PART iv PRESERVED MILK 197 



taste, shows no other signs of having been heated, but it is far 

 from being absolutely sterile. All the vegetative forms of 

 bacterial life, together with the less resistant spores, are killed, 

 but some of the most resistant spores, particularly the so-called 

 peptonising bacteria, remain alive, although they develop only 

 very slowly when the milk, after " sterilisation," is kept at a 

 low temperature. Such milk will keep for a considerable 

 length of time, although it may not be actually sterile. 

 Absolutely sterile milk cannot be obtained commercially. The 

 only way to obtain a sterile milk (unless it is heated for a long 

 time at a high temperature, which, as already seen, renders it 

 unfit for use) is to subject it to fractional sterilisation as 

 recommended by Tyndall. For this purpose, the milk is heated 

 on consecutive days for 2 hours at 65 70 C. ; in the interval the 

 temperature is kept between 25 and 55 C. By this means the 

 spores which remain alive grow daily into fresh bacteria, which 

 are then killed on the following day by the higher temperature, 

 and so the milk finally becomes sterile. The taste and outward 

 appearance of the milk hardly undergo any change by the 

 treatment, but sterilisation in this way is very costly and 

 tedious, and has not found any considerable application. 



Even at 100 C., and still more above that temperature, the 

 properties of milk are altered, although, judging by the appear- 

 ance, the milk is unchanged. The most important of these 

 changes are the following : 



The greater part of the albumen coagulates at about 70 C. 

 and the casein is changed at a temperature above 100 C., so that 

 the milk acquires a brownish tinge, and this alteration in colour 

 is therefore not always due to the caramelising of the milk 

 sugar. 



The acidity of milk decreases on heating to 100 C., because 

 of the separation of carbon dioxide. When the temperature 

 exceeds 100 C. the acidity increases, owing to the splitting off 

 of phosphorus-containing acids (nucleic acid) from the casein. 

 The loss of carbon dioxide on heating leads also to the precipi- 

 tation of lime salts (phosphates and citrates). Milk becomes 

 less susceptible to the coagulating action of rennet the higher 

 the temperature rises, until finally coagulation ceases if the 

 milk has been heated for half an hour at 110 C. 



The enzymes of milk are in general destroyed by moment- 



