Aids to the analysis of food and drugs / by T.H. Pearmain and C.G. Moor.

  • Thomas Hames Pearmain
Date:
[1899]
Catalogue details

Licence: Public Domain Mark

Credit: Aids to the analysis of food and drugs / by T.H. Pearmain and C.G. Moor. Source: Wellcome Collection.

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    When milk is allowed to stand for some time, a separation of the fat takes place. This is known as the rising of the cream, the larger fat globules of the milk naturally rising first. Absolutely fresh milk is ampho- teric in reaction—that is to say, blue litmus is turned red. and red litmus blue ; but usually by tbe time milk reaches the consumers it is faintly acid, owing to some slight fermentative changes taking place, resulting in the pro- duction of lactic acid. When allowed to stand still longer, milk undergoes fermentative changes, due to micro-organisms, usually the Bacillus acidi lactici. This decomposes the milk- sugar, resulting in the production of lactic acid, which eventually causes the milk to become sour. This change is largely dependent upon the time the milk has been allowed to stand, but more particularly upon the tem- perature ; milk, of course, becomes sour more quickly in the summer than in the winter. When the fermenta- tive change has resulted in the production of about 0'4 per cent, of lactic acid, the milk can be distinctly recognised by the taste to be sour. Finally, when the acidity reaches 0'6 per cent., the milk curdles, whereby it spontaneously separates into a solid, known as ‘curd,’ which consists of the fatty and proteid constituents of the milk, and a clear liquid known as ‘ whey,’ which consists essentially of a solution of milk-sugar and mineral salts. This change is also brought about artificially, as in the manufacture of cheese, by the addition of rennet. The further changes which take place in the composition of the milk after curdling depend upon the nature of the bacteria which have gained access, various organisms giving rise to different fermentations. As would be expected from its origin, milk is subject to great valuations in its composition, on account of the age, breed, food, changes in health, and the general conditions of the animal yielding it. The average com- position of genuine cow’s milk is as follows : Water 87’6 per cent. Fat ... Milk-sugar ... Proteids Mineral matter
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