192 NORMAL MILK : ITS ADULTERATIONS, ETC. 



readily by rennet than fresh milk, but there are good grounds 

 for the view that this is due to a change in the distribution of the 

 calcium salts as well as possibly to a change in the casein. It 

 has been claimed that sterilised or boiled milk is more easy of 

 digestion than unboiled milk, but this, again, is possibly due to 

 the fact that it is not curdled so easily in the stomach, and does 

 not produce so firm a clot. There appears to be no evidence 

 that healthy adults digest boiled milk either more or less readily 

 than unboiled milk. In one respect boiled milk is less to be 

 preferred than fresh milk. From the evidence adduced by 

 Barlow, and since fully confirmed, it seems that children fed 

 exclusively on sterilised milk have a scorbutic tendency. It has 

 long been known that the absence of fresh food of any descrip- 

 tion is a predisposing cause of scurvy, but no substance has yet 

 been identified as the agent which confers anti-scorbutic pro- 

 perties. 



It is of considerable importance to be able to distinguish 

 between fresh milk, on the one hand, and " pasteurised " or 

 " sterilised " milk, on the other. 



Sterilised Milk. 



Milk is a product which affords all the necessary nourishment 

 for the growth of micro-organisms ; these not only develop 

 products which cause alteration of the milk — e.cj., lactic acid and 

 proteolytic enzymes — but also are in some cases injurious to 

 health. 



They are destroyed by heat. Hence milk is frequently 

 " sterilised " by heat, the object being to bring about the de- 

 struction of the micro-organisms. 



Many processes are used. Pasteur originally recommended 

 heating to 70° C. for a short time, a process which was sufficient 

 to destroy all adult forms of pathogenic organisms and, practi- 

 cally, all others. The spores, however, were left untouched and 

 retained their vitality ; on cooling to the mean air temperature 

 these developed into the adult forms and resumed their activity. 

 To destroy the spores, a process of continued " pasteurisation " 

 has been used. This consists of alternately heating to 70° C. 

 for, say, twenty minutes ; cooling to a lower temperature and 

 keepiiig at this temperature for a sufficient length of time tO' 

 allow the spores to develop ; again heating to 70° ; and repeating 

 this process many times. By this process, which is very tedious, 

 the taste and composition of the milk undergoes but little alter- 

 ation. 



It has been found that most spores can be killed by continued 



