SOME CHEESE DEFECTS 375 



with animal faeces ; cleanliness at the farm will go a 

 long way towards preventing it. 



The lactic acid fermentation inhibits the organisms 

 which cause it, and the addition of sour milk " lange 

 wei," or a specially clean starter, is advisable where 

 sponginess of curd is expected. 



Ripening at low temperatures is also useful. The 

 addition of . I per cent of potassium nitrate (saltpetre) to 

 the milk for cheesemaking is beneficial. Boekhout 

 and De Vries have shown that the gas-producing 

 bacteria break up the milk-sugar in the milk or cheese 

 with the evolution of carbon dioxide and hydrogen gases, 

 but when potassium nitrate is present they obtain the 

 oxygen which they need from this compound, leaving 

 the milk-sugar to be changed by the ordinary lactic 

 acid organisms. 



Ex. 162. Make a fermentation or Wisconsin curd-test with 

 clean milk, and one to which a small amount of cow-dung or 

 pure culture of Bact, lactis aerogenes has been added. 



Take two glass jars or tin vessels each holding about a pint ; 

 they must have tin or glass covers or lids. Sterilize them in the 

 hot-air sterilizer ; when cool fill the vessels about two-thirds full 

 of the milks to be tested, and put on the lids. 



Now place them in an incubator or water bath, kept at 38 C, 

 and when the milk has reached this temperature add to each vessel 

 ten drops of commercial rennet diluted with two or three cubic 

 centimetres of boiled water. Shake or rotate the vessels gently 

 so as to mix the rennet with the milk, and when curdling is 

 complete, allow them to stand twenty minutes. Cut the curd in 

 each into small pieces with a separate sterilized knife, or with 

 one which is sterilized each time before use by dipping in boiling 

 water, and cover up for half an hour. Then open the vessel and 

 smell ; note pleasant or disagreeable odour. 



