. Several 

 workers, including E. PRIOR (II.) have found that copious aeration 

 increases the quantity of acids formed in yeast cultures ; but it 

 is not yet certain how far these are to be regarded as products of 

 the purely chemical action of oxygen on the constituents of the 

 medium or as the result of hyperstimulation of yeast metabolism. 

 According to the comparative experiments of G. KORFF (I.) with 

 Saaz, Frohberg and Logos yeasts, the aerated cultures contain 

 a higher proportion of fixed acids, whilst those traversed by a 

 current of hydrogen furnish a higher yield of volatile acids. In 

 this connection we may recall the remarks made on p. 126, vol. ii. 

 with reference to the divergent chemico-physiological behaviour 

 of the cells of sedimental and film yeasts. The nature of the 

 influence of external conditions on the coefficient of respiration 

 of yeasts (see p. 79, vol. ii.) has not yet been sufficiently investi- 

 gated. Under favourable circumstances the amount of heat 

 liberated by respiration may give rise to a considerable increase in 

 temperature, and in a case observed by EFFRONT (VIII.), in 

 which 2 kilos, of pressed yeast were crumbled down and exposed 

 to the air (20 C.) as a layer 37 cm. in depth, this amounted to 

 36 0. in three hours. 



The utility of aerating the nutrient medium as regards the 

 development of the yeast to be grown therein has long been 

 recognised in practice. During the early stages of rousing (aera- 

 tion), whilst it is still very hot, the wort absorbs oxygen freely, 

 fixing it by chemical combination and retaining it partly in the 

 form of carbon dioxide. Afterwards, when the temperature has 

 fallen, a considerable amount of the gas is also retained physically 

 (in solution). The quantities were found by PASTEUR (III.), in 



