No microbial activity can occur unless water is available. Water activity is the ratio between the vapour pressure of the food and that of pure water. Pure water has aW = 1.0 and a relative humidity of 100%. This means that aW x 100 indicates the equilibrium relative humidity, which the particular food would produce if enclosed in a sealed container at a constant temperature. Organisms have their own characterstic optimal aW and range of aW for their growth.
The lowest aW values permitting growth of spoilage organisms are
- Normal bacteria
- xalophilic 0.77
- Normal yeast
- xalophilic fungi 0.65
- osmophilic yeasts 0.60
As can be seen from the above values dry foods are most likely spoiled by moulds.
Syrups and honey favours the growth of osmophilic yeasts.
Moist neutral foods like milk and meats are spoiled by bacteria.
Microorganism of importance in food bacteriology
Micro-organisms of interest in food bacteriology are: