Water activity and hurdle technology
Why water activity, not moisture content, decides whether food spoils, the limits for bacteria, yeasts and moulds, and how hurdle technology works.
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Two foods can contain the same amount of water and keep completely differently. Honey is about 17 per cent water and keeps for years at room temperature; a soft cheese with more water lasts days. The difference is water activity: how much of the water is free to support microbes and chemical reactions, rather than how much water there is.
Water activity is one of the central ideas of food preservation, and hurdle technology, the deliberate combination of several mild preserving factors, is how much of the world's food is kept safe without sterilising or freezing it. Both are standard topics in food science, dairy and post-harvest exams.
What water activity means
Water in food is held to different degrees: some is bound tightly to proteins, sugars and salts; some is loosely held in capillaries; some is effectively free. Water activity (aw) measures the energy status of that water, its availability, on a scale from 0 to 1:
Here p is the vapour pressure of water above the food and p0 the vapour pressure of pure water at the same temperature. Pure water has aw = 1.0. A food at aw 0.85 is in equilibrium with air at 85 per cent relative humidity: placed in more humid air it gains moisture; in drier air it loses it.
Dissolved solutes and solid surfaces lower water activity. Salt and sugar are the classic tools: they bind water and lower its vapour pressure, which is why salting, sugaring and drying have preserved food for thousands of years.
Moisture content is not the same thing
Moisture content is how much water a food contains (per cent by weight). Water activity is how available it is. The relation between them is the moisture sorption isotherm, a curve specific to each food. Isotherms are usually S-shaped and show hysteresis: a food drying out holds more water at a given aw than the same food taking water up. They are used to predict how a dried food will behave in storage and packaging.
Limits for microbial growth
Every microbe has a minimum water activity below which it cannot grow. These are among the most frequently examined numbers in food microbiology:
| Group | Typical minimum aw for growth |
|---|---|
| Most spoilage bacteria | about 0.91 |
| Clostridium botulinum (proteolytic strains) | about 0.94 |
| Staphylococcus aureus (aerobic) | about 0.86, the most tolerant common pathogen |
| Most yeasts | about 0.88 |
| Most moulds | about 0.80 |
| Halophilic (salt-loving) bacteria | about 0.75 |
| Xerophilic moulds and osmophilic yeasts | about 0.60–0.61 |
| No microbial growth | below about 0.60 |
Values vary with temperature, pH and other conditions, and toxin production often needs a higher aw than growth. The broad pattern matters most: bacteria need the most water, yeasts less, moulds least. This is why a jam or a dried fruit that spoils usually spoils by mould or yeast, not bacteria.
Water activity and chemical stability
Water activity also controls chemical and physical changes:
- Lipid oxidation is slowest at low water activity, around 0.2–0.4, where a thin layer of water protects fats; it is faster in very dry foods and again at higher aw.
- Non-enzymatic (Maillard) browning peaks at intermediate water activity, roughly 0.6–0.8, where reactants are concentrated but still mobile.
- Enzyme activity generally increases with water activity.
- Texture: crisp foods such as crackers and breakfast cereals lose crispness above a critical water activity, often around 0.4–0.5; dry powders cake.
The amount of water held as a single layer on the food's surfaces, the monolayer value (estimated with the BET or GAB models of the isotherm), is often the target for maximum stability of dried foods.
Typical foods
| Water activity | Examples | Main spoilage risk |
|---|---|---|
| 0.98 and above | fresh meat, fish, milk, fruit and vegetables | most bacteria |
| 0.93–0.98 | bread, cooked sausages, many soft cheeses | bacteria, moulds |
| 0.85–0.93 | dry-cured sausages and ham, mature cheeses | Staph. aureus, yeasts, moulds |
| 0.60–0.85 (intermediate moisture) | jams, dried fruit, salted fish, soft dried meat, many confectionery fillings | yeasts and moulds, xerophiles |
| below 0.60 | dried pasta, biscuits, milk powder, dried spices, honey (about 0.5–0.6) | no microbial growth; oxidation and texture changes |
Estimating water activity from a recipe
For dilute solutions, Raoult's law gives an estimate: aw = moles of water ⁄ (moles of water + moles of dissolved particles).
Sugar syrup: 50 g sucrose (342 g/mol) in 100 g water.
- Water: 100 ⁄ 18 = 5.56 mol. Sucrose: 50 ⁄ 342 = 0.146 mol.
- aw ≈ 5.56 ⁄ (5.56 + 0.146) = 0.974.
Brine: 10 g salt (NaCl, 58.4 g/mol) in 100 g water. Salt dissociates into two ions, so the particles are 2 × 10 ⁄ 58.4 = 0.342 mol.
- aw ≈ 5.56 ⁄ (5.56 + 0.342) = 0.942.
Gram for gram, salt lowers water activity far more than sugar, because its particles are smaller and it dissociates. Real concentrated solutions deviate from Raoult's law, so water activity is measured with an instrument (dew-point or capacitance hygrometers) rather than calculated, but the estimate shows why so much sugar is needed in jams.
Hurdle technology
A single preserving factor strong enough to make a food stable on its own, such as drying it to aw 0.6, heavy salting or sterilising it, usually damages taste, texture and nutrition. Hurdle technology, developed and named by the German meat scientist Lothar Leistner in the 1970s and 1980s, combines several milder factors, each a hurdle that microbes must jump. Together they stop growth, though none could do it alone.
The main hurdles are:
- Water activity (drying, salt, sugar, humectants)
- Acidity (pH), through fermentation or added acids
- Temperature: heating (F value) and chilling (t)
- Preservatives: salts of organic acids, nitrite, smoke, spices
- Redox potential (Eh) and modified atmospheres
- Competitive microflora: lactic acid bacteria in fermented foods
- Packaging
Many traditional foods are hurdle foods whose makers never named the principle: fermented sausages (low aw, low pH, nitrite, competitive flora, smoke), cheeses, pickles, soy sauce and many intermediate-moisture products across Asia, Africa and Latin America.
The idea has a biological basis. Stressed microbes must spend energy maintaining their internal balance (homeostasis): pumping out protons at low pH, accumulating compatible solutes at low aw. Several stresses at once exhaust them. Hurdles that attack different targets work better together than the same total stress from one direction; this synergy is called multi-target preservation.
Designing with hurdles
A shelf-stable meat snack needs to be safe at room temperature without retorting. Pathogens of concern include Staph. aureus (grows down to about aw 0.86 and pH 4.5 on its own) and spore-formers.
A design might combine: aw of about 0.85 (drying with salt and sugar), pH of about 5.0 (fermentation or a marinade), a cooking step that kills vegetative pathogens, and vacuum packaging. Staph. aureus could still grow at either aw 0.85 or pH 5.0 alone near its limits, but the combination, after the kill step, prevents it. A product like this must still be validated by challenge testing, because published limits are for single factors under ideal conditions.
Key points
- Water activity aw = p ⁄ p0 = ERH ⁄ 100; pure water is 1.0.
- Moisture content measures how much water; water activity measures how available it is. The sorption isotherm links them.
- Approximate minimum aw: most bacteria 0.91, yeasts 0.88, moulds 0.80, xerophiles 0.61; no growth below about 0.60. Staph. aureus is the most tolerant common pathogen (about 0.86).
- Lipid oxidation is slowest around aw 0.2–0.4; Maillard browning peaks at about 0.6–0.8; intermediate-moisture foods lie between about 0.60 and 0.85.
- Salt lowers aw more than the same weight of sugar.
- Hurdle technology (Leistner) combines mild factors (aw, pH, temperature, preservatives, Eh, competitive flora) that together stabilise food.
What the exams ask
- School-leaving and entrance papers: the definition of water activity, why salting and sugaring preserve food, and examples of foods by aw.
- Degree and research-fellowship entrances in food and dairy technology: microbial aw limits, sorption isotherms and hysteresis, the stability map, monolayer values, intermediate-moisture foods and hurdle technology.
- Professional examinations: designing shelf-stable products, combining hurdles, measuring water activity, and the role of aw in food safety rules for low-acid foods.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why doesn't honey spoil?
Its water activity, about 0.5 to 0.6, is too low for almost all microbes, and it is also acidic and contains hydrogen peroxide produced by an enzyme. If honey absorbs moisture and its water activity rises, osmophilic yeasts can ferment it.
Does freezing lower water activity?
Effectively, yes. As water turns to ice, the remaining liquid becomes more concentrated in solutes, so its water activity falls. That, along with low temperature, is part of why frozen food is stable.
Is a low water activity enough to make food safe?
It prevents growth but does not necessarily kill. Pathogens such as Salmonella can survive for months in dry foods like flour, peanut butter and spices, and cause illness if eaten, so dry foods still need hygienic production and, often, a kill step.
Sources
- Fennema, O. R., Damodaran, S. and Parkin, K. L. (eds) Fennema's Food Chemistry, 5th edition. CRC Press, 2017. Chapter on water and ice.
- Labuza, T. P. "The effect of water activity on reaction kinetics of food deterioration." Food Technology 34, 36–41 (1980).
- Leistner, L. and Gorris, L. G. M. "Food preservation by hurdle technology." Trends in Food Science and Technology 6, 41–46 (1995).
- Barbosa-Cánovas, G. V., Fontana, A. J., Schmidt, S. J. and Labuza, T. P. (eds) Water Activity in Foods: Fundamentals and Applications, 2nd edition. Wiley-Blackwell, 2020.
- Jay, J. M., Loessner, M. J. and Golden, D. A. Modern Food Microbiology, 7th edition. Springer, 2005.
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