44 PHYSIOLOGY OF NUTRITION 



Beer- wort is the best nutrient medium for the culture of yeast. 1 Other 

 liquids are used, however, among which may be mentioned Pasteur's solution 

 as given above, grape juice, the juice of various other fruits and berries, and other 

 materials containing sugar. Hansen has carried out very exhaustive studies 

 upon yeasts and has established, among others, the following important species. 2 



Saccharomyces cerevisia I. Hansen. An English top -fermentation yeast, 

 which produces, in beer-wort at room temperature, from 4 to 6 per cent, of 

 alcohol. In the resting condition the plant consists of single cells, which begin 

 to multiply by budding when placed in beer-wort. The young generation con- 

 sists of large spherical or oval cells (Fig. 21). After the temination of the 

 primary fermentation a scum appears on the surface of the fermenting liquid 

 and on this a continuous membrane of yeast-cells is formed. The general 

 appearance of these cells is different from that of the sedimentary forms; much 

 elongated cells are found here (Fig. 22). In the surface membrane of old cul- 

 tures .occur very much elongated cells that are entirely unlike the young sedi- 

 ment cells from which they have developed (Fig. 23). This film formation 



Fig. 21. — Saccharomyces cerevisice I. Fig. 22. — Saccharomyces cerevisia I. Sur- 

 Young cells from the sediment of the beer- face film at i5-i6°C. {After E. Hansen.) 



vat. {After E. Hansen.) 



furnishes a striking example of the great variability in form, that is 

 characteristic of yeast cells. 



In order to obtain ascospores young cultures must be used, and it is 

 also essential that air be plentifully supplied. Little plaster of Paris disks 

 prepared with special moulds are used for this purpose. These are placed 

 in small, shallow glass pans (Petri dishes), covered with similar pans of slightly 

 greater diameter, and then sterilized. A few drops from a day-old culture of 

 yeast cells are placed upon one of these plaster disks. Sterilized water is 

 poured into a dish around the disk, to keep the latter constantly moist. After 

 some time the ascospores are formed. Temperature exerts a pronounced in- 

 fluence upon their formation. With the same temperature, ascospores of 

 different species develop at different rates, and this fact is made use of in indenti- 



1 Jörgensen, Alfred P. C, Die Mikroorganismen der Gärungsindustrie. 4te Aufl. Berlin, 1898. Idem, 

 Microorganisms and fermentation. Philadelphia, 19 n. Lindner, Paul, Mikroskopische Betriebskontrolle 

 in den Gärungsgewerben. 2te Aufl., Berlin, 1898. (ste Aufl., Berlin, 1909.) [Hansen, Emil Chr., Prac- 

 tical studies in fermentation. Transl. by Alex. K. Miller. 227 p. London and New York, 1896. — See 

 also the references on brewing, etc., given on p. 181.] 



The Carlsberg Laboratory in Copenhagen is especially interested in the study of fermentation organisms. 

 It publishes a journal devoted to this study, entitled " Meddeledser fra Carlsberg Laboratories" 



? More information upon top and bottom fermentation will be found in Chapter VIII of this Part. 



