i MILK 5 



fat globules vary, however, very considerably in the different 

 breeds, and also in individuals at different stages of the period 

 of lactation. In the same volume of milk towards the end of 

 the period of lactation, the number of fat globules is greater 

 than at the beginning of the period. 



The debated question as to whether the fat globules are 

 surrounded by a membrane of any sort is not definitely decided, 

 but the present views are that none exists, and that the surface 

 tension keeps the globules separate from one another, as is seen 

 in other emulsions. 



Milk fat is fluid at ordinary temperatures and is in an under- 

 cooled state. It is only in the process of butter-making that 

 the fat passes into the solid form. As the specific gravity of 

 milk fat is 0'93 at 15 C., the globules are lighter than the 

 liquid in which they are present, and so they tend to rise to 

 the surface. As is well known, milk when left to stand 

 gradually gains a layer of these fat globules on the upper 

 surface, giving what is known in daily life as cream. 



The fat of cow's milk is, at ordinary temperatures, a soft 

 yellowish mass with a pleasant taste and only a faint smell. 

 It melts between 31 and 36 C., and the point of solidification 

 is from 24-19 C. According to Gutzeit, 1 the refractive index 

 of butter fat is generally 1'462 at 40 C. 



The chemical composition of milk fat shows it to be com- 

 posed of a mixture of the triglycerides of volatile and non-volatile 

 fatty acids. It is characteristic of other animal fats that they 

 only contain a small number of the different glycerides, whereas 

 in milk fat there are not less than ten and it is possible that as 

 many as twelve different glycerides are present. 



Of these there are the glycerides of four volatile fatty acids, 

 viz., butyrin, capronin, caprylin, and caprin, and the glycerides 

 of six non-volatile fatty acids, viz., myristin, palmitin, stearin, 

 arachin, laurin, and olein. The constant presence of formin 

 and acetin has not yet been satisfactorily proved. It is 

 probable that there is no difference in the composition of the 

 small and large fat globules. 



The quantity of fat found in milk is very variable ; none of 

 the other constituents of the milk is subject to such large 

 fluctuations as is the fat. In the case of individual cows the 



1 Ref. p. 4. 



