352 MILK 



that do liquefy gelatin and digest casein; it forms no gas from 

 carbohydrates, although rare forms also have been reported that 

 form gas; the colonies are small. The tendency to anaerobiosis 

 accounts for the fact that Streptococcus lacticus is most abundant 

 in deep portions of the milk. 



The bacteria of the coli-aerogenes group attack both carbo- 

 hydrates and protein, but as long as carbohydrate is available 

 proteins are protected, so that relatively small amounts are broken 

 down. Deep cleavage products from protein may appear, al- 

 though usually in small quantity. The lactose in milk is first 

 inverted to dextrose and galactose, and then lactic, acetic, formic, 

 propionic, succinic, and sometimes small amounts of valeric acids 

 are formed. Some strains produce relatively large amounts of 

 lactic acid, others less lactic and more acetic acid. The gases 

 formed are chiefly carbon dioxid, hydrogen, and traces of marsh- 

 gas. The members of this group not infrequently produce a 

 peculiar taste and odor in milk. At first the taste is aromatic 

 and not unpleasant, but later it becomes sharp, and finally re- 

 minds one of stable odor. 



The streptococcus group attacks chiefly carbohydrates, while 

 proteins are decomposed to a very small degree. It is probable 

 that proteins are not utilized for food to a considerable extent by 

 streptococci before they have been partially broken down by 

 other bacteria, such as the liquefying micrococci, members of 

 the hay bacillus group, and other peptonizing bacteria. Jensen 

 has shown that streptococci attack chiefly peptones or caseones, 

 the latter being even more suitable than the former. From the 

 lactose in milk they produce nearly pure lactic acid, although the 

 amount produced by different strains varies within rather wide 

 limits. The lowest amount is about 0.3 pen cent., while the 

 largest is about 1.3 per cent. Those strains that produce the 

 smaller amounts of acid do not coagulate milk, but the strains 

 which usually coagulate milk in typical fashion produce up to 

 0.9 per cent. 



The quantity of acid produced by lactic acid bacteria depends 

 upon several factors. Since Streptococcus lacticus forms more 

 acid than Bacillus aerogenes, predominance of the former will 

 result in a higher percentage of acid. As stated before, conditions 

 in perfectly fresh milk are not as suitable for growth of Strepto- 

 coccus lacticus as during later stages when some of the protein 

 has been broken down and rendered more easily assimilable. 

 Marshall has shown by his work on Bacterial Associations in the 

 Souring of Milk that saprophytic bacteria may or may not favor 

 growth of lactic acid bacteria. Marshall states, "the means by 

 which acceleration of lactic acid fermentation is produced is not 



