Equilibrium Relative Humidity
The relative humidity of the air in equilibrium with a food at a specified temperature, numerically related to water activity when true equilibrium is established.
Equilibrium relative humidity describes the vapour environment above a sample after net moisture exchange has stabilised at a defined temperature.
Under equilibrium, water activity multiplied by one hundred corresponds numerically to equilibrium relative humidity. The relation is conditional on temperature and true equilibration.
A chamber controller measures local air, not the vapour state inside the product. Local surface conditions can differ from both the sensor and the centre.
Sample preparation, sealed volume, temperature control, sensor response and volatile compounds affect the determination. Traceable standards and instrument checks are necessary.
When chamber humidity approaches the product equilibrium value, net drying slows. This explains process behaviour but does not by itself define safety or sensory readiness.
State temperature, method and sample location with the result. Use product water activity for product decisions and psychrometric measurements to manage the environment.
Related in the Codex
References
- https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
- https://www.nist.gov/calibrations/traceability
- https://www.fsis.usda.gov/guidelines/2023-0002