The alcoholic solution filtered off from the combined casein and phosphate of sodium, contains the milk sugar and soluble chlorides.
It is evaporated to dryness on a water-bath, and the residue with the vessel containing it, is weighed. It is then gently ignited, and the weight of the remaining residue being deducted from the total weight before ignition, gives the yield of milk sugar. Or the milk sugar may be determined by titration with a standard copper solution.
For the determination of the ash it is only necessary to ignite the milk solids from 5 c.c. of milk, in the small platinum dish, by which operation all the organic matter being driven off, that which remains behind constitutes the ‘ash’ and is weighed as such.
It will be obvious that in order to determine with anything like rigid accuracy the quality of any sample of milk by analysis, not only must a normal standard for the purpose of comparison be adopted, but such normal standard must represent very closely and with but little variation the definite composition of all sound and genuine milk.
Professor Wanklyn says that “the following, which is the result of several concordant analyses of country-fed milk, may be taken as representing normal milk. In 100 grammes of milk—
| Solids (dry at 100° C) | 12·5 | grammes. |
| Water | 87·5 | |
| ——— | ||
| 100·0 |
“The 12·5 grammes consist of 9·3 grammes of ‘solids which are not fat,’ and 3·2 grammes of fat.” The above data, which are founded on the examination of a very large number of different samples of milk, are confirmed by the researches of Müller and Eisenstuck, who were employed by the Royal Agricultural Society of Sweden in a similar investigation. The labours of these chemists extended over a twelvemonth, and the result of them was to show that the milk yielded day by day, for a whole year, by a herd of cows was remarkably constant in composition.
Professor Wanklyn gives the following formulæ for the calculation and statement of the results of milk analysis. He says, treating the question quite rigidly, which I believe is the proper way of dealing with it, we arrive at the following:—
Problem 1. Given the percentage of ‘solids, not fat’ (= a), in a specimen of sophisticated milk (i. e. milk, either watered, or skimmed, or both)—required the number of grammes of genuine milk which was employed to form 100 grammes of it.
Answer. Multiply the percentage of ‘solids, not fat’ by 100, and divide by 9·3.