Professor Wanklyn has devised and published in his excellent little manual ‘Milk
Analysis’[37] a process by which a very thorough chemical examination of milk may be accomplished with great facility and expedition.
[37] Trubner and Co.
In his preliminary remarks he condemns, as utterly unreliable and misleading, the inferences to be drawn from those hydrometric instruments, the lactometer or lactodensimeter, and creamometer. “A very little consideration,” he says “will suffice to make intelligible the obliquity of the indications of the lactometer and to show how untrustworthy it must be. The lactometer, as of course will be understood, is simply the hydrometer applied to milk; and readings of the instrument are neither more nor less than specific gravities. The more milk-sugar, and casein, and mineral matter there is in a given specimen of milk, the greater (other things being equal) will be its density or specific gravity, and the higher the lactometer reading.
“If, however, fat globules (as happens in the instance of milk) be diffused through the fluid, then, because fat is lighter than water, the effect of the other milk solids on the gravity of the liquid, will be more or less neutralised. The density of milk-fat is about 0·9, water being 1·0. Now, if a solution of casein and milk-sugar, of specific gravity 1·030, be sufficiently charged with fat globules, its specific gravity may be sent down even below the gravity of water. How much would be required to bring about such a result is a matter of simple calculation.
“This being understood, it will be obvious that if the specimens of milk differ in specific gravity, there must be two distinct and equally valid ways of accounting for the difference.
“The milk with the lower gravity may be milk let down with water, or let down with fat, i. e. milk let down by being enriched.”
In support of this last assertion Professor Wanklyn quotes corroborative instances afforded by the examination of different specimens of milk known as ‘strippings,’ these being the last portions of milk yielded by the cow at the termination of the milking. All these ‘strippings’ had a lower specific gravity than normal milk.
Further, Professor Wanklyn points out that the specific gravity of organic fluids is a fallacious index of the amount of solids they may contain, as is illustrated by the fact, that whilst a 10 per cent. solution of chloride of potassium has a specific gravity of 1·065 at 15° C., and a 10 per cent. solution of casein and milk sugar, has a specific gravity of only about 1·035.
The creamometer meets with equal condemnation in Professor Wanklyn’s little book, since different specimens of milk vary considerably in their yield of cream, and a perfectly pure sample of milk may yield less cream than one which has been tampered with.