Relationship between RH & aw The water activity of a food can be related to an equilibrium relative humidity in the air around the product. That is, at only one relative humidity will the air be in moisture equilibrium with the food product where the food neither gives up or adsorbs water. This relative humidity is the "equilibrium relative humidity" or ERH
Relationship between RH & aw The water activity of a food can be related to an equilibrium relative humidity in the air around the product. That is, at only one relative humidity will the air be in moisture equilibrium with the food product where the food neither gives up or adsorbs water. This relative humidity is the "equilibrium relative humidity" or ERH
Free" water & "bound" water In the past people simplified the state of water in foods by denoting two types: "free" water or "bound" water. "free" water or "bound" water. The working definition for these terms is: Free water is that which gives water activity of 1, bound water gives water activity less than 1
Free" water & "bound" water In the past people simplified the state of water in foods by denoting two types: "free" water or "bound" water. "free" water or "bound" water. The working definition for these terms is: Free water is that which gives water activity of 1, bound water gives water activity less than 1
Free water is relatively easy to remove from a food product while bound water takes more energy to release from the food. Thus, the latent heat required to remove a molecule of water from a food increases as the water activity decreases. This is important to those who design drying operations, since the energy requirement to provide sufficient driving force for drying is related to the latent energy of vaporization
Free water is relatively easy to remove from a food product while bound water takes more energy to release from the food. Thus, the latent heat required to remove a molecule of water from a food increases as the water activity decreases. This is important to those who design drying operations, since the energy requirement to provide sufficient driving force for drying is related to the latent energy of vaporization
physical changes As a food product dries out and the water molecules become less mobile, physical changes also occur in the food. As water is removed, the remaining product generally becomes increasingly viscous. The product may go through several regions of properties, where viscosities are intermediate between a pumpable liquid and a stationary solid
physical changes As a food product dries out and the water molecules become less mobile, physical changes also occur in the food. As water is removed, the remaining product generally becomes increasingly viscous. The product may go through several regions of properties, where viscosities are intermediate between a pumpable liquid and a stationary solid
The state diagram For a simple system of solute and solvent The glass transition curve represents a metastable transition where viscosity is so high that the product does not "flow". Below this curve, the food is stable to diffusionlimited processes for extremely long times
The state diagram For a simple system of solute and solvent The glass transition curve represents a metastable transition where viscosity is so high that the product does not "flow". Below this curve, the food is stable to diffusionlimited processes for extremely long times