As the moisture content of a food decreases, a restraint is observed on the vapor pressure which is evident from the water activity.

All of the mentioned impose restraint on the behavior of water.

Just like the vapor pressure on an individual component is higher than when it is in bulk, the water activity is low when in bulk. Water activity is directly proportional to the surface tension and molecular weight of water and inversely proportional to the density of water.

Raoult’s law gives the relationship between the moles of water and the moles of solute in a solution and its water activity.

When a solute is added to water, concentration of water is reduced and interaction with solute may break or increase the water structures.

If a dehydrated porous absorptive food is soaked in a solution whose viscosity is not too high to penetrate, then the food will absorb an amount of solution approximately equal to the amount of water it would normally absorb.

When there is an equilibrium between the vapor pressure and the percentage relative humidity, water activity is directly proportional to the percentage relative humidity. Even a water activity as high as 0.85 for food items is considered to be very dry for substances.

Rice has a lesser water activity than apples. Apples contain higher water per 100 grams of dry material.

Sodium chloride has a higher water activity than sucrose. This is because sucrose creates structures in water and reduces water activity below what they should be based on concentration alone.

All of the mentioned are advantages of semi moist food.