Water Content and Water Activity: Two Factors That Affect Food Safety

• free or available water
• water that is bound to different molecules such as proteins and carbohydrates

Water Content in Food

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:

Mw = moisture content on a wet per cent basis
w = wet weight
d = dry weight

Water Activity

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 (a_w) 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.

Relationship between Water Content and Water Activity

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.

Water Activity of Common Food Products

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. Determining the water activity of a product or ingredient is necessary when conducting a hazard analysis for many products.

Water Content of Common Food Products

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: 

Typical Water Activity Limits for Organisms

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.

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.

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:

Safe Storage Timeframes for Wet Grain

Finished product moisture (FPM) levels will minimize the risk of mycotoxin development due to high-moisture grain. FPM levels are:

• wheat <14.6%
• soybeans <14%
• beans <16.1%
• lentils <13.1%
• peas <16.1%
• mustard seed <9.6%
• oats <13.6%
• barley-general purpose <14.9%
• barley-select <13.6%
• canola <10.1%
• flax <10.1%

When moisture level of incoming grain exceeds FPM levels, safe storage timeframes are:

• if moisture is > FPM and <=18% dry to below FPM within 40 days
• if moisture is > 18% and <=20% dry to below FPM within 20 days
• if moisture is > 20% and <= 22% dry to below FPM within 10 days
• if moisture is >22% and <= 24% dry to below FPM within five days
• if moisture is >24% dry to below FPM within three days

Source: Canadian Grain Commission Good operating practices for: Grains, oilseeds and pulses grain handling and processing facilities,

Factors that Influence Water Activity

• Drying: Water activity is decreased by physically removing water (Ex: beef jerky).
• Solutes: Water activity is decreased by adding solutes such as salt or sugar (Ex: jams, cured meats).
• Freezing: Water activity is decreased by freezing (Ex: water is removed in the form of ice).
• Combination: One or more of the above can be combined for a greater influence on water activity (Ex: salting and drying fish).

When to Test for Water Activity or Water Content

Water activity values are useful when:

• determining the safety or shelf stability of a product

Water content values are useful when:

• establishing the dry weight of a food or ingredient
• determining yield
• confirming the end point of a drying process

How to Test for Water Content (Moisture) and Water Activity

There are a variety of test methods and equipment available for determining water content (moisture) and water activity in food and other commodities, including:

Water content:

• Official Methods of Analysis of AOAC International
• Moisture Analyzers

Water Activity (a_w):

• 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 Food Safety and Inspection Branch or call 204-795-8418 in Winnipeg.