its proper relative proportion. I ought at once to disabuse your minds of the idea that milk is absolutely constant in composition; it varies to some considerable extent, but in talking of it to-day, for popular purposes, I shall assume a fair average composition, and explain the extent to which the variations occur afterwards.
Fat is the most variable and probably the most important of the ingredients of milk. It certainly is the most important from a dairy farmer’s point of view, because if the milk contains but little fat, he can procure but a smal proportion of cream and make but little butter from it. From a dietetic point of view also, it is, I say, the most important, or most important but one, of milk constituents. The butter fat is a peculiar one, differing in many respects from all the vegetable fats with which we are acquainted, and differing also to a considerable extent from other animal fats. The most marked point of difference is in the fact that butter contains three volatile fatty acids, the principal one of which is Butyric, which, as far as we know, is not met with in any other fat, either animal or vegetable.
It by no means follows, however, that because butter fat possesses these peculiarities it is easy to identify it; on the contrary, it is only by a tedious and somewhat difficult process of chemical, analysis, that the butyric acid can be so far separated as to enable a definite statement to be made as to the purity, or otherwise, of the sample of so-called butter fat.
Commercially we get this fat in a state of what may be called "semi-purity," as Cream. Good cream contains from 50 to 60 or 65 per cent, of butter fat, the remainder consisting of water and a small proportion of the other constituents of milk. When cream is churned into butter the envelopes of the fat globules are broken, and a large number of these tiny little spheres of fat, originally of microscopic size, adhere together, while a large proportion of the water and the soluble constituents are washed away with the butter milk. We thus get butter fat in a still higher state of purity.
