Water activity basics for safety and quality in food products
WATER ACTIVITY'S USEFULNESS as a food quality and safety measurement was suggested when it became evident water content could not adequately account for microbial growth fluctuations. The water activity (aw) 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. Biological cells and tissues are not homogeneous solutions; neither are food products. Food products derive most of their perceived consumer benefits (apart from nutritional value) from their physical and chemical differences. Water is not uniformly distributed, nor can it be disregarded exclusively as solvent.
Protecting against food spoilage.
Since yeast, molds and bacteria require a certain amount of available water to support growth, designing a product with an aw below 0.6 provides an effective control. Water activity is defined as the equilibrium relative humidity (ERH) divided by 100. Some common spoilage organisms and their aw limits are listed in Table 1. The simplest way to reduce water activity is with a process which drives off water - cooking, baking or dehydration. The high-heat processes also use the lethal properties of heat, while dehydration or freeze drying only work by lowering the aw to a level that curbs growth.
Binding free water and food design.
The second method involves tying up the free water by the addition of solutes, usually sugars or sodium chloride. This creates an imbalance in osmotic pressure which draws the water from cells. Food designers face new challenges in maintaining sufficiently low aw with many of today's fat replacers. Fat, which does not contribute to the free water, is replaced by water or a gel to provide lubricity. These gels do not reduce aw and additional control methods are necessary to prevent spoilage.
Water activity concepts - over 40 years old.
Until recently, water activity as a physiochemical parameter was mainly discussed in two scientific disciplines: physical chemistry and food microbiology. In the former, it measures the thermodynamic free energy of water and in the latter it is used to define the lower limits of growth of food spoiling microorganisms. Microbiologists turned to water activity measurements upon discovering that microbial spoilage of food occurs at widely varying levels of water content. Scott†, in the 1950's, applied the water activity concept to describe the water availability for microbial growth.
The definition of water activity.
Scott defined water activity (aw) as the ratio of the water vapor pressure over a food (P) to that over pure water (P0).
aw = P/P0
Thus, multiplication of the water activity by 100 gives the relative humidity of the atmosphere in equilibrium with the food.
R.H. (%) = 100 x aw
Water activity is a better index for microbial growth than water content.
Water activity better predicts the growth of microorganisms because microorganisms can only use "available" water, which differs considerably depending on the solute. On average, ions bind the most water, whereas polymers bind the least water; sugars and peptides fall into an intermediate position. At the same molecular concentration, salt lowers the water activity more than sugar.
When a substance is added to lower water activity, the result can be complicated. Ideally, an inert material could be added which would decrease water activity without any other effects such as increased ionic strength and decreased surface tension. In reality, the choice of substance can have a profound effect. For instance, salt could be added to one reaction mixture and sugar to another. The amounts can be controlled so that the resulting compounds have identical water activity. But the results of the reaction will differ because of the differing influences of salt and sugar on biological reactions. Salt and sugar form different additional hurdles.
Water activity and hurdle technology.
Water activity should be regarded as an external parameter like pH or temperature. Under certain conditions, it will act synergistically with other environmental parameters. Under other conditions it will be the sole parameter determining the outcome of a certain process.