It is now generally accepted that aw is more closely related to the physical, chemical, and biological properties of foods and other natural products than is total moisture content. Specific changes in color, aroma, flavor, texture, stability, and acceptability of raw and processed food products have been associated with relatively narrow aw ranges.”
In 1953, William James Scott showed that microbial growth in food is governed not by water content, as most people thought, but by water activity. Four years later, he established the concept of a minimum water activity for microbial growth. Water activity is now routinely used by food manufacturers to determine whether or not a product is susceptible to microbial proliferation. Water is recognized as being very important, if not critical, to the stability of most products. Controlling the water within a product, by some method of drying or by chemically/structurally binding (salting or sugaring) has long been used by man for preservation. This not only controls microbial spoilage, but also chemical and physical stability.
Let’s use a thought experiment to better understand water activity. Take a glass of water, and a dry sponge. Dip the corner of the sponge into the glass of water. The water will, of course, move from the glass into the sponge.
What is the difference between the water in the glass and the water in the sponge?
The answer is that the water in the glass is free, while that in the sponge is, to some extent, bound. It has a lower energy state than the water in the glass. We know that, because to retrieve the water from the sponge we need to do work on it (squeeze the sponge). That reduction in the water’s energy reduces its vapor pressure, increases its boiling point, and reduces its freezing point. In other words, the water in the sponge is different from the water in the glass in measurable ways
Let’s consider the reduction in vapor pressure.
We can calculate the change in energy that accompanies a change in pressure using the first law of thermodynamics. If we let the symbol U represent the energy in a system, and calculate the change in U that occurs when we change the volume, at constant pressure (we assume no heat is added or removed) we can write:
dU represents a small change in energy, and dV represents a small change in volume. The relationship between pressure and volume, called the ideal gas law, is
where n is the number of moles of gas, R is a constant, known as the gas constant (8.31 J/mol K) and T is the temperature of the gas in kelvins. We can differentiate the ideal gas law to get dV:
Combining this with the first law we get:
Now, the energy required to go from the vapor pressure of the pure water in the glass, which we call the saturation vapor pressure or p0, to the vapor pressure of the water in the sponge is
The ratio p /p0 is called the water activity, aw,
when we are talking about the water in the sponge, or water in foods or other solids or liquids. We call it the relative humidity when we apply it to water in the air, and sometimes multiply it by 100 to express it as a percent.The ratio U/n is the energy per mole of water and is called the water potential, with the symbol y. Water potential has units of Joules/mole. With this substitution we finally arrive at the equation relating the energy of the water in the sponge and its water activity:
The equation tells us that we can express the energy state of the water in a product either as a water potential or as a water activity. Some fields of science use water potential and others use water activity (some also use freezing point depression or osmolality, but these are all equivalent concepts). There are advantages and disadvantages to each, but the important thing to understand is that both are measures of the energy state of the water and have a strong theoretical basis. We focus on water activity here because that is the measure most widely used in food science and engineering.
What determines water activity?
Now consider what factors influence water activity. We can lower the water energy by adsorbing the water in the sponge. Water adsorbed onto any surface lowers its energy state. The water is bound by hydrogen bonds, capillary forces and van der Waals - London forces, so it has less energy than free water. We call these effects matrix effects. The water energy can be decreased in another way as well. We can dilute the water with solutes. Since work is required to restore the water to its pure, free state, this also reduces the water activity and water potential. We call these effects osmotic effects. We sum the reduction in energy from matric and osmotic effects to get the total change in energy.
Water Content Alone is Not a Reliable Predictor:
Traditionally, discussions about water in products or ingredients focus on moisture or water content, which is a quantitative or volumetric analysis that determines the total amount of water present. Water content of a product is a familiar concept to most people. One measures the water content by loss on drying, infrared, NMR or Karl Fisher titration. Moisture content determination is essential in meeting product nutritional labeling regulations, specifying recipes and monitoring processes. However, water content alone is not a reliable predictor of microbial responses and chemical reactions in materials.
Chemically Bound Water is Unavailable to Microbes
The limitations of water content measurement as an indicator of safety and quality are attributed to differences in the intensity which water associates with other components in the product. The water content of a safe product varies from product to product and from formulation to formulation. One safe, stable product might contain 15% water while another containing just 8% water is susceptible to microbial growth. Although the wetter product contains proportionally more water, its water is chemically bound by other components, making it unavailable to microbes. Using only water content values, it’s impossible to know how “available” the water in the product is to support microbial growth or influence product quality.
Water Activity is Most Relevant for Quality and Safety Issues
Another more important type of water analysis is water activity (aw). Water activity describes the energy status or escaping tendency of the water in a sample. It indicates how tightly water is “bound,” structurally or chemically, in products. Both the water content and the water activity of a sample must be specified to fully describe its water status. However, water activity is the property most relevant for quality and safety issues. Water activity is closely related to the partial specific Gibbs free energy of the system. Thus, water activity is a thermodynamic concept and has requirements for measurements. These requirements are that the system be in equilibrium, the temperature defined, and a standard state specified. Pure water is taken as the reference or standard state from which the energy status of water in food systems is measured. The Gibbs free energy of free water is zero; thus, the water activity is 1.0.
Water Activity and Growth of Microorganisms in Food:
(Range of aw- Microorganisms Generally Inhibited by Lowest aw in This Range- Foods Generally within This Range).
- Pseudomonas, Escherichia, Proteus, Shigella, Klebsiella, Bacillus, Clostridium perfringens, some yeasts.
- Highly perishable (fresh) foods and canned fruits, vegetables, meat, fish, milk, and beverages.
- Salmonella, Vibrio parahaemolyticus, C. botulinum, Serratia, Lactobacillus,Pediococcus, some molds, yeasts (Rhodotorula, Pichia).
- Some cheeses (Cheddar, Swiss, Muenster, Provolone), cured meat (ham), bread, tortillas.
- Many yeasts (Candida, Torulopsis, Hansenula), Micrococcus.
- Fermented sausage (salami), sponge cakes, dry cheeses, margarine.
- Most molds (mycotoxigenic penicillia),Staphyloccocus aureus, most Saccharomyces (bailii) spp., Debaryomyces.
- Most fruit juice concentrates, sweetened condensed milk,syrups, jams, jellies,soft pet food.
- Most halophilic bacteria,mycotoxigenic aspergilli.
- Marmalade, marzipan, glacé fruits, beef jerky.
- Xerophilic molds (Aspergillus chevalieri, A. candidus, Wallemia sebi), Saccharomyces bisporus.
- Molasses, raw cane sugar, some dried fruits, nuts, snack bars, snack cakes.
- Osmophilic yeasts (Saccharomyces rouxii), few molds (Aspergillus echinulatus, Monascus bisporus).
- Dried fruits containing 15-20% moisture; some toffees and caramels; honey, candies.
- No microbial proliferation.
Dry pasta, spices, rice, confections, wheat.
- No microbial proliferation.
- Whole egg powder, chewing gum, flour, dry beans.
- No microbial proliferation.
- Cookies, crackers, bread crusts, breakfast cereals, dry pet food, peanut butter.
- No microbial proliferation.
- Whole milk powder, dried vegetables, freeze dried corn.
Water activity is a thermodynamic measure of the energy of water in a product. It is directly related to the microbial susceptibility of food products. It is also well-correlated with degradative chemical and physical reactions that end shelf life in foods. It can be used to predict and maximize shelf life, to make packaging decisions, to avoid glass transition, and in many other facets of formulation. Because it is measured on a scale with a known standard, it is particularly well suited to being a safety and quality specification. It is cited in several FDA regulations and guidelines, and is the only measurement that can be used as a HACCP critical control point.
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