148 PHARMACEUTICAL BACTERIOLOGY. 



Yeast organisms may be studied very conveniently in the hanging drop. 

 The development of mould may be observed between two sterile slides. 

 Since these organisms are much larger than bacteria there is little diificulty 

 in examining them under the low power of the microscope. Mount in 

 water or in a weak solution (10 per cent.) of caustic potash or soda. In 

 looking for yeasts and moulds in liquids, centrifugalizing may be desirable. 

 Staining methods will rarely be necessary. 



While it is true that not all moulds are pathogenic, yet it must be remem- 

 bered that many are decidedly so, besides most of them are very objection- 

 able on account of the disagreeable mouldy odor and taste, if for no other 

 reason. Mouldy food substances are not fit for consumption and moulds 

 should not occur in any of the pharmaceuticals, syrups, soda fountain 

 preparations and fruit juices. Most of the yeasts are non-pathogenic. The 

 common yeast has even been used as an intestinal disinfectant in typhoid 

 fever, yet no preparations in the drug store should be allowed to undergo 

 yeast fermentation for the reason that the process changes the quality and 

 flavor of the substances thus attacked. Fruit pulp, fruit juices and syrups 

 of all kinds are peculiarly liable to the attacks of the yeast organisms and 

 every precaution should be taken to guard against such infection. This is 

 not a simple matter because the yeast cells and the yeast spores are found 

 everywhere and develop very readily in all saccharine, slightly acid substances. 

 Moist heat sterilization or pasteurization are the most effectual means for 

 preventing yeast fermentations. 



The yeast cakes used by the housewife in making bread consist simply 

 of pure cultures of Saccharomyces. The cakes must be kept dry and in the 

 cold (ice chest) to prevent ready decomposition. Even under the most 

 favorable conditions they soon become worthless. As soon as the cake 

 is mixed with the bread dough with adequate warmth, the yeast cells 

 begin to feed upon the various available food substances present and 

 multiply rapidly (by budding), resulting in the formatin of alcohol and 

 liberation of COj gas, which latter in an attempt to escape, causes the so- 

 called rising of the bread. If the dough is not thoroughly mixed, the gas 

 liberation is uneven and the bread will be unsatisfactory, because there will 

 be large cavities in some parts of the loaf and in other parts the loaf will be 

 solid. Bread must be baked quickly, after the rising has reached the 

 proper degree, othervnse the loaf will be flat and doughy. The housewife 

 in the country simply prepares sour dough cakes which take the place of the 

 manufactured yeast cakes used in the city. In biscuit making the desired 

 CO2 gas liberation is brought about by the use of baking soda and sour 

 milk or by means of baking powder alone. 



The alcoholic fermentation in the manufacture of beer is caused by the 

 several varieties and forms of Saccharomyces cerevisecB {Torula cerevisea). 



