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644 MILK.
peroxidase and a proteolytic enzyme which, according to Hildebrandt, 1
occurs to a much greater extent in the active gland as compared with
the inactive one.
As human milk and the milk of animals are essentially of the same
constitution, it seems best to speak first of the one most thoroughly
investigated, namely, cow’s milk, and then of the essential properties
of the remaining important kinds of milk.2
Cow’s Milk.
Cow’s milk, like every other kind, forms an emulsion which consists
of very finely divided fat suspended in a solution consisting principally
of protein bodies, milk-sugar, and salts. Milk is non-transparent, white,
whitish-yellow, or in thin layers somewhat bluish-white, of a faint, insipid
odor and mild, faintly sweetish taste. The specific gravity is 1.028
to 1.0345 at 15° C. The freezing-point is -0.54-0.59° C., average
—0.563° C, and the molecular concentration 0.298.
The reaction of perfectly fresh milk is generally amphoteric toward
litmus. The extent of the acid and alkaline part of this amphoteric
reac:ion has been determined by different investigators, especially
Thorner, Sebelien, and Cotjrant.3
The results differ with the indi-
cators used, and moreover the milk from different animals, as well as
that from the same animal at different times during the lactation period,
varies slightly. Courant determined the alkaline part by N/10 sul-
phuric acid, using blue lacmoid as indicator, and the acid part by N/10
caustic soda, using phenolphthalein as indicator. He found, as an average
for the first and last portions of the milking of twenty cows, that 100
cc. milk had the same alkaline reaction toward blue lacmoid as 41 cc.
N/10 caustic soda, and the same acid reaction toward phenolphthalein
as 19.5 cc. N/10 sulphuric acid. The actual reaction of cow’s milk,
which follows from the electrometric estimation, is, on the contrary,
Foa 4
claims, nearly neutral, like the reaction of animal fluids and
tissues in general.
Milk gradually changes when exposed to the air, and its reaction
becomes more and more acid. This depends on a gradual transforma-
tion of the milk-sugar into lactic acid, caused by micro-organisms.
’Bert, Compt. Rend., 98; Thierfelder, Pfliiger’s Arch., 34, and Maly’s Jahresber.,
13; Hildebrandt, Hofmeister’s Beitrage, 5.
* A very complete reference to the literature on milk may be found in Raudnitz’s
"Die Bestandteile der Milch," in Er{r’’bnisse der Physiol., 2, Abt. 1. The literature
of the last few years may be found in the references by Raudnitz, Monatsschrift f.
Kinderheilkunde.
’ Thorner, Maly’s Jahresber., 22; Sebelien, ibid., Courant; Pfliiger’s Arch., 50.
* Compt. rend. soc. biolog. (58), 59, 51.
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