A fruit processor in north-east Brazil turns one mango harvest into three products. The pasteurised juice must be kept chilled and drunk within days of opening. The jam sits on an ambient shelf for months. The dried slices, sealed in a foil pouch, also keep for months without refrigeration. Same fruit, same factory, same hygiene team. The difference is not how much water each product contains but how available that water is, and how acidic the product is. Water activity and pH are the two measurements that explain most of the gap between food that spoils in days and food that keeps for years.
In short
- Water activity (aw) measures how much of a food’s water is free for microbes, from 0 to 1.00; pH measures acidity on a 0 to 14 scale, where 7 is neutral.
- Most bacteria need aw above about 0.91, most yeasts above about 0.88 and most moulds above about 0.80; below about 0.60, microbial growth effectively stops.
- pH 4.6 is the key boundary: at or below it Clostridium botulinum cannot grow, so acid foods need a much milder heat process than low-acid foods.
- The two work together. Codex treats a canned food as low-acid only if its pH is above 4.6 and its aw is above 0.85.
- Low aw and low pH stop growth but do not reliably kill: Salmonella survives for months in dry foods.
What is water activity?
Water activity (aw) is the ratio of the water vapour pressure of a food to that of pure water at the same temperature. It tells you how much of the water in a food is free to dissolve substances, take part in reactions and support microbes, rather than how much water the food contains.
aw = p ÷ p₀ and ERH (%) = aw × 100
Here p is the vapour pressure above the food and p₀ the vapour pressure above pure water, so pure water has an aw of 1.00. A food sealed in a small chamber brings the air above it to its equilibrium relative humidity (ERH), which is aw × 100. A cracker at aw 0.30 has an ERH of 30%, so unwrapped in air at 75% relative humidity it absorbs moisture and goes soft.
Water content and water activity are different things. Fresh bread is roughly 35 to 40% water and has an aw of about 0.95, so it moulds within days. Honey holds about 17 to 18% water, yet its aw is around 0.6 because dissolved sugars hold that water tightly. Sugar and salt lower aw by binding water; drying lowers it by removing water.
What is pH, and why does pH 4.6 matter?
pH is a measure of acidity on a scale from 0 to 14, where 7 is neutral and lower numbers are more acidic. The scale is logarithmic: each whole unit is a tenfold change in hydrogen ion concentration, so pH 4 is ten times more acidic than pH 5 and a hundred times more acidic than pH 6.
The line food scientists watch most closely is pH 4.6. At or below it, Clostridium botulinum, a spore-forming bacterium whose toxin causes the paralytic illness botulism, cannot grow and produce toxin. Foods at pH 4.6 or below are called acid or high-acid foods; those above it are low-acid foods. Acidified foods, such as pickled vegetables, are low-acid foods brought to an equilibrium pH of 4.6 or below by adding acid; equilibrium pH is the pH once the acid has spread evenly through solids and liquid.
Most pathogenic bacteria grow best close to neutral, at about pH 6.5 to 7.5, and below about pH 4.0 few can grow at all. Yeasts and moulds are far more tolerant of acid, which is why they, not bacteria, usually spoil fruit juices, pickles and yoghurt.
How do water activity and pH control microbial growth?
Every microorganism has a minimum water activity and a minimum pH below which it cannot grow. Push a food below those minimums and the microbe stops multiplying, even when the temperature and nutrients suit it.
| Microorganism or group | Approximate minimum aw for growth |
|---|---|
| Clostridium botulinum, non-proteolytic strains | 0.97 |
| Clostridium botulinum, proteolytic strains | 0.94 |
| Most bacteria | 0.91 |
| Most yeasts | 0.88 |
| Staphylococcus aureus | 0.83 to 0.86, depending on conditions such as oxygen |
| Most moulds | 0.80 |
| Halophilic (salt-loving) bacteria | 0.75 |
| Xerophilic (dry-loving) moulds and osmophilic (sugar-tolerant) yeasts | 0.61 |
| Any microbial growth | Effectively stops below about 0.60 |
Bacteria are generally the most demanding about water and moulds the least, which is why forgotten jam grows mould rather than turning slimy. Staphylococcus aureus is the most dry-tolerant of the common bacterial pathogens, although its toxin production generally needs a higher water activity than its growth.
Water activity does not explain everything in dry and semi-dry foods. Sorption isotherms and glass transition, which explain why crisps go soft and powders cake, are covered in Food Science for Industry Professionals.
What are the water activity and pH of common foods?
These approximate values vary with recipe, variety and process, but they explain most everyday shelf lives.
| Food | Typical aw | Typical pH | Usual spoilers | Keeps for |
|---|---|---|---|---|
| Fresh meat | About 0.99 | 5.5 to 6.2 | Cold-tolerant bacteria such as Pseudomonas | Days, chilled |
| Pasteurised milk | About 0.99 | About 6.7 | Lactic acid bacteria, cold-tolerant bacteria | Days to weeks, chilled |
| Bread | About 0.95 | 5.0 to 6.0 | Moulds | Days |
| Yoghurt | About 0.99 | 4.0 to 4.6 | Yeasts and moulds | Weeks, chilled |
| Jam | 0.75 to 0.85 | 3.0 to 3.5 | Xerophilic moulds and osmophilic yeasts, once opened | Months unopened |
| Honey | About 0.6 | About 3.9 on average | Osmophilic yeasts, if it absorbs water | Years, sealed |
| Dry pasta, biscuits, milk powder | Below 0.60 | Not the controlling factor | Rancidity or softening, not microbes | Months to years, kept dry |
| Canned beans (low-acid) | High | Above 4.6 | None while the can is sound | Years |
The canned beans show a third route to a long shelf life: heat that destroys the spores that matter, and a hermetic seal that keeps new microbes out.
How do water activity and pH work together?
Water activity and pH reinforce each other. A microbe stressed by mildly low aw tolerates acid less well, and the reverse, so two moderate barriers can stop growth where neither would alone. This is the basis of hurdle technology, in which several mild controls replace one severe one.
Codex writes the interaction into its definition of a low-acid canned food: one in which any component has a pH above 4.6 and a water activity above 0.85 (Code of Hygienic Practice for Low and Acidified Low Acid Canned Foods, CXC 23-1979). Shelf-stable versions need a botulinum cook; foods outside the definition still need a process designed for whatever can grow in them. National rules differ, so check your national legislation.
- Measure the equilibrium pH of every component, not just the liquid, and the water activity of a representative sample at a controlled temperature, commonly 25 °C.
- If any component has a pH above 4.6 and an aw above 0.85, treat the product as low-acid. A shelf-stable version needs a validated botulinum cook: a 12D process, usually a minimum F0 of about 3 minutes, where F0 is the lethal heat expressed as equivalent minutes at 121.1 °C.
- If the pH is 4.6 or below, or the aw is 0.85 or below, design the process around the organisms that can still grow, such as yeasts, moulds and acid-tolerant bacteria.
- Set targets inside the limits so that normal batch variation never crosses them, and monitor every batch.
Worked example
A sauce maker checks three new products against the Codex low-acid definition.
Product A, tomato and basil sauce: pH 4.2, aw 0.97. The pH is below 4.6, so it is an acid food. C. botulinum cannot grow, and a hot-fill or pasteurisation step aimed at yeasts, moulds and acid-tolerant bacteria is usual.
Product B, bean and vegetable soup: pH 6.1, aw 0.98. Both values exceed the limits, so it is low-acid and, if sold at room temperature, needs a validated botulinum cook.
Product C, beef jerky: pH 5.8, aw 0.80. The pH is above 4.6 but the aw is below 0.85, so it falls outside the definition. Cooking must still destroy pathogens from the raw meat, and aw 0.80 is above the 0.61 floor for xerophilic moulds, so the pack must keep moisture out.
How much more acidic is A than B? The difference is 6.1 − 4.2 = 1.9 pH units:
[H⁺] ratio = 10^(6.1 − 4.2) = 10^1.9 ≈ 79
Product A has about 79 times the hydrogen ion concentration of product B.
Do low water activity and low pH kill microbes?
No. Low water activity and low pH stop or slow growth, but they do not reliably kill. Many microbes survive in a dormant state and grow again when conditions change, and some pathogens cause illness without growing in the food at all.
- Salmonella has caused outbreaks linked to peanut butter, chocolate, spices and flour: dry foods in which it cannot grow but can survive for months.
- Shiga toxin-producing E. coli has caused outbreaks linked to unpasteurised apple juice despite its low pH.
- Bacterial spores survive drying and acid. When a dry soup mix is rehydrated and kept warm, spores of Bacillus cereus and Clostridium perfringens can germinate and multiply.
- Moulds that grew before drying can leave mycotoxins, such as aflatoxins, which drying and normal cooking do not destroy.
Both values can also drift: dry foods pick up moisture in humid storage, and mould growth can use up acid and raise the local pH of an acid food. Packaging and shelf-life studies must allow for this.
How do you measure water activity and pH?
Water activity is measured with a water activity meter: a small sample is sealed in a chamber at a controlled temperature, and a dew-point or capacitance sensor reads the humidity of the air once it reaches equilibrium with the food. The instrument should be checked regularly against standard salt solutions of known water activity.
pH is measured with a pH meter and glass electrode, calibrated with at least two buffer solutions that bracket the expected value, for example pH 4.0 and 7.0. Liquids are measured directly; solid and particulate foods are blended first, and acidified foods should be measured once they have reached equilibrium. Test strips are too imprecise for decisions close to pH 4.6. Kitchen versions of these ideas, including a red cabbage pH indicator, are part of Introduction to Food Science.
Frequently asked questions
What is a safe water activity for shelf-stable food?
There is no single safe value, because it depends on pH, processing and packaging. As a guide, few bacteria grow below about 0.91, Staphylococcus aureus is limited at about 0.83 to 0.86, and 0.85 is widely used as a reference point, for example in the Codex definition of a low-acid canned food. Xerophilic moulds and osmophilic yeasts grow down to about 0.61, so foods between 0.61 and 0.85 still need mould control. Check your national legislation.
Why is pH 4.6 the cut-off between acid and low-acid foods?
Because Clostridium botulinum, the spore-forming bacterium that causes botulism, does not grow or produce toxin at pH 4.6 or below. Its spores are very heat resistant, so low-acid canned foods need a severe botulinum cook, typically at around 121 °C under pressure. Acid foods can be made shelf-stable with much milder heat aimed at less resistant yeasts, moulds and bacteria.
Why does jam go mouldy but not slimy?
Jam has a water activity of about 0.75 to 0.85 and a pH of about 3.0 to 3.5. Those conditions stop almost all bacteria, which need more available water and prefer near-neutral pH. Xerophilic moulds and osmophilic yeasts, however, grow down to about aw 0.61 and tolerate acid well. Once the jar is opened, mould spores land on the surface, where condensation can also raise the local water activity.
Does freezing change water activity?
Yes. As food freezes, water turns into ice and is no longer available, so the water activity of the food falls as the temperature drops. That is one reason microbes stop growing in frozen food. Freezing does not reliably kill them, though: when the food thaws, the water becomes available again and surviving microbes can start to grow.
Can a food have a low pH and still be unsafe?
Yes. Low pH stops C. botulinum and slows most pathogens, but some survive in acid foods long enough to cause illness. Salmonella and Shiga toxin-producing E. coli have caused outbreaks linked to unpasteurised juices, and norovirus and hepatitis A, which do not need to grow in food, have been spread by frozen berries. Acid foods still need good hygiene and often a kill step.
Next step. Introduction to Food Science teaches water activity and pH from first principles, then applies them to microbial growth with FAT TOM, to classifying foods by acidity, and to the preservation methods that drying, salting, pickling and canning rely on. The course ends with a proctored final assessment and an ASC certificate, and you can see all eleven food science and technology courses to choose your next level.
Sources. ICMSF, Microorganisms in Foods 5: Characteristics of Microbial Pathogens (1996). J. M. Jay, M. J. Loessner and D. A. Golden, Modern Food Microbiology, 7th edition (Springer, 2005). S. Damodaran and K. L. Parkin (eds), Fennema’s Food Chemistry, 5th edition (CRC Press, 2017). Codex Alimentarius Commission, Code of Hygienic Practice for Low and Acidified Low Acid Canned Foods, CXC 23-1979 (Codex Alimentarius). World Health Organization, Five Keys to Safer Food Manual (2006) (WHO).
This article is general guidance only and is not a substitute for the applicable standard, legislation or the advice of a qualified food safety professional.