Milk, its Chemical and Microscopic Examination
November 24, 1886
Mr. M.H. Styles read a paper on “Milk, its Chemical and Microscopical Examination,” of which the following is a summary.
The value of milk as an article of food, especially for the young, is so universally recognised, that its examination with the view of forming an opinion of its quality and genuineness, becomes a matter of very considerable importance. Average country milk contains from 12 ½ caseine (curds) 4, sugar 4 ¾ and ash (mineral matter) ¾ per cent. Many things influence its quality, more particularly the mode of feeding, the age of the cow, and the length of time she has been in milk.
Still the variation takes place within limits which experience shows are fairly well defined, and thus analysts are enabled to decide with tolerable certainty whether a sample is genuine or not.
The fat of milk, so well known in the form of butter, occurs as tiny globules enclosed in an membrane partaking of the nature of a cell. These globules vary considerably in size ranging from 1-1,500th to 1-10,000th of an inch in diameter. When milk is allowed to stand they gradually rise to the surface, forming cream.
Caseine is the nitrogenous portion of the milk, and consists of caseine proper and albumen, the latter being that part which forms the scum when the milk is boiled. Caseine is easily separated by adding a little vinegar or a small quantity of rennet and gently heating. The curds thus produced contain a large proportion of the fat, which may be removed by washing with ether. The curds containing the fat when pressed into a mould and allowed to undergo a slow fermentation ultimately become cheese.
Sugar is left in the whey after the removal of the caseine and fat, the liquid is boiled down to a syrup and set aside for the sugar to crystalise; the crystals are afterwards refined.
In the analysis of milk, the first operation is the estimation of the solids. These are obtained by evaporating a small measured or weighed quantity n a platinum capsule over a vessel of boiling water for three hours. The dry residue is weighed together with the dish and the known weight of the later subtracted from the total. This residue, heated to redness until the organic matter is burnt off, leaves a nearly white ash consisting chiefly of lime, soda, potash, phosphoric acid, and chlorine, with a small trace of iron.
Probably the most important work in connection with the milk-analysis is the estimation of fat. By Wanklyn’s method, which, substantially, is that generally adopted throughout this country, the dry solids are boiled with successive quantities of ether, and the solution of fat thus obtained gently evaporated in a weighed dish until the ether is driven off. The weight of the residue consisting of the fat, is then taken, or the remaining solids in the platinum dish may be thoroughly dried and weighed, furnishing the solids not fat, the most important factor in determining the amount of water that has been added to adulterated milk.
By Wanklyn’s method it has been found that the solids not fat of genuine milk never fall below 9 per cent, and seldom lower than 9.3. Nine per cent, has accordingly been taken as the limit, and any sample of milk yielding less than 9 per cent when analysed by Wanklyn’s process is declared adulterated, the amount of adulteration being determined by the deficiency in the non-fatty solids. The removal of cream is evidenced by the finding of a low percentage of fat, the limit in this case being 2 ½ per cent, although an average milk should certainly give from 3 to 3 ½.
The author’s estimation of fat were made by a process differing from Wanklyn’s, a process which he found gave rather higher results, as by it the fat was more completely extracted. The weighed or measured quantity of milk was mixed with about three-fifths of its weight of clean powdered pumice stone in a porcelain dish, and the whole evaporated to dryness; the contents of the dish were removed, powdered in a mortar, then packed in a percolator and tested with cold ether until all the fat was extracted. The ethernal solution of fat is evaporated and weighed as described above. In working this method there was considerable saving both of time and ether.
Mr. Stiles also showed how the specific gravity – the comparative weight of equal bulks of milk and water – was taken, a delicate balance being used for this purpose. He had constructed a table showing the results of his own analysis of 12 samples of milk obtained in the town and neighbourhood, the average of these agreeing in a most remarkable manner with the average obtained by various workers in operating upon may thousands of samples throughout the country.
Slides were exhibited under the microscope, showing fresh milk and the way in which measurements of the fat globules were taken, and also sour milk containing bacterium lactis.
A discussion followed, in which Messrs. Kirk, Furnival, J.G. Walker, and the chairman took part.
A vote of thanks to the lecturer terminated the proceedings.