Availability of Water for Microbial Growth in Foods
By John Lindquist. Based on material in the Laboratory Manual for the Food Microbiology Laboratory (1998 edition, edited by John L. and published at the University) which was used in Bacteriology/Food Science 324 at the University of Wisconsin – Madison.
Regarding the growth of microorganisms in foods, the negative aspects of spoilage and disease-producing microorganisms cause a great deal of concern to the food microbiologist. In preventing the growth of these organisms, one can seek to control various extrinsic factors – those which are associated with the conditions in which the food is kept such as temperature, O2 availability and relative humidity. Intrinsic factors – those associated with the food itself – can also be manipulated to some extent and include pH, O/R potential, inhibitory compounds, water content and nutrients potentially usable by microorganisms.
Many foods serve as good media for microorganisms. One particularly rich medium which comes to mind is milk. Organisms which are involved in the development of food products – such as fermented milk products – must not be impeded by overgrowth of undesirable organisms or unsuitable properties of the raw starting material. Relevant intrinsic and extrinsic factors can be manipulated in the proper development of different fermented foods such as sausage, sauerkraut and yogurt.
In this discussion, we focus on water as both an intrinsic and extrinsic factor. Most foods contain a substantial amount of water – the milieu in which nutrients for microorganisms are dissolved and their biochemical reactions take place. Indeed, the growth of bacteria is always associated with an aqueous environment. Some bacteria are accorded the freedom of lakes and streams and some are trapped in drops of moisture in the soil – to give a couple examples from nature.
The water content of a food product can be determined by weighing the food, drying it in a 105°C oven overnight, and then weighing the dried food. The moisture content is calculated by determining the difference in weights between the "fresh" and the dried product.
As the relative amount of solutes in the water increases – by direct addition and/or a drying process – the water which is free and available to microorganisms decreases. Some examples: Foods A and B may contain the same high amount of water but differ significantly in the amount of NaCl. Foods C and D may have the same amount of NaCl but different amounts of water. As NaCl in food is always dissolved in the water (up to the saturation point), the concept of available water (aw) becomes more important than simply the amount of water in a food!
To give aw some numerical meaning, consider pure water in a beaker as having the maximum aw value of 1.00. As the water becomes loaded up with solutes, this numerical value will decrease as will the vapor pressure of the solution. (A little physics and meteorology begin to enter in.) The numerical value for aw cannot be determined from the solute content but rather by the following equation: aw = p/po where p = the vapor pressure of the solution (in a closed system), and po = vapor pressure of pure water (in the same closed system). The next equation relates aw to equilibrium relative humidity (ERH) which is traditionally expressed as a percentage (100% being associated with air saturated with water): aw = ERH/100.
So, there is an interplay between the solution in the beaker and the atmosphere – as there would be between a food and the atmosphere – in order for an equilibrium to be achieved. Put a soda cracker (aw=0.45) in a humid room (ERH=99%), and it will take up water, becoming less crisp. Put a slice of bread (aw=0.96) in a dry room (ERH=50%), and it will dry out. Microbial activity becomes more probable in the moist cracker and less so in the dry bread.
One of the oldest methods for preserving food is that of drying. As the aw of a food product is lowered, fewer and fewer organisms will be able to grow. Bacteria tend to require more available water for growth than do the yeasts, and the yeasts tend to require more available water for growth than do the molds. The aw of most fresh foods is 0.99 or greater. Most spoilage bacteria require an aw greater than 0.91 for growth while most spoilage molds can grow at aw levels as low as 0.80.
It is often necessary to determine the aw of a product. This can be difficult, particularly with solid foods. One can estimate the aw of the food by the use of the salt-crystal method and then determine the aw more precisely with expensive electronic sensing devices. The salt-crystal method makes use of the fact that saturated salt solutions or salt crystals have a particular aw. Should saturated salt solutions be exposed to an atmosphere with an aw greater than that of the salt, the salt solution will take up water from the atmosphere. For example, if a dry piece of filter paper containing crystals of sodium carbonate (whose aw we know is 0.87) is exposed to mayonnaise (aw =0.90-0.97) in a closed system (such as a sealed beaker or petri dish), the filter paper containing the salt crystals will take up water from the more moist atmosphere created by the mayonnaise and become visibly wet. A filter paper containing crystals of sodium sulfate (aw =0.98) in the same closed system would remain dry. This is the basis of the salt-crystal method. Although it is not too sensitive nor very accurate, it does allow one to estimate the aw of a product and can make for a very interesting and instructive laboratory exercise.
The following table gives the approximate aw at room temperature for a number of salts which can be used in a salt-crystal experiment.
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These general microbiology pages have copyright by John Lindquist