As formulators build the latest creations, they must ensure that each part of the product meets its target and predetermined standards. Although characteristics such as aesthetics, fragrance, performance and color are market-dependent, preservative efficacy is not and must remain high. Preservatives are required to ensure that organisms do not grow and contaminate the product, and while it is necessary to ensure that all steps leading up to the development of the formula follow proper protocol—such as sanitary environment and microbe-free starting materials—a proper preservative system is crucial to ensure the long-term stability and safety of the product.
Across the industry, most companies have different methods and criteria to gauge their preservative system’s ability to inhibit microbe growth. Despite these variations, however, all formulators must ensure they develop a product that is not only effective at protecting against bacteria and fungi, but also is safe when applied to the skin. Irritation issues often arise from the improper use of preservatives, i.e. unnecessarily high levels of preservatives combined with materials that penetrate them deeper into the skin, which can lead to certain skin sensitization issues.1
Challenge testing: The test used to measure the efficacy of a preservative system is referred to as a challenge test. To perform this test, a product is inoculated with various organisms such as bacteria, yeast and fungi, then their rate of kill is measured during a defined length of time. Although challenge test protocols are slightly different throughout the personal care industry, they provide an effective benchmark of the strength of the preservative system.
There is no official preservative efficacy test for cosmetics, thus many larger companies have their own in-house challenge method and most of these methods have criteria that are much more stringent than either compendial or industry standards. Companies that do not have their own method may use the USP or Personal Care Products Council’s method as a way to determine preservative efficacy.2, 3
Preservation vs. sensitization: In an age where the consumer has put strong emphasis on safe but effective products, optimizing preservative systems has never been more important. Insufficient preservation obviously can lead to the proliferation of organism growth; on the other hand, too much preservative may lead to skin sensitization issues as well as product stability concerns. Clearly, a balance must be maintained between the preservative system and the potential for skin sensitization. This is especially true for eye area products, where the risk of potential contamination is high but at the same time, the risk of sensitivity in the thin skin around the eyes is a major concern. Unfortunately, eye products and others that are packaged in jars pose the greatest risk of contamination because unlike pumps or dispensed products, jars require consumers to continuously open them and touch the product to use it.
To ensure that a broad-spectrum of fungi and bacteria are inactivated, a combination of preservatives is required. The preservative blend obviously must be compatible with the formula and not change any of its characteristics, such as odor or color. One trend for combining preservatives has aimed to lower the need for traditional preservatives, which can be accomplished by blending them with multifunctional ingredients, natural ingredients or essential oils.
In the past, parabens and various formaldehyde donors such as imidiazolidinyl urea and DMDM hydantoin were widely and safely used. But due to recent regulatory and safety issues around the globe, their popularity has greatly diminished. Specifically, studies have found a correlation between skin sensitivity (allergy) and levels of formaldehyde;4, 5 there are also issues in both Europe and California regarding the safety of formaldehyde. Further, regulations have been published that classify formaldehyde as a carcinogen.6, 7
Consumer concern over these ingredients has led manufacturers to other combinations of preservatives. Ingredients such as phenoxyethanol and chlorophenisin have become more common, and combining them with organic acids typically yields strong results. Organic acids such as sodium benzoate and potassium sorbate are industry favorites for their strong mold- and yeast-inhibiting abilities.8 Suppliers also have provided formulators with countless blends of preservatives that provide broad-spectrum activity. Such materials have been balanced for efficacy in various systems. This allows formulators to combine several components by adding only one material.
Many natural preservatives work via antioxidant abilities and play a role in preventing lipid ingredients from turning rancid. Vitamin E and countless plant extracts have flooded the market in the past years and are marketed to formulators as unconventional pathways to help preserve formulas. In fact, some of the traditional preservative blends now contain glycols or even natural antioxidants. Marketers have used this to their advantage to gain market share by appealing to the naturals trend. The basics are still needed though, as most water-based formulas cannot be preserved without some conventional preservative.
Deterring Microbial Growth
There are multiple actions that formulators can take to create a formula that is hostile to microbial growth. For instance, optimizing an emulsion’s stability as well as its processing can assist the preservative profile. Formulas produced at a lower pH are more effective to combat microbe issues since there is more free acid at a lower pH, thus making the preservatives more available. In addition, keeping the preservatives active in the water phase (where microbes grow) is a key factor. This can be accomplished by adding glycerin and glycols to the system. Since these materials are not lipophilic, they help to partition the aqueous and oil phase and lower the water activity. They also boost the preservative activity at the water/oil interface.
There are also times where formulators find that preservative levels can be modified simply by changing the sequence or temperatures at which the preservatives are added. Experimentation with re-sequencing the formula and adding portions of the preservatives at various stages is therefore helpful—especially in a new system. As many formulators have experienced, a preservative level that has been effective in past systems is sometimes no longer as effective in a new system.
Besides preservatives, materials that have multifunctional activities including preservation can be used. These are not just preservatives or emulsifiers, they play multiple roles when added to a formulation. The list includes ingredients such as caprylyl glycol, ethylhexylglycerin, EDTA and numerous phospholipids. By incorporating these molecules into a formulation, formulators can create an emulsion that is hostile to microbial growth. In addition, if at the same time one pays close attention to both the pH and water activity, the need for conventional preservation will be significantly reduced. The challenge with this approach is that it takes time, patience and some creativity while formulating.
Further, most preservatives are effective in the normal pH range of 4–8. So if the formulator can ensure that the water phase of the emulsion is as acidic as it can be, they will not only create an environment in which it is difficult for microbes to grow, but also create a condition that is optimum for long-term preservative efficacy.
Concerning formula processing, the order of addition has become a popular skill to build in order to optimize the preservative system. It is essential that formulators not only add the preservatives in the water phase, but also post-emulsion to maintain contact in the aqueous phase and prevent migration to the interface. It is also important to not overheat the phase in which the preservatives are added since some preservatives may degrade at higher temperatures; they are best added in at lower temperatures (> 60°C). Further, what might begin at the bench as 30 min of mixing at 65°C could turn into 95 min at 80°C during manufacturing, so it is important to watch processing times and temperatures. Ensuring that temperatures and cooling rates during production are comparable to the conditions at the bench is crucial. Also, confirming that during production, the turnover rate of mixing batches is similar to that at the bench can also play a role. This will yield consistent challenge test results in both the bench and production batch.
In the end, it all comes down to the experimental design and a willing microbiology lab. Testing systems with various combinations and sequencing will provide the proper preservative system. Throughout all this preservative experimentation, the formulator must stay cognizant that the product maintains stability and its original aesthetic characteristics. Sometimes the product’s viscosity and color may shift with the addition of certain preservatives, so making even slight changes in preservatives requires a new set of stability experiments and aesthetic feedback. There is no magic bullet for each system; formulators will always be busy finalizing their preservative system while making sure their product is safe and has a low chance of skin sensitization.
1. MD Lundov, L Moesby, C Zachariae and JD Johansen, Contamination versus preservation of cosmetics: A review on legislation, usage, infections and contact allergy, Contact Dermatitis 60 70–78 (2009)
4. AC de Groot, T van Joost, JD Bos, HLM van der Meerenand JW Weyland, Patch test reactivity to DMDM hydantoins, Contact Dermatitis 18 197–201 (1988)
5. W Uter, and PJ Frosch (for IVDK study group and the German Contact Dermatitis research group, DKG) Contact allergy from DMDM hydantoin, 1994–2000, Contact Dermatitis 47 57–58 (2002)
6. Scientific Committee on Cosmetic Products and Non-food Products, Oct. 16, 2009, Opinion concerning a clarification on the formaldehyde and para-formaldehyde entry in Directive 76/768/EEC on cosmetic products, Opinion: European Commission (2002)
7. Formaldehyde, 2-butoxyethanol and 1-tert-butoxypropan-2-ol, IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, International Agency for Research on Cancer: Lyon, France 88 39–325 (2006)
8. DC Steinberg, Preservatives for cosmetics, Cosmetics & Toiletries ingredient resource series, Cosm & Toil 6, 16 (1996)