198 MILK AND DAIRY PRODUCTS PART 



arily heating it at 80 C. Lecithin is partly destroyed when 

 the milk is heated. 1 Milk sugar suffers practically no change 

 from such temperatures as are met with in practice. 



When milk is heated the fat tends to flow together into 

 visible drops, which, on standing, impart an unpleasant appear- 

 ance and a thick consistency to the layer of cream. Such a 

 layer does not easily mix again with the milk. Latterly this 

 drawback has been removed by " homogenising " the milk, that 

 is, subjecting it to energetic mechanical treatment. The process 

 devised by A. Gaulin is most convenient for this purpose. The 

 milk is heated to 85 C., and then forced through fine metal 

 capillary tubes under a pressure of 250 atmospheres against a 

 conical surface of agate. The fat globules of the milk are then 

 split up into extremely fine particles, so fine, in fact, that when 

 the milk stands they are no longer able to rise to the surface 

 and form cream. Homogenised milk cannot either be separated 

 by means of a separator. Thus the inconvenience due to the 

 fat globules running together is prevented, and the unpleasant 

 deposit of particles of cream in the neck of the bottle no longer 

 takes place. 



In the pasteurising of milk the heating is for a longer or 

 shorter time at a temperature below 100 C. Generally the 

 temperature varies between 65 and 90 C., and the time from 

 several hours to a few minutes. The object of the pasteurisa- 

 tion is to obtain better keeping properties, and to destroy any 

 pathogenic germs, for these are, in general, easily destroyed by 

 heat, except the anthrax bacillus, which, owing to its ability to 

 form spores, is very resistant. Pathogenic bacteria which 

 accidentally find their way into milk are killed at 80 C. where 

 the sample is normal. 



Milk is pasteurised either by heating it in glass bottles in a 

 water-bath, or allowing it to flow through a continuously 

 working pasteurising plant, such as is usually found in large 

 dairies. In the first case the time can be as long as desired, 

 but in the latter case each particle of milk is only heated for a 

 very few minutes. 



Pasteurised milk usually keeps much better than unpasteur- 

 ised, if, directly after pasteurising, the milk is cooled to as low 



1 Bordas et de Rackowski, Compt. rend de I'Acad. des Sciences, Vol. 

 CXXXVI, p. 56. 



