More than any other ingredients, preservatives are the most vilified ingredients and most often called out as the worst ingredients you can use in a formula. However, preservation is mandated by the FDA stating that products sold to the general public must be safe for use when it is applied to the body and must be free of contamination.
Since personal care products are not made and used in sterile conditions microbes will inevitably grow making the use of preservatives a must for almost all personal care products. Even if a product may be free of contaminants at the time of production, the product must still be preserved sufficiently to kill microbes that are introduced later by the consumers themselves.
Natural, insufficiently preserved cream overgrown with mold.
Factors to Minimize Contamination
Key factors for the exclusion or minimization of microorganisms throughout a product’s life cycle include the following measures:
- Clean Equipment: Sanitize equipment thoroughly with isopropyl alcohol, chlorine or another proven sanitizer. Heat steam (water above 100°C) works also.
- Clean Water: Use sterile, distilled or deionized water, but not tap water. Lower the water activity (amount of free available water in a formula) by adding small amounts of glycerin, sorbitol, propylene glycol, or butylene glycol to the water phase.
- Formula Optimization: Create a hostile environment for microorganisms. For example, adjust the pH of the water phase to the level where preservatives act best and enhance susceptibility of microbes towards preservatives by adding EDTA, citric acid, sodium citrate, etridonic acid, sodium gluconate, or phytic acid.
- Preservatives: Use a preservative or better a combination of preservatives that offer broad-spectrum protection (kill bacteria and mold) and add the preservatives to the water-phase or at the end of the the formula, but not the oil phase. Consider adding co-preservatives including essential oils and fragrances that have antimicrobial properties including clove, cumin, eucalyptus, lavender, lemon, thyme, sage, sandalwood, neem and tea tree oils.
- Packaging Optimization: Use packaging that effectively protects the formula and minimizes contamination by the user. Consider filling products into containers that have a dispensing system (e.g. airless dispenser bottles) and do not require the consumer to use their fingers.
Below is a list of the most commonly used preservatives for personal care products. Most preservatives are not used alone but in combination with others.
- Parabens: Parabens including methylparaben, propylparaben, and butylparaben are the most commonly used preservatives. They are very effective against most bacteria and fungi. Their efficacy can be limited at pH values outside the range of 4-8. As research has shown that some parabens can have an estrogen-like effect, this finding has fueled a fear that the parabens in underarm deodorants and other cosmetics can migrate into breast tissue and contribute to the development of tumors. However, no direct evidence of a causal link between parabens and cancer, however, has been shown. A review of the data available concluded it is biologically implausible that parabens could increase the risk of any estrogen-mediated effects. The American Cancer Society also concluded that there was insufficient scientific evidence to support a claim that use of cosmetics such as antiperspirants increase an individual's risk of developing breast cancer, and that larger studies are needed to find out what effect, if any, parabens might have on breast cancer risk.
- Urea Derivatives: Urea derivatives including imidazolidinyl urea and diazolidinyl urea are effective against all types of microbes including bacteria, fungi, and mold. They work in a wider ph range (3-9) than parabens and are therefore often combined with parabens. They are also often used in surfactant systems.
- Phenol Derivatives: Phenol derivatives like phenoxyethanol have been used in cosmetics for many years and can be effective against a range of microbes. As they are not as effective as parabens and urea derivatives they are often combined with other preservatives. Phenoxyethanol should not be used in formulas containing HE-cellulose or non-ionic surfactants as its efficacy can be compromised.
- Alcohols: Ethanol is a good preservative but it has to be used at high levels (at least 20%). Other alcohol derivatives like benzylalcohol, dichlorobenzyl alcohol are more effective but are also typically combined with other preservatives. Like phenoxyethanol, benzylalcohol should not be used in formulas containing non-ionic surfactants as its efficacy can be compromised.
- Quats: Quats (quaternary ammonium compounds) are ingredients typically used as hair conditioning agents. However, many of them can kill microbes. This include ingredients like benzalkonium chloride, methene ammonium chloride, benzethonium chloride and quaternium-15. Quats work in a wide ph range (4-10) but their cationic nature makes them less compatible with anionic surfactants.
- Isothiazolones: Isothiazolones including methylchloro-isothiazolinone and methyl-isothiazolinone are effective at already low levels. They work in a wide pH range and are useful in a variety of formulation systems.
- Other Preservatives: There are a variety of other preservatives including potassium sorbate, sorbic acid, sodium benzoate, chlorocresol, chloroxylenol, chloracetamide, triclosan, DMDM Hydantoin, and iodopropynyl butylcarbamate that all are effective against some but not all types of microorganisms, or face other limitations.
- Natural Preservatives: Based on the increasing demand for natural preservatives the cosmetic industry is continuously searching for new natural compounds able to limited the growth of microorganisms. So far, only a few effective natural preservatives have found their way to the market including Orange-Lemongrass Blend, Honeysuckle Blend, and a solution of peptides isolated from the bacteria Leuconostoc kimchii upon fermentation of radishes.
Expected Shelf Life
Unfortunately, it is not possible to accurately predict the shelf-life of a product that has been preserved with a certain preservative since the shelf-life depends on various factors including:
- The type of formula and the ingredients used (whether and how much it contains free water)
- The circumstances under which the product has been produced (how well the tools have been disinfected)
- The purity of ingredients (whether microbes have been introduced by contaminated ingredients)
- The storage conditions (temperature, sun exposure, humidity)
- Whether or not the product will be dispensed "finger-free" (e.g. by a pump, disk dispenser, or airless pump)
If a product has been produced under optimum conditions, contains a broad-spectrum preservation system, is stored adequately, and dispensed "finger-free" a shelf-life of 2-3 years can be expected. However, the determination of a shelf-life should never be based on assumptions but solely on bacterial stability testing.