198 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



such sugars they are aerobic. Dextrose, sucrose, maltose, 

 and lactose are all fermented with the development of acid 

 and gas. A total of one to one and one-quarter per cent of 

 acid may develop in milk, but as mentioned above, it is 

 only in part lactic acid, acetic and other volatile acids being 

 present. Usually the lactic ■ acid formed is levorotatory. 

 The gas developed is a mixture of hydrogen and carbon 

 dioxide. When grown in milk the curd produced is usually 

 torn by gas bubbles and shrinks, expelling a considerable 

 proportion of the whey. When present in milk in unusually 

 large quantities, such organisms may interfere with the pro- 

 duction of the best quality of cheese. 



Technical Utilization op Lactic Acid 

 Fermentation 



Associative Action.' — It has frequently been observed, 

 particularly in the development of bacteria in milk, that 

 mixed cultures are able to bring about changes which pure 

 cultures of the constituent organisms could not produce. 

 In some cases there is an increased production of acid. In 

 other cases the type of the change in milk is altered materi- 

 ally. It will be noted below that several of these associative 

 actions, particularly those among the desirable bacteria, 

 bring about changes of considerable economic importance. 

 Inasmuch as bacteria rarely occur in pure culture in nature, 

 the spontaneous changes brought about in milk and the 

 various fermented foods must, therefore, in general be the 

 result of associative action. 



Lactic Acid Bacteria in Milk 



Milk when freshly drawn, practically always contains 

 some lactic acid bacteria, usually streptococci and lacto- 

 bacilli. These may have come in part from the udder, from 

 the coat of the cow and very largely, usually, from the milk 

 utensils. When the milk is allowed to stand, particularly in 



