MICRO-ORGANISMS AND ENZYMS 3OI 



complete the extraction. The sohition thus obtained 

 is filtered through clean straw, sand, and charcoal and 

 then treated with enough salt to prevent decomposi- 

 tion; a brine containing 6 or 7 pounds of salt to lOO 

 pounds of solution is about the proper strength. 

 Rennet-extract properly prepared is dark in color, but 

 clear. The appearance of turbidity in the extract is 

 an indication of the beginning of decomposition. It 

 must be kept in a cool, dark place. In some cases, 

 whey was once used as a medium for preparing ren- 

 net-extract, a practice that would insure a large num- 

 ber of objectionable micro-organisms in the extract. 

 It can readily be seen how home-made rennet-extract 

 may be a source of serious bacterial contamination 

 in milk. The preparation of home-made extracts is^ 

 fortunately, much less common now. The serious 

 objections to their use are (i) liability to bacterial 

 contamination and (2) variation in strength of dif- 

 ferent lots, usually requiring the use of quite variable 

 amounts of one preparation as compared with another. 



Commercial rennet-extract. — The general substi- 

 tution of commercial for home-made rennet-extracts 

 is of distinct advantage in cheese-making, because the 

 commercial forms are much more uniform in strength 

 and less liable to bacterial contamination. Commer- 

 cial rennet-extracts contain about 16 per cent of salt 

 and a trace of boric acid. Some have expressed the 

 fear that the boric acid used as a preservative in ren- 

 net-extract might injure the value of cheese as a pure 

 food. There need be absolutely no alarm felt, when 

 we consider the small amount of rennet-extract used 

 in cheese-making and the very small proportion of 

 this that goes into cheese. In fact, the amount of 



