208 BACTERIA 



makes little or no difference to the practical desirability of 

 cooling milk, yet it is obvious that less cooling will be re- 

 quired in the cold season. 



We now come to the protective processes known as steril- 

 isation and pasteurisation. As we have already seen, steril- 

 isation indicates a complete and final destruction of bacteria 

 and their spores. As applied to methods of preserving milk, 

 sterilisation means the use of heat at, or above, boiling- 

 point, or boiling under pressure. This may be applied in 

 one application of one to two hours at 250 F., or it may 

 be applied at stated intervals at a lower temperature. The 

 milk is sterilised that is to say, contains no living germs- 

 is altered in chemical composition, and is also boiled or 

 " cooked," and hence possesses a flavour which to many 

 people is unpalatable. 



Now, such a radical alteration is not necessary in order to 

 secure non-infectious milk. The bacteria causing the dis- 

 eases conveyable by milk succumb at much lower temper- 

 atures than the boiling-point. Advantage is taken of this 

 in the process known as " pasteurisation." By this method 

 the milk is heated to 167-185 F. (75-85 C.). Such a 

 temperature kills harmful microbes, because 75 C. is de- 

 cidedly above their average thermal death-point, and yet 

 the physical changes in the milk are practically nil, because 

 85 C. does not relatively approach the boiling-point. 

 There is no fixed standard for pasteurisation, except that 

 it must be above the thermal death-point of pathogenic 

 bacteria, and yet below the boiling-point. As a matter of 

 fact, 158 F. (70 C.) will kill all souring bacteria as well as 

 disease-producing organisms found in milk. If the milk is 

 kept at that temperature for ten or fifteen minutes, we say 

 it has been " pasteurised." If it has been boiled, with or 

 without pressure, for half an hour, we say it has been 

 " sterilised." The only practical difference in the result is 

 that sterilised milks have a better keeping quality than 



