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moderately flexible tube, which joins two bells each submerged in its own bath. The
immersed in the 25Úc water bath contains the sample and the other bell at a lower
temperature, surrounds a dish of pure water. The temperature of each water bath is
measured with great accuracy by means of a copper constant thermocouple. The
difference in temperature of two baths is calculated following equilibrium and this is
taken as the temperature of sample and pure water. The concentration of water in the
solution to be determined and vapours pressure of both solution are read from
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established tables.aw is then calculated from the ratio of two vapours pressure as noted
above.
The method is cumbersome and lengthy. It was originally used to determine the aw
of various saturated solutions. Presently, this method is seldomly used.
3. Vapour pressure manometer (manometry)
The moisture content of food related directly to its vapour pressure at constant
temperature. This pressure may be measured accurately by manometric procedures.
Relatively high precision (±0.002 aw) has been claimed for this technique.
The sample to be measured is ground and introduced into flask, which is attached
through a trap to a simple manometer. The manometer assembly (figure 15.2) is then
evacuated and during this time the sample chamber is maintained out approximately -
80Úc. Following evacuation, the sample is warmed to room temperature, while one side
of manometer is maintained at essentially zero pressure. The fluid level in the sample
arm of the manometer is then deflected by increase in pressure caused by vapour
pressure of the sample itself. Air entrapped in the sample also contributes to deflection
of the manometer, but this can be compensated for relatively easily. Humidity
determination can be carried out on the same sample at various moisture levels by
allowing the sample moisture to distill into freeze trap by reweighing the sample.
4. Hair hygrometer
The principle underlying this instrument is that the keratinaceous protein of hair
absorbs moisture from the atmosphere with commensurate stretching. It the hair strands
(usually 3 or more strands are breided) are fixed at one end, attached to an indicating
lever arm on the other end and allowed to come into equilibrium with the substance to
be measured, ERH can be read directly.
5. Isopiestic hygrometry
(Isopiestic equilibration) like hair hygrometry, this technique requires only very
inexpensive equipment. With this procedure, the weight gain of some absorptive material
such as microcrystalline cellulose or a protein is measured at various established aw levels.
The equilibration usually is carried out in a series of desiccators, each containing a
saturated solution and each at different aw levels. From this the standard sorption
isotherm is calculated. The unknown to be measured then is placed in desiccators with
one of the absorptive material for a given time, usually 24 hr and the weight gain at the
end of this period is compared to the standard isotherm.
Comparisons between aw levels of various food determined by this technique and by
electric hydrometric procedure has given good correlation at aw levels > 0.09 and
superior precision has been claimed at aw levels
6. Electric hygrometry
Ratronic electric hygrometer (in our college) has operation similar to above (figure15.3)
one. In this, A.C. current can be passed through as saturated solution of LiCl2 suspended
into an insert carrier such as glass wool. A current potential difference of 25v, which
heats the cell, is provided across the solution. The water vapour pressure (WVP) of
solution rises and upon reaching the WVP of environment, water evaporation occurs.
The dried lithium chloride residues remaining after evaporation no longer conducts
current and heating ceases. As the residue cools, water is once again taken up from the
environment and the cycle is repeated at reduced amplitude. Eventually, a temperature is
reached at which the WVP of environment. This temperature is then measured and
related to the WVP of saturned lithium chloride and hence the environment from which
the ERH can be calculated.
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The other method used for measuring water activity includes:
7. Freezing point depression
8. Dew point methods, and
9. Chemical methods
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EXERCISE 16
DEMONSTRATION OF POLARIMETER
Principle:
Certain organic substance possesses the property of rotating the plane of polarized
light. Such substances are said to be optically active. In ordinary light, the vibrations are
confined to a single direction or plane. Polarized light may be obtained by passing light
through a nicol prism. The instrument by which optical activity of a liquid is determined by
inserting nicol prism (figure 16.1) in the path of a ray of light before and after passing
through a liquid is called polarimeter. This instrument is chiefly used to determine the optical
rotation of a solution. A beam of monochromatic light is passed through nicol prism to
produce plane polarized light, and through a tube containing the optically active material
(e.g. Sugar solution) and finally through a second nicol prism (figure16.2). When these
prisms are placed at right angles with no optically active material intervening, the only light
passed by first prism is stopped by the second prism and the rear field appears dark. When
optically active material is introduced between the two prisms, light appears and the field
becomes bright. The second prism must be rotated in order to prevent the passage of light.
The number of degrees through which the 2nd prism must be rotated to the rear prism must
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