Microorganisms, including bacteria, require water, nutrients, and a suitable temperature range to grow and multiply. While total moisture content (the percentage of water in a product) is easy to measure, it is not the most accurate predictor of microbial growth potential. The more precise measure used in food science is Water Activity (Aw), which determines the amount of water actually available to support microbial growth. Aw measures the energy status of water in a system, indicating how tightly water molecules are bound to non-water components like salts or sugars. Only this unbound, available water can be utilized by microbes for their metabolic processes and subsequent proliferation.
Understanding the Water Activity Scale
Water Activity is quantified on a scale that runs from 0.0 to 1.0, where 1.0 represents the Aw of pure, distilled water, and 0.0 represents a completely dry substance. The value is a thermodynamic property, measured as the ratio of the water vapor pressure of the food to the vapor pressure of pure water under the same conditions. This measurement makes Aw a more reliable metric than simple moisture content; two different foods can have the same total water content, yet the one containing more dissolved solutes will have a lower Aw. For example, fresh meat typically has an Aw of 0.98 to 0.99, while dried pasta or crackers often have an Aw below 0.30.
The Critical Threshold for Pathogenic Bacteria
The level of water activity required for bacteria to grow varies, but most common foodborne pathogenic bacteria are unable to multiply below a specific threshold. For the vast majority of spoilage and disease-causing bacteria, growth is inhibited at water activity levels below 0.91. The widely accepted regulatory limit for controlling the growth of pathogens in non-refrigerated foods is 0.85 Aw. This lower threshold is necessary because Staphylococcus aureus is known to be slightly more tolerant of dry conditions, capable of growing down to an Aw of 0.86.
Other specific pathogens require higher Aw levels for multiplication. For instance, Clostridium botulinum, which produces a dangerous toxin, is generally inhibited at an Aw of 0.93 and below. Common contaminants like Salmonella and Escherichia coli typically require an Aw of 0.95 or higher to proliferate. Fresh and moist foods, such as produce, meat, and milk, typically range between 0.98 and 1.0 Aw, making them highly susceptible to rapid bacterial spoilage.
Specialized Microbes That Thrive in Low Water Activity
While an Aw of 0.85 prevents the growth of pathogenic bacteria, it does not guarantee complete microbial stability. Certain specialized microorganisms are adapted to grow in environments with much lower available water content. These organisms include xerophilic molds and osmophilic yeasts, which cause spoilage in many dried or high-sugar products. Molds are the most tolerant group, with some species able to grow at an Aw as low as 0.65 to 0.70.
Osmophilic yeasts thrive in high-solute solutions and can multiply in foods with water activity levels down to 0.60. They are commonly found as spoilage agents in high-sugar products like dried fruits, fruit cakes, jams, and concentrated syrups.
How Reducing Water Activity Preserves Food
The principle of reducing water activity forms the basis of many traditional and modern food preservation methods. This technique makes water metabolically unavailable to microorganisms by binding the water molecules or physically removing them. The two primary methods employed to achieve a lower Aw are the addition of solutes and the physical removal of water through drying.
Adding solutes involves incorporating high concentrations of substances like salt or sugar into the food product. The salt or sugar molecules compete with microbial cells by strongly binding the water, a process known as salting or sugaring. Curing meats with salt or preserving fruit in a high-sugar jam or jelly are common examples of this technique.
Dehydration physically removes water through processes like sun drying, hot-air drying, or freeze-drying. Creating beef jerky or dried fruits relies on this process to lower the Aw to a point where microbes cannot proliferate, extending the shelf life without refrigeration.