Challenging times for preservation

It is not sufficient to merely ensure the finished product is sterile or contains an acceptable level of organisms. Further steps must be taken to minimise contamination and spoilage of the product during storage and use.1 Spoilage due to inadequate preservation can include loss of viscosity, breakdown of emulsions, colour change, malodor, the production of harmful toxins and potentially skin irritation and disorders. The importance of preservation after the product is placed into the market place is emphasised by the recommendation of setting a date of durability, or period after opening (POA). According to the article 6(1) (c) of the Cosmetic Directive (76/768/EEC) the period of time after opening shall be indicated for all the products which are not single-use products or that cannot lead in case of deterioration to damages to the human health according to article 2 of the same directive. The expiry date of a product is obviously influenced by the preservatives used in the formulation.2 To give an idea of the potential scale of contamination, a study by Baird3 investigated 232 products intended for use on babies and found that 53 of these were contaminated (23%). The study included products which had not been used, products used in the domestic environment, and products used at a maternity ward in a hospital; contamination was found in all three groups. Staphylococcus spp. and Pseudomonas spp. were among the isolated bacteria. A preservative is defined as a substance used to protect food, wood etc. against decay, discoloration or spoilage.4 Although there are many hundreds of chemicals which meet this definition, the cosmetic scientist is restricted by legislation to a limited number of chemistries, and further still by the level at which these chemistries are allowed to be included in product. In a perfect world we would incorporate a single preservative which possessed broad spectrum activity, efficacy at low concentrations, excellent water solubility, compatibility with all other cosmetic ingredients, no significant odour or colour, pH tolerance, global approval and heat stability. Of course, being perfectly safe, easy to handle and cheap would also be welcome attributes. Unfortunately, at the present time, this material simply does not exist. Skin itself provides a virulent medium upon which microorganisms can grow so has adapted various defence mechanisms to prevent or inhibit microbial invasion and growth.5 As always, certain microorganisms have developed ways of circumventing these mechanisms, and while generally innocuous, these can prove dangerous if a person’s skin is damaged or the immune system compromised. For example, Propionibacterium acnes is a common, usually innocuous, resident of the skin but can, in some cases, contribute to the development of acne.6 It is also important that any product applied to the skin poses no threat to the natural system but supports the natural defence mechanisms already in place.

New approaches

Modern personal care products are becoming increasingly complex and this complexity brings with it further challenges. While some ingredients, such as chelating agents, can boost preservative efficacy, many modern ingredients have a negative effect. Actives and vitamins provide nutrients for growth, nonionic ingredients can inactivate preservatives and powders, such as talc, can absorb them. It is advisable to keep preservative levels as low as possible, so bearing this in mind, many formulations utilise the principals of “Hurdle Technology”.7 Hurdle technology is a term used to describe the intelligent combination of different preservation factors, or hurdles, to deteriorate the growth of microorganisms, a system originally developed in the food industry. This includes good manufacturing practice (eg. water filtration), appropriate packaging (single application if possible), emulsion form (W/O is preferred), low water activity, pH control and using antimicrobial ingredients (eg. certain essential oils).8 It can not be stressed too much that the preservative is in the formulation to protect the product during its lifetime and not to remove heavy microbial contamination arising from poor manufacturing, storage and packing procedures. High levels of preservative are to be avoided as the incidence of contact allergy to preservatives is concentration-dependent, so overpreservation may lead to more cases of cosmetic related contact allergy.9