Water Activity (aw) Measurement– Why is it important?

Water activity plays an important role in product quality in various fields, including:

• Food industry
• Pharmaceutical industry, Cosmetic industry
• Seed storage
• Tobacco industry
• Industrial manufacturing

As explained in a previous Blog-Post , all forms of life depend on water. Water activity indicates the amount of water which is biologically available to microorganisms. Each species of microorganism (bacteria, yeast, mould…) has a minimum water activity value below which growth is no longer possible.

Water activity	Contaminant
aw = 0.91...0.95	Many bacteria
aw = 0.88	Many yeasts
aw = 0.80	Many mildews
aw = 0.75	Halophile bacteria
aw = 0.70	Osmiophile yeasts
aw = 0.65	Xerophile mildew

The US Food and Drug Administration (FDA) has adopted the concept of water activity for establishing limits beyond which certain types of foods are considered susceptible to mold and bacteria and have set the following regulations: The water activity level of 0.85 aw is used as a point of definition for determining whether a low-acid canned food or an acidified food is covered by the regulations. Low-acid canned foods can be preserved by controlling water activity at levels above 0.85 aw. The minimum aw level for the growth of C. botulinum is approximately 0.93 aw. Depending on various product characteristics this minimum level can be as high as 0.96 aw.

The regulations (21 CFR 113.3(e) (1) (ii)) state that commercial sterility can be achieved by the control of water activity and the application of heat. The heat is generally necessary at aw levels above 0.85 aw to destroy dormant cells of microorganisms of public health significance (e.g. staphylococci) and spoilage microorganisms which can grow in a reduced aw environment. See also the following other sections of the regulations which deal with aw controlled products:

• 21 CFR 113.10
  Attendance at an approved school giving instruction appropriate to the preservation technology involved.
• 21 CFR 113.40(i)
  Equipment and procedures for thermal processing of foods where critical factors such as water activity are used.
• 21 CFR 113.81(f)
  Additional factors to be controlled to prevent the growth of microoganisms not destroyed by the thermal process.
• 21 CFR 113.100(a) (6)
  Record keeping requirements for aw determinations.

Some examples of water activity controlled low-acid canned foods, that may have an aw of greater than 0.85 aw, are: canned cake, bread, bean paste, some chutney, salted vegetables, salted fish, guava paste, lupini beans, syrup, toppings, puddings, and some oriental specialty sauces. Water activity is usually controlled by the use of salt or sugar. There are situations where routine aw determinations need not be made during production. For example, if salt is the preservative, percent salt determinations alone may be sufficient to document control of water activity and commercial sterility. However, the processor or the processing authority would need to have datawhich consistently relates salt levels in the particular product to aw levels. Water activity could also be controlled by formulation as long as the formulation is related to a given aw level by sufficient data. Since changes in ingredients suppliers may change the aw of the finished product, periodic aw determinations by the processor would be appropriate. (Source: U.S. Food and Drug Administration)

Water activity (aw) of some common foods

• Caviar: 0.92 aw
• Cheese Spread: 0.95 aw
• Dry Milk 8% total moisture: 0.70 aw
• Fudge Sauce: 0.83 aw
• Liver Pate: 0.96 aw
• Peanut Butter 15% total moisture: 0.70 aw
• Red Bean Paste: 0.93 aw
• Salami: 0.82 aw
• Soft Moist Pet Food: 0.83 aw
• Soy Sauce: 0.80 aw

Water is the major component of most living organisms. From a biological standpoint, water has many distinct properties that are critical for the proliferation of life. It carries out this role by allowing organic compounds to react in ways that ultimately allow replication. All known forms of life depend on water.

What products contain water?

Two broad categories exist:

• Hygroscopic: substances that absorb water from their surroundings
• Salts
• Vegetal fibres (wood, paper, produce…)
• Meat
products

• Hydrophobic: substances that don't absorb water from their surroundings
• Gases
• Sand, glass in powder form
• Metals in powder form

Water can have different effects in various products and is recognized in the food industry as being critical for the microbiological, enzymatic, chemical and composition stability of most products. Variations in water activity can significantly affect:

• Color
• Taste
• Nutritional value
• Protein and vitamin content
• Shelf life
• Storage and packaging
• Solubility

Water migration within a product

The water activity of a product will always try to reach equilibrium with the surrounding atmosphere. Free water will migrate from a region within a product with a high water activity to regions of low water activity. Water will migrate until equilibrium is reached. Equilibrium relative humidity (ERH) = 100 x aw.

What is static equilibrium?

Static equilibrium is a set of conditions under which a product and the surrounding atmosphere have the same water activity levels.

• Product and environment are at the same temperature
• The partial pressure of water vapor in the environment is the same as in the product.

How to control water activity

The water activity in products can be controlled by using various additives (humectants), by using satisfactory packaging materials or by maintaining favourable maturation and storage conditions.

How can water activity measurements be applied in the real world?

Water activity provides better information than the total moisture content regarding:

• Microbial, chemical and enzymatic stability of perishable products such as foods, seeds and pharmaceutical goods
• Cohesion and flow properties of hygroscopic powders
• Adherence of coatings
• Control of caking and compaction properties
• Dimensional stability of materials such as paper
• Tensile strength and elastic recovery
• Yield pressure and compaction

Total water in a product

So we have seen that the water activity is part of the total water in a product; specifically it is the free water. However, there also exists bound water within a product. The bound water is usually measured by infrared or loss on drying: this is known as the moisture content!

What is moisture content?

Moisture content is the percentage of water by weight in the product compared to the dry or wet weight of the product and includes both the bound water plus the free water. Moisture content is often confused with the water activity.

Is it possible to correlate water activity and moisture content?

At equilibrium, the relation between the percentage of water and the water activity of a hygroscopic material can be graphically represented by a curve: the sorption isotherm. For each water activity value, the sorption isotherm shows the corresponding moisture content at a given constant temperature. Each product has its own sorption isotherm.

With the help of a sorption isotherm, the moisture content of a product can be determined by measuring water activity. This is beneficial as water activity measurements are quick and non-destructive. The sorption isotherm (usually non-linear) is determined by experiment for each different product and at different temperatures.

What should I measure: water activity or moisture content?

Both measurements have advantages and disadvantages. The disadvantages of measuring moisture content:

• Moisture content does not tell you how much free water is available to microorganisms
• Provides no information regarding how water will migrate in or out of a product
• Measuring the moisture content destroys the sample
• Moisture content alone may not mean anything
• Measurement techniques are either costly or time-consuming
• Moisture content is unaffected by sample preparation

Measuring water activity, however, provides many benefits:

• Directly measures free water
• Non-destructive
• Quick and simple measurement
• Measures differences in sample preparation (vital for products affected by external spoilage such as tablets, most food products and seeds)

How to carry out a test procedure

However, since water activity is a qualitative measurement, used alone it does not define:

• shelf life
• growth potential of specific organisms
• physical properties (clumping etc.)

In order to carry out a meaningful measurement for water activity, it is also important to check the actual parameter that you are looking to control.

1^st Test: Actual parameter and water activity

• Shelf life tests
• Presence of spoilage organisms
• Physical properties
• Taste tests
• Efficacy of products (drugs, cosmetics)
• Costly tests
• Long time required
• Often required for compliance

Initial tests provide the detailed information on the parameter you are aiming to control. The tests may take more time with greater costs. They will provide details that allow you to relate, for example, shelf life to water activity for your specific product. At the same time you can test the water activity of raw ingredients and mid-production samples.

2^nd Test: Test for water activity only

• Raw ingredients (check consistency)
• Production processes (do recipes remain the same)
• End products (for shelf life and quality)
• Quick and easy
• Performed on site/in field
• Lower cost

This means, moving forward, subsequent water activity measurements alone can be used as an indicator of shelf life for end products, but also as a check on raw ingredients and processes to catch deviations before they can impact products. As water activity measurements are faster and available at lower cost this reduces overall costs and testing times.

Example: using water activity for shelf life testing

For unique food products where published water activity data is not available shelf life cannot be assessed accurately by testing water activity alone. Also, where production processes can vary it may be interesting to refine the process to provide the best compromise between product costs and shelf life. In this example, Product Awas produced in four batches, each was immediately tested for water activity and then subsequently underwent a long-term shelf life test.

Product A	Shelf Life	Water Activity
Batch 1	9 months	0.83 aw
Batch 2	5 months	0.85 aw
Batch 3	3 months	0.88 aw
Batch 4	2.5 months	0.90 aw

Testing demonstrates that to meet a three-month shelf life products should be produced with awater activity reading equal to or below 0.88 aw. Quick and easy water activity measurements can therefore be used alone moving forward to confirm that products meet this specification. When can water activity measurement be used alone? This can be carried out on known products with defined standards:

• Grains, cheese, salami…

For these products, health organisations or industrial bodies have already performed detailed testing and defined standard water activity limits for specified products. Water activity measurements can also be used alone to simply monitor product changes. The water activity provides a quick way to determine if an end product has changed:

• Processing change (drying time, particle size)
• Raw ingredient change (supplier inconstancy)
• Contamination (chemical or organic)

Learn more about water activity in the following video:

Other related blogs: How to measure water activity