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Water content, or moisture content, is a measurement of the total water contained in a food. It is usually expressed as a percentage of the total weight:
|w - d|
|Mw (wet basis) =||-----||x (100)|
Mw = moisture content on a wet per cent basis
w = wet weight
d = dry weight
Water activity is a measurement of the availability of water for biological reactions. It determines the ability of micro-organisms to grow. If water activity decreases, micro-organisms with the ability to grow will also decrease.
Water activity (aw) is expressed as the ratio of the vapour pressure in a food (P) to the vapour pressure of pure water (P0). It predicts whether water is likely to move from the food product into the cells of micro-organisms that may be present.
For example, a water activity of 0.90 means the vapour pressure is 90 per cent of that of pure water. Water activity increases with temperature due to changes in the properties of water such as, the solubility of solutes such as salt and sugar, or the state of the food.
Water content on its own is not enough information to determine food safety or predict product shelf life. The relationship between water content and water activity is complex and related to the relative humidity of the food and its water content. This relationship must be determined for each specific food item.
It is easy to assume that foods with higher water content will have a higher water activity than dry foods. This is not always correct. It is possible to have products with the same water content but very different water activities. For example, salami and cooked beef have similar water content of approximately 60 per cent. However, the water activity of salami is 0.82 and cooked beef is approximately 0.98.
Most food has a water activity greater than 0.95 which supports the growth of bacteria, yeast and mould. Knowing the water activity of a food is important when preparing a Hazard Analysis Critical Control (HACCP) plan. The water activity of a product or ingredient is necessary when conducting a hazard analysis and completing Form 1 for many products.
|fresh meat and fish||0.99|
|raw vegetables (ex: carrots, cauliflower, peppers)||0.99|
|raw fruits (ex: apples, oranges, grapes)||0.98|
|cooked meat, bread||0.91-0.98|
|moist cakes (ex : carrot cake)||0.90-0.95|
|flours, rice, beans, peas||0.80-0.87|
|jams, marmalades, jellies||0.75-0.80|
|dried spices, milk powder||0.20-0.60|
Water content measurement may be useful in some situations such as determining yield. Water activity, however, is a more suitable measurement for predicting food safety and quality. The approximate water content of some typical food is listed below:
|bread, commercially prepared||36|
By lowering water activity, food can be made safe to store. The table below shows water activity levels that can support the growth of particular groups of organisms.
|Group of Micro-Organisms||Minimum aw required for growth|
|most gram-negative bacteria||0.97|
|Staphylococcal toxin production (by Staphylococcus aureus)||0.93|
|most gram-positive bacteria||0.90|
|Halophilic bacteria (grow best at high salt concentrations)||0.75|
|Xerophillic moulds (can grow on dry foods) and Osmophillic yeasts (can grow in the presence of high concentrations of organic compounds, ex: sugars)||0.62-0.60|
Moulds have minimum water activities for growth and toxin production. Most moulds require a higher water activity than the minimum requirement for growth to produce mycotoxins. The table below shows a few common mycotoxins and minimum water activities for mould growth and toxin production.
|Mycotoxin||Mould||Minimum aw requirement|
The grain industry often tests for water content when receiving and storing grain. The Canadian Grain Commission developed the document: Good operating practices for: Grains, oilseeds and pulses grain handling and processing facilities, which has useful information :
Finished product moisture (FPM) levels will minimize the risk of mycotoxin development due to high-moisture grain. FPM levels are:
When moisture level of incoming grain exceeds FPM levels, safe storage timeframes are:
Water activity values are useful when:
Water content values are useful when:
There are a variety of test methods and equipment available for determining water content (moisture) and water activity in food and other commodities, including:
- Official Methods of Analysis of AOAC International
- Moisture Analyzers
Water Activity (aw):
- Health Canada - The Compendium of Analytical Methods (Volume 3)
- Water Activity Meters
There are a number of companies that manufacture and supply equipment for water activity testing. Also, many laboratories can conduct water content (moisture) and water activity testing on a fee-for-service basis. This is a good option for processors without access to testing equipment in their own facility.
For more information, email the CVO/Food Safety Knowledge Centre or call 204-795-8418 in Winnipeg.