280 YEASTS AND YEAST-LiKE FUNGI 



It is a bottom yeast although it does not ferment melibiose. Certain 

 asporogenous yeasts, Cryptococcus kefyr and C. sphaerica are also 

 frequently found in fermented milk preparations. Another asporog- 

 enous lactose-fermenting species of yeast is well known. It forms 

 pseudomycelium and has been called Torula cremoris. Lactose- 

 fermenting yeasts have been found in souring figs also. Only a small 

 amount of alcohol (4 per cent or less) is ordinarily formed, but 

 carbon dioxide is formed in abundance. The resultant beverages 

 are effervescent rather than intoxicating. In cream they are fre- 

 quently the cause of considerable economic loss as "foamy cream.'' 

 It has been shown by Rugosa ^^ that lactose-fermenting yeasts and 

 yeast-like fungi require an exogenous source of nicotinic acid for 

 growth. One hundred fourteen strains of lactose fermenting yeasts 

 and related fungi were studied. Strains of Saccharomyces and 

 Cryptococcus, lactose non-fermenters, did not require nicotinic acid. 



Zygosaccharomyces. (See Fig. 111.) This is a large genus and 

 has been much studied since conjugation takes place regularly. It is 

 also of some economic importance. Stelling-Dekker did not recog- 

 nize the importance of the distinction between yeasts in which the 

 usual growth form is diploid from those in which it is haploid and 

 so she placed Zygosaccharomyces as a subgenus of Saccharomyces. 

 Morphologically they are much alike, except that spores of Saccharo- 

 myces are formed after proliferation of diploid yeast cells and Zygo- 

 saccharomyces forms spores immediately after conjugation of ordi- 

 nary haploid cells in most cases, although heterogamous conjugation 

 is sometimes seen. It has been suggested that Zygosaccharomyces 

 is merely a growth form of haploid Saccharomyces strains and that 

 the spores formed are non-viable, but this has by no means been 

 established. 



A considerable number of the species of Zygosaccharomyces are 

 able to withstand very high sugar concentrations and are therefore 

 frequently found as the causative agent in the spoilage of honey. 

 Lockhead and associates have made a careful study of honey yeasts. 

 Many species are unable to grow in a medium containing less than 

 32 per cent honey. They are osmophilic, and other sources of sugar 

 than honey, or salts may be used to produce a high osmotic pressure. 

 Osmophilic yeasts including species of Zygosaccharomyces and as- 

 porogenous yeasts were found in all 191 samples of honey examined 

 from all parts of Canada. Dilution, numbers of cells present, and 

 an activating substance in honey were factors in the development of 

 the yeasts. Similar organisms were isolated from the nectar of most 

 flowers examined, but they were not found in any of the soils exam- 

 ined save those about apiaries. 



