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ABOUT THE VSA
2. ABOUT THE VSA
The VSA Isotherm Generator is an automatic moisture sorption isotherm generator that can
generate both dynamic and equilibrium moisture sorption isotherms.
2.1 MOISTURE SORPTION ISOTHERMS
A moisture sorption isotherm defines the relationship between water activity (a
w
) and
moisture content at a given temperature. This relationship is complex and unique for each
product due to different interactions (colligative, capillary, and surface effects) between the
water and the solid components at different moisture contents. An increase in a
w
almost
always accompanies an increase in water content, but in a nonlinear fashion. Moisture
sorption isotherms are sigmoidal in shape for most foods, although foods that contain large
amounts of sugar or small soluble molecules have a J-type isotherm curve shape.
Isotherms provide information about product quality and safety. A few uses for
isotherms include:
1. Monolayer moisture content determination.
2. Determine critical water activity or moisture content.
3. Limits for crispness, hardness, and flow properties.
4. Optimize moisture contents at a safe water activity that maximizes moisture and avoids
over drying.
5. Determine shelf-life and storage stability of a product.
6. Predict packaging requirements based on sorption properties of a product.
7. Determine kinetics of sorption and water vapor diffusion coefficients
8. Determine the equilibrium water activity of a mixture of dry ingredients.
9. Determine the degree of crystallinity of powders.
10. Determine the level of amorphous material in a product.
11. Determine critical water activities for phase transitions.
12. Determine the relationship between water activity and glass transition temperature.
13. Determine the relationship between water activity and crystallization.
14. Determine hysteresis levels for a product.
15. Determine the moisture sensitivity of a product.
16. Determine the equilibrium moisture content at a given water activity.
17. Allow rapid moisture content determination from water activity analysis through an
isotherm curve.
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