Temperature for rennin action. The optimum coagulation by calf rennin
is about 40° to 42°C. Below this temperature coagulation is less rapid
and no clotting occurs below 10° to 15°C. Also no clotting occurs above
60° to 65 °C. The clot is softer at low temperatures and tougher and
stringy at high temperatures. By optimum is meant the temperature at
which coagulation takes place most rapidly for a definite concentration of
rennin and milk.
Effect of previously boiling the milk upon rennin coagulation. If milk
is boiled and then cooled before the rennin is added, the rate of coagulation
is retarded and a much softer, more flocculent clot is obtained. Pasteuriza-
tion also affects the rate of coagulation of the milk and the type of clot
formed by rennin but not to the extent that boiling does.
Richardson and Palmer found by electrokinetic evidence that heat in-
creased the electric charge on the casein micelles or the cataphoretic velocity
of the casein solution. The fact that rennin does not form as firm a clot
with milk that has been previously heated indicates that rennin reduces
the charge on the casein particles but not sufficiently to form a firm clot.
This offers a colloidal explanation of why the addition of active cations
(as calcium chloride) to heated milk causes the rennin to coagulate the
milk normally.
Hydrogen-ion concentration. The reaction of the milk affects the rapid-
ity of coagulation and the character of the curd formed. Ordinarily when
the reaction of the milk is alkaline coagulation does not occur. This is
shown by the addition of a small amount of soda to milk before the addi-
tion of junket. The optimum hydrogen-ion concentration for rennin activity
has been reported to lie in the zone between pH 5.99 and 6.40.
Character of cations. In addition to rennin, cations are necessary to
bring about coagulation of milk. Because casein and calcium are so closely
involved in milk, the cation calcium is important in bringing about coagula-
tion. Hence, Rogers states that it is to be expected that the concentration
of both casein and calcium markedly affect both the rate of coagulation and
the character of the clot. If milk is diluted with sufficient water, clotting
is both delayed and incomplete, the clot being soft. If calcium chloride
is added to the water, diluted milk clotting properties are restored, which
suggests that the concentration of calcium ions is more important than
that of the casein ions.
Rogers states that any metallic ion can replace the calcium in coagulation.
However, it is generally accepted that the sodium and potassium salts of
paracasein are soluble. Monovalent ions are less effective than divalent
ones in replacing the calcium. Rogers reports that all monovalent ions did
not bring about coagulation in some instances. The divalent ions were not
all equally effective, calcium and barium being more efficient than mag-
nesium.
304 MILK AND CHEESE
Sugar. Sugar tends to prevent the coagulation of milk by rennin.
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Temperature for rennin action. The optimum coagulation by calf rennin
is about 40° to 42°C. Below this temperature coagulation is less rapid
and no clotting occurs below 10° to 15°C. Also no clotting occurs above
60° to 65 °C. The clot is softer at low temperatures and tougher and
stringy at high temperatures. By optimum is meant the temperature at
which coagulation takes place most rapidly for a definite concentration of
rennin and milk.
Effect of previously boiling the milk upon rennin coagulation. If milk
is boiled and then cooled before the rennin is added, the rate of coagulation
is retarded and a much softer, more flocculent clot is obtained. Pasteuriza-
tion also affects the rate of coagulation of the milk and the type of clot
formed by rennin but not to the extent that boiling does.
Richardson and Palmer found by electrokinetic evidence that heat in-
creased the electric charge on the casein micelles or the cataphoretic velocity
of the casein solution. The fact that rennin does not form as firm a clot
with milk that has been previously heated indicates that rennin reduces
the charge on the casein particles but not sufficiently to form a firm clot.
This offers a colloidal explanation of why the addition of active cations
(as calcium chloride) to heated milk causes the rennin to coagulate the
milk normally.
Hydrogen-ion concentration. The reaction of the milk affects the rapid-
ity of coagulation and the character of the curd formed. Ordinarily when
the reaction of the milk is alkaline coagulation does not occur. This is
shown by the addition of a small amount of soda to milk before the addi-
tion of junket. The optimum hydrogen-ion concentration for rennin activity
has been reported to lie in the zone between pH 5.99 and 6.40.
Character of cations. In addition to rennin, cations are necessary to
bring about coagulation of milk. Because casein and calcium are so closely
involved in milk, the cation calcium is important in bringing about coagula-
tion. Hence, Rogers states that it is to be expected that the concentration
of both casein and calcium markedly affect both the rate of coagulation and
the character of the clot. If milk is diluted with sufficient water, clotting
is both delayed and incomplete, the clot being soft. If calcium chloride
is added to the water, diluted milk clotting properties are restored, which
suggests that the concentration of calcium ions is more important than
that of the casein ions.
Rogers states that any metallic ion can replace the calcium in coagulation.
However, it is generally accepted that the sodium and potassium salts of
paracasein are soluble. Monovalent ions are less effective than divalent
ones in replacing the calcium. Rogers reports that all monovalent ions did
not bring about coagulation in some instances. The divalent ions were not
all equally effective, calcium and barium being more efficient than mag-
nesium.
304 MILK AND CHEESE
Sugar. Sugar tends to prevent the coagulation of milk by rennin.