Safety · Beginner ·
Preservatives 101: why water-based products need one
What a preservative does, how to pick one, and why the usual DIY shortcuts do not work.
Key takeaways
- Any product with water, or touched by wet fingers, can grow bacteria, yeast and mold.
- Choose a broad-spectrum preservative and respect its pH range, usage rate and maximum temperature.
- Antioxidants (vitamin E) stop oils going rancid. They are not preservatives.
- A preservative reduces risk. Clean technique is still essential.
Why water is the problem
Microbes need water to grow. Lotions, creams, toners, serums, gels and shampoos all contain it. Once it is in your jar, even tiny numbers of bacteria and mold can multiply into millions. Contaminated products can cause skin and eye infections, and the product often looks and smells fine until it is badly contaminated.
What "broad-spectrum" means
A preservative system should protect against bacteria, yeast and mold. Some ingredients only cover one of these, so they must be paired. The table shows some options from the Formularium library.
| System | Typical use | Works at pH | Good to know |
|---|---|---|---|
| Phenoxyethanol + ethylhexylglycerin (e.g. Euxyl PE 9010) | 0.5-1.1% | 3-12 | Broad, widely used and very pH-tolerant |
| Sodium benzoate + potassium sorbate (e.g. Euxyl K712) | 0.5-1.5% | 3-5.5 | Fails above pH 5.5, so you must measure pH |
| Benzyl alcohol + salicylic acid + sorbic acid (e.g. Geogard ECT) | 0.6-1.0% | 3-8 | Almond-like smell; add below 45°C |
| Radish root ferment (e.g. Leucidal Liquid) | 2-4% | 3-8 | Weak against mold; pair with an antifungal |
| Sodium dehydroacetate | 0.1-0.6% | up to 6.5 | Only fights mold and yeast; needs a partner for bacteria |
| Phenoxyethanol on its own | 0.3-1.0% | 3-10 | Strong against bacteria and very pH-tolerant; weaker against mold, so many formulas add ethylhexylglycerin. The EU limit is 1% |
| Benzyl alcohol | 0.5-1.0% | 3-8 | Accepted in natural formulas; mainly antibacterial, so pair it with sodium dehydroacetate or a sorbate |
| Sorbic acid and benzoic acid (the acid forms of sorbate and benzoate) | 0.1-0.6% and 0.1-0.5% | 3-6 and 3-5 | Barely dissolve in water: dissolve them in a little alcohol or propanediol first, and use them only in acidic products |
Boosters and helpers
Pentylene glycol, caprylyl glycol, glyceryl caprylate and 1,2-hexanediol make a preservative work better, and they also hydrate or soften the skin. They are boosters, not preservatives on their own. Adding a chelator such as disodium EDTA (0.05-0.2%) also helps, because metal ions can weaken a preservative.
Myths to leave behind
- "Essential oils preserve it." At safe usage levels they do not.
- "Vitamin E is a preservative." It is an antioxidant that protects oils from rancidity. Microbes are a different problem.
- "Honey, vinegar or alcohol is enough." Not at amounts that are safe and pleasant on skin.
- "It's natural, so it will keep." Natural and fresh ingredients (fruit, aloe, herbal infusions) actually feed microbes.
Using a preservative correctly
- Read the supplier's datasheet for the usage range, pH range and maximum temperature.
- Weigh it accurately, since it is usually only 0.5-1.5% of the batch. The preservative calculator gives the exact grams.
- Add it during the cool-down phase, below its heat limit, and mix thoroughly.
- Check the final pH is inside the preservative's working range.
- Use clean, sanitised tools and containers. A preservative cannot rescue a dirty batch.
What about anhydrous products?
Balms, body butters and oil serums contain no water, so microbes do not grow in them easily. They still benefit from an antioxidant, and they must stay dry: one wet finger in a jar of body butter is enough to introduce water.