Protection of Foods by Drying
449
For example, a liter of water contains 55.5 moles. Assuming that the water is pure,
a w =
55.5
55.2 + 0
= 1.00
If, however, 1 mole of sucrose is added,
a w =
55.5
55.5 + 1
= 0.98
This equation can be rearranged to solve the number of moles of solute required to give a specified a w
value. Although the foregoing is not incorrect, it is highly oversimplified, as food systems are complex
by virtue of their content of ingredients that interact with water and with each other in ways that are
difficult to predict. Sucrose, for example, decreases a w more than expected, so that calculations based
on Raoult’s law may be meaningless.
4
In preparing IMF, water may be removed either by adsorption or desorption. By adsorption, food
is first dried (often freeze dried) and then subjected to controlled rehumidification until the desired
composition is achieved. By desorption, the food is placed in a solution of higher osmotic pressure so
that at equilibrium, the desired a w is reached.
28 Although identical a w values may be achieved by these
two methods, IMF produced by adsorption is more inhibitory to microorganisms than that produced
by desorption (see below). When sorption isotherms of food materials are determined, adsorption
isotherms sometimes reveal that less water is held than for desorption isotherms at the same a w . The
sorption isotherm of a food material is a plot of the amount of water adsorbed as a function of the
relative humidity or activity of the vapor space surrounding the material. It is the amount of water
that is held after equilibrium has been reached at a constant temperature.
21 Sorption isotherms may be
either adsorption or desorption, and when the former procedure results in the holding of more water
than the latter, the difference is ascribed to a hysteresis effect. This, as well as other physical properties
associated with the preparation of IMF, has been discussed by Labuza,
21 Sloan et al.,
33 and others,
and will not be dealt with further here. The sorption properties of an IMF recipe, the interaction of
each ingredient with water and with other ingredients, and the order of mixing of ingredients, all add
to the complications of the overall IMF preparation procedures, and both direct and indirect effects
on the microbiology of these products may result.
The following general techniques are employed to change the water activity in producing an IMF
20 :
1. Moist infusion. Solid food pieces are soaked and/or cooked in an appropriate solution to give the
final product the desired water level (desorption).
2. Dry infusion. Solid food pieces are first dehydrated and then infused by soaking in a solution
containing the desired osmotic agents (adsorption).
3. Component blending. All IMF components are weighed, blended, cooked, and extruded or
otherwise combined to give the finished product the desired a w .
4. Osmotic drying. Foods are dehydrated by immersion in liquids with a water activity lower than
that of the food. When salts and sugars are used, two simultaneous countercurrent flows develop:
solute diffuses from solution into food, and water diffuses out of food into solution.
The foods in Table 18–4 were prepared by moist infusion for military use. The 1-cm-thick slices
equilibrated following cooking at 95–100
◦ C in water and holding overnight in a refrigerator. Equilibration is possible without cooking over prolonged periods under refrigeration.
6 IMF deep-fried
449
For example, a liter of water contains 55.5 moles. Assuming that the water is pure,
a w =
55.5
55.2 + 0
= 1.00
If, however, 1 mole of sucrose is added,
a w =
55.5
55.5 + 1
= 0.98
This equation can be rearranged to solve the number of moles of solute required to give a specified a w
value. Although the foregoing is not incorrect, it is highly oversimplified, as food systems are complex
by virtue of their content of ingredients that interact with water and with each other in ways that are
difficult to predict. Sucrose, for example, decreases a w more than expected, so that calculations based
on Raoult’s law may be meaningless.
4
In preparing IMF, water may be removed either by adsorption or desorption. By adsorption, food
is first dried (often freeze dried) and then subjected to controlled rehumidification until the desired
composition is achieved. By desorption, the food is placed in a solution of higher osmotic pressure so
that at equilibrium, the desired a w is reached.
28 Although identical a w values may be achieved by these
two methods, IMF produced by adsorption is more inhibitory to microorganisms than that produced
by desorption (see below). When sorption isotherms of food materials are determined, adsorption
isotherms sometimes reveal that less water is held than for desorption isotherms at the same a w . The
sorption isotherm of a food material is a plot of the amount of water adsorbed as a function of the
relative humidity or activity of the vapor space surrounding the material. It is the amount of water
that is held after equilibrium has been reached at a constant temperature.
21 Sorption isotherms may be
either adsorption or desorption, and when the former procedure results in the holding of more water
than the latter, the difference is ascribed to a hysteresis effect. This, as well as other physical properties
associated with the preparation of IMF, has been discussed by Labuza,
21 Sloan et al.,
33 and others,
and will not be dealt with further here. The sorption properties of an IMF recipe, the interaction of
each ingredient with water and with other ingredients, and the order of mixing of ingredients, all add
to the complications of the overall IMF preparation procedures, and both direct and indirect effects
on the microbiology of these products may result.
The following general techniques are employed to change the water activity in producing an IMF
20 :
1. Moist infusion. Solid food pieces are soaked and/or cooked in an appropriate solution to give the
final product the desired water level (desorption).
2. Dry infusion. Solid food pieces are first dehydrated and then infused by soaking in a solution
containing the desired osmotic agents (adsorption).
3. Component blending. All IMF components are weighed, blended, cooked, and extruded or
otherwise combined to give the finished product the desired a w .
4. Osmotic drying. Foods are dehydrated by immersion in liquids with a water activity lower than
that of the food. When salts and sugars are used, two simultaneous countercurrent flows develop:
solute diffuses from solution into food, and water diffuses out of food into solution.
The foods in Table 18–4 were prepared by moist infusion for military use. The 1-cm-thick slices
equilibrated following cooking at 95–100
◦ C in water and holding overnight in a refrigerator. Equilibration is possible without cooking over prolonged periods under refrigeration.
6 IMF deep-fried