Of all the formulation decisions you will make, choosing and correctly using a preservative system is the most important one for safety. An unpreserved or inadequately preserved water-containing cosmetic product can become contaminated with bacteria, yeast, and mould — sometimes within days of manufacture — and applying a contaminated product to the skin can cause serious harm.
Yet preservatives are also one of the most misunderstood topics in DIY skincare. There is significant confusion about which products need preserving, which preservatives work for which formulas, what "natural" preservation actually means, and how to use preservatives correctly. This guide addresses all of those questions clearly and practically.
Does Your Formula Need a Preservative?
The short answer: if your formula contains water — or will come into contact with water during use — it needs a preservative. This includes:
| Product Type | Needs Preservative? | Reason |
|---|---|---|
| Lotions and creams (emulsions) | Yes | Water phase supports microbial growth |
| Serums (water-based) | Yes | Water phase supports microbial growth |
| Toners and mists | Yes | Predominantly water; high contamination risk |
| Shampoos and conditioners | Yes | Water-based; used in wet environments |
| Aloe vera gel | Yes | High water activity; very susceptible to contamination |
| Anhydrous face oils (no water) | No* | No water = no microbial growth medium; antioxidant recommended instead |
| Body butters (anhydrous) | No* | No water; antioxidant recommended to prevent rancidity |
| Lip balms (anhydrous) | No* | No water; antioxidant recommended |
| Sugar/salt scrubs (used in shower) | Recommended | Water introduced during use; high contamination risk over time |
*Anhydrous products do not require a preservative but do benefit from an antioxidant (such as Vitamin E / Tocopherol at 0.1–0.5%) to prevent oxidative rancidity of the oils and butters.
How Preservatives Work
Cosmetic preservatives work by inhibiting the growth of microorganisms — bacteria, yeast, and mould — that can contaminate a product during manufacture or use. They do this through several mechanisms depending on the preservative type: disrupting microbial cell membranes, inhibiting enzyme activity, or creating an inhospitable environment for microbial growth.
No single preservative is effective against all types of microorganisms across all formula types. This is why many professional formulas use a preservative system — a combination of two or more preservatives that work synergistically to provide broad-spectrum protection against bacteria (gram-positive and gram-negative), yeast, and mould.
The effectiveness of a preservative system is also influenced by several formula variables: pH, water activity, the presence of other ingredients (some can neutralise or bind preservatives), and the physical form of the product. This is why preservative efficacy testing (also called challenge testing) is the only way to confirm that a preservative system is working in your specific formula.
Common Cosmetic Preservatives: A Complete Comparison
| Preservative | INCI Name | Usage Rate | Effective pH | Spectrum | Natural / Synthetic | Notes |
|---|---|---|---|---|---|---|
| Phenoxyethanol + Ethylhexylglycerin | Phenoxyethanol, Ethylhexylglycerin | 0.75–1.0% | 3–8 | Broad | Synthetic | The most widely used system in professional formulation. Reliable, well-studied, broad pH range. The industry standard for a reason. |
| Optiphen Plus | Phenoxyethanol, Caprylyl Glycol, Sorbic Acid | 0.75–1.5% | 3–6 | Broad | Synthetic | Paraben-free; good for emulsions. pH must be below 6 for sorbic acid component to be effective. Can cause emulsion instability at high use rates. |
| Leucidal Liquid SF | Lactobacillus Ferment | 2–4% | 3.5–5.5 | Bacteria + some yeast | Natural / COSMOS | Derived from radish fermentation. COSMOS-approved. Narrow spectrum — often combined with AMTicide Coconut for mould coverage. Higher use rate increases cost. |
| Naticide | Parfum (Fragrance) — listed as fragrance | 0.3–1.0% | 3–8 | Broad | Natural-derived | ECOCERT/COSMOS-approved. Has a sweet almond/vanilla scent that can affect fragrance profile. Listed as "Parfum" on label — note this for transparency. |
| Geogard Ultra | Gluconolactone, Sodium Benzoate | 0.5–1.0% | 3–5 | Broad | Natural / COSMOS | ECOCERT-approved. Requires acidic pH to work — ineffective above pH 5. Also functions as a mild PHA (polyhydroxy acid) exfoliant. |
| Sodium Benzoate + Potassium Sorbate | Sodium Benzoate, Potassium Sorbate | 0.5% each | 3–5 | Bacteria + yeast + mould | Synthetic (food-grade) | Low cost; widely available. Strictly pH-dependent — must be below pH 5 to be effective. Often used in toners and low-pH serums. |
| Germall Plus | Propylene Glycol, Diazolidinyl Urea, Iodopropynyl Butylcarbamate | 0.1–0.5% | 3–9 | Very broad | Synthetic | Very effective at low use rates. Contains a formaldehyde-releasing component (diazolidinyl urea) — some consumers prefer to avoid. Not suitable for leave-on rinse-off products in some markets. |
Four Factors That Affect Preservative Efficacy
1. pH
pH is the single most important variable affecting preservative performance. Many preservatives — particularly organic acid-based systems like sodium benzoate, potassium sorbate, and sorbic acid — are only effective in their undissociated (acidic) form, which means they lose efficacy rapidly above pH 5. Phenoxyethanol is more pH-tolerant (effective up to pH 8) but still performs best in the pH 4–7 range.
Always measure and adjust the pH of your formula before adding your preservative, and always test the pH of the finished product. A pH meter is an essential piece of equipment for any serious formulator — pH strips are not accurate enough for this purpose.
2. Water Activity
Water activity (Aw) is a measure of how much "free" water is available in a formula for microbial growth. Humectants like glycerin, propylene glycol, and sorbitol reduce water activity by binding water molecules, making them less available for microorganisms. A formula with high glycerin content (above 20%) may have sufficiently reduced water activity to inhibit microbial growth — but this is not a substitute for a proper preservative system in most formulas.
3. Ingredient Interactions
Some ingredients can bind to or neutralise preservatives, reducing their effective concentration in the formula. High concentrations of proteins (e.g., hydrolysed proteins in hair care), certain surfactants, and some plant extracts can interact with preservatives. If you are adding unusual or high-concentration ingredients, this is another reason why challenge testing is important.
4. Manufacturing Hygiene
A preservative system is not a substitute for good manufacturing practice. If you introduce a high microbial load during manufacturing — through contaminated equipment, unclean hands, or non-sterile water — even a well-chosen preservative system may be overwhelmed. Always use distilled or deionised water, sanitise all equipment before use, and work in a clean environment.
Which Preservative System Should You Choose?
The right preservative system depends on your formula type, target pH, whether you need natural/COSMOS certification, and your customer's preferences. Use this decision guide as a starting point.
| If your formula is… | Recommended Starting Point |
|---|---|
| A standard lotion or cream (pH 5–7) | Phenoxyethanol + Ethylhexylglycerin at 0.75–1.0% |
| A low-pH serum or toner (pH 3.5–5) | Sodium Benzoate + Potassium Sorbate (0.5% each) or Geogard Ultra |
| A natural / COSMOS-certified product | Naticide (0.3–1.0%) or Leucidal Liquid SF + AMTicide Coconut |
| A shampoo or conditioner (pH 4.5–5.5) | Phenoxyethanol + Ethylhexylglycerin or Optiphen Plus |
| An aloe vera gel or water gel (pH 4–5) | Phenoxyethanol + Ethylhexylglycerin or Geogard Ultra |
| An anhydrous body butter or face oil | No preservative needed — use Vitamin E at 0.1–0.5% as antioxidant |
The Five Most Common Preservative Mistakes in DIY Skincare
1. Using Vitamin E as a preservative. Vitamin E (Tocopherol) is an antioxidant — it prevents oxidative rancidity of oils and fats. It does not prevent microbial growth in water-containing formulas. This is one of the most common and potentially dangerous misconceptions in DIY skincare.
2. Using essential oils as preservatives. While some essential oils have antimicrobial properties in laboratory tests, they do not provide reliable broad-spectrum preservation in finished cosmetic formulas at safe use rates. Tea tree oil, for example, would need to be used at concentrations that cause skin sensitisation to provide meaningful preservation.
3. Adding the preservative at too high a temperature. Many preservatives — particularly phenoxyethanol — should be added to the formula at or below 40°C. Adding them to hot phases can cause them to volatilise or degrade, reducing their effectiveness.
4. Assuming a product is "safe" because it smells fine. A contaminated product does not always smell bad or look different. Gram-negative bacteria in particular can contaminate a product without any visible or olfactory signs. The only way to know if a product is adequately preserved is through challenge testing.
5. Using the wrong preservative for the formula's pH. As discussed above, many preservatives are pH-dependent. Using sodium benzoate in a formula with a pH of 6.5 provides essentially no protection. Always check the effective pH range of your chosen preservative and adjust your formula accordingly.
Frequently Asked Questions
Can I use a preservative in a product I sell commercially?
Yes — in fact, you are required to adequately preserve any water-containing cosmetic product you sell commercially in Australia. The preservative must be listed by its correct INCI name on the product label. Ensure you are using the preservative within its approved usage rate range and that it is listed on the Australian Industrial Chemicals Introduction Scheme (AICIS) register.
What is challenge testing and do I need it?
Challenge testing (also called preservative efficacy testing or PET) involves deliberately introducing specific microorganisms into a finished product and measuring whether the preservative system reduces them to safe levels within a defined timeframe. For products you sell commercially, challenge testing is strongly recommended — it is the only way to confirm your preservative system is working in your specific formula. Several Australian cosmetic testing laboratories offer this service.
Are parabens safe to use?
Parabens are among the most extensively studied cosmetic preservatives in existence. The current scientific consensus from regulatory bodies including the EU Scientific Committee on Consumer Safety (SCCS) and the Therapeutic Goods Administration (TGA) is that commonly used parabens (methylparaben, ethylparaben, propylparaben) are safe at approved concentrations in cosmetic products. Consumer concern about parabens is largely based on misinterpreted research. That said, paraben-free formulation is a legitimate market preference, and there are effective alternatives available.
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All products from The Skin Science Company are intended for cosmetic use only. Nothing in this article constitutes regulatory, legal, or safety advice. Formulators and product manufacturers are responsible for ensuring their products are adequately preserved and comply with all applicable Australian cosmetic regulations. Consult a qualified cosmetic chemist or regulatory consultant for advice specific to your products.