In the food manufacturing industry, water activity (aw) is an important property to test and regulate. As water activity can affect a large range of factors that can alter how a product tastes to how long a shelf life it will have. Many smaller or new producers may not be aware of the importance of water activity on their products.

According to Chambers Dictionary of Science and Technology, water activity (Food Sci) is:

"An expression of the amount of water present in a food, raw material of product which is available to support microbial growth. As it is reduced, the rate of growth of micro-organisms declines. The key food preservation principles are based on the reduction of water activity by removing water or by adding solutes such as sugar or salt."

This guide will discuss all the aspects relating to water activity testing and monitoring.

water activity formula

What is water activity (aw)?

Water activity (aw) is a measurement of the amount of available water in a product, and it has a significant impact on food production. Water activity is measured on a scale from 0 to 1, with 0 being completely dry and 1 being pure water. Water activity plays a critical role in determining product quality, safety, and shelf life.

Water activity influences:

  • Texture abnormalities
  • Enzymatic reactions
  • Flavour abnormalities
  • Browning reactions
  • Microbiological stability
  • Oxidation reactions
  • Protein & vitamin content
  • Powder caking
  • Chemical stability
  • Shelf life
  • Enzymatic stability
  • Storage
  • Water migration
  • Packaging

Food Regulations

Food Standards Agency (FSA)

Provides guidance on water activity (aw) for food businesses to ensure the safety and quality of food products. Some of the key advice provided by the FSA includes:

Measuring and monitoring water activity: Food businesses should measure and monitor the water activity of their products to ensure that they are safe for consumption and have a sufficient shelf life. This is particularly important for products that are susceptible to microbial growth, such as meats, dairy products, and baked goods.

Controlling water activity: Food businesses should take steps to control the water activity of their products, such as by reducing the water content or adding preservatives. This can help to prevent microbial growth and ensure the safety and quality of the product.

Assessing risk: Food businesses should assess the risk of microbial growth based on the water activity level of their products. This can help to identify potential hazards and take appropriate action to prevent them.

Training and education: Food businesses should ensure that their staff are trained and educated on the importance of water activity and how to measure and control it effectively.

Compliance with regulations: Food businesses must comply with relevant regulations and standards related to water activity, such as the European Union Regulation on Microbiological Criteria for Foodstuffs.

By following these guidelines, food businesses can ensure that their products are safe for consumption and meet quality standards.


HACCP (Hazard Analysis and Critical Control Points) is a food safety management system that focuses on identifying and controlling potential hazards in food production. Water activity is an important factor to consider in HACCP as it can impact the growth of microorganisms in food products.

Water activity is a measure of the amount of free water available in a food product for microorganisms to grow. It is expressed as a ratio of the vapour pressure of the food to the vapour pressure of pure water at the same temperature. The higher the water activity of a food product, the more likely it is to support the growth of microorganisms such as bacteria, yeasts, and moulds.

In HACCP, water activity is considered a critical control point (CCP) because it can directly affect the safety of the food product. Controlling water activity can be achieved through various methods such as drying, adding preservatives, or modifying the packaging. By monitoring water activity as a CCP, food processors can identify and prevent potential hazards before they occur, ensuring the safety of the food product for consumers.

Desk with HACCP file on it
Isotherm chart of water activity in food preservation

Properties affected by water activity

Microbial growth: Water activity is an essential factor that determines microbial growth in food. Bacteria, yeasts, and moulds require water to grow, and they have a minimum water activity level below which they cannot grow. Controlling water activity in food can help prevent spoilage and the growth of harmful bacteria.

Texture: Water activity affects the texture of food. Foods with low water activity tend to be dry and hard, while those with high water activity tend to be soft and moist. In food production, water activity can be controlled to achieve the desired texture of a product.

Flavours: Water activity affects the flavour of food by influencing the concentration of flavour compounds. Foods with high water activity tend to have a more of an intense flavour, while those with low water activity tend to be milder in flavour.

Preservation: Controlling water activity is an important factor in food preservation. Low water activity inhibits microbial growth and helps preserve the quality and safety of the product. Many preservation techniques such as drying, salting, and smoking rely on controlling water activity to preserve food by removing the water content.

Enzymatic stability and reactions:

Enzymatic stability and reactions have a significant impact on food production. Enzymes are biological catalysts that accelerate chemical reactions. 

Their reactions play a vital role in many food processes, including fermentation, baking, and cheese making.

Enzymatic stability refers to the ability of an enzyme to maintain its activity under specific conditions.

Here are some ways that enzymatic stability and reactions affect food production:

Quality: Enzymes can have a positive or negative effect on the quality of the final product. For example, enzymes used in cheese making can improve texture and flavour, while enzymes that cause spoilage can reduce shelf life and product quality.

Processing efficiency: Enzymes can increase processing efficiency and reduce production costs. For example, enzymes used in bread baking can speed up the fermentation process and reduce proofing time.

Nutritional value: Enzymes can improve the nutritional value of food by breaking down complex molecules into simpler forms that are easier to digest. For example, enzymes used in the production of soy sauce can break down proteins and increase the availability of amino acids.

Stability: Enzymatic stability is critical in food production because enzymes can be sensitive to environmental factors such as temperature, pH, and the presence of other substances. Enzymatic instability can lead to reduced activity, decreased processing efficiency, and reduced product quality.

Safety: Enzymes can affect food safety by breaking down toxins or harmful substances, or by producing harmful by-products. Enzymatic reactions can also lead to the formation of allergenic compounds, which can cause adverse reactions in some individuals.

Read our onsite microorganism hub page for more information on testing for unwanted microorganisms. Alternatively, click here to view our microorganism product range, which includes products like Petrifilm™ from 3M™ and MicroSnap™ and InSite™ tests from Hygiena™, both of which are leaders in microorganism rapid testing.

Where and when to test or monitor water activity?

Manufacturing Stages

Water activity should be monitored at various stages in the food manufacturing process to ensure the safety and quality of the final product. Some of the key stages where water activity should be monitored include:

Ingredient receiving and storage: Incoming ingredients should be checked for their water activity levels and stored appropriately to prevent spoilage or contamination.

Mixing and blending: When mixing and blending ingredients, it's important to monitor the water activity of the mixture to ensure that it is safe for further processing.

Cooking and heating: The water activity of the food product should be monitored during cooking and heating to ensure that it is heated to a safe temperature to kill any harmful microorganisms that may be present.

Cooling and chilling: After cooking, the food product should be cooled and chilled to prevent the growth of microorganisms. Monitoring the water activity during this stage can help ensure that the product is cooled and chilled to the appropriate temperature.

Packaging: The water activity of the final product should be checked before packaging to ensure that it is safe for consumption and has a sufficient shelf life.

Storage and distribution: The water activity of the final product should be monitored during storage and distribution to ensure that it remains safe and meets quality standards.

By monitoring water activity at each stage of the food manufacturing process, food processors can identify potential hazards and take appropriate action to prevent them, ensuring the safety and quality of the final product.

Factory worker checking the seal of a industrial boiling vessel.
Image of a water activity meter and probe and examples of testable foods

How a water activity meter works?

A water activity meter is a device used to measure the water activity (aw) of a food product. The instrument typically consists of a sensor, a temperature control system, and a display unit.

The sensor measures the partial vapour pressure of water in the food sample using a thin-film capacitance sensor or a dew point sensor. The thin-film capacitance sensor measures the dielectric constant of the food sample, which changes with the amount of free water in the sample. The dew point sensor measures the temperature at which water vapour begins to condense on a chilled mirror, which is a function of the water activity of the sample.

The temperature control system maintains a constant temperature inside the instrument, typically between 20 and 25°C, to ensure that the water activity measurement is accurate and repeatable.

The display unit shows the water activity reading on a digital screen, usually ranging from 0.00 to 1.00. The water activity meter may also have additional features, such as data logging capabilities, calibration options, and the ability to measure temperature and humidity.

To use a water activity meter, a small sample of the food product is placed in a sample cup or chamber and inserted into the instrument. The sensor then measures the water activity of the sample, and the result is displayed on the screen. The measurement can be used to determine if the food product is safe for consumption, has a sufficient shelf life, or requires further processing to reduce the water activity level.


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