Water Activity: Thermodynamics in Food
PROCESSING
Water activity is an important thermodynamic concept that is often misunderstood and explained poorly, but can be critical to understanding food preservation, food texture, and other properties. Pathogenic and spoilage microorganisms cannot grow or produce toxins below certain values of water activity, and so maintaining foods below these critical values can become a critical control point.
Water activity is another name for the chemical potential of the water in a system that may include dissolved solids, ionic species, and insoluble material. It may seem obvious to say, but water activity is a property of the water in a system. Some published literature refers to the water activity of humectants, such as glycerin or sugars, but what they mean is the effect these solutes may have on the water activity of the water in a system. It makes no sense to speak of the water activity of sugars.
Other discussions make an effort to account for water activity values by imagining the physical state and location of the water in a system. These are hard to know exactly, especially in complex systems, and really are unnecessary, because most manipulations of water activity are empirical. The most significant points to understand are that water activity is distinctly different, though clearly related to, water content and that water activity is an equilibrium property.
Water Content
Water content is typically measured by weight loss upon extended vacuum drying and can be expressed as a percent of wet or dry weight. Commercially available instruments use heat lamps or microwaves to dry small samples placed on digital scales to give good measures of moisture content within about 10 min. These are valuable for process control, but the standard for accuracy is typically overnight drying.
Other measures of moisture include Karl Fischer titration, near infrared absorption (NIR), and nuclear magnetic resonance (NMR). Foods vary widely in their natural moisture content and, sometimes, in moisture distribution within a food system.
Moisture content is usually expressed on a weight basis, but as will be seen, what matters in understanding water activity is the molecular basis, in which composition is expressed as mole fractions.
Mole fraction is the ratio of the number of moles of a substance to the total number of moles of all substances present. A mole is the amount of a substance equal to its atomic weight. Atomic weights are based on the weight of carbon 12 (C12) defined as 12.0. Water has a molecular weight of 18. All moles of any substance, expressed in grams, have exactly the same number of molecules, known as Avogadro’s number, equal to 6.02252 x 1023. Calculation of mole fractions is straightforward for solutions in which the solute is a pure, known material, but becomes more challenging when mixtures of large molecules are involved and when ionic substances are present. Ionic substances are materials, such as salts, which disassociate in solution to varying degrees. Each separate ion behaves as a distinct entity, as does any undisassociated combination. This can make theoretical predictions of properties that depend on mole fractions difficult, but remembering the concept can help one to understand qualitative trends.
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In particular, it is important to know that the water activity of water in an ideal solution is equal to the mole fraction of water in solution. (Not, as some literature says, the mole fraction of solutes. See Leung, cited later.) Because water has a relatively low molecular weight compared with many of the other materials that make up foods, the number of moles present, and hence the mole fraction, is generally greater than might be expected from the moisture content. For example, a processed meat with moisture content of about 10% (dry basis) has a water activity of about 0.4.
Equilibrium
Equilibrium exists in a given system when the chemical potential of all substances in all phases is equal. In such a situation, there is no movement among phases of any material and no change in temperature or pressure. It is a difference in chemical potential that induces transfer of a material from one phase to another. So, for example, if a dry food is placed in a humid atmosphere, the food picks up moisture from the air, and the air, in a closed system, loses moisture to the food. The reverse—in which a wet food loses moisture to a dry atmosphere—can also happen. Most people have experienced these phenomena without necessarily understanding the underlying physics or thermodynamics.
Understanding the concept of equilibrium provides a convenient means of measuring water activity directly. The gas phase in equilibrium with a solid or liquid normally behaves more ideally than the liquid or solid phase. In the gas phase, the chemical potential or water activity is just the mole fraction, which is also the ratio of the partial pressure to the pure component vapor pressure at the same temperature. In the case of water, this is known as the relative humidity, sometimes expressed as a percent. Relative humidity is easy to measure using instruments that measure a known electrical or physical response of a substance to a change in moisture content or other means to measure humidity. For instance, the temperature at which water condenses from a mixture of air and water vapor is called the dew point and can be related to the relative humidity of the original mixture. (Dew points are often provided in weather reports as measures of humidity and, hence, indicators of comfort levels. Low dew points correspond to low humidities.) Some instruments use a cooled mirror to measure dew point and report relative humidity or water activity.
There are instruments that measure relative humidity in a closed chamber over a food sample and water content of the sample simultaneously. The plot of moisture content against water activity at a constant temperature generates a curve known as an isotherm. There are numerous collections of experimentally determined isotherms for foods. Some examples are Handbook of Food Isotherms by H.A. Iglesias and J. Chirife (Academic Press, 1982) and “Water Activity and Other Colligative Properties of Foods” by H.K. Leung in Physical and Chemical Properties of Food, ed. M.R. Okos (American Society of Agricultural Engineers, 1986).
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Another technique is to equilibrate food samples in a closed vessel, such as a bell jar, which also contains a solution with a previously established and known water activity. Solutions of sulfuric acid or saturated salt solutions may be used for this purpose. After the passage of time, the sample, the atmosphere, and the known reference material are all in equilibrium at the given temperature and so the water activity of the sample is known and can be related to its measured water content.
It has been observed that isotherms are slightly different when they are determined by moisture absorption or by desorption. This phenomenon is called hysteresis. Often, the desorption curve lies above the absorption curve, meaning that at the same water activity, the equilibrium water content is higher for measuring by desorption than it is for measuring by absorption. Upon repetition of the measurements on the same materials, hysteresis often is reduced. This may occur because the materials are modified by the experiments; for example, crystallinity may be changed. There may also be mass transfer effects, in which desorption from the interior of a large sample may be incomplete. The practical impact is that one needs to know how isotherms were determined to understand their reliability and usefulness. For many purposes, a rough average in between the absorption and desorption curves is adequate.
Temperature has a relatively small effect on most isotherms because solubility of solutes and the vapor pressure of water increase with temperature in much the same way, so with an increase in temperature, the partial pressure of water should increase, but the mole fraction at equilibrium may decrease, creating a very small net effect on water activity.
Practical Applications of Water Activity
As previously mentioned, pathogens and spoilage microbes do not grow or produce toxins below certain water activities. A value of 0.85 is generally considered sufficient for food safety, though some very hardy microbes can grow down to water activity of 0.60. Most pathogens are inhibited below water activities of 0.95.
Intermediate-moisture foods are considered those with water activities between 0.60 and 0.85, and include materials such as dried fruits, jams and jellies, salted fish, and aged cheese.
Reducing water activity can be one of several hurdles used in food preservation, often combined with mild heating and added preservatives. The net effect of using several hurdles to spoilage can be foods with better taste and texture than their counterparts preserved with one more-severe treatment, such as canning.
Water activity can be used to formulate icings of baked goods that will not attract moisture from the crumbs, which generally are higher in water activity. Humectants are relatively low molecular weight substances that can be added to foods to reduce water activity while maintaining soft texture that might otherwise be damaged by removing water. Some examples are glycerin, propylene glycol, sorbitol, and corn syrup.
Dried fruits packaged with cereals often dry further and get hard while the cereal may become soft due to moisture migration among components with differing water activity. Earlier cereal packaging was more permeable to water and so some moisture escaped during storage, reducing the impact, but newer packaging materials are more effective at retaining the moisture in the package. Solutions include adding solids, such as cereal fines, to the fruit and using freeze-dried fruit instead of intermediate-moisture fruit.