LACTIC ACID FERMENTATION 203 



Industry seemed to indicate that commercially pasteurized 

 milk practically always contains resistant strains of 

 Streptococcus lacticus which will bring about normal sour- 

 ing, and that putrefactive changes in pasteurized milk are 

 little if any more common than in milk which has not been 

 pasteurized. 



ScGondary Changes in Soured Milk. — ^When soured milk 

 is allowed to stand, lactic acid formation eventually ceases 

 and very little change occurs in the milk for a considerable 

 period of time unless air is allowed access to it. This has 

 been taken advantage of in certain districts in northern 

 Europe to preserve meat for a time in soured milk. 



In certain of the Scandinavian countries, particularly in 

 Norway, milk is kept from the v/inter months for use in the 

 home during the summer months while the cattle are away 

 in the mountain pastures. The milk is first heated and then 

 stored in casks after inoculation with a suitable starter. 

 The organisms present are lactic acid bacteria and a species 

 capable of producing a considerable degree of viscosity. 

 This ropy milk will keep for a considerable period of time 

 if air is excluded. 



When air gains access to soured milk, certain aerobic 

 forms, particularly certain molds such as O'idium lactis, 

 begin to grow upon the surface. These organisms rapidly 

 oxidize the lactic acid to carbon dioxide and water, thus 

 decreasing the acidity of the solution. At the same time 

 certain of the molds produce proteolytic ferments which 

 bring about partial or complete digestion of the casein. 

 When the acidity of the solution has been sufficiently 

 decreased, certain of the aerobic and anaerobic spore- 

 producing bacteria find conditions favorable for develop- 

 ment and bring about proteolytic or even putrefactive 

 changes. 



Lactic Acid in Cheese Manufacture. — ^Rennet curd 

 cheeses are in part ripened through the activity of lactic 



