In making sorghum
molasses, in order to have a delicate-flavored product, one must have
"a good boil" and remove the scum forming on the surface. In this way,
tannins, which would increase the bitterness of the sorghum, proteins, and
other substances are removed.
Rahn has shown analytically the results of the application of "the law of
Gibbs and Thomson which states that substances which cause a depression
of surface tension will accumulate in the surface. In a gelatin solution,
there is more gelatin in the very surface layer than in the center of the
solution." In a milk foam the concentration of protein is greater in the
foam than in the solution. The analytical results of Rahn are given in
Table 2.
Rahn states that "If protein is concentrated on the surface it has a
tendency to become solid, but all proteins do not behave alike. Some
solidify rapidly, others slowly, and some do not solidify at all. Quite often,
this solidification is irreversible, and the protein, when put back into the
solution, will not dissolve again." When milk foams, solid walls form
around the air bubbles and, when the foam settles, these walls of protein
can be seen with the aid of a microscope.
Interfacial tension of liquids/liquids. When two non-miscible
liquids are poured together, one liquid forms a layer on top of the other,
thus making a liquid/liquid boundary. The less the solubility of the two
liquids in each other, the greater their interfacial tension; but most liquids
16 RELATION OF COOKERY TO COLLOID CHEMISTRY
TABLE 2
Composition of Skimmed Milk and Its Foam (Rahn)
Constituent of
Average of 9 experiments
with intensely foaming
skimmed milk
Average of 6 experiments
with slightly foaming
skimmed milk
the milk
Liquid part
%
Foam
%
Liquid
%
Foam
%
Protein
Fat and lactose
Ash
3.09
4.85
0.75
3.51
4.73
0.78
3.01
4.92
0.74
3.24
4.92
0.76
Total solids
Protein increase in the
foam
8.69
9.02
13.6
8.67
8.92
7.6
are not completely insoluble in each other. Just as a substance may con-
centrate at a liquid/gaseous interface, so likewise those substances which
decrease interfacial tension tend to concentrate at a liquid/liquid or a
liquid/solid interface. With the addition of a third substance soluble in
water, Traube's rule holds, but if it is more soluble in the second liquid
then the lowering of the interfacial tension is small and Traube's rule
scarcely applies.
Adsorption. Adsorption has been defined as the concentration of a
substance at an interface. The increased concentration at the interface is
designated as positive adsorption ; a decrease in the boundary layer is called
negative adsorption. The amount adsorbed depends on the concentration
of the material being adsorbed and the extent of the surface at which it
can be adsorbed. The importance of surface reactions or phenomena can-
not be over-emphasized in food preparation. Lowering of interfacial tension
aids in the forming of emulsions. Fat is strongly adsorbed by sugar crystals,
and when the two are mixed this aids in distributing the fat throughout
the batter.
Page 16
Presented as published in 1932. Historical recipes may not meet modern food-safety standards. Cook from the modern interpretation, not the original instructions.
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In making sorghum
molasses, in order to have a delicate-flavored product, one must have
"a good boil" and remove the scum forming on the surface. In this way,
tannins, which would increase the bitterness of the sorghum, proteins, and
other substances are removed.
Rahn has shown analytically the results of the application of "the law of
Gibbs and Thomson which states that substances which cause a depression
of surface tension will accumulate in the surface. In a gelatin solution,
there is more gelatin in the very surface layer than in the center of the
solution." In a milk foam the concentration of protein is greater in the
foam than in the solution. The analytical results of Rahn are given in
Table 2.
Rahn states that "If protein is concentrated on the surface it has a
tendency to become solid, but all proteins do not behave alike. Some
solidify rapidly, others slowly, and some do not solidify at all. Quite often,
this solidification is irreversible, and the protein, when put back into the
solution, will not dissolve again." When milk foams, solid walls form
around the air bubbles and, when the foam settles, these walls of protein
can be seen with the aid of a microscope.
Interfacial tension of liquids/liquids. When two non-miscible
liquids are poured together, one liquid forms a layer on top of the other,
thus making a liquid/liquid boundary. The less the solubility of the two
liquids in each other, the greater their interfacial tension; but most liquids
16 RELATION OF COOKERY TO COLLOID CHEMISTRY
TABLE 2
Composition of Skimmed Milk and Its Foam (Rahn)
Constituent of
Average of 9 experiments
with intensely foaming
skimmed milk
Average of 6 experiments
with slightly foaming
skimmed milk
the milk
Liquid part
%
Foam
%
Liquid
%
Foam
%
Protein
Fat and lactose
Ash
3.09
4.85
0.75
3.51
4.73
0.78
3.01
4.92
0.74
3.24
4.92
0.76
Total solids
Protein increase in the
foam
8.69
9.02
13.6
8.67
8.92
7.6
are not completely insoluble in each other. Just as a substance may con-
centrate at a liquid/gaseous interface, so likewise those substances which
decrease interfacial tension tend to concentrate at a liquid/liquid or a
liquid/solid interface. With the addition of a third substance soluble in
water, Traube's rule holds, but if it is more soluble in the second liquid
then the lowering of the interfacial tension is small and Traube's rule
scarcely applies.
Adsorption. Adsorption has been defined as the concentration of a
substance at an interface. The increased concentration at the interface is
designated as positive adsorption ; a decrease in the boundary layer is called
negative adsorption. The amount adsorbed depends on the concentration
of the material being adsorbed and the extent of the surface at which it
can be adsorbed. The importance of surface reactions or phenomena can-
not be over-emphasized in food preparation. Lowering of interfacial tension
aids in the forming of emulsions. Fat is strongly adsorbed by sugar crystals,
and when the two are mixed this aids in distributing the fat throughout
the batter.