=427. Composition of Milk.=—The composition of milk not only varies
with the genus and species of the mammal from which it is derived, but
also depends in a marked degree on idiosyncrasy.[400]
Milk is a mixture containing water, proteids, fat, carbohydrates,
organic and inorganic acids and mineral salts. There have also been
observed in milk in minute quantities ammonia, urea, hypoxanthin,
chyme, chyle, biliverdin, cholesterin, mucin, lecithin, kreatin, leucin
and tyrosin. In the fermentation which milk undergoes in incipient
decomposition there is sometimes developed from the proteid matter, as
pointed out by Vaughn, a ptomaine, tyrotoxicon, which is a virulent
poison.[401] The presence of these last named bodies is of interest
chiefly to the physiologist and pathologist and can receive no further
attention here.
From a nutritive point of view, the important components of milk are
the fats, proteids and sugar, but especially in the nourishment of the
young the value of lime and phosphoric acid must be remembered. The
mean composition of the most important milks, as determined by recent
analyses, is given below:
Water. Sugar. Proteids. Fat. Ash.
Per cent. Per cent. Per cent. Per cent. Per cent.
Cow 86.90 4.80 3.60 4.00 0.70
Human 88.75 6.00 1.50 3.45 0.30
Goat 85.70 4.45 4.30 4.75 0.80
Ass 89.50 6.25 2.00 1.75 0.50
Mare 90.75 5.70 2.00 1.20 0.35
Sheep 80.80 4.90 6.55 6.85 0.90
The mean composition of milk, as given by Watts and König, is given in
the following tables:
WATTS.
Mineral
Water. Solids. Proteids. Fats. Sugar. Salts.
Woman 87.65 12.35 3.07 3.91 5.01 0.17
Ass 90.70 9.30 1.70 1.55 5.80 0.50
Cow 86.56 13.44 4.08 4.03 4.60 0.73
Goat 86.76 13.24 4.23 4.48 3.91 0.62
Sheep 83.31 16.69 5.73 6.05 3.96 0.68
Mare 82.84 17.16 1.64 6.87 8.65
KÖNIG.
Casein and Milk
Water. Fat. albumin. sugar. Ash.
Woman 87.41 3.78 2.29 6.21 0.31
Mare 90.78 1.21 1.99 5.67 0.35
Ass 89.64 1.63 2.22 5.99 0.51
Cow 87.17 3.69 3.55 4.88 0.71
The average composition of 120,540 samples of cow milk, as determined
by analysis, extending over a period of eleven years, was found by
Vieth to be as follows:[402]
Per cent.
Total solids 12.9
Solids not fat 8.8
Fat 4.1
The quantity of solids and fat in milk is less after longer than after
shorter periods between milkings.
The quantity of solids and fat in cow milk is less in the spring than
in the autumn.
The chief organic acid naturally present in milk is citric, which
exists probably in combination with lime.
The mean content of citric acid in milk is about one-tenth of one per
cent.[403]
Citric acid is not found in human milk, and probably exists only in the
mammary secretions of herbivores.
Among the mineral acids of milk, phosphoric is the most important, but
a part of the phosphorus found as phosphoric acid in the ash of milk
may come from pre-existing organic phosphorus (lecithin, nuclein).
The sulfuric acid, which is found in the ash of milk, is derived from
the sulfur of the proteid matter during ignition.
Lactic acid is developed from lactose during the souring of milk as the
result of bacterial activity.
Gases are also found in solutions of milk, notably carbon dioxid, which
gives to freshly drawn milk its brothy appearance.
The ash of milk has the following composition expressed as grams per
liter of the original milk:[404]
Grams Probable form Grams
Component. per liter. of combination. per liter.
{ sodium chlorid 0.962
Chlorin 0.90 { potassium chlorid 0.830
{ KH₂PO₄ 1.156
{ K₂HPO₄ 0.853
Phosphoric acid 2.42 { MgHPO₄ 0.336
{ CaHPO₄ 0.671
{ Ca₃(PO₄)₂ 0.806
Potassium 1.80 (as shown above)
and as potassium citrate 0.495