(1) Condensed Milk
The compound known as "condensed milk " is an illustration of the application of the drying or desiccation theory, accomplished by evaporating the excess of moisture, adding sugar, and packing in hermetically-sealed vessels. The milk, as received from the dairies, is placed, in vessels having a capacity of 750 to 1000 gal., where it is maintained at a slightly raised temperature by means of steam-heat, and undergoes evaporation in vacuo. The duration of the process varies from 2 to 5 1/2 hours. Refined sugar in powder is added in the proportion of about 1/3 by weight of the total condensed product; and when the mass assumes the consistency of thick honey, it is put into tin boxes, and hermetically sealed. The proper conduct of the operation is by no means easy. There is much danger of a decomposition of the caseine in the presence of heat and sugar, especially if the milk has been in the slightest degree "turned "; also much of the fatty constituents will distil with the water, if the temperature is allowed to exceed 100° F. (38° C). Attention has recently been called, in the Analyst and elsewhere, to the fact that these unfavourable conditions do frequently come into play, and that the loss of nitrogenous matter by decomposition, and the loss of equally important fat, partly volatilized, partly decomposed, so generally sustained by condensed milk, render it unfit to replace new milk in the nursery.
Small quantities are prepared (almost solely for the American market) without the addition of sugar, in which case the evil is lessened; but the product does not keep so well.
(2) Mabrun's Process
This simple process was probably the foundation of the preceding. The milk is warmed at a moderate temperature, in a tin vessel furnished with a leaden tube for the expulsion of the air. The tube is then compressed, and the orifice is soldered up. After 6 months' keeping, the milk is as good as new. The process received a prize of 1500 fr. from the French Academy of Sciences.
(3) Morfit's Process
In 1 gal. milk at 130° to 140° F. (55° to 60° C.) is dissolved 1 lb. gelatine; the mixture is left to cool to a jelly, when it is cut into slices and dried. The compound is used to gelatinize more milk, and this is repeated till the gelatine is in the proportion of 1 lb. to 10 gal. of milk.
(4) Neumann points out that the electric state of milk, as affected by the bodies with which it comes in contact, exerts an unquestionable influence over Its keeping. Milk which has stood in a tin vessel, and is turned out into glass or pewter, will not keep sweet so long as if left in the tin. Milk will keep well in zinc, antimony, bismuth, copper, brass, or iron vessels; but iron is apt to impart a disagreeable taste; and copper, after a while, is found in notable proportion in the milk. Caution is therefore requisite with utensils of this metal. Block-tin vessels are best; but the milk should not be shifted from vessel to vessel, and the latter should be filled as full as possible.
(5) When milk contained in wire-corked bottles is heated to the boiling-point in a water-bath, the oxygen of the included small portion of air under the cork seems to be carbonated, and the milk will, it is said, keep fresh for a year or two.
According to Dr. Be-sana, this substance, which has met with so much favour in England and elsewhere as an antiseptic, especially for the preservation of milk, has the following composition: - Boracic acid, 18 parts; borax, 9; sugar, 9; glycerine, 6.
(7) Amixtureof 2 dr. boracic acid with 3 dr. common salt, of which an addition of § dr. to 1 gal. of milk is said to increase its keeping qualities for 24 hours.
(8) According to Prof. Caldwell, boracic acid is the best antiseptic for preserving milk or keeping it sound for an unusual length of time. When the temperature was 80° F. (27° C), and the milk soured in 20 to 22 hours, 1 part boracic acid, added to 500 of milk, caused it to remain sweet for 50 hours. Again, he found that 1 part boracic acid, added to 1000 of milk by weight, kept it sweet for a space of 50 hours when the temperature was 72° F. (22° C). When applied to milk warm from the cow, it kept it sweet and sound twice as long as milk not treated with it. No injury occurs to the milk in using 1 part boracic acid for 1000 of milk. Boracic acid, he stated, was not poisonous. He had partaken of milk thus preserved, and no harm resulted from the taking of such milk into the stomach.
(9) Portch states that salicylic acid cannot be considered a success for preserving milk or butter, as it conveys an unpleasant sweetish flavour, which increases till decomposition ensues.