The role of salts in heat coagulation of milk. In heat coagulation of
milk, the milk salts play an important role, for the salt equilibrium is
altered by heat. When milk is boiled precipitation of part of the calcium
phosphate occurs. Sommer and Hart have concluded that salts are the
main factor in heat coagulation of fresh milk. Electrolytes have a marked
effect upon the stability of colloids. In precipitating a hydrophilic colloid
divalent and trivalent ions are generally more effective than monovalent
ones. In the milk are found the monovalent cations, sodium and potas-
sium; the monovalent anion, chlorine; the divalent cations, calcium and
magnesium; and the trivalent anions, phosphate and citrate. Sommer and
Hart concluded that the coagulation of milk on heating may be due to an
excess or a deficiency of calcium and magnesium. They explain this as
follows. "The casein of the milk is most stable with regard to heat when
it is in combination with the calcium. If the calcium combined with the
casein is above or below this optimum, the casein is not in its most stable
condition. The calcium of the milk distributes itself between the casein,
citrates, and phosphates chiefly. If the milk is high in citrate and phosphate
content, more calcium is necessary in order that the casein may retain its
optimum calcium content after competing with the citrates and phosphates.
If the milk is high in calcium there may not be sufficient citrates and
phosphates to compete with the casein to lower its calcium content to the
optimum. In such cases the addition of citrates or phosphates makes the
casein more stable by reducing its calcium content. The magnesium func-
tions by replacing the calcium in the citrates and phosphates."
Heat coagulation of casein endothermic. Leighton and Mudge have
shown that an endothermic reaction accompanies the appearance of visible
curds when milk is coagulated by heat. This is accompanied by precipita-
tion of calcium and magnesium as phosphate and citrates.
A similar reaction occurs in custard. In cooking custard, the ingredients
of which are milk, egg, and sugar, the temperature drops or does not rise
for a period of time during coagulation or setting of the custard, a condition
particularly noticeable just before curdling takes place.
Coagulation of milk by cooking meat or vegetables in it. Fresh
milk is seldom coagulated by heating for home use. The temperature
attained in ordinary heating is not great enough to cause coagulation, nor
is the milk heated for the long period required for coagulation at boiling
temperatures. But with the addition of other foods to milk in food prepara-
tion, coagulation often occurs with a very short period of heating. One
of the factors in this coagulation is undoubtedly the salt content of the
food added to the milk as well as the salt content of the milk. The balance
of the milk salts for greatest stability may be upset and coagulation occurs
when the food is heated in the milk.
COOKING OF VEGETABLES IN MILK 307
Page 249
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The role of salts in heat coagulation of milk. In heat coagulation of
milk, the milk salts play an important role, for the salt equilibrium is
altered by heat. When milk is boiled precipitation of part of the calcium
phosphate occurs. Sommer and Hart have concluded that salts are the
main factor in heat coagulation of fresh milk. Electrolytes have a marked
effect upon the stability of colloids. In precipitating a hydrophilic colloid
divalent and trivalent ions are generally more effective than monovalent
ones. In the milk are found the monovalent cations, sodium and potas-
sium; the monovalent anion, chlorine; the divalent cations, calcium and
magnesium; and the trivalent anions, phosphate and citrate. Sommer and
Hart concluded that the coagulation of milk on heating may be due to an
excess or a deficiency of calcium and magnesium. They explain this as
follows. "The casein of the milk is most stable with regard to heat when
it is in combination with the calcium. If the calcium combined with the
casein is above or below this optimum, the casein is not in its most stable
condition. The calcium of the milk distributes itself between the casein,
citrates, and phosphates chiefly. If the milk is high in citrate and phosphate
content, more calcium is necessary in order that the casein may retain its
optimum calcium content after competing with the citrates and phosphates.
If the milk is high in calcium there may not be sufficient citrates and
phosphates to compete with the casein to lower its calcium content to the
optimum. In such cases the addition of citrates or phosphates makes the
casein more stable by reducing its calcium content. The magnesium func-
tions by replacing the calcium in the citrates and phosphates."
Heat coagulation of casein endothermic. Leighton and Mudge have
shown that an endothermic reaction accompanies the appearance of visible
curds when milk is coagulated by heat. This is accompanied by precipita-
tion of calcium and magnesium as phosphate and citrates.
A similar reaction occurs in custard. In cooking custard, the ingredients
of which are milk, egg, and sugar, the temperature drops or does not rise
for a period of time during coagulation or setting of the custard, a condition
particularly noticeable just before curdling takes place.
Coagulation of milk by cooking meat or vegetables in it. Fresh
milk is seldom coagulated by heating for home use. The temperature
attained in ordinary heating is not great enough to cause coagulation, nor
is the milk heated for the long period required for coagulation at boiling
temperatures. But with the addition of other foods to milk in food prepara-
tion, coagulation often occurs with a very short period of heating. One
of the factors in this coagulation is undoubtedly the salt content of the
food added to the milk as well as the salt content of the milk. The balance
of the milk salts for greatest stability may be upset and coagulation occurs
when the food is heated in the milk.
COOKING OF VEGETABLES IN MILK 307