METHODS COMMONLY USED IN HEATING MILK 303 



conducted. The feeling of safety engendered by the belief in 

 pasteurisation is very unsatisfactory if the farmer regards it as 

 enabling him to use dirty methods in collecting the milk. The 

 remarks in connection with the preliminary filtration which are 

 given on pp. 275-7 show that this tendency cannot be neglected in 

 considering the conditions of the public milk supply. 



BOILED MILK 



The border-line between pasteurised and boiled milk may 

 become so fine as to be almost absent. Strictly speaking, a milk 

 which is boiled must be supposed to have reached the temperature 

 corresponding to the boiling-point of milk, which for the present 

 purpose may be considered to be that of water. Where milk is 

 heated in a water bath without pressure, the temperature of the 

 milk will not reach that of the surrounding water. Where the 

 water is only just raised to the boiling-point, the temperature of 

 the milk is probably several degrees lower than that of the water, 

 but where the water is kept at the boiling-point for some minutes, 

 the temperature of the two fluids will approximate more closely, 

 although the temperature of the milk will remain below that of the 

 boiling water. 1 For all practical purposes, however, the milk may 

 be regarded as having been boiled. 



Milk is frequently boiled without a surrounding water-jacket, 

 in which case the temperature will evidently reach 100 C. and 

 will fulfil the technical requirement for using the term boiled. 

 Domestically, it is probable that if this method is used the tempera- 

 ture is not maintained at boiling-point for any length of time, 

 owing to the physical fact that the milk will boil over and tends 

 to burn. Usually, therefore, the temperature will only be kept at 

 this level for one or two minutes, and the milk subsequently cooled. 

 Commercially, however, or on a large scale, the temperature could 

 be maintained at this height for any desired length of time by 

 using steam under pressure, although actually it is not attempted. 

 Here also, as in the case of pasteurised milk, the length of time at 

 which the temperature is maintained should be mentioned. 



STERILISED MILK 



Strictly speaking, milk thus designated should be free from all 

 bacteria and from their spores. In actual fact such milk is very 

 difficult to obtain. The early work of Fliigge showed many years 

 ago that the spores found in milk are extremely resistant to heat, 

 and that there is considerable difficulty in obtaining a really sterile 

 milk. Spores are not killed by a temperature of 100 C., and in 

 order to destroy them it is necessary to heat the milk considerably 

 over 100 C. on successive days. Very little milk thus treated is 

 used for infant feeding. 



1 Cp. the data on pp. 296-7. 



