The substance casein (the curd which forms the cheese) resembles the gluten of vegetables, the fibrin of meat, and the albumen of the egg in nutritive qualities ; it possesses the same value, weight for weight, as food ; but when eaten alone as cheese, is, like eggs, a constipating food.
When the curd and cream have been taken from the milk, and the whey is evaporated to dryness, sugar of milk is obtained; this sugar represents the starch of wheaten bread, while the butter and curd or casein represent the fibrin and fat of meat ; consequently milk partakes of the nature both of animal and vegetable food.
The power of retaining the globules of air or steam in moistened flour when baking, possessed by the white of egg, belongs, though in a less degree, to the curd of milk ; consequently bread is much improved in lightness and appearance when it is made with milk.
1 lb. of Cows Milk contains -
Mineral matter . .
Milk is full of very minute globules of oil, which may be seen by the microscope floating on a transparent liquid. It would take 10,000 of them to cover an inch if placed side by side. These globules are transparent in themselves, but they cause the opacity of the milk by scattering and bending the rays of light which reach them. If milk is left standing the greater number of these oil globules rise to the surface and are called cream. The milk beneath (which becomes skim milk) is nearly transparent, and would be quite if all the globules had risen. Churning cream breaks these globules (each of which is enclosed in a thin membrane or bag) and the oil united into a mass is easily made into butter. The Devonshire clotted cream is made by heating the ordinary cream nearly to the boiling-point, when the envelopes of the oil globules burst; the rich oily cream after standing some hours is skimmed off.
Asses' milk is considered excellent for invalids. 1 lb. of it contains :
Mineral matter . .
The casein or flesh-former in milk is held in solution in it by an alkali which is known to have the power of doing this ; an acid, by neutralizing the alkali, solidifies the casein into CURD. When milk is drunk, the acid of the gastric juice at once renders the casein of the milk solid in the stomach ; and milk soured by keeping will, as we all know, curdle.
An acid thrown into milk produces the same effect, and changes it to curd and whey.
Rennet, by producing acid and rendering the casein insoluble, helps us to obtain it for the manufacture of cheese; the substance next perhaps to animal food in nourishing properties.
The milk of ewes and goats is the richest, possessing as much as nine per cent, of casein and ten per cent, of butter.
Milk is adulterated with water, chalk, and starch, and is sometimes sold as pure milk when it is quite skim milk.
The Lactometer is the instrument used for testing milk, and should find a place in every well-furnished house. It is a glass tube, closed at one end, and marked with fine divisions on the outside. The milk is left for twenty-four hours in the tube, and the depth of the cream which forms on it can be ascertained by means of the graduations marked.
Chalk leaves a white sediment at the bottom of the glass if milk is left standing. The milk should be poured off without disturbing this sediment, and water should be poured on it, and it should again be allowed to settle ; repeat this a second time and a white powder will be left. If it effervesces with an acid it is chalk.
Cream may be kept by putting two or three lumps of sugar into it; boiling it first. It may be kept, if scalded, without sugar.