You reduce water activity in food by removing water physically (drying, freezing) or by binding it chemically with solutes like salt, sugar, and humectants so it’s no longer available for microbial growth. Water activity, expressed as a value between 0 and 1.0, measures the vapor pressure of a food relative to pure water. A reading of 0.80 means the food’s vapor pressure is 80% that of distilled water. Most bacteria need a water activity above 0.91 to grow, so getting below that threshold is the core goal of most preservation strategies.
Why Water Activity Matters More Than Moisture Content
Total moisture content tells you how much water is in a food, but not how much of that water is actually available to support microbial growth and chemical reactions. Water activity specifically measures the “free” water, the portion not bound to proteins, sugars, or other molecules. A food can have relatively high moisture content yet still be shelf-stable if enough of that water is locked up by solutes or structural components.
Free water gives off vapor to the surrounding air whenever the air’s vapor pressure is lower than the food’s. This exchange is what allows bacteria, yeasts, and molds to access moisture for growth. Reducing the amount of available water, rather than simply total water, is what actually inhibits spoilage organisms and pathogens.
Adding Salt or Sugar
Dissolving solutes into a food’s water phase is one of the most straightforward ways to lower water activity. Salt and sugar both work by binding free water molecules, but they differ dramatically in efficiency. A 13% salt solution (by weight) drops water activity to roughly 0.91, enough to suppress most common bacteria. To reach the same water activity with sugar, you need a 55% solution. On a weight basis, salt is roughly four times more effective at binding water than sugar.
This is why heavily salted products like cured meats and brined vegetables have long shelf lives, while sugar-preserved foods like jams and syrups require very high sugar concentrations (often 60% or more) to stay safe. The practical tradeoff is flavor: salt at preservation-level concentrations can overwhelm a product’s taste, which is why many preserved foods use a combination of salt, sugar, and other methods rather than relying on a single solute.
Using Humectants
Humectants are ingredients that attract and hold water molecules through hydrogen bonding, effectively pulling free water out of circulation. Glycerol, sorbitol, and xylitol are among the most common in commercial food production, and all carry “generally recognized as safe” (GRAS) status from the FDA. You’ll find them used heavily in products like jerky, soft-baked snacks, and protein bars where the goal is a lower water activity without a bone-dry texture.
Concentration matters, but more isn’t always better. A meta-analysis of humectant use in jerky found that levels below 5% significantly increased moisture retention (keeping the product tender), while levels above 5% showed no additional benefit. In commercial jerky formulations, glycerol and sorbitol are typically used at concentrations between 2.5% and 15%, depending on the target texture and shelf life. Other ingredients like konjac, egg albumin, and isolated soy protein can also function as humectants at much lower inclusion rates, around 0.2%.
Drying and Dehydration
Physically removing water is the most direct route to lowering water activity. Air drying, oven drying, sun drying, freeze-drying, and spray drying all accomplish this by evaporating free water from the food matrix. The key variable is how much water you remove and how uniformly you remove it. Uneven drying can leave pockets of high water activity inside a seemingly dry product, creating localized zones where spoilage organisms thrive.
Osmotic dehydration offers a gentler alternative, especially for fruits and vegetables. The process involves soaking food pieces in a concentrated sugar syrup or salt brine. Water migrates out of the food cells into the surrounding solution through the cell membranes, while some solute migrates inward. Submerging banana slices in a 70% sugar syrup, for example, reduces the fruit’s weight by about 50%. For pear fruit, increasing syrup concentration from 35 to 45 degrees Brix and the syrup-to-fruit ratio from 1:1 to 2:1 boosted water loss from 18% to 23%.
Osmotic dehydration alone typically leaves products with water activity between 0.94 and 0.97, which isn’t low enough for shelf stability without refrigeration. It’s best used as a preliminary step before conventional drying or as part of a combined preservation approach. The upside is that it preserves color, flavor, and nutrients better than aggressive heat drying, since the process runs at lower temperatures.
Freezing
Freezing reduces water activity by converting free water into ice crystals. As the temperature drops below the freezing point of the food (typically between -1°C and -5°C), free water and loosely held water in the outer layers crystallize first, while tightly bound water remains liquid at the molecular level. This phase transition concentrates the remaining solutes in the unfrozen liquid, further depressing water activity in that fraction.
The result is a product where most of the water is locked in solid form and unavailable for microbial use. Freezing doesn’t sterilize food or kill all microorganisms, but it effectively halts their growth by denying them accessible water. The practical limitation is that water activity rises back to its original level the moment the food thaws, so freezing is a preservation method that requires maintaining the cold chain, not a permanent reduction.
Combining Methods
Most shelf-stable foods rely on more than one water activity reduction strategy at once, an approach food scientists call “hurdle technology.” A beef jerky, for example, might use salt and sugar to bind water, glycerol as a humectant, and heat drying to remove moisture. Each method contributes a partial reduction in water activity, and together they reach a target that would be impractical or unpalatable with any single technique.
Combining methods also lets you moderate each individual ingredient. Instead of needing 13% salt (which most people would find inedible), you might use 3% salt plus 5% glycerol plus thorough drying to hit the same water activity target. This flexibility is especially valuable in product development where taste, texture, and shelf life all have to balance.
Measuring Water Activity
You can’t manage what you can’t measure, and water activity requires dedicated instruments. The industry standard is a chilled-mirror dew point meter, which works by cooling a mirror inside a sealed chamber until condensation forms, then calculating water activity from the dew point temperature. These instruments, like the commonly used AquaLab series, deliver accuracy within 95% to 105% of expected values and can measure across a working range from about 0.17 to 1.0.
Calibration is essential. Standard practice uses saturated salt solutions with known water activity values to verify the instrument before each batch of readings. Temperature also matters: water activity increases as temperature rises, so measurements should be taken at a consistent, documented temperature to be comparable over time. For quality control in production, testing should happen both during formulation and on finished products, since processing steps like cooking, cooling, and packaging can all shift water activity in unexpected directions.
Water Activity Targets for Common Foods
The target you’re aiming for depends on what organisms you need to control. Most bacteria stop growing below 0.91, which is the threshold that separates fresh-like foods from intermediate-moisture products. Dropping below 0.85 inhibits nearly all bacterial pathogens and is the cutoff many regulators use to define a shelf-stable food that doesn’t require refrigeration. Molds are hardier and can grow down to about 0.70, which is why dried fruits and nuts with water activity in the 0.70 to 0.80 range can still develop mold if not properly packaged. A few extremely tolerant molds survive down to 0.60, but below that threshold, virtually no microbial growth occurs.
- Fresh meat, fruits, vegetables: 0.95 to 0.99
- Cured meats, aged cheeses: 0.85 to 0.91
- Dried fruits, jams, jerky: 0.60 to 0.85
- Crackers, dried spices, powdered milk: 0.20 to 0.60
Matching your reduction method to your target water activity, your ingredient constraints, and your desired product texture is ultimately what determines which combination of strategies works best for any given food.