Water activity is not moisture content

Water activity (aw) is the ratio of a food’s equilibrium vapor pressure to that of pure water at the same temperature. Foods with similar moisture content can have different aw because of composition and how water interacts with salts, sugars, proteins, and other components. FDA’s technical guide explains measurement and sorption behavior; it also mentions aw 0.85 in the context of determining coverage under particular U.S. low-acid canned and acidified-food regulations. That does not mean all pathogens disappear below 0.85. Lower aw can prevent growth of many bacteria without killing them, and yeasts, molds, and product-specific conditions still matter. FDA technical guide

Solutes do not affect growth identically

A 2026 study by Sun and colleagues tested *Salmonella Enteritidis* in experimental systems adjusted with sodium chloride, sucrose, or glycerol. The authors reported solute-dependent minimum growth aw ranges: 0.95–0.96 with sodium chloride, 0.96–0.97 with sucrose, and 0.92–0.93 with glycerol. Transcriptomic and NMR analyses examined differences in water status and bacterial response. These are results for the tested strain, media, temperature, and design, not universal growth limits for all Salmonella or real foods. Study abstract and conditions

Growth can stop while a pathogen remains viable

The practical distinction in low-moisture foods is between growth and survival. Salmonella may persist for extended periods even when low aw prevents it from multiplying. It can become a hazard again if conditions change during rehydration, formulation, or preparation. An aw reading alone therefore does not establish that an ingredient is pathogen-free or that a heat process was effective. Controls need to address contaminated inputs, a validated lethality step, recontamination, and storage. The cited experiment also shows why equal aw values cannot automatically be assumed to produce equal microbial responses when the solute differs.

An expert perspective on using measurements

Water activity depends on temperature and equilibrium conditions. Control the measurement temperature, instrument calibration, sample preparation, and variation within and between product lots. Use measurements from safety-relevant product locations rather than relying only on a convenient average. Any process limit should be supported by validation for the actual formulation. Water activity is one hurdle to combine with pH, preservatives, heat, packaging, and storage; it is not a sterilization method. For dry foods, do not infer the absence of pathogens from low moisture. Maintain hazard analysis and suitable process monitoring for ingredients and finished product.

Sources and references

FDA: Water Activity (aw) in Foods · Mechanistic Insights into Salmonella Growth Kinetics Under Solute-Induced Water Activity Stresses via Transcriptomics and NMR Analysis