THE MILK IN ITS RELATION TO CHEESE 21 



seldom become active, because they are commonly an- 

 aerobic and so will not develop in milk kept under ordinary 

 conditions, and the rapid growth of the lactic acid-forming 

 bacteria prevents their growth. These organisms act 

 on the milk-fat, decomposing it. Butyric acid fermenta- 

 tions are more common in old butter and cheese. In 

 these, the fermentation causes a rancid flavor. 



32. Molds and yeasts. The cattle feed and the air 

 of the barn always contain considerable numbers of yeasts 

 and mold spores. Yeasts have been found by Hastings 1 

 to cause an objectionable fermentation in Wisconsin 

 cheese. No further study of this group as factors in 

 cheese-handling has been reported. Mold spores, es- 

 pecially those of the blue or green molds (Penicillum sp.) 

 and the black molds (Mucors), are always abundant 

 in milk. These spores are carried into all cheeses made 

 from unpasteurized milk, in numbers sufficient to cover 

 the cheeses with mold if they are permitted to grow. 

 Pasteurization 2 kills most of them. The border-line 

 series commonly referred to as the streptothrix-acti- 

 nomyces group are also very abundant in all forage and 

 are carried in large numbers into all milk and its products. 



33. Bacterial contamination of milk. When drawn 

 from the cow, milk is seldom if ever sterile. Organisms 

 usually work their way from the tip of the teat into the 

 udder and multiply there. The fore milk usually con- 

 tains more organisms than does that drawn later. Most 

 of the bacterial contamination of the milk is due to the 

 handling after it is drawn from the cow. 



1 Hastings, E. G., Distribution of lactose-fermenting yeasts 

 in dairy products, Wis. Exp. Sta. Kept. 23, pages 107-115. 



2 Thorn, C., and S. H. Ayers, Effect of pasteurization upon 

 mold spores, Jour. Agr. Research 6 (1916), no. 4, pages 153-156. 



