274 THE CHEMICAL CONTROL OP THE DAIRY. 



a fairly permanent foam wten the cream is whipped. Of these, 

 gum tragacanth added in the proportion of O'l per cent, is the 

 most effective. A careful microscopic examination of the cream 

 after the addition of a little iodine solution will reveal the presence 

 of particles of gum tragacanth, in which starch granules can be 

 detected. Agar, which has also been used as a thickening agent, 

 gives Cayaux's resorcinol test for cane sugar, but not any of the 

 other tests. 



Homogenised Milk. — In the equations given on p. 256 the 

 fat globules have been considered as being free from any con- 

 densed layer ; this is not the case, as the surface energy of small 

 globules condenses round them a layer of serum, which may, for 

 physical considerations, be included in the globule ; this will 

 decrease the value of ds — df, retard, and in extreme cases stop, 

 the rising of very small globules. 



In the case of the globules of cows' milk the influence of the 

 layer is sufficiently small, though not absolutely negligible, to 

 be left out of consideration. When, however, the globules of 

 fat are reduced to a diameter below that of the smallest naturally 

 occurring globules it becomes of importance, ' and the rate of 

 rising of cream is much less than that indicated by the formulae. 



By forcing milk, heated to such a temperature* that surface 

 energy is reduced to a minimum, whUe chemical change in the 

 milk is prevented, under a high pressure through very small 

 openings, the fat globules are reduced to a very small size. The 

 condensed layer bears such a relation to the globule that the 

 cream rises with extreme slowness, and practically speaking 

 remains mixed with the milk. This process is termed homo- 

 genising. Owing to the fact that the condensed layer is held 

 so firm by the great surface energy of small particles, it is im- 

 possible to churn milk or cream that has been homogenised ; 

 as the effective diameter of the globules is increased by the 

 condensed layer, homogenised mUk and, especially, cream are 

 thicker for the same percentage of fat than fresh milk or cream. 

 Surface energy varies considerably with temperature, and con- 

 sequently the thickness of the layer ; for this reason the thick- 

 ness of homogenised cream varies more with temperature than 

 the thickness of ordinary cream. Homogenised cream can 

 neither be churned nor whipped. 



* The temperature should not exceed 60° C, as the mechanical work 

 done in forcing the milk through small openings is partly converted into 

 heat, which raises the temperature of the milk some degrees. 



