The freezing-point depends on the concentration of those constit uents which are in perfect solution. It is consequently affected by the proportion of fat only so far as the space for the molecules in solution is decreased by bulk of the milk globules held in suspension. That is, the higher the content of fat, the less the space for the molecules in solu tion; e.g., in cream with 14 per cent. by weight of fat there remain available 85 space units; in unskimmed milk with 4 per cent., 9G space units. In like manner the other constituents in suspension diminish the space for the solution. No conclusion as to the state of the constit uents in suspension can at present be drawn from the freezing-point. Its depression depends not only on the number of the molecules hut on their nature, since mixed solutions act differently from solutions of a single salt. Solution and suspension, too,-are states that have much in common and differ in degree rather than in kind.
The electrical resistance depends on the concentration of the dis sociated molecules in solution and not on the proportion of sugar and urea. The constituents which are wholly or partially in suspension, increase by the electrical resistance their friction and cohesion. The reciprocal of electrical resistance is conductivity.
Its specific heat depends partly on the state of the fat globules. The more rigid they have become, the more heat is required to liquefy them.
The viscosity (internal friction) is a very complex property, which depends chiefly on the proportion of casein.
The reaction with litmus paper depends on the proportion of the dissolved components, especially of the phosphates and caseins. Since the presence of monophosphates alone would precipitate the casein, and diphosphates would turn red litmus paper blue, fresh milk can give only amphoterie or alkaline reactions. The capacity of the bases and acids to form combinations is measured by their ability to react with alkalies and acids, so as to produce changes of color in an indicator. For the alkali-combining power phenolphthalein and caustic soda are used. In testing the acid-combining power we titrate, using alizarin red, methyl orange, or blue litmus paper, with hydrochloric or sulphuric acid. Each of these indicators gives a different figure, the last named the highest, because the change to red follows only when all the casein has been combined with acids. All titrations are conventional. They give other figures if the degree of concentration of the titrated fluids is changed, as when diluted milk is used. If, instead of caustic soda, baryta is employed, much higher values will be obtained, because of the precipitation of the insoluble phosphates.
On standing, CO, is given off and 0 is taken up. The acidity con sequently decreases and with it the tendency to coagulation. The milk globules now rise and carry with than a liquid abounding in pro teids (especially perhaps in lecith-albtunins and lactomucins), in certain salts, superoxidases and reductases. The calcium phosphates, the casein, and the impurities, on the other hand, sink gradually to the bottom. By centrifugal force this separation is hastened and perfected. During this process micro-organisms pass both into the cream and into the slime. If bacterial decomposition can be warded off without boiling, the ferments contained in the milk gradually accomplish their function.
Filtration not only frees the milk from impurities and cellular ele ments, but removes some casein and apparently some lactomucin, even when only a dense paper filter is used. If the milk is drawn or forced
through a filter of clay, or the like, the filtrate will contain only the lactalbumill, traces, at most, of a proteid (which acetic acid will precip itate), the whole of the milk-sugar, the citric acid, the coloring matter, all the Cl and K, and nearly all the Na, but only a part, greater or less, of the Mg, Ca, Fe, Superoxidases, reductases, and proteolytic ferments do not pass the clay filter; the peroxidases pass only slowly and in traces. Data for other ferments are wanting. The alexins that have been tested (typhoid agglutinins and bactericides for typhus and colon) do not pass the filters.
In dialysis those constituents in solution which have a small molec ular weight, such as the dissolved salts, milk-sugar, and citric acid, are removed. (Human milk is reported not to surrender chlorine.) In protracted dialysis the alkali is withdrawn from the casein compounds and the casein is precipitated.
On freezing, a highly concentrated solution collects below while the fat rises in cream. The fat globules stiffen and such milk is easier to churn. If the milk is kept frozen for a long time its efficacy as a bac tericide is increased. Albuminous flakes of an unknown nature separate out on thawing, and may be dissolved by heating.
Heating, boiling, and superheating over 100° C. (•12° F.), cause fundamental changes in all the constituents. Slow heating to 70°—S0° C• (15S°-176° F.) has nearly the same effect as rapid heating to 100° C. (212° F.) 'Alilks very rich in lactalbumin or lactoglobulin (ass's milk, or colostrum in its first stage) coagulate on boiling, because the casein is carried clown by the precipitation of the proteids, which arc coagulated by heat. (The same occurs when cove's milk is boiled with S per cent. of egg-albumin.) Cow's milk is the only other kind whose behavior under heat has been accurately examined. First the odorous matter, about 90 per cent. of the CO„ and half of the () and are expelled. Consequently the acidity* falls about 17.5 c.c. MO sodium hydrate solution per litre and phosphates of the alkaline earths are precipitated. Even without the loss of the the phosphates are separated into soluble and insolu ble; but this change is subject to reversal, in the course of time, when ever the phosphates, insoluble in water, may he held in solution by the other salts. A part of the citric acid, however, becomes insoluble by precipitation of tricalcitun citrate (1 per cent. by heating to 75° C. (167° during 15 minutes; as high as:32 per cent. by heating to 100° C. (212° F.) during 5 minutes). Consequently the filtrate from a clay filter contains less calcium (5-14 mg. of ('a0 per litre) and less phosphoric acid (11 mg. of and the rennin coagulation is retarded by this and by the loss in acidity. The coagulates at 55° C. (131°F.) and upwards, but not completely until it has been kept, a long time at boiling heat. By this process both simple and alkali sulphuretted hydrogen (mercaptan) are split off from it, and these give rise to a part of the accompanying odor. The precipitated lactalbutnin forms, in connection with calcium salts, the foundAion with which the casein, after its dissociation, also unites.