The clear liquid which separates when milk is curdled with rennet is called whey, and contains the milk sugar and mineral salts. The sugar is manufactured from it on a limited scale, and is used as an ingredient in infant foods, and as a convenient medium in certain medical preparations. In Sweden a kind of cheese is made from whey, but the great bulk of it everywhere is used for feeding pigs.
The comparative composition of different varieties of milk is given in the following table:
────────────────┬──────┬───────┬──────┬───────┬──────┬──────┬────────┬────── Human│ Cow │Buffalo│ Goat │ Sheep │ Mare │ Ass │Reindeer│ Whale ────────────────┼──────┼───────┼──────┼───────┼──────┼──────┼────────┼────── Water 88.32│ 87.75│ 82.57│ 86.34│ 81.08 │90.38 │90.30 │ 67.7 │ 60.47 Fat 3.43│ 3.40│ 7.63│ 4.25│ 7.67 │ 1.00 │ 1.30 │ 17.1 │ 20.00 Protein 1.55│ 3.50│ 4.69│ 4.40│ 6.08 │ 1.98 │ 1.80 │ 10.9 │ 12.42 Milk Sugar 6.44│ 4.60│ 4.30│ 4.26│ 4.26 │ 6.28 │ 6.20 │ 2.8 │ 5.63 Salts 0.26│ 0.75│ 0.81│ 0.75│ 0.91 │ 0.36 │ 0.40 │ 1.5 │ 1.48 ────────────────┼──────┼───────┼──────┼───────┼──────┼──────┼────────┼────── Total 100.00│100.00│ 100.00│100.00│100.00 │100.00│100.00│ 100.00 │100.00 ────────────────┼──────┼───────┼──────┼───────┼──────┼──────┼────────┼────── Specific 1.032│1.0315│ 1.033│ 1.033│ 1.038 │ 1.034│ 1.033│ ... │ ... Gravity │ │ │ │ │ │ │ │ ────────────────┴──────┴───────┴──────┴───────┴──────┴──────┴────────┴──────
[Illustration: FIG. 4 is a photograph of two Petri dishes, which have been inoculated with ordinary milk (A), and milk that has been subjected to sterilisation (B). The whitish bacterial colonies on A are due to enormous numbers of organisms, while B is quite free from such growth.
For the production of a reliable lactic food, it is essential that certain precautions as to the treatment of the milk, and the maintenance of a suitable temperature during the growth of the lactic bacteria, should be observed.
In the first place, milk immediately after extraction from the cow contains only a few organisms, but these multiply so rapidly that in a few hours the bacterial content may amount to many millions per ounce. In preparing a pure culture of any specific organism, then, care must be taken to destroy all the bacteria that have accidentally found their way into the milk, inoculating with the organisms it is desired to cultivate. This is best accomplished by heating the milk to the boiling-point of water for about thirty minutes, by which time almost all the undesirable bacteria have been killed.]