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At equilibrium, the relationship between water content and equilibrium relative humidity of a material can be displayed graphically by a curve, the so-called moisture sorption isotherm.
For each 
He found that Type II and Type III isotherms require highly porous materials or desiccants, with first monolayer adsorption, followed by multilayer adsorption and finally leading to capillary condensation, explaining these materials high moisture capacity at high relative humidity.
Care must be used in extracting data from isotherms, as the representation for each axis may vary in its designation. Brunauer provided the vertical axis as moles of gas adsorbed divided by the moles of the dry material, and on the horizontal axis he used the ratio of partial pressure of the gas just over the sample, divided by its partial pressure at saturation. More modern isotherms showing the sorption of water vapor, on the vertical axis, provide the ratio of the weight of water adsorbed divided by its dry weight , or that ratio converted into a percentage. On the horizontal axis they provide relative humidity or water activity of the air presented to the material.
Sorption Isotherms are named as such because the equilibrium established must be for a constant temperature and this temperature should be specified. Normally, materials hold less moisture when they are hotter, and more moisture when they are colder. Occasionally, a set of isotherms are provided on one graph that shows each curve at a different temperature. Such a set of adsorption isotherms is provided in Figure 3 as measured by Dini on a Type V silica gel.Dini, S., “Analysis of Cross-Cooled Desiccant Cooling Systems”, Ph.D. Thesis, Illinois Institute of Technology; 1981
At equilibrium, the relationship between water content and equilibrium relative humidity of a material can be displayed graphically by a curve, the so-called moisture sorption isotherm.
For each humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
value, a sorption isotherm
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
indicates the corresponding water content
Water content or moisture content is the quantity of water contained in a material, such as soil (called soil moisture), rock, ceramics, crops, or wood. Water content is used in a wide range of scientific and technical areas, and is expressed as ...
value at a given, constant temperature. If the composition or quality of the material changes, then its sorption behaviour also changes. Because of the complexity of sorption process the isotherms cannot be determined explicitly by calculation, but must be recorded experimentally for each product.
The relationship between water content and water activity
Water activity (''aw'') is the partial vapor pressure of water in a solution divided by the standard state partial vapor pressure of water. In the field of food science, the standard state is most often defined as pure water at the same tempe ...
(aw) is complex. An increase in aw is usually accompanied by an increase in water content, but in a non-linear fashion. This relationship between water activity and moisture content at a given temperature is called the moisture sorption isotherm. These curves are determined experimentally and constitute the fingerprint of a food system
The term food system describes the interconnected systems and processes that influence nutrition
Nutrition is the biochemical and physiological process by which an organism uses food to support its life. It provides organisms with nutrients ...
.
BET theory
Brunauer–Emmett–Teller (BET) theory aims to explain the physical adsorption of gas molecules on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of materials. The obs ...
(Brunauer-Emmett-Teller) provides a calculation to describe the physical adsorption of gas molecules on a solid surface. Because of the complexity of the process, these calculations are only moderately successful; however, Stephen Brunauer was able to classify sorption isotherms into five generalized shapes as shown in Figure 2. He found that Type II and Type III isotherms require highly porous materials or desiccants, with first monolayer adsorption, followed by multilayer adsorption and finally leading to capillary condensation, explaining these materials high moisture capacity at high relative humidity.
Care must be used in extracting data from isotherms, as the representation for each axis may vary in its designation. Brunauer provided the vertical axis as moles of gas adsorbed divided by the moles of the dry material, and on the horizontal axis he used the ratio of partial pressure of the gas just over the sample, divided by its partial pressure at saturation. More modern isotherms showing the sorption of water vapor, on the vertical axis, provide the ratio of the weight of water adsorbed divided by its dry weight , or that ratio converted into a percentage. On the horizontal axis they provide relative humidity or water activity of the air presented to the material.
Sorption Isotherms are named as such because the equilibrium established must be for a constant temperature and this temperature should be specified. Normally, materials hold less moisture when they are hotter, and more moisture when they are colder. Occasionally, a set of isotherms are provided on one graph that shows each curve at a different temperature. Such a set of adsorption isotherms is provided in Figure 3 as measured by Dini on a Type V silica gel.Dini, S., “Analysis of Cross-Cooled Desiccant Cooling Systems”, Ph.D. Thesis, Illinois Institute of Technology; 1981
See also
* Desiccant *Food chemistry
Food chemistry is the study of chemical processes and interactions of all biological and non-biological components of foods. The biological substances include such items as meat, poultry, lettuce, beer, milk as examples. It is similar to biochemist ...
*Moisture vapor transmission rate
Moisture vapor transmission rate (MVTR), also water vapor transmission rate (WVTR), is a measure of the passage of water vapor through a substance. It is a measure of the permeability for vapor barriers.
There are many industries where moisture c ...
*Shelf life
Shelf life is the length of time that a commodity may be stored without becoming unfit for use, consumption, or sale. In other words, it might refer to whether a commodity should no longer be on a pantry shelf (unfit for use), or no longer on a ...
References
{{reflist Food technology Pharmaceutical industry