344 KEPHIE PRESERVATION OF MILK 



kept for future use. Their origin appears to be unknown. Many 

 organisms have been found in kephir grains, including bacteria and 

 yeasts. It has been stated that the kephir contains an enzyme lactase 

 which has the power of hydrolysing milk sugar, thereby producing 

 glucose and galactose, and that yeast then attacks the former as in 

 ordinary alcoholic fermentation. Kephir can be prepared from cows' 

 milk by the simultaneous action of beer yeast and Bacillus bnlgaricus. 

 The reactions resemble those of the preparation of Koumiss but do 

 not proceed so far (Ginzberg, I. c.). 



Kephir resembles koumiss in composition, but contains less alcohol 

 and albumoses and more casein. 



Milk Preservation. Milk is peculiarly liable to undergo change 

 under the influence of micro-organisms, for the growth of which it 

 serves as an excellent medium. In the udder, milk is normally free 

 from micro-organisms, but, unless special precautions are taken, within 

 a very short time after milking it absolutely teems with them. 



In many cases examined, milk, within a few minutes of milking, 

 has been found to contain thousands of organisms per cubic centi- 

 metre. The organisms, or their spores, are derived from the air, the 

 hands of the milker, the hair or teats of the cow, and particularly from 

 the vessels in which the milk is received. 



The rate at which the bacteria multiply is largely dependent 

 upon the temperature. It was found that after fifteen hours at 15 C , 

 milk contained 100,000 bacteria per cubic centimetre, while the same 

 milk kept fifteen hours at 25, contained 72,000,000 per cubic centi- 

 metre, and at 35, 165,000,000 per cubic centimetre. 1 



The importance of quickly cooling milk is thus evident, since usually 

 it is desirable to hinder as much as possible the growth of bacteria. 

 The micro-organisms which find their way into milk are of various 

 types ; in most cases, perhaps, the majority are such as convert milk 

 sugar into lactic acid, while organisms of almost all kinds may be 

 found, many objectionable and some highly dangerous. The first 

 evident effect of the growth of organisms is usually the souring and 

 consequent curdling of the milk. 



It is obvious, therefore, that if the milk is to be kept for any length 

 of time some means of preventing the growth of these organisms must 

 be taken. Two chief methods suggest themselves : (1) To prevent the 

 entrance of the germs, or (2) to use some means which will either kill 

 them, or prevent their growth. 



The first method is almost impracticable on the large scale, but the 

 second is often adopted. This is either by " sterilisation " or " Pasteur- 

 isation ". 



In the former process the milk is heated to a sufficiently high 

 temperature (about 115 C.) to destroy all germs. This is usually 

 effected in steam under pressure. The milk is then kept excluded 

 from the air, or air which has filtered through a thick layer of cotton 

 wool may be admitted. Milk so prepared will not sour and may be 

 kept indefinitely. Unfortunately certain undesirable changes are pro- 



1 Miquel, Central-B. fiir Agricult.-Chem., 1890, 575. 



