Chapter III 

 Yeasts and Moulds 



UNLIKE the bacteria, the yeasts and moulds prefer acid nutrient 

 media, their natural habitat being soft, juicy fruits. They also 

 thrive in sour milk and dairy products, in which they can produce 

 a variety of changes owing to the number of enzymes which they 

 secrete. They generally oxidise lactic acid, and thus render the 

 medium better adapted for the nutrition of bacteria ; it is only 

 with the co-operation of yeasts and moulds that putrefactive 

 fermentation can become established in sour milk. Yeasts and 

 moulds play an important part in the ripening of many sour milk 

 and soft rennet cheeses. As most moulds hydrolyse fats, they 

 contribute largely towards the development of rancidity in butter. 

 Most yeasts and moulds can grow at comparatively low tempera- 

 tures. While bacteria only develop in media containing at least 

 20 to 30 per cent, of water, moulds require only 14 per cent., and 

 will therefore grow on comparatively dry feeding stuffs, whence 

 they find their way with the stable dust into milk. 



A. YEASTS 



The best-known property of these organisms is the power to 

 ferment sugars with the production of alcohol and carbon dioxide. 

 They find technical application in the manufacture of beer, wine 

 and spirits, while pressed yeast is used in baking to raise the 

 dough. The sporing and the non-sporing forms are known as 

 Saccharomycetes and Torulce, respectively. The non-sporing 

 Mycodermce occupy a special position ; like the moulds, they only 

 produce alcohol quite exceptionally, but they are able to oxidise 

 this substance. 



1. Saccharomycetes. These are the most typical of the alcohol- 

 producing yeasts (Figs. 2, 4 and 7). As the form of the cells is 

 not very characteristic, and subject to considerable variation, the 

 various species are best distinguished by the scheme of Emil 

 Christian Hansen, which is based on the investigation of the spore 

 formation and observation of the time which elapses before spores 

 are formed under certain conditions. Moist plaster of Paris 

 blocks are particularly well adapted for this purpose. The pure 

 cultures used in breweries are generally round or oval yeasts, which 



