Effect of heating milk on acidity. When milk is heated at the boil-
ing point or at temperatures above or near the boiling point the titratable
acidity at first decreases owing to the loss of carbon dioxide, and then
increases. Whittier and Benton report that the hydrogen-ion concentra-
tion increases continuously. They find the hydrogen-ion increase and the
later increase in titratable acidity is due to the formation of acids from
constituents of the milk. The amount of acid produced depends upon the
time and temperature of heating, a greater amount of acid being produced
with a longer heating period and with higher temperatures. From their
experiments they conclude that the acid is produced from the lactose of the
milk. They have shown that, the greater the concentration of lactose pres-
ent, the greater the amount of acid formed at a definite temperature and
for a definite time.
Coagulation of Milk
Under certain conditions, the addition of alcohol as well as the appli-
cation of heat may cause coagulation of milk. Milk may be coagulated by
the addition of rennin or by bringing the acidity oi the milk to the isoelec-
tric point of the casein. When milk is combined with other foods, the salt
content of the food or the tannin content of the food may be factors that
aid coagulation.
Alcohol coagulation. The addition of 70 per cent alcohol to milk may
cause coagulation. As the pH of the milk decreases, it becomes susceptible
to alcohol precipitation, though this varies with different milks. Usually
the milk is precipitated by alcohol while it is still stable to heat and
sterilizing temperatures. Some freshly secreted milk is coagulated by
alcohol, but this milk is usually abnormal in some way. About 2 cc. of
alcohol are added to an equal quantity of milk for the test. Casein is
precipitated by alcohol as calcium caseinate, calcium is not released as by
acid coagulation.
Rennet coagulation. Milk may be coagulated by the addition of
rennet. Rennet is an extract that is usually obtained from the inner lining
of the stomachs of calves and lambs. The rennet contains an enzyme called
rennase or rennin.
The clotting of the milk is generally believed to be the direct action of
the rennin on the casein. But the manner in which these changes is pro-
duced is not fully understood. A very small amount of rennin is capable
of coagulating a large amount of milk. At favorable or optimum hydrogen-
ion concentrations for clotting, 1 part of the fairly pure enzyme prepara-
tion is able to coagulate 3,000,000 or more parts of milk.
Mechanism of clotting. It is usually stated that the casein is changed to
paracasein by the action of the rennin. It is also often stated that the
302 MILK AND CHEESE
Page 244
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Effect of heating milk on acidity. When milk is heated at the boil-
ing point or at temperatures above or near the boiling point the titratable
acidity at first decreases owing to the loss of carbon dioxide, and then
increases. Whittier and Benton report that the hydrogen-ion concentra-
tion increases continuously. They find the hydrogen-ion increase and the
later increase in titratable acidity is due to the formation of acids from
constituents of the milk. The amount of acid produced depends upon the
time and temperature of heating, a greater amount of acid being produced
with a longer heating period and with higher temperatures. From their
experiments they conclude that the acid is produced from the lactose of the
milk. They have shown that, the greater the concentration of lactose pres-
ent, the greater the amount of acid formed at a definite temperature and
for a definite time.
Coagulation of Milk
Under certain conditions, the addition of alcohol as well as the appli-
cation of heat may cause coagulation of milk. Milk may be coagulated by
the addition of rennin or by bringing the acidity oi the milk to the isoelec-
tric point of the casein. When milk is combined with other foods, the salt
content of the food or the tannin content of the food may be factors that
aid coagulation.
Alcohol coagulation. The addition of 70 per cent alcohol to milk may
cause coagulation. As the pH of the milk decreases, it becomes susceptible
to alcohol precipitation, though this varies with different milks. Usually
the milk is precipitated by alcohol while it is still stable to heat and
sterilizing temperatures. Some freshly secreted milk is coagulated by
alcohol, but this milk is usually abnormal in some way. About 2 cc. of
alcohol are added to an equal quantity of milk for the test. Casein is
precipitated by alcohol as calcium caseinate, calcium is not released as by
acid coagulation.
Rennet coagulation. Milk may be coagulated by the addition of
rennet. Rennet is an extract that is usually obtained from the inner lining
of the stomachs of calves and lambs. The rennet contains an enzyme called
rennase or rennin.
The clotting of the milk is generally believed to be the direct action of
the rennin on the casein. But the manner in which these changes is pro-
duced is not fully understood. A very small amount of rennin is capable
of coagulating a large amount of milk. At favorable or optimum hydrogen-
ion concentrations for clotting, 1 part of the fairly pure enzyme prepara-
tion is able to coagulate 3,000,000 or more parts of milk.
Mechanism of clotting. It is usually stated that the casein is changed to
paracasein by the action of the rennin. It is also often stated that the
302 MILK AND CHEESE