Optimizing Formula Preservation

By: Eric S. Abrutyn, TPC2 Advisors Ltd.

Posted: February 26, 2010, from the March 2010 issue of Cosmetics & Toiletries.

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Nonionics such as monoesters of fatty acids and alkyl glycosides can enhance preservatives at concentrations below the critical micelle concentration (CMC) by improving their solubility, since at low concentrations a low interfacial tension exists, which allows increased interaction between the preservative and the microbe present in the aqueous phase.

Natural ingredients as preservatives: Essential oils and fragrances have some antimicrobial properties but typically require high concentrations in order to be effective; examples are: clove, cumin, eucalyptus, lavender, lemon, thyme, sage, sandalwood, neem and tea tree oils. It should be noted that in the presence of some pure botanical oils, caprylyl glycol and other long chain glycols are known to interfere with viscosity by reducing it over time. They can also cause discoloration.

Raw material/preservative interactions to consider: It should be noted that UV sunscreens can deactivate formaldehyde-releasing preservatives, and surfactant micelles tend to capture and inhibit preservative activity. Preservatives can also be compromised in the presence of solid particles as they may absorb onto the surface of the particles and thus become unavailable within the aqueous phase. Examples of solid particles could include: talc, inorganic sunscreens, clays and pigments. Proteins and highly ethoxylated compounds (strong hydrogen binders) may also deactivate preservative activity.

At > 15%, silicones can also make a formulation more difficult to preserve due to the creation of a separate phase and the retardation of microbe killing during insult testing. This is due to surface tensions, and interferes with current microbial assays designed to measure preservation efficacy. Finally, heat may inactivate preservatives such as formaldehyde donors and isothiazolinones by accelerating hydrolysis and breaking them down. It can also degrade and dissipate those that are more volatile. Thus, heating formaldehyde donors to above 60°C is not recommended; in this case, it is instead suggested that they be added after emulsification, as noted above.

Conclusion
It should be cautioned that all preservatives have some level of allergenic potential because they are bioactive in nature. Preservative suppliers have the latest safety data on their preservatives and this should be consulted before using a preservative in a finished product. In addition, consumer marketers should ensure they have properly tested the safety of their products.