Water Activity
Water activity is defined as the ratio of the vapor pressure of food samples to the vapor pressure of pure water.
It is denoted by aw.
Mathematically, aw = P/Po
Where,
P = Vapor pressure of food contents
Po = Vapor pressure of pure water
Water activity may also be defined as the Equilibrium Relative Humidity divided by 100.
aw = ERH/100
Importance of Water Activity
Water activity is important in food preservation. It controls the microbial growth on foods. It also controls the self life of foods. We know microorganisms cannot grow at water activity 0.6 and below. At 0.7 most of the molds growth are inhibited, at 0.8 most of the yeasts growth are inhibited and at aw 0.9 the bacterial growth are inhibited. So, water activity is important to preserve food for long time.
• Principal groups of food and their water activity:
| Water Activity | Foods |
| 0.98 | Fresh fruits and vegetables, meats, fishes, milk and most beverages |
| 0.93-0.98 | Evaporated milk, tomato paste, processed cheese, bread, canned cured meats etc |
| 0.85-0.93 | Dry or fermented sausage, dried beef, raw ham, aged cheddar cheese, sweetened condensed milk etc |
| 0.60-0.85 | Dried fruit, flour, cereals, jams and jellies, nuts, some aged cheeses |
| 0.60 and below | Chocolate, confectionery, honey, biscuits, crackers, potato chips, dried eggs, milk and vegetables |