Water Activity – Why do we need to control it?
A Close Look at Water in your Products
Predicting Food Spoilage:
Water activity (Aw) has its most useful application in predicting the growth of bacteria, yeasts and molds.
For a food to have a useful shelf life without relying on refrigerated storage, it is necessary to control either its acidity level (pH) or the level of water activity (Aw) or a suitable combination of the two.
This can effectively increase the product’s stability and make it possible to predict its shelf life under known ambient storage conditions. Food can be made safe to store by lowering the water activity to a point that will not allow dangerous pathogens such as Clostridium botulinum and Staphylococcus aureus to grow in it.
This is why there are specific guide-lines for product like summer sausage having a pH of less than 5.2 and a Aw of less then 0.91.
What is pH?
The pH is a measurement of acidity in a given solution. The non scientific meaning of pH is the “potential hydrogen” or the concentration of hydrogen ions in a given solution the stronger the solution the smaller the number
| Product | PH | Logarithmic value | Concentration of hydrogen |
| Stomach acid | 2.0 | 1 x 10 -2 | 0.01 |
| Lemon juice | 3.5 | 1 x 10 –3.5 | 0.0005 |
| Summer sausage | 5.2 | 1 x 10 –3.5 | 0.000012 |
| Milk | 6.5 | 1 x 10 –6.5 | 0.0000005 |
| Distilled water | 7 | 1 x 10 –7 | 0.0000001 |
| Milk of magnesia | 9 | 1 x 10 –95 | 0.000000001 |
| Sodium bicarbonate (baking soda) | 10 | 1 x 10 –10 | 0.0000000001 |
| Lye | 11 | 1 x 10 –11 | 0.00000000001 |
As you can see a small difference in the pH number indicates a large change in its concentration. The addition of phosphates to meat corrects the pH to a
pre-rigor state, which gives the meat greater water holding and binding capacity.
Adding acids such as lactic acid (fermented milk) and citric acid (lemon juice) not only give products flavour, but also increase shelf life. Adding buffered acids such as potassium lactate or sodium nitrate which have a neutral pH are a more effective means to control shelf life when using 1 to 3 % in the finished product.
Another example controlling pH is the use of GDL (glucono delta lactone) and starter culture in dry and semi-dry products. They lower the pH gradually killing off unwanted bacteria resulting in a tangy product that is shelf stable if the water activity (Aw) is low enough.
Alas, changing the pH of meat significantly can cause unwanted side effects, such as poor binding in restructured jerky or “cooking out” the protein in chicken and fish products.
What is water activity ?
Water activity is a measure of the free water or water that is not bound in your food product, this water support growth of microorganisms such as yeast mold and bacteria. These microorganisms cause food to spoil and people to get sick. Water activity and moisture content are not the identical, two products can have the same water content yet have very different water activity.
The Typical Water Activity of Some Foodstuffs
| Type of Product | Water Activity (AW) |
| Fresh meat and fish | 0.99 |
| Bread | 0.95 |
| Summer sausage | 0.89-0.91 |
| Jams and jellies | 0.8 |
| Beef jerky | 0.85 |
| Dried fruit | 0.6 |
| Biscuits | 0.3 |
| Milk powder | 0.2 |
| Instant coffee | 0.2 |
The water activity scale extends from 0 (bone dry) to 1.0 (pure water) but most foods have a water activity level in the range of 0.2 for very dry foods to 0.99 for moist fresh foods. Water activity is in practice usually measured as equilibrium relative humidity (ERH).