XV. MILK AKD MILK PEODUCTS 335 



slowly sinks to the bottom, the warmer and therefore lighter milk 

 rising in the more central portion of the vessel to make way for it, 

 while the milk nearer the surface is slowly drawn outwards towards 

 the walls of the vessel and sinks. In this way, a very slow circulation 

 probably takes place, and, during the whole time, the fat globules are 

 tending to rise to the surface, from which, on account of their levity, 

 they will not be moved by the gentle downward currents. In this 

 way, the fat globules accumulate quickly at the surface, behaving in 

 much the same way as if the milk were set in a very shallow vessel, 

 the buoyancy of the globules having, so to speak, only to do the work 

 of raising them out of the slow current of milk which is continually 

 passing beneath the cream layer. Another factor which may aid in 

 the process is the persistence of the fat, during rapid cooling, in its 

 liquid, and therefore lighter, form, while the aqueous portions of the 

 milk are rendered denser as they cool. As has already been stated, 

 liquid fat is of lower specific gravity than solid fat at the same tem- 

 perature, and there is some evidence that super-cooling of fat globules 

 readily occurs. In ordinary setting, assuming that the same period 

 elapses before the fat solidifies, the aqueous portion of the milk will 

 not differ so much in density from the fat globules for so long a time, 

 as when the milk is quickly cooled. 



Another possible way in which the quick cooling facilitates the 

 rise of the fat, may be in its preventing or delaying the coagulation of 

 the small quantity of fibrin which, according to Babcock, is present 

 in milk and which, by entangling the fat globules, hinders their rise 

 under ordinary conditions of setting. 



Many other advantages attend the practice of cold setting, among 

 which the very important one of lessening the fermentation of the milk 

 sugar and of hindering all bacterial growth, both in the cream and in 

 the skim-milk, by the low temperature and shorter time of setting, 

 may be mentioned. 



The composition of cream is liable to enormous variation, the pro- 

 portion of fat fluctuating between 9 or 10 per cent and 60 or even 70 

 per cent. 



If obtained at low temperature, the amount of fat is usually small 

 about 20 per cent ; by shallow setting, it may vary from 15 to 40 per 

 cent ; whilst with the separator, by adjusting the rate at which the 

 milk passes away, almost any richness of cream may be obtained. 



The aqueous portion of cream contains the usual solids of milk 

 almost in the same proportion as in milk itself. The amount of solids 

 not fat is usually slightly higher than in milk, due probably to evap- 

 oration of water during the setting. This is especially the case with 

 "clotted cream," prepared by the Devonshire method, and whose 

 composition is more uniform than that of ordinary cream. According 

 to Richmond, the average composition of this substance is water 

 34-26 per cent, fat 58-16, ash 0-60, solids not fat 7-52. 



The specific gravity of cream can only conveniently be directly de- 

 termined if its fat is below 30 per cent. If stiffer than this, it must be 

 diluted with an equal volume of separated milk and the specific gravity 

 of the mixture taken. Richmond gives the following numbers : 



