600x600px |Moisture sorption isotherm 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 value, a sorption isotherm indicates the corresponding water content 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 processes, the isotherms cannot be determined explicitly by calculation, but must be recorded experimentally for each product. The relationship between water content and water activity (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 .1 BET theory (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. 2640x180px |Figure 2. Isotherm Curve Types 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. 3605x480px |Figure 3. Type V Isotherm Curve Set
350x310px |Moisture sorption isotherm 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 value, a sorption isotherm indicates the corresponding water content 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 processes, the isotherms cannot be determined explicitly by calculation, but must be recorded experimentally for each product. The relationship between water content and water activity (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 .Bell, L.N., and Labuza, T.P. 2000. "Practical Aspects of Moisture Sorption Isotherm Measurement and Use". 2nd Edition AACC Eagan Press, Eagan, MN BET theory (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. Brunauer, S. “The Absorption of Gasses and Vapors, Volume 1,” Oxford University Press, 1943, p.49640x180px |Figure 2. Isotherm Curve Types 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; 1981605x480px |Figure 3. Type V Isotherm Curve Set
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.Bell, L.N., and Labuza, T.P. 2000. "Practical Aspects of Moisture Sorption Isotherm Measurement and Use". 2nd Edition AACC Eagan Press, Eagan, MN
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Brunauer, S. “The Absorption of Gasses and Vapors, Volume 1,” Oxford University Press, 1943, p.49640x180px
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Dini, S., “Analysis of Cross-Cooled Desiccant Cooling Systems”, Ph.D. Thesis, Illinois Institute of Technology; 1981605x480px