chloric
acid
Combinations of proteins with acids or alkalies in food prep-
aration. Many combinations of proteins with acids or alkalies are formed
in food preparation. Most alkaline salts of the proteins are soluble. Some of
the acid salts are soluble; others are difficultly soluble. Casein, the protein
present in milk in the largest quantity, has a pH of 4.7 at its isoelectric
point. Casein in sweet milk is found as an alkaline salt. Fresh milk has a
pH of 6 to 7. If an acid is added to the milk the casein will be precipitated
when the reaction of the milk reaches the isoelectric point of the casein,
pH 4.7. This occurs in natural souring by the formation of lactic acid
in the milk. Familiar examples of combinations of acid with milk are the
addition of lemon juice to milk for sherbet or the addition of tomatoes to
milk for cream of tomato soup. If enough acid is added to lower the reac-
tion of the milk below the isoelectric point of the casein, an acid salt is
formed. If this salt is soluble, the curds of casein will dissolve. This change
of the protein from an alkaline to an acid salt often occurs in making may-
onnaise and other salad dressings. The addition of a small amount of acid
to egg yolk will curdle it, but upon the addition of a little more acid the
curd may dissolve.
Stoichiometrical combination. Stoichiometrical combination means
that the reaction between compounds is carried out according to the laws
of valence. Loeb and others working with dilute solutions of proteins,
acids, alkalies, and salts showed that proteins combine with acids and al-
kalies in stoichiometrical relationship. But Hoffman and Gortner have
shown that proteins in stronger concentrations of acids or alkalies adsorb
SURFACE TENSION 13
acid or alkali*. This means that owing to the surface area and the physical
property of adsorption the proteins can combine with larger quantities of
acids or alkalies than is possible in stoichiometrical combination alone.
Boundary Phenomena
Because of the size of micelles, surface phenomena assume an important
place in colloidal reactions. Surface tension, the formation of foams, inter-
facial tension, adsorption, formation of surface skins, orientation of mole-
cules, cohesion, and adhesion all have application in food preparation. Dif-
ferent authorities use a different terminology to designate the chemical and
physico-chemical processes taking place at the interface between two phases.
Kruyt calls them boundary phenomena, Freundlich designates them as
capillary chemistry, and other authorities use other terms.
Total surface area increases in proportion to the increase in number and
decrease in size of the micelles. Molecular systems have proportionally a
greater surface area than colloidal ones, but, on account of the small size
of the particles, other forces have a greater effect than surface ones in
molecular systems.
Freundlich states that '*The subject of capillary chemistry may be divided
into natural subdivisions, according to the nature of the interfaces which
separate the various possible pairs of phases. We can distinguish the follow-
ing interfaces: liquid/gaseous, liquid/liquid, solid/gaseous, solid/liquid,
solid/solid. Because of the complete rigidity of the interface between two
solids, the section relating to this pair drops out."
Page 13
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chloric
acid
Combinations of proteins with acids or alkalies in food prep-
aration. Many combinations of proteins with acids or alkalies are formed
in food preparation. Most alkaline salts of the proteins are soluble. Some of
the acid salts are soluble; others are difficultly soluble. Casein, the protein
present in milk in the largest quantity, has a pH of 4.7 at its isoelectric
point. Casein in sweet milk is found as an alkaline salt. Fresh milk has a
pH of 6 to 7. If an acid is added to the milk the casein will be precipitated
when the reaction of the milk reaches the isoelectric point of the casein,
pH 4.7. This occurs in natural souring by the formation of lactic acid
in the milk. Familiar examples of combinations of acid with milk are the
addition of lemon juice to milk for sherbet or the addition of tomatoes to
milk for cream of tomato soup. If enough acid is added to lower the reac-
tion of the milk below the isoelectric point of the casein, an acid salt is
formed. If this salt is soluble, the curds of casein will dissolve. This change
of the protein from an alkaline to an acid salt often occurs in making may-
onnaise and other salad dressings. The addition of a small amount of acid
to egg yolk will curdle it, but upon the addition of a little more acid the
curd may dissolve.
Stoichiometrical combination. Stoichiometrical combination means
that the reaction between compounds is carried out according to the laws
of valence. Loeb and others working with dilute solutions of proteins,
acids, alkalies, and salts showed that proteins combine with acids and al-
kalies in stoichiometrical relationship. But Hoffman and Gortner have
shown that proteins in stronger concentrations of acids or alkalies adsorb
SURFACE TENSION 13
acid or alkali*. This means that owing to the surface area and the physical
property of adsorption the proteins can combine with larger quantities of
acids or alkalies than is possible in stoichiometrical combination alone.
Boundary Phenomena
Because of the size of micelles, surface phenomena assume an important
place in colloidal reactions. Surface tension, the formation of foams, inter-
facial tension, adsorption, formation of surface skins, orientation of mole-
cules, cohesion, and adhesion all have application in food preparation. Dif-
ferent authorities use a different terminology to designate the chemical and
physico-chemical processes taking place at the interface between two phases.
Kruyt calls them boundary phenomena, Freundlich designates them as
capillary chemistry, and other authorities use other terms.
Total surface area increases in proportion to the increase in number and
decrease in size of the micelles. Molecular systems have proportionally a
greater surface area than colloidal ones, but, on account of the small size
of the particles, other forces have a greater effect than surface ones in
molecular systems.
Freundlich states that '*The subject of capillary chemistry may be divided
into natural subdivisions, according to the nature of the interfaces which
separate the various possible pairs of phases. We can distinguish the follow-
ing interfaces: liquid/gaseous, liquid/liquid, solid/gaseous, solid/liquid,
solid/solid. Because of the complete rigidity of the interface between two
solids, the section relating to this pair drops out."