YEAST. 289 



''must" in the preparation of wine. The methods of pure culture 

 are essentially the same as for bacteria. A nutrient gelatine, pre- 

 pared by dissolving from five to six parts gelatine in " wort " of 

 beer (i.e., infusion of malt), which has been " hopped," is recom- 

 mended as a solid transparent medium for culture. We prepare a 

 cover-glass with the nutrient gelatine, inoculate with the yeast, 

 invert it upon a moist chamber, and determine by direct micro- 

 scopical observation whether the colonies developing in the culture 

 material are actually derived from single cells. In contradistinc- 

 tion to bacteria, no conclusion as to the nature of the yeast can be 

 drawn from the habits or appearances of the flecks, since different 

 species can have the same appearance, and different colonies from 

 the same species can have differing appearances. From cultures 

 controlled under the microscope yeast cells can be removed with 

 the looped platinum wire, previously heated red-hot, and with them 

 flasks containing sterilised wort, or other selected nutrient solution, 

 can be inoculated. For laboratory purposes yeast can be grown, 

 either in bulk or in moist chamber cultures, in Pasteur's fluid. In 

 slide cultures very beautifully- branched chains of cells can be 

 obtained. The simplest' way of preparing Pasteur's fluid is to keep 

 dry in a bottle, very finely powdered and most thoroughly mixed, 

 10 gram, potassium phosphate, 1 calcium phosphate, 1 magnesium 

 sulphate, and 50 ammonium tartrate, or larger or smaller quantities, 

 so long as proportional. For use dissolve in the proportion 1 

 gram, of this mixture with 12 of sugar in 70 cc of water. 



The Saccharomycetes generally hibernate in the soil, but the 

 yeast of beer is only known in a state of cultivation. By suitable 

 cultures they can be led to produce spores, a number of which 

 appear in each cell. This is easily effected when sterilised plates 

 of plaster of Paris, placed in a flat vessel so as to be half -immersed 

 in water, and covered over with a bell- jar, have strained or filtered 

 yeast smeared upon them. Kept at about 30 C., spore formation 

 in Saccharomyces cerevisece may follow even in about twenty-four 

 hours, but more usually in from four to eight days. Pieces of 

 fireclay have also been recommended for the culture of yeast- 

 spores. They have the advantage that they can be cleaned with 

 a brush, sterilised in a dry chamber at 150 C., and are then 

 again ready for use. 



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