How to Choose a Food Preservative: Sorbate, Benzoate, Propionate, Nisin and Natamycin Compared
A product developer's guide to selecting preservatives by pH, target organism, product type and regulation, with the pKa science that explains why sorbate and benzoate fail in low-acid foods.
A preservative that works in a lemonade can do almost nothing in a cream sauce. The reason is chemistry, and once you understand it, choosing a preservative becomes systematic.
The key concept: undissociated acid and pKa
Benzoic, sorbic and propionic acids are weak acids. In solution, they exist partly as undissociated acid (HA) and partly as the dissociated anion (A⁻). The undissociated form is uncharged, crosses the microbial cell membrane, then releases protons inside the cell, where the pH is near neutral. This acidifies the cytoplasm and forces the cell to spend energy pumping protons out.
The proportion of undissociated acid depends on pH relative to the acid's pKa:
| Preservative | pKa (approx.) | Undissociated at pH 3.0 | at pH 4.0 | at pH 5.0 | at pH 6.0 |
|---|---|---|---|---|---|
| Benzoic acid | 4.2 | 94 % | 61 % | 14 % | 2 % |
| Sorbic acid | 4.76 | 98 % | 85 % | 37 % | 5 % |
| Propionic acid | 4.87 | 99 % | 88 % | 43 % | 7 % |
At pH 6, almost all benzoic acid is in the inactive form. That is why benzoate is a poor choice for near-neutral products and a good choice for soft drinks at pH 3.
Comparison of common preservatives
| Preservative (E-number) | Best against | Effective pH range (practical) | Typical applications | Watch-outs |
|---|---|---|---|---|
| Sorbic acid, potassium sorbate (E200, E202) | Yeasts and moulds; some bacteria | Up to about 6–6.5 | Cheese, baked goods, beverages, sauces, dressings, dried fruit | Some moulds can degrade sorbate to a compound with a kerosene-like off-odour; can inhibit yeast fermentation in dough |
| Benzoic acid, sodium benzoate (E210, E211) | Yeasts and moulds; some bacteria | Up to about 4.5 | Soft drinks, juices, pickles, sauces | Can react with ascorbic acid, under heat and light, to form trace benzene; taste at high levels |
| Propionic acid, calcium propionate (E280, E282) | Moulds; rope-forming Bacillus | Up to about 5.5–6 | Bread and baked goods | Little effect on baker's yeast, which is why it suits yeast-raised bread |
| Nisin (E234) | Gram-positive bacteria, including spore-formers such as Clostridium and Bacillus | Most stable and active under acidic conditions | Processed cheese, dairy desserts, some canned foods | Not effective against yeasts, moulds or Gram-negative bacteria on its own |
| Natamycin (E235) | Moulds and yeasts | Wide range | Surface treatment of cheese and dry sausages | Use is restricted to specific applications in many markets |
| Sulphites (E220–E228) | Bacteria, yeasts, moulds; also antioxidant and anti-browning | Most effective at low pH | Wine, dried fruit, some shellfish | Regulated allergen in EU/UK (above 10 mg/kg); prohibited on fresh fruit and vegetables for raw consumption in the US |
| Nitrite (E249, E250) | C. botulinum in cured meats | Meat pH | Cured and processed meats | Strict limits; nitrosamine formation concerns |
A selection process
- Identify the spoilage risk. Use your product's water activity, pH, process and packaging to identify which organisms can survive and grow. Shelf-life trials or spoiled samples tell you who is actually causing problems.
- Check pH. If the product is above pH 5, benzoate is unlikely to work at legal levels. Consider sorbate, or redesign the hurdles (for example, lower pH).
- Check the matrix. Fat can partition lipophilic preservatives out of the water phase, where microbes live. High protein can bind some preservatives. Dose is calculated on the whole product, but activity depends on the water phase.
- Check processing. Is the preservative stable to your heat treatment? Will it interfere with fermentation?
- Check regulations in every target market. Permitted foods and maximum levels differ. In the EU and UK, see Regulation (EC) No 1333/2008 and its Annex II. In the US, many preservatives are covered by GRAS listings or food additive regulations, for example sodium benzoate in 21 CFR 184.1733, which sets a 0.1 % limit. Codex's General Standard for Food Additives (GSFA) is the reference for many other countries.
- Check labelling and customer requirements. Many retailers have "free from" or clean-label policies. Fermentates, cultured dextrose and vinegar-based ingredients are sometimes used as label-friendly alternatives, but they are still functional ingredients that need validating.
- Validate with a challenge or shelf-life study. See shelf-life testing.
Common mistakes
- Raising preservative dose instead of fixing pH. A 0.2 pH drop can do more than doubling the dose of a weak acid.
- Combining benzoate and ascorbic acid in a beverage without assessing benzene risk. Formulation, heat and light exposure matter; FDA and industry tested for this after the issue was identified in 2005–2006.
- Assuming the preservative replaces hygiene. Preservatives are designed to inhibit low numbers of organisms. A heavily contaminated product will still spoil.
- Forgetting carry-over. A preservative in an ingredient (for example, a sauce in a ready meal) can carry over into the final product and may need declaring or may breach limits.
Sources
- Davidson, P. M., Taylor, T. M. and David, J. R. D. (eds.). Antimicrobials in Food, 4th edition. CRC Press, 2020.
- European Union. Regulation (EC) No 1333/2008 on food additives, Annex II.
- U.S. Food and Drug Administration. 21 CFR 184.1733, Sodium benzoate.
- U.S. Food and Drug Administration. Data on Benzene in Soft Drinks and Other Beverages.
- Codex Alimentarius. General Standard for Food Additives (CXS 192-1995).
This guide is educational. Safety-critical limits (thermal processes, pH limits, allergen claims) must be confirmed with your process authority, certification body or regulator for your product and market. Spotted an error? Tell the editors.