EXTRACTS OF PURE DRY YEAST 



91 



To the extract from 1 per cent of dry yeast, 1 per cent peptone 

 was added, the reaction adjusted to pH 7.5, and the medium then 

 heated and filtered, tubed and sterilized. 



Most of the streptococci grew readily in this medium and some, 

 vigorously. It was observed, however, that the hydrogen ion 

 concentration changed from pH 7.5 to from pH 6.0 to 6.3 and it 

 was evident that the dried fresh yeast contained some fermentable 

 material. 



This preparation of dry fresh yeast could not be used for fer- 

 mentation tests because of the change in hydrogen ion concen- 

 tration which occurred without the presence of a test sugar or 

 other similar substance. The yeast peptone medium described 

 above is lightly buffered for this is necessary in studying fer- 



TABLE 1 



One per cent extract of dry fresh 

 yeast* 



TOTAL 

 NITROGEN 



per cent 



0.0209 



AMINO 



NITROGEN 



per cent 



0.0040 



PROTEIN 

 OTHER 

 THAN 

 AMINO 

 ACIDS 



per cent 



0.1078 



REDUCING 

 SUGAR 



per cent 



0.008 



pH 



6.3 



* Moisture in dry yeast 5.9 per cent. 



mentations, at least with the streptococci; and this is probably 

 true of other bacteria. It has been found that the limiting hy- 

 drogen ion concentration of some of the streptococci is from about 

 pH 6.2 to 6.5 so it is at once evident that a medium cannot be 

 used in which the hydrogen ion concentration without a test 

 sugar may reach the pH values mentioned. The yeast peptone 

 medium could be easily buffered so as to take care of the increase 

 in acidity but then the fermentations of such organisms as those 

 previously mentioned would be entirely missed. 



The extract of 1 per cent of dry fresh yeast contained nitroge- 

 nous material including amino acids, some reducing sugar, and 

 other fermentable material as is shown in table 1. 



Although a 1 per cent yeast extract made from dry fresh yeast 

 contained a relatively small amount of amino acid it evidently 

 contained something valuable to support the growth of deli- 



