Bound water often requires the application of heat and suction to drive
it off. Burns says, "An alumina gel cannot be dried by heating it for 2 or 3
FLUIDITY AND VISCOSITY 23
days at 500°C., yet some relationship does exist between the 'free' and
'bound' water. Under certain conditions as yet undefined, bound water
may become free again, and the reverse." The greater power of some
samples of meat to retain moisture and yet appear firm and less juicy than
other samples cooked under the same conditions is known. Ostwald states
that pork can be distinguished from other meats by the fact that its water-
holding capacity suffers the least change when cooked or dried. In his work
with angel cake Barmore speaks of the crumb of cake baked at higher oven
temperatures as containing as much moisture but apparently binding the
water more securely for it appears more dry.
Fluidity, Viscosity, and Plasticity of Colloidal Systems
Fluidity and viscosity. Bingham uses the term fluidity to express
the opposite of viscosit)^ A fluid like water yields readily to any force that
tends to change its form, whereas a viscous substance shows some resistance
to flow. Viscosity is one of the important properties of colloidal systems.
As a general rule, the lyophobic colloids show a viscosity but little greater
than that of the dispersion medium, the viscosity increasing only slightly
with increasing concentration of the micelles. But the lyophilic colloids may
show very high viscosities or even plasticity with very low concentrations
of the micelles.
Bingham states that "a mixture of liquids may have an indefinite number
of fluidities dependent upon the method of mixing, in other words, upon
the structure of the liquid." He also states that colloidal solutions show
differences in fluidity due to differences in structure. Thus it is possible that
cake or other batters made with the same materials and the same propor-
tion of materials may show differences in the structure of the finished cake
on account of different methods of mixing, giving different viscosities to
the batter.
Some substances flow readily; others resist flow; and some must have
weight applied to start flow. When a substance tends to resist a shearing
force it may exhibit a flow that is characterized as viscous, turbulent, or
plastic. If the substance entirely regains its original shape, when the shear-
ing stress is removed, it shows perfect elasticity. If the original shape is
not entirely regained and the substance is deformed to an extent directly
proportional to the shearing force, then the substance is said to show
viscosity. This flow that is directly proportional to the shearing force is
called linear flow. By this is meant that if a weight of 1 pound produces
a definite deformation, a weight of 2 pounds produces twice that deforma-
tion. Turbulent flow is the flow obtained when the ratio of the shearing
force to the deformation decreases.
A pure liquid at a given temperature and pressure has a definite fluidity.
The viscosity of water is approximately six times as great at 0° as at
100°C. The viscosity of sols usually decreases with an increase in tem-
24 RELATION OF COOKERY TO COLLOID CHEMISTRY
Page 23
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.
Bound water often requires the application of heat and suction to drive
it off. Burns says, "An alumina gel cannot be dried by heating it for 2 or 3
FLUIDITY AND VISCOSITY 23
days at 500°C., yet some relationship does exist between the 'free' and
'bound' water. Under certain conditions as yet undefined, bound water
may become free again, and the reverse." The greater power of some
samples of meat to retain moisture and yet appear firm and less juicy than
other samples cooked under the same conditions is known. Ostwald states
that pork can be distinguished from other meats by the fact that its water-
holding capacity suffers the least change when cooked or dried. In his work
with angel cake Barmore speaks of the crumb of cake baked at higher oven
temperatures as containing as much moisture but apparently binding the
water more securely for it appears more dry.
Fluidity, Viscosity, and Plasticity of Colloidal Systems
Fluidity and viscosity. Bingham uses the term fluidity to express
the opposite of viscosit)^ A fluid like water yields readily to any force that
tends to change its form, whereas a viscous substance shows some resistance
to flow. Viscosity is one of the important properties of colloidal systems.
As a general rule, the lyophobic colloids show a viscosity but little greater
than that of the dispersion medium, the viscosity increasing only slightly
with increasing concentration of the micelles. But the lyophilic colloids may
show very high viscosities or even plasticity with very low concentrations
of the micelles.
Bingham states that "a mixture of liquids may have an indefinite number
of fluidities dependent upon the method of mixing, in other words, upon
the structure of the liquid." He also states that colloidal solutions show
differences in fluidity due to differences in structure. Thus it is possible that
cake or other batters made with the same materials and the same propor-
tion of materials may show differences in the structure of the finished cake
on account of different methods of mixing, giving different viscosities to
the batter.
Some substances flow readily; others resist flow; and some must have
weight applied to start flow. When a substance tends to resist a shearing
force it may exhibit a flow that is characterized as viscous, turbulent, or
plastic. If the substance entirely regains its original shape, when the shear-
ing stress is removed, it shows perfect elasticity. If the original shape is
not entirely regained and the substance is deformed to an extent directly
proportional to the shearing force, then the substance is said to show
viscosity. This flow that is directly proportional to the shearing force is
called linear flow. By this is meant that if a weight of 1 pound produces
a definite deformation, a weight of 2 pounds produces twice that deforma-
tion. Turbulent flow is the flow obtained when the ratio of the shearing
force to the deformation decreases.
A pure liquid at a given temperature and pressure has a definite fluidity.
The viscosity of water is approximately six times as great at 0° as at
100°C. The viscosity of sols usually decreases with an increase in tem-
24 RELATION OF COOKERY TO COLLOID CHEMISTRY