perature, part of this being due to the effect of temperature upon the
intermicellar liquid. Gortner states that in "colloid systems changes due
to temperature are influenced not only by the viscosity of the dispersion
medium but likewise by the effect of temperature on solvation." Thus
gelatin and agar-agar form sols with rather low viscosity at high tempera-
tures when compared to the viscous liquid or plastic gels they form at low
temperatures. Starch usually forms a suspension at low temperatures, and
its decided increase in viscosity or plasticity comes with rapid hydration at
the gelatinization point. Gortner states that heating a starch paste beyond
the gelatinization temperature causes a decrease in viscosity or plasticity.
Electrolytes added to lyophilic systems, often even in traces, cause great
changes in the viscosity of the sol.
The factors affecting the viscosity of lyophilic systems. Gort-
ner adds an eleventh factor, that of rate of shear, pointed out by Sharp
and Gortner, to the ten given by Ostwald that cause variation in the
viscosity of lyophilic systems. They are as follows: (1) concentration,
(2) temperature, (3) degree of dispersion, (4) solvation, (5) electrical
charge, (6) previous thermal treatment, (7) previous mechanical treat-
ment, (8) the presence or absence of other lyophilic colloids, (9) the age
of the lyophilic sol, (10) the presence of both electrolytes and non-elec-
trolytes, and (11) the rate of shear.
Viscosity is closely related to the consistency of the finished product in
food preparation. So close is this relation in many cases that the ten factors
listed by Ostwald may nearly be taken as ten commandments of food
preparation. Thus the consistency of a custard is influenced by the concen-
tration of egg or the protein micelles; the temperature to which it is
cooked ; the degree of dispersion of the micelles, which is influenced by the
reaction and other factors; the degree of hydration, which is influenced by
reaction, the kind and concentration of salts present, etc. ; the beating
of the egg; the use of milk or w^ater; how long the custard has aged in
addition to the age of the eggs and milk when used ; the kinds and con-
centration of salts in the egg and milk as well as the addition of sodium
chloride and the non-electrolyte sugar.
Since the line of demarcation between sols and gels is not a definite one,
fruit jellies, gelatin, milk, cream, as well as egg dishes, may be added to
the group of foods in which the consistency of the finished product is related
to viscosity. But this does not end the application, for the structure or type
of product in baked goods is closely related to the viscosity of the batter or
dough, which in turn is influenced by all these factors. Of course these
factors or nearly the same ones affect other properties as well as viscosity of
food materials. Thus the extensibility of gluten, the heat coagulation of
proteins, etc., are influenced by many or all of these factors.
Plasticity. Bingham defines plasticity as "a property of solids in virtue
of which they hold their shape permanently under the action of small
shearing stresses but they are readily deformed, worked or molded, under
ENERGETICS 25
Page 24
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AI-modernized reading of the original text
A modernized reading is not available for this page yet. You are seeing the original text.
perature, part of this being due to the effect of temperature upon the
intermicellar liquid. Gortner states that in "colloid systems changes due
to temperature are influenced not only by the viscosity of the dispersion
medium but likewise by the effect of temperature on solvation." Thus
gelatin and agar-agar form sols with rather low viscosity at high tempera-
tures when compared to the viscous liquid or plastic gels they form at low
temperatures. Starch usually forms a suspension at low temperatures, and
its decided increase in viscosity or plasticity comes with rapid hydration at
the gelatinization point. Gortner states that heating a starch paste beyond
the gelatinization temperature causes a decrease in viscosity or plasticity.
Electrolytes added to lyophilic systems, often even in traces, cause great
changes in the viscosity of the sol.
The factors affecting the viscosity of lyophilic systems. Gort-
ner adds an eleventh factor, that of rate of shear, pointed out by Sharp
and Gortner, to the ten given by Ostwald that cause variation in the
viscosity of lyophilic systems. They are as follows: (1) concentration,
(2) temperature, (3) degree of dispersion, (4) solvation, (5) electrical
charge, (6) previous thermal treatment, (7) previous mechanical treat-
ment, (8) the presence or absence of other lyophilic colloids, (9) the age
of the lyophilic sol, (10) the presence of both electrolytes and non-elec-
trolytes, and (11) the rate of shear.
Viscosity is closely related to the consistency of the finished product in
food preparation. So close is this relation in many cases that the ten factors
listed by Ostwald may nearly be taken as ten commandments of food
preparation. Thus the consistency of a custard is influenced by the concen-
tration of egg or the protein micelles; the temperature to which it is
cooked ; the degree of dispersion of the micelles, which is influenced by the
reaction and other factors; the degree of hydration, which is influenced by
reaction, the kind and concentration of salts present, etc. ; the beating
of the egg; the use of milk or w^ater; how long the custard has aged in
addition to the age of the eggs and milk when used ; the kinds and con-
centration of salts in the egg and milk as well as the addition of sodium
chloride and the non-electrolyte sugar.
Since the line of demarcation between sols and gels is not a definite one,
fruit jellies, gelatin, milk, cream, as well as egg dishes, may be added to
the group of foods in which the consistency of the finished product is related
to viscosity. But this does not end the application, for the structure or type
of product in baked goods is closely related to the viscosity of the batter or
dough, which in turn is influenced by all these factors. Of course these
factors or nearly the same ones affect other properties as well as viscosity of
food materials. Thus the extensibility of gluten, the heat coagulation of
proteins, etc., are influenced by many or all of these factors.
Plasticity. Bingham defines plasticity as "a property of solids in virtue
of which they hold their shape permanently under the action of small
shearing stresses but they are readily deformed, worked or molded, under
ENERGETICS 25