Water activity is not an intuitive concept for most people. But once you understand it, it can give you the power to control moisture in food. Preventing microbial growth is just the start.
Get the essentials of water activity condensed in this 20 minute webinar. You’ll learn:
-What water activity is
-How it’s different from moisture content
-Why it controls microbial growth
-How understanding water activity can help you control moisture in your product.
Mary Galloway has been a lead scientist in the METER Food Research & Development lab for eight years. She specializes in using and testing instruments that measure water activity and its influence on physical properties. She has worked with many customers to solve their moisture-related product issues and frequently gets the chance to answer the question, “What is water activity?”
How do food manufacturers maintain the texture of a tender raisin and a crispy brand flake, avoid caking and clumping, or determine if a product is susceptible to spoilage? All of these issues are controlled by water activity. If you understand how water activity works, you can develop products that are desirable and predict and prevent potential storage problems.
Principles of thermodynamics:
Water activity (aw) is the measure of the energy status of water in a system. It is a principle of thermodynamics and follows the same rules. A higher water activity means more energy, and the water can do more work, such as microbial growth, moisture migration, or chemical and physical reactions. Differences in water activity will dictate how moisture moves (in terms of energy, not concentration). Water at a higher water activity has more energy than the water at a lower water activity. How will water lower its energy status to become more stable? It will migrate to a lower water activity.
In energy systems, we can adapt the Gibbs free energy equation (Equation 1) to determine the activity of the water in a system at a given temperature. The energy of water in a system is equal to the energy of pure water (𝜇o) plus the gas constant (R) times the temperature constant (T) and the natural log of fugacity. Note the only variable that’s dependent in this equation to determine the energy of the water is fugacity.
Fugacity (f/f0) is the escaping tendency of a material or what vapor can escape out of a sample.
Fugacity is measured by partial pressures, or the vapor pressure of water above a sample at a specific temperature, divided by the vapor pressure of pure water at the same temperature. And relative vapor pressure (sometimes called partial vapor pressure) is exactly water activity. So, if we determine the partial vapor pressure of a sample, we can calculate the water activity.
Figure 1 illustrates what vapor pressure is. There is a sample of food in a container on the left. Water molecules escape out of the sample into the headspace. These molecules produce a specific pressure inside the sealed container. This pressure is compared to the pressure produced by pure water as demonstrated in the container on the right. Since water activity is a ratio between two pressures, it is unitless and measured on a scale of 0 (no energy) to 1 (the same energy as pure water).
It’s important to point out that pressure needs to be allowed to equilibrate, and the temperature and pressure need to be constant. Water activity at 25 ℃ will be different than the water activity 35 ℃. Generally, it will be higher. So if you take a reading at 25 ℃ one day, and the next day you take it at a different temperature, they will not be the same water activity because water activity is dependent on temperature being constant.
Primary method: loss-on drying (Equation 2)
Primary method: Titration (Equation 3)
Unlike water activity, which is an energy status, moisture content is a qualitative measure, or an amount, of water. It is not a driving force. It does affect texture, but it is not a driving force for reactions or changes in a product. There are two primary methods used to measure it.
One issue with measuring moisture content is there is no standard. There is nothing that has an intrinsic moisture content of X to which we can compare. We can get an answer for percent moisture content, but we don’t know if it’s accurate.
|Water Activity||Moisture Content|
|Driving force||Not a driving force|
|Known standards (salt solutions)||Empirical measurement with no standard|
|Must define wet basis or dry basis (LOD)|
Compare the moisture content of a cookie versus honey, and you’d expect the moisture content of the honey to be higher. This is true: Honey is 18% MC, and a cookie is 5% MC. But these two products have the same water activity (0.60aw), which means if you submerged the cookie in the honey for a week, the cookie would not become soft. Why? Because water activity, not moisture content, is the driving force behind reactions (in this case moisture migration). Nothing would happen because the energies (or water activities) are the same.
There are applications for both water activity and moisture content (Table 2). Water activity is a more accurate way to predict and prevent storage issues, but note that moisture content does affect texture. You can use moisture content to improve texture, depending on what kind of product you want. It can also be used to determine ingredient concentrations or nutritional content which is important for labeling requirements. And, if your product has a moisture content limit, for instance pet food at 10%, you must determine moisture content to know if the product is in compliance.
|Water Activity||Moisture content|
|Control microbial growth||Adjust texture at a given water activity|
|Control moisture migration||Determine ingredient concentrations|
|Avoid caking and clumping||Determine nutritional content|
|Formulate profitable products||Labeling requirements|
|Control chemical reaction rates|
|Model dry ingredient mixing|
|Predict effects of temperature abuse|
|Achieve optimal texture|
|Conduct shelf life testing|
|Predict packaging needs|
Each product has its own unique relationship between water activity and moisture content. Figure 2 shows the relationship between water activity and moisture content in products we’ve tested. They are all completely different, and each graph has a different shape.
The relationship between water activity and moisture content is called a moisture sorption isotherm, and it can be used to determine a critical water activity. This is where the moisture sorption properties physically change and can take up more moisture. A critical water activity is is determined by a change in the slope of the curve. At the water activity where the slope changes, the product will change in texture or experience other types of reactions.
You can determine the effect of formulation by comparing different isotherms from one formulation to the next. For instance you can model dry ingredient mixing to predict the water activity when you mix two new ingredients. You can also determine the effect of temperature abuse; if a product is shipped and stored in a hot truck or warehouse, what will happen to the product when it gets to the retailer? You can perform isotherms at different temperatures and predict what the effect will be. Isotherms also are essential for predicting shelf life.
Microorganisms need water for growth, and they get water from their surroundings. When an organism is surrounded by a lower water activity than its interior, it experiences osmotic stress. In figure 3, the water activity inside of a cell is 0.95 aw. Outside of the cell, the water activity is 0.90 aw. Since a high water activity wants to become a low water activity, the water inside the cell will move out, and when it does, the cell will lose turgor pressure. The cell will try to adapt by changing its metabolic process to reduce its internal water activity. If it can match the environment, it will have enough water or energy to grow and reproduce.
But what if it can’t match the environment? Another cell in Figure 3 has a water activity of 0.93 but does not match the environment at 0.90. In this case, there is not enough energy for the cell to grow and reproduce, and it will go dormant.
How well a microbe can adapt and lower its water activity dictates what its water activity limit is. In the 1950’s Dr. William James Scott showed that microorganisms have a water activity level below which they will not grow (Table 3). There is a specific water activity for each microorganism that will inhibit growth, and they cannot grow in an environment below that limit.
|Microorganism||Minimum Water Activity|
|Clostridium botulinum E||0.97|
|Clostridium botulinum A, B||0.94|
|Staphylococcus aureus (anaerobic)||0.90|
|Staphylococcus aureus (aerobic)||0.86|
Table 3 shows that salmonella has a water activity limit of 0.95. This means if a product has a water activity of 0.95, and conditions are ideal for pH, temperature, nutrients, and there are no competing species, salmonella cannot grow. If any of these conditions change or are less than ideal for microbial growth, then the limiting water activity could be increased. Bacteria can grow at a higher water activity than this limit, but they can’t ever grow at a lower water activity. And it doesn’t matter what matrix the bacteria are in, a cookie, a powder, or a pet food, if these bacteria are present, they will not grow below that limit.
Note that water activity is not a kill step or a removal of the bacteria. It’s a control step that prevents the growth of microorganisms, which means the product is safe but not sterile. The bacteria are still there. If these foods were in the presence of a higher water activity than what their limit is, they could grow. That is a potential problem, but if you formulate to keep the water activity low enough, there won’t be a problem.
Table 3 also shows that aerobic staph bacteria has a minimum water activity of 0.86. That means everything above 0.86 aw is considered a potentially hazardous food. If these bacteria start growing, they make people sick, thus foods above that water activity are deemed potentially hazardous. Everything below 0.85 aw is a limit where that cannot happen.
|Range of Water Activity||Microorganisms Generally Inhibited by Water Activity in this Range||Foods Generally Within this Range|
|0.95-1.00||Pseudomonas, Escherichia, Proteus, Shigella, Klebsiella, Clostridium|
perfringens, Clostridium botulinum, and Salmonella
|Fresh fruits, canned fruits and vegetables, & fish|
|0.90-0.95||Saccharomyces cerevisiae, Vibrio parahaemolyticus, Serratia,|
Lactobacillus, Pediococcus, Bacillus cereus, and Listeria monocytogenes
|Some cheeses (cheddar, swiss, provolone, muenster),|
and cured ham
|0.85-0.90||Staphylococcus aureus, Micrococcus and many yeasts (Candida|
|Salami, sponge cakes, dry cheeses, and margarine|
|0.85 AND UP||POTENTIAL HAZARDOUS FOODS|
|0.80-0.85||Mycotoxigenic pennicilia (Penicillum expansum, Penicillum islandicum),|
and some yeasts (Saccharomyces bailii and Debaromyces hansenii)
|Most fruit juice concentrates, condensed milk,|
|0.75-0.80||Halophilic bacteria, and mycotoxigenic Aspergilli (Aspergillus niger, Asper-|
gillus ochraceous, and Aspergillus candidus)
|Jam, marmalade and marzipan|
|0.65-0.75||Xerophilic molds (Erotium chevalieri, Erotium amstelodami, Wallemia|
sebi), and Saccharomyces bisporus
|Jelly, molasses, raw cane sugar, nuts and some|
|0.60-0.70||NO MOLDS FOR SPOILAGE|
|0.60-0.65||Osmophilic yeasts (Zygosaccharomyces rouxii), and a few molds|
(Aspergillus enchulatus and Monascus bisporus)
|Dried fruits containing 15-20% moisture, some|
candy, and honey
|0.60 AND LOWER||NO MICROBIAL GROWTH|
|0.50-0.60||No microbial proliferation||Dry pasta and spices|
|0.40-0.60||No microbial proliferation||Whole egg powder|
|0.30-0.40||No microbial proliferation||Cookies, crackers, and bread crusts|
|0.20-0.30||No microbial proliferation||Roasted ground coffee and table sugar|
Table 4 is a graph showing the entire water activity range for various organisms, including molds and yeasts. It also shows typical foods found in each water activity range. Notice above 0.85 are the potentially hazardous foods. Molds have lower water activity limits, but molds generally involved in spoilage are at or above 0.7. There is no growth for any microbe below 0.6. You can use this information to produce products that are not potentially hazardous or susceptible to spoilage molds.
One grower dried his pecans to a 4% moisture content. He wasn’t sure if 4% was dry enough to prevent microbial growth, but historically he’d never had issues with this spec. If he were to look at a moisture sorption isotherm to determine the relationship between water activity and moisture content, he would see that a water activity of 0.68 in his pecans relates to a 4% moisture content. 0.68 is below the microbial limit for mold growth. So if his water activity stays at 0.68, a 4% moisture content is sufficient to prevent mold.
But the grower’s product did mold. Why?
His moisture content measurement was only accurate to one half of a percent. When the pecans measured 4%, they were actually closer to 4.5%, which meant the water activity exceeded the safe limit for mold. Moisture content was not an adequate quality spec because the pecans could be anywhere from 3.5 to 4.5%, and the grower would never know.
If the pecan grower’s moisture content fluctuates from 3.5 to 4.5% not only are the pecans susceptible to mold, they could make less profit. A lower moisture content means a lower quality (harder) nut, and it means more nuts go in each bag (overpack). However, if he used a more accurate water activity spec, he could prevent both issues. He could keep the moisture content at exactly 4% by using a water activity spec of 0.68.
A dry soup manufacturer processed a mix to a 3% moisture content. He received new pepper to add to the mix which also measured at 3% moisture content. However, when he mixed the two ingredients together, the entire batch clumped. What happened? Even though the moisture contents were the same, the water activities were different.
The soup mix was 0.28 aw, and the water activity of the pepper was 0.69 aw which was above the critical water activity of the soup. Higher water activity always moves toward a lower water activity, so moisture migrated from the pepper into the soup and caused the mixture to clump. If the manufacturer had measured the water activity before adding it to the soup he could have predicted the caking and clumping because he knew that 0.69 aw was above the critical limit for the soup. By tracking water activity of incoming ingredients, the manufacturer could monitor quality in their suppliers and set an acceptance spec that would be below the critical water activity. They could use this information to achieve consistency in their incoming ingredients.
Water activity is also critical to product formulation. If you manufactured a snack cake and generated isotherms for icing, cream filling, and cake, you would see that each ingredient has a different relationship between water activity and moisture content. Each curve is a different shape (Figure 4).
At a water activity just under 0.7 (vertical line), the ingredients all have different moisture contents. The icing is 5%, the cream filling is close to 15%, and the cake is at 20%. Each moisture content gives a different texture when a customer bites into the snack cake. You can formulate each ingredient to this exact water activity, and each component will maintain its moisture content and texture. Because the water activity of every component is the same, moisture will not migrate from one component to another.
A pet food manufacturer produced to a moisture content of 6.5% because he’s never had any spoilage with that spec. He created an isotherm and found at a 6.5% moisture content his product had a water activity of 0.4, which was well below any microbial limits. But was his moisture spec too low? Since pet food is allowed a maximum moisture content of 10%, he could safely increase moisture content and water activity to increase his profit margin and improve texture.
After using isotherm data to identify a critical water activity limit and performing shelf-life calculations, the pet food manufacturer set a new water activity spec of 0.6, which corresponds to a moisture content of 9.5%. Both of these values were within safety and regulatory limits. By increasing water activity and the moisture content spec, he reduced raw ingredient cost. He used less ingredients to produce the same amount of pet food, essentially replacing those ingredients with water. He also reduced electricity and heat because of less time in the ovens. And the product was better because the moisture content was higher. By understanding water activity, the manufacturer was able to consistently increase profit without compromising quality or safety.
Water activity can influence the reaction rates of various types of chemical reactions that occur in food and pharmaceuticals.
Figure 5 is a graph developed by Dr. Ted Labuza which shows most reaction rates increase at water activities near 0.6. The graph illustrates where bacteria, yeast, and mold grow. And it also shows where enzymatic activity increases. Browning reactions peak around 0.6 and then fall off because there is more water in the matrix at that point, and they get diluted. Lipid oxidation follows an unusual trend where it’s high at low water activities and high again at higher water activities. Interestingly, it’s more stable at water activities between 0.3-0.4 which is important for some products like potato chips that contain a lot of fat and oil.
Water activity is the energy of water in a system. It’s qualitative and inherent in the product itself. It’s a driving force that enables things to happen like microbial growth, moisture migration and physical and chemical changes. Moisture content is just the amount of water. It’s not a driving force, so it won’t tell you what water is going to do, it only tells you how much is there.
Water activity is the right specification for preventing microbial growth, maintaining physical and chemical stability, formulating products, and for predicting shelf life.
Labuza, Ted P., K. Acott, S. R. TatiNl, R. Y. Lee, Jv Flink, and W. McCall. “Water activity determination: a collaborative study of different methods.” Journal of Food Science 41, no. 4 (1976): 910-917.
Scott, W. J. “Water relations of food spoilage microorganisms.” In Advances in food research, vol. 7, pp. 83-127. Academic Press, 1957.