S74 Microbes and You 



for just a few minutes, and their ascospores are only slightly more 

 resistant. This is in contrast to bacterial spores, some of which 

 may be boiled for an hour without being inactivated. 



DISTRIBUTION IN NATURE 



Yeasts are probably not as abundant in nature as molds because 

 of differences in nutritional requirements. Yeasts may be found in 

 places where sugar is available, and even though the substrate 

 contains high concentrations of sugar, as in jam, jelly, and syrup, 

 the osmotic pressure does not materially affect the growth of these 

 cells. Soil is not the natural habitat of yeasts the way it is for 

 bacteria and for molds. Yeasts find their way into the ground 

 when they are washed or blown from the surface of fruits, partic- 

 ularly grapes. Insects might be considered to be reservoirs of 

 yeasts, and these arthropods are undoubtedly important in the dis- 

 semination of fungi of all kinds. Healthy persons may serve as 

 carriers of yeasts. The nectar of flowers and the exuding sap of 

 trees and plants may contain large numbers of yeasts which have; 

 been transported to these localities by the action of wind or by; 

 insects. --'f^ ■ ^^ 



ECONOMIC IMPORTANCE 



INDUSTIIAL APPLICATIONS 



Because of their high fermentative abilities members of the 

 genus Saccharomyces are employed extensively in the baking and 

 in the brewing industries, with carbon dioxide and alcohol the chief 

 end products of their action. The yeast cake that you buy at the 

 grocery store is composed of about 95% true yeast cells (Saccha- 

 romyces) and about 5% starch which serves as a binding agent to 

 help maintain the shape of the yeast cakes and the viability of 

 yeast cells. As the enzymes liberated by the yeasts attack the 

 fermentable constituents of dough, the carbon dioxide gas released 

 in the process makes the dough "rise." The yeast, therefore, serves 

 as a leavening agent. 



Yeasts are particularly rich in vitamin Bi (thiamine) and Bo 

 ( riboflavine ) , and by irradiating cells with ultra-violet light, they 

 become fortified with "sunshine vitamin" D. When the diet of 



