great enough, nor the boiling temperature high enough, nor the boiling
long enough continued for these factors to be very important. The salt
and the tannin contents of the vegetables are probably the principal causes
of coagulation. Some vegetables contain larger amounts of tannin than
others. Tannin is a dehydrating agent and brings about denaturation of
hydrophilic sols, like gelatin, starch, and agar-agar. After denaturation the
hydrophile is sensitive to small amounts of electrolytes and precipitation
occurs readily. Kruyt states that tannins do not bring about dehydration
of the protein in an alkaline medium. Hence the addition of soda in small
amounts to the milk in w^hich the vegetable is cooked prevents coagulation
of the milk. Tannins lower the surface tension, which results in foaming
of vegetables containing tannin when they are cooked in water. It is rather
interesting that the vegetables that usually foam the most when cooked
in water are the ones that have the greatest tendency to coagulate milk.
Tomato soup. When tomatoes are combined with milk to make cream of
tomato soup, coagulation may occur. The acidity of tomatoes varies some-
what, but is about pH 4.4 to 4.6. If the amount of tomato added to the
milk is great enough to lower the /)H of the mixed milk and tomato to
4.8 to 4.6, the casein is precipitated without heating. This may happen if
the milk is already fairly acid.
Since a longer time of heating milk increases the tendency to curdle it is
preferable to heat tomato soup for only a short time. Heating slowly also
increases the tendency to curdle, which may be due to the longer time
required. In Experiment 55 in the laboratory outline several different
methods of combining the tomato juice and milk are given. The tomato
is usually added to the milk by stirring, for in this w^ay the milk is diluted
with a smaller amount of acid substance during the first part of the mixing.
There is less tendency to curdle when the hot tomato is added to cold milk,
than when the cold tomato is added to hot milk. Probably the slight
denaturation brought about by heating the milk may partially account
for this. Occasionally some milk is acid enough and the tomato is acid
enough to cause curdling with all methods of combining unless soda is
added. At other times curdling does not occur with any method of com-
bining as outlined in this experiment.
Fruits and milk. When cream is added to fruit, clotting often occurs.
This is usually due to the acidity of the fruit, but may also be due to an
enzyme in it. Raw pineapple contains an enzyme, bromelin, that brings
about clotting of milk. However, the pineapple juice not only brings about
clotting but also peptization, for after a time the clot formed is less firm
and the flavor is similar to that of peptized meat.
Boiling and Heating of Milk
The physical and chemical properties of the constituents of milk account
for the behavior of milk during its use in food preparation. Thus sub-
SUGAR REACTIONS WITH PROTEINS OF MILK 309
Page 251
Presented as published in 1932. Historical recipes may not meet modern food-safety standards. Cook from the modern interpretation, not the original instructions.
AI-modernized reading of the original text
A modernized reading is not available for this page yet. You are seeing the original text.
great enough, nor the boiling temperature high enough, nor the boiling
long enough continued for these factors to be very important. The salt
and the tannin contents of the vegetables are probably the principal causes
of coagulation. Some vegetables contain larger amounts of tannin than
others. Tannin is a dehydrating agent and brings about denaturation of
hydrophilic sols, like gelatin, starch, and agar-agar. After denaturation the
hydrophile is sensitive to small amounts of electrolytes and precipitation
occurs readily. Kruyt states that tannins do not bring about dehydration
of the protein in an alkaline medium. Hence the addition of soda in small
amounts to the milk in w^hich the vegetable is cooked prevents coagulation
of the milk. Tannins lower the surface tension, which results in foaming
of vegetables containing tannin when they are cooked in water. It is rather
interesting that the vegetables that usually foam the most when cooked
in water are the ones that have the greatest tendency to coagulate milk.
Tomato soup. When tomatoes are combined with milk to make cream of
tomato soup, coagulation may occur. The acidity of tomatoes varies some-
what, but is about pH 4.4 to 4.6. If the amount of tomato added to the
milk is great enough to lower the /)H of the mixed milk and tomato to
4.8 to 4.6, the casein is precipitated without heating. This may happen if
the milk is already fairly acid.
Since a longer time of heating milk increases the tendency to curdle it is
preferable to heat tomato soup for only a short time. Heating slowly also
increases the tendency to curdle, which may be due to the longer time
required. In Experiment 55 in the laboratory outline several different
methods of combining the tomato juice and milk are given. The tomato
is usually added to the milk by stirring, for in this w^ay the milk is diluted
with a smaller amount of acid substance during the first part of the mixing.
There is less tendency to curdle when the hot tomato is added to cold milk,
than when the cold tomato is added to hot milk. Probably the slight
denaturation brought about by heating the milk may partially account
for this. Occasionally some milk is acid enough and the tomato is acid
enough to cause curdling with all methods of combining unless soda is
added. At other times curdling does not occur with any method of com-
bining as outlined in this experiment.
Fruits and milk. When cream is added to fruit, clotting often occurs.
This is usually due to the acidity of the fruit, but may also be due to an
enzyme in it. Raw pineapple contains an enzyme, bromelin, that brings
about clotting of milk. However, the pineapple juice not only brings about
clotting but also peptization, for after a time the clot formed is less firm
and the flavor is similar to that of peptized meat.
Boiling and Heating of Milk
The physical and chemical properties of the constituents of milk account
for the behavior of milk during its use in food preparation. Thus sub-
SUGAR REACTIONS WITH PROTEINS OF MILK 309