moisture content refers to the amount of water that a sample contains it can be expressed as a percentage mass or other type of ratio water activity is the amount of Unbound water present in the sample in other words part of the water is normally chained to chemical groups in the sample and cannot move freely to be used for example by bacteria which would cause food spoilage water activity is based on a scale of 0 to one with pure water having a value of one it is defined as the vapor pressure of water in a sample divided by the vapor pressure of pure water at the sample temperature as the chemically bound water cannot Escape only the free water will exert vapor pressure and therefore this characteristic can be used to determine microbial spoilage chemical stability and physical stability for example in the majority of food samples a water activity of 0.5 will mean the likelihood of my microbial growth is very low