“Which preservative should I add to my cream?” is one of the most common questions from beginner formulators. And the way the question is phrased already contains a mistake. Preservation is not an ingredient you toss in at the end of mixing like a pinch of salt. It is a system that begins long before you open your first jar of raw materials. It includes the pH of your cream formula, water activity, incorporation temperature, compatibility with other components, and even the type of packaging. Today, we will break this system down brick by brick so that your products are not only beautiful but also safe.
Why preservation is necessary and what happens without it
Any cosmetic product containing water — cream, lotion, toner, gel — is a breeding ground for microorganisms. Bacteria, yeast, and mold enter the product from the air, hands, tools, and raw materials. Without an adequate preservation system, a colony Pseudomonas aeruginosa can double in 20 minutes at room temperature.
The consequences of microbial contamination range from unpleasant odors and emulsion separation to serious skin and eye infections. Products for the eye area and damaged skin are particularly dangerous. This is not a theoretical risk — laboratories record thousands of cases of contamination in homemade and even industrial cosmetics every year.
The main rule: if there is water (or an aqueous phase, hydrosols, extracts) in the formulation, a preservative is mandatory. Anhydrous products (balms based on butters, anhydrous oil-based serums) do not formally require preservation, but they are still susceptible to oxidation.
Main groups of preservatives
Preservatives are divided into several categories based on their mechanism of action. Understanding these groups is the first step toward intelligent blending.
Preservative (INCI) | Group | pH range | Dosage, % | Features |
|---|---|---|---|---|
Phenoxyethanol | Glycol ether | 3.0–8.0 | 0.5–1.0 | Broad spectrum, but weak against fungi. The basis of most systems |
Ethylhexylglycerin | Booster / co-emulsifier | 4.0–8.0 | 0.3–1.0 | Not a preservative on its own! Enhances the effect of Phenoxyethanol |
Potassium Sorbate | Organic acid (salt) | 2.0–5.5 | 0.15–0.3 | Antifungal. Works ONLY in an acidic environment |
Sodium Benzoate | Organic acid (salt) | 2.0–5.0 | 0.1–0.5 | Antibacterial. Often paired with Potassium Sorbate |
Benzisothiazolinone (BIT) | Isothiazolinone | 2.0–9.0 | 0.01–0.05 | Very effective, but allergenic. Banned in leave-on products in the EU |
Dehydroacetic Acid (DHA) | Organic acid | 3.0–6.5 | 0.05–0.6 | Good antifungal. Often in a blend with Benzyl Alcohol |
Benzyl Alcohol | Aromatic alcohol | 3.0–8.0 | 0.5–1.0 | Mild bacteriostatic. Works perfectly paired with DHA |
Organic acids
Sorbic Acid, Benzoic Acid, Dehydroacetic Acid. Work only in an acidic environment (pH < 5.5). Mechanism: the non-dissociated form of the acid penetrates the microbe's membrane.
Glycol ethers
Phenoxyethanol, Caprylyl Glycol, Ethylhexylglycerin. Wide pH range, but each has its own activity profile. Phenoxyethanol is the industry's “workhorse”.
Multifunctional
Glyceryl Caprylate, Pentylene Glycol, p-Anisic Acid. Formally not preservatives, but boosters and emollients with antimicrobial properties. They strengthen the main system.
pH is the key to everything
This is perhaps the most underestimated topic in home cosmetic formulation. Most recipes online do not mention the pH of the final product at all, yet it is exactly what determines whether your preservative will work. Organic acids (sorbic, benzoic, dehydroacetic) exist in solution in two forms: dissociated (ionized) and undissociated. Only the undissociated form possesses antimicrobial activity.
As the pH increases, the equilibrium shifts towards the ionized form — and the preservative stops working. This is not a gradual decline, but a sharp drop in efficacy near the pKa value of the specific acid.
Preservative | pKa | Working pH | Optimal pH | % of active form at pH 5.5 |
|---|---|---|---|---|
Sorbic Acid | 4.76 | < 5.5 | 4.0–4.5 | ~15% |
Benzoic Acid | 4.20 | < 5.0 | 3.5–4.0 | ~5% |
Dehydroacetic Acid | 5.27 | < 6.5 | 4.5–5.0 | ~37% |
Phenoxyethanol | — | 3.0–8.0 | 4.0–6.0 | Independent of pH |
Practical example: Potassium Sorbate at pH 4.0 has ~85% active (sorbic) acid. At pH 6.0, it has only ~5%. This means that the same preservative at the same dosage can be effective or completely useless, depending on the pH of your cream formula. Always measure the pH of your final product!
A systematic approach: combining preservatives
One preservative almost never provides complete protection. Bacteria, yeast, and mold have different sensitivities to different substances. Therefore, the professional approach at Walker Formulation Academy is to build a preservation system from several components with different mechanisms of action.
Antibacterial component — Phenoxyethanol, Sodium Benzoate or Benzyl Alcohol. This is the base that keeps the bacterial load in check.
Antifungal component — Potassium Sorbate, Dehydroacetic Acid. Yeast and mold are often more resistant than bacteria and require a separate agent.
Booster / synergist — Ethylhexylglycerin, Caprylyl Glycol, Pentylene Glycol. Increases the permeability of microbial membranes to the main preservatives.
pH control — citric acid or lactic acid. Keeps the pH within the working range of the preservatives throughout the entire shelf life.
Chelator — Disodium EDTA or Sodium Phytate. Binds metal ions that bacteria need for growth and increases the efficiency of the entire system.
Classic combination
Phenoxyethanol 0.8% + Ethylhexylglycerin 0.4%. pH 4.5–6.0. The most common combination for emulsions. Reliable, proven, and suitable for most formulations.
Acidic pair
Sodium Benzoate 0.3% + Potassium Sorbate 0.2%. pH 4.0–5.0. Ideal for toners, micellar waters, and low-pH products. Requires strict acidity control.
Alcohol-based system
Benzyl Alcohol 0.7% + Dehydroacetic Acid 0.3%. pH 4.0–6.0. Works well in natural cosmetics. Broad spectrum: bacteria + fungi. Gentle on the skin.
Multifunctional
Phenoxyethanol 0.5% + Glyceryl Caprylate 0.5% + Pentylene Glycol 2.0%. pH 4.5–6.5. A modern approach using boosters. Allows for a reduction in the dose of the main preservative.
Incompatible combinations: Do not combine Phenoxyethanol with high concentrations of non-ionic surfactants (Polysorbate 80 > 2%) — the surfactant can bind the preservative into micelles and render it ineffective. Similarly, protein ingredients (silk hydrolysates, collagen) can inactivate certain preservatives through adsorption.
Challenge test: how to verify efficacy
The only reliable way to ensure your preservation system is working is a challenge test (microbiological stability test). Standards: ISO 11930 (European) and USP <51> (American). The essence: a known quantity of microorganisms is intentionally introduced into the finished product, and their decline is monitored over 28 days.
Staphylococcus aureus — a Gram-positive bacterium, a common contaminant in skin products
Pseudomonas aeruginosa — a Gram-negative bacterium, the most “resilient” and dangerous in cosmetics
Escherichia coli — an indicator of faecal contamination, a mandatory test organism
Candida albicans — a yeast fungus, resistant to many preservatives
Aspergillus brasiliensis — a mould fungus, especially relevant for products with botanical extracts
Practical recommendations
Problem | Probable cause | Solution |
|---|---|---|
Cream formula spoiled after 2 weeks | pH too high for the chosen preservative | Measure pH, adjust with citric acid to the working range |
Mould appeared on the surface | No antifungal component in the system | Add Potassium Sorbate or DHA to the main preservative |
Unpleasant odour after a month | Bacterial contamination during production | Disinfect tools, work with gloves, sterilise packaging |
Preservative “not working” in a new formulation | Interaction with surfactants or protein ingredients | Check compatibility, replace preservative or reduce surfactant concentration |
Skin irritation from the product | Overdose of preservative or allergenic component | Reduce dose, replace with a milder option, add a booster |
Always measure the pH of the finished product — indicator strips have an error margin of ±0.5; for precise work, you need a pH meter with an electrode
Add the preservative to a cooled emulsion — most preservatives lose activity at temperatures above 50°C. Optimal: 35–40°C
Do not trust “natural” preservatives blindly — grapefruit seed extract (GSE) contains synthetic benzalkonium chlorides as an impurity, and essential oils irritate the skin at bactericidal concentrations
Sterilise packaging and tools — 70% isopropyl alcohol, treating packaging with boiling water or a UV steriliser. The preservative protects against contamination during use, not against dirty production
Consider the type of packaging — wide-neck jars require more robust preservation than pumps and airless bottles, as the user introduces microbes with their fingers every time
Add a chelating agent — Disodium EDTA (0.05–0.15%) or Sodium Phytate (0.1–0.3%) binds metal ions and significantly boosts the entire preservation system
Keep control samples — retain a sample of every batch and check it after 1, 3, and 6 months for odour, colour, consistency, and pH
Advice from Oksana Walker: I always tell my students: preservation is not the last step in a cream formula, but the first. Start designing your preservation system before you choose your active ingredients. The emulsion pH, packaging type, and storage conditions must all be aligned with your protection system. One good preservative in the right conditions is better than three in the wrong ones.
Preservation is a topic that does not forgive a superficial approach. But if you master the principles (pH-dependence, synergy of components, correct dosage), you will be able to create products that not only look beautiful and smell pleasant but also remain safe throughout their entire stated shelf life. And that is the main indicator of a formulator's professionalism.
Read also: pH in cosmetics • From kitchen to cosmetics
Oksana Walker
Cosmetic chemist, founder of the Walker Formulation Academy
IFSCC • SCS • IAA • IAC
