Fundamentals of Water Activity: Product Safety & Quality

Water activity's usefulness as a quality and safety measurement was suggested when it became evident moisture content could not adequately account for microbial growth fluctuations. Water activity is a measure of the energy status of the water in a system. The water activity (a_w) concept has served the microbiologist and food technologist for decades and is the most commonly used criterion for safety and quality. Its usefulness cannot be denied.

Predicting Safety and Stability
Water activity predicts safety and stability with respect to microbial growth, chemical and biochemical reaction rates, and physical properties. Figure 1 shows stability in terms of microbial growth limits and rates of degradative reactions as a function of water activity. Therfore, by measuring and controlling the water activity, it is possible to:

Predict which microorganisms will be potential sources of spoilage and infection
Maintain the chemical stability of products
Minimize nonenzymatic browning reactions and spontaneous autocatalytic lipid oxidization reactions
Prolong the activity of enzymes and vitamins
Optimize the physical properties of products such as moisture migration, texture, and shelf life.

diagram

Microbial Growth
Microorganisms have a limiting water activity level below which they will not grow. Water activity, not moisture content, determines the lower limit of "available" water for microbial growth. Since bacteria, yeast, and molds require a certain amount of "available" water to support growth, designing a product below a critical aw level provides an effective means to control growth. Water may be present, even at high content levels, in a product, but if its energy level is sufficiently low the microorganisms cannot remove the water to support their growth. This 'desert-like' condition creates an osmotic imbalance between the microorganisms and the local environment. Consequently, the microbes cannot grow and its numbers will decline until it eventually dies.

Limiting Microorganism Growth
While temperature, pH, and several other factors can influence whether an organism will grow in a product and the rate at which it will grow, water activity is often the most important factor. Water activity may be combined with other preservative factors (hurdles), such as temperature, pH, redox potential, etc., to establish conditions that inhibit microorganisms. The water activity level that limits the growth of the vast majority of pathogenic bacteria is 0.90 a_w, 0.70 a_w for spoilage molds, and the lower limit for all microorganisms is 0.60 a_w. View a chart listing the water activity limits for growth of microorganisms significant to public health.

Chemical/Biochemical Reactivity
Water activity influences not only microbial spoilage but also chemical and enzymatic reactivity. Water may influence chemical reactivity in different ways; it may act as a solvent, reactant, or change the mobility of the reactants by affecting the viscosity of the system. Water activity influences nonenzymatic browning, lipid oxidization, degradation of vitamins and other nutrients, enzymatic reactions, protein denaturation, starch gelatinization, and starch retrogradation (see Figure 1). Typically, as the water activity level is lowered, the rate of chemical degradative reactions decreases.

Physical Properties
Besides predicting the rates of various chemical and enzymatic reactions, water activity affects the textural properties of foods. Foods with high water activity have a texture that is described as moist, juicy, tender, and chewy. When the water activity of these products is lowered, undesirable textural attributes, such as hardness, dryness, staleness, and toughness, are observed. Low water activity products normally have texture attributes described as crisp and crunchy, while these products at higher water activity levels change to soggy texture. Critical water activities determine where products become unacceptable from a sensory standpoint.

Caking, Clumping, Collapse and Stickiness
Water activity is an important factor affecting the stability of powders and dehydrated products during storage. Controlling water activity in a powder product maintains proper product structure, texture, stability, density, and rehydration properties. Knowledge of the water activity of powders as a function of moisture content and temperature is essential during processing, handling, packaging and storage to prevent the deleterious phenomenon of caking, clumping, collapse and stickiness. Caking is water activity, time, and temperature dependent and is related to the collapse phenomena of the powder under gravitational force.

Moisture Migration
Because water activity is a measure of the energy status of the water, differences in water activity between components is the driving force for moisture migration as the system comes to an equilibrium. Thus, water activity is an important parameter in controlling water migration of multicomponent products. Some foods contain components at different water activity levels, such as filled snacks or cereals with dried fruits. By definition, water activity dictates that moisture will migrate from a region of high water activity to a region of lower water activity, but the rate of migration depends on many factors. Undesirable textural changes can result from moisture migration in multicomponent foods. For example, moisture migrating from the higher a_w dried fruit into the lower water activity cereal causes the fruit to become hard and dry while the cereal becomes soggy.

Water Adjusts Between Materials Until Water Activity Equilibrium
Differences in water activity levels between components or a component and environmental humidity are a driving force for moisture migration. Knowledge of whether water will absorb or desorb from a particular component is essential to prevent degradation, especially if the substance is moisture sensitive. For example, if equal amounts of component 1 at 2% and component 2 at 10% moisture content must be blended together, will there be moisture exchange between the components? The final moisture content of the blended material would be 6%, but did any moisture exchange between component 1 and 2? The answer depends on the water activities of the two components. If the water activities of the two components are the same, then no moisture will exchange between the two components. Also, two ingredients at the same moisture content may not be compatible when mixed. If two materials of differing water activities but the same water content are mixed, the water will adjust between the materials until an equilibrium water activity is obtained.

Shelf-life/Packaging
Water activity is a critical factor in determining the shelf life of products. Critical upper and lower water activity levels can be established with respect to microbial, texture, flavor, appearance, aroma, nutritional, and cooking qualities for food products. Rates of exchange of moisture through the package and the rate of change in water activity of the food towards a critical limit will determine the shelf life of a product. Knowledge of the temperature, ambient relative humidity and critical water activity values will aid in selection of a package with the correct barrier properties to optimize quality and shelf life.