HOMOGENISATION. .427 



Bolton and Revis test for agar-agar by diluting 50 c.c. of cream 

 with 100 c.c. of water, adding 5 c.c. of 10 per cent, calcium 

 chloride solution, boiling and filtering ; to the cooled filtrate 

 one-half to two-thirds of strong alcohol is added, the precipitate 

 is filtered off and boiled with a small quantity of water till no 

 more dissolves, filtered hot and evaporated to 5 c.c. ; in the 

 presence of agar-agar the solution gelatinises. If gelatine be 

 present, it must be removed by adding tannin to the filtrate, 

 preferably evaporated to 25 c.c., till no more is precipitated. 

 After cooling below 60 a little white of egg is added, and the 

 whole heated in boiling water for 30 minutes, and the filtrate 

 evaporated to 5 c.c. as before. 



Cream has also been thickened by adding a strong solu- 

 tion of casein in alkalies, condensed milk, or milk powder. 

 These may be detected by the solids not fat, being appreciably 

 higher than the figures given on p. 187, and also by the alde- 

 hyde figure calculated to the cream devoid of fat being much 

 above 22. 



Homogenisation of Milk. In the equations given on p. 409 

 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, while 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 ; 



* 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. 



