LACTIC AND BUTYRIC FERMENTATIONS. 279 



a density greater than 1'036, otters have a less density. The 

 top cultivation was made from the portion immediately xmder- 

 neath the thin layer of cream, so that it is not probable that 

 they were carried up by the cream. 



Lactic Fermentation. — The most commonly observed effect 

 of the action of micro-organisms is the souring of milk. This 

 is due to a numerous class of organisms, chiefly bacteria and 

 bacilli, which convert the milk-sugar into lactic acid. The 

 chemical equation for this change usually given is 



CiaHjjO,, -f OHa = iCjHsOs. 



The change, however, never proceeds in this delightfully simple 

 manner, certain quantities of carbon dioxide and often alcohol 

 being always produced ; by keeping up a free supply of oxygen 

 a very large proportion of carbon dioxide can be obtained. Some 

 lactic ferments give an amount of lactic acid agreeing approxi- 

 mately with the above equation ; others produce notable amounts 

 of alcohol and other products. Other organisms, again, produce 

 very small quantities of lactic acid and large amounts of other 

 substances. Hueppe has studied this class of organisms minutely 

 and has described many species ; few of these form spores and 

 are destroyed with comparative ease by heat ; generally speaking, 

 their optimum temperature is about 35° C. (97° F.). 



A number of organisms, grouped under the designation Bacillus 

 of Massol, have been recently studied, which produce very large 

 quantities of lactic acid (up to 3 per cent.). They do not ferment 

 cane sugar. These all form chains of long rods, which stain 

 often unequally by Gram. 



Butyric Fermentation. — When this takes place the milk 

 coagulates without the development of acidity, but the milk 

 becomes alkaline ; a bitter taste is acquired, the precipitated 

 casein is redissolved, and butyric acid is formed ; an unpleasant 

 smell is usually developed. This fermentation, which does not 

 readily occur if lactic acid is developed, appears to be also caused 

 by many micro-organisms, which attack both milk-sugar and 

 casein. When this fermentation takes place, the solid portion 

 of the milk is reduced to a very much greater extent than by 

 the lactic fermentation. 



There is another butyric fermentation which takes place with 

 development of a very high acidity, and in which the lactic 

 acid produced by the lactic ferments is converted into butyric 

 acid. Large quantities of gases — carbon dioxide and hydrogen 

 — are produced, and other volatile acids, acetic and, more rarely, 

 propionic, are produced at the same time. This fermentation 

 does not develop till the milk has stood for some time, and appears 



