~226 MICRO-ORGANISMS AND FERMENTATION. 



paragraphs a review is given of the special features which 

 yeasts present in this respect. 



The inorganic substances phosphorus, potassium, mag- 

 nesium, and sulphur have been enumerated by A. Mayer as 

 indispensable for the nutrition of yeasts. His statement is 

 based on the results of his analyses of yeast (not pure cultures) 

 and of his nutritive experiments. As is well known, yeast 

 contains considerable quantities of phosphoric acid. In 

 Munich brewery yeast, the proportion of phosphoric acid 

 represents about 3| per cent, of the dry residue. If malt- 

 wort or other nutrient medium contains too small a quantity 

 of phosphoric acid, this defect may be remedied by adding 

 . potassium phosphate. Potassium is also an essential food 

 element for yeast. It is readily absorbed, both in the form 

 of phosphate and sulphate. Magnesium is an element of 

 equal importance. Sulphur can always be detected in yeast, 

 and must, therefore, be regarded as essential to its metabolism. 

 Calcium, on the contrary, does not appear to be necessary for 

 the propagation of yeast. This element plays an important 

 part, however, in the fermentation process, for it has been 

 shown by Seyffert that brewery yeast quickly degenerates 

 in a wort poor in lime. According to recent work carried out 

 by Delbriick, Lange, Henneberg, Hayduck, Seyftert, and 

 others, calcium carbonate is of importance by rendering 

 certain poisonous substances innocuous which are present in 

 the raw materials, and are believed to be of an albuminoid 

 character (see Chap. i.). In the preparation of artificial 

 nutrient solutions for yeast, these substances should be added 

 in the form of salts, the total quantity not exceeding about 

 1 gramme per litre.* 



* Ad. Mayer made use of 



Acid potassium phosphate, KH.,P0 4 , 

 Crystallised magnesium sulphate, MgS0 4 , 7HoO, 

 Tribasic calcium phosphate, Ca 3 P.,O s , 

 beginning with greatly reduced quantities. 



Laurent made use of a solution containing per litre 

 0*75 gramme potassium phosphate. 

 5'0 grammes ammonium phosphate or sulphate. 

 O'l gramme magnesium sulphate. 

 I'O tartaric acid. 



