Coagulation occurs over a wider range of pH when milk is used with
the egg than when distilled water is used. If the pH is adjusted with
hydrochloric acid and sodium hydroxide, it occurs from pH 0.2 to 8.6 or
over even wider ranges. The reaction of a milk-egg custard mixture, with
no added acid or alkali, is between pH 6 to 7 with an average of about
6.5. This custard mixture was made up in a large quantity, and divided
into different portions. To these portions hydrochloric acid or sodium hy-
droxide was added to adjust the custard to the desired pH. As the pH
was lowered the firmness of the custard increased, until at about pH 5
curdling occurred. All custards with a pH below 5 curdled, the curd be-
coming very fine in texture, and forming a dense layer in the bottom of
the container. This layer decreased in amount as the pH was lowered
below the isoelectric point of egg albumin. At a pH 0.2 the custard was
badly charred, and the curd was very slight and fine, the custard quite soft.
With increased alkalinity above pH 6.5 the coagulum was less firm. The
custards could be placed in the order of acidity by the depth of color: the
greater the acidity the lighter the color, the greater the alkalinity the
deeper the color. The color ranged from light cream, through yellow, to a
deep orange-yellow.
Acids. Chick and Martin state that acid solution hastens the second
part of the heat-coagulation process, that is, the clotting or coagulation,
but does not hasten the first part of the process, the denaturation. They
have reported that acid accelerates the rate of coagulation. They state
that the influence of acid in accelerating the coagulation rate of a neutral
solution is at first relatively small, but with each successive addition of acid
its influence becomes disproportionately greater. Loeb using isoelectric crys-
talline egg albumin in a 1 per cent solution at a /)H 4.8 found that it
coagulated at a temperature not far from 60°C. When acid was added
and the /)H lowered to 4.39 the coagulation did not occur until about
80°C. With pH 4.25 coagulation did not occur at 95°C.
Fruits that do not have a high acidity such as dates and figs may be
used in custards and tend to give a firmer custard, because they lower the
/)H slightly. More acid fruits, such as lemon juice, cannot be used in
very large quantities. The addition of quite acid fruit juices tends not only
to coagulate the casein and albumin but also to hasten curdling. Custards
that are made of milk that is slightly sour will curdle more readily during
heating. If the acidity has not reached the stage at which curdling occurs,
the custard is firmer.
Alkalies. Chick and Martin have found that in alkaline solution the
second part of the coagulation process, the aggregation or coagulation of
CUSTARDS 337
the protein, does not occur. If after heating the alkali is neutralized with
acid, coagulation occurs. Thus, if enough of an alkali or of an alkaline
salt is added to a custard to render the solution sufficiently alkaline, the
custard will not coagulate on heating. But if acidified after heating the
custard will **set."
Page 274
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Coagulation occurs over a wider range of pH when milk is used with
the egg than when distilled water is used. If the pH is adjusted with
hydrochloric acid and sodium hydroxide, it occurs from pH 0.2 to 8.6 or
over even wider ranges. The reaction of a milk-egg custard mixture, with
no added acid or alkali, is between pH 6 to 7 with an average of about
6.5. This custard mixture was made up in a large quantity, and divided
into different portions. To these portions hydrochloric acid or sodium hy-
droxide was added to adjust the custard to the desired pH. As the pH
was lowered the firmness of the custard increased, until at about pH 5
curdling occurred. All custards with a pH below 5 curdled, the curd be-
coming very fine in texture, and forming a dense layer in the bottom of
the container. This layer decreased in amount as the pH was lowered
below the isoelectric point of egg albumin. At a pH 0.2 the custard was
badly charred, and the curd was very slight and fine, the custard quite soft.
With increased alkalinity above pH 6.5 the coagulum was less firm. The
custards could be placed in the order of acidity by the depth of color: the
greater the acidity the lighter the color, the greater the alkalinity the
deeper the color. The color ranged from light cream, through yellow, to a
deep orange-yellow.
Acids. Chick and Martin state that acid solution hastens the second
part of the heat-coagulation process, that is, the clotting or coagulation,
but does not hasten the first part of the process, the denaturation. They
have reported that acid accelerates the rate of coagulation. They state
that the influence of acid in accelerating the coagulation rate of a neutral
solution is at first relatively small, but with each successive addition of acid
its influence becomes disproportionately greater. Loeb using isoelectric crys-
talline egg albumin in a 1 per cent solution at a /)H 4.8 found that it
coagulated at a temperature not far from 60°C. When acid was added
and the /)H lowered to 4.39 the coagulation did not occur until about
80°C. With pH 4.25 coagulation did not occur at 95°C.
Fruits that do not have a high acidity such as dates and figs may be
used in custards and tend to give a firmer custard, because they lower the
/)H slightly. More acid fruits, such as lemon juice, cannot be used in
very large quantities. The addition of quite acid fruit juices tends not only
to coagulate the casein and albumin but also to hasten curdling. Custards
that are made of milk that is slightly sour will curdle more readily during
heating. If the acidity has not reached the stage at which curdling occurs,
the custard is firmer.
Alkalies. Chick and Martin have found that in alkaline solution the
second part of the coagulation process, the aggregation or coagulation of
CUSTARDS 337
the protein, does not occur. If after heating the alkali is neutralized with
acid, coagulation occurs. Thus, if enough of an alkali or of an alkaline
salt is added to a custard to render the solution sufficiently alkaline, the
custard will not coagulate on heating. But if acidified after heating the
custard will **set."