A complete analysis of milk involves the determination of the water, the fat (the essential
constituent of the cream), the casein, milk-sugar, and ash.
The following is an outline of Professor Wanklyn’s neat and ingenious method of analysis:—
By means of an accurately graduated pipette, he first places 5 cubic centimetres of the milk in a small weighed platinum dish (about 14 grammes in weight) just previously ensuring the sample from which the milk is taken being thoroughly mixed.
The dish is then placed over a water-bath (the water in which must be kept vigorously boiling the whole time) for three hours, at the end of which time all the water having been driven off, there will remain in the dish a completely dried up residue.
The increase in weight between the empty dish and the residue, will give the weight of the ‘milk solids’ from 5 c.c. of milk. Of course, if this weight be multiplied by 20, the yield from 100 c.c. of milk will be obtained.
To reduce this to a percentage statement it is necessary to remember that 100 c.c. of average milk weigh 102·9 grammes. The next proceeding consists in the determination of the fat. This is done by treating the dried milk solids resulting from the 5 c.c. of milk with ether. There are several important minutiæ necessary to be observed in connection with this part of the process, for the particulars of which the reader is referred to Professor Wanklyn’s book. Suffice it to say, that if properly performed, the whole of the fat is dissolved by the ether, and being separated from the non-fatty portion of the residue is weighed and calculated as ‘fat.’
If, then, the amount found as fat be deducted from the whole of the milk solids previous to their treatment with ether, the ‘milk solids, not fat,’ will be arrived at. Professor Wanklyn estimates the casein[38] as follows:—He treats the milk solids, not fat, with hot alcohol, by which means he dissolves out from them, and removes the milk-sugar and the soluble chlorides. The remaining residue, consisting of casein and phosphate of sodium (chemically combined with the casein), is dried on a water-bath until it ceases to lose weight. It is then weighed along with the vessel containing it, and ignited. The combined weight of the vessel and phosphate of sodium remaining after ignition being deducted from the weight previous to ignition, the difference is the casein.
[38] Under the head “Casein” Prof Wanklyn includes the entire nitrogenous materials of milk.
Another and quicker method, recommended by Professor Wanklyn, for the determination of the casein, is to measure it by the amount of albuminoid ammonia it is capable of yielding when subjected to the ‘albuminoid ammonia process,’ invented by Messrs Wanklyn, Chapman, and Smith.