The microbial safety of cosmetic products is an important consideration. It is integral to ensuring the stability of the products for the entire shelf- life and to preventing damage to health of the consumers. Currently, the number of accepted substances for microbial stabilisation is limited due to legal restrictions or public discussion.
Hurdle technology and the intelligent combination of multifunctional additives help to design microbiologically stable products. The synergistic blend of phenethyl alcohol and ethylhexylglycerin is an effective stabilisation system, while consisting of 90% nature-identical material.
The regulatory framework for preservatives varies in different regions. According to the EU Cosmetics Directive 76/768/EEC preservatives are substances ‘for the primary purpose of inhibiting the development of microorganisms’ in cosmetic products. These substances are listed in Annex VI of the EU Cosmetics Directive.1 Japan lists preservatives in Annex III of the Japanese Standard of Cosmetics.2 Under the Federal Food, Drug, and Cosmetic Act in the US, cosmetic ingredients, with the exception of colour additives, ‘do not require FDA approval before they go on the market. Companies who market cosmetics have the legal responsibility to ensure the safety of their products’.3 In addition to legal requirements, the options for microbial stabilisation are restricted by marketing demands. The movement to preservative-free or natural products and restrictions on certain actives, i.e. formaldehyde-releasers, isothiazolinones, etc., are increasing. The desire to avoid cosmetic preservatives extends to all categories of traditional preservatives, leading to a limited number of accepted actives. Parabens, for example, are some of the most commonly used preservatives due to their excellent efficacy and low sensitising potential.4,5 Public discussion has caused some manufacturers to avoid using these materials in new formulations. The claim ‘paraben-free’ on personal care products first appeared in 2005. According to market research company Mintel, in 2011 this claim was used for 9% of total beauty and personal care launches worldwide; mainly in North America, Europe, and Japan. In contrast, the ‘preservative-free’ claim is still of minor importance. Less than one per cent of new cosmetic products were launched worldwide with this claim; mainly in Japan (3% of total launches).6 It is debatable whether these claims meet the consumer needs or increase their uncertainty of the safety of cosmetic products. In addition, the trend to natural and sustainable products is ongoing. According to Kline & Company, the global natural personal care segment in 2010 is about USD 23 bn. This is a small segment in the personal care market (~USD 300 bn). However, the sales growth of 15% from 2009 to 2010 and the expected annual sales growth of 12% through 2015 are remarkable.7,8 There is still no universal definition of ‘natural’ within the personal care market. Looking into the ingredients of natural products, Kline differs in ‘truly natural’ and ‘natural-inspired’ products and states that 76% of global sales of natural products are in the group of ‘natural-inspired’. The majority of ingredients in naturalinspired products are synthetic. These are combined with some natural ingredients. The ‘truly natural’ criteria often include higher portions of natural ingredients and mainly avoid less desirable ingredients such as silicones, ethoxylates, chemical UV filters, and synthetic preservatives.7 However, products including high percentages of natural components are more susceptible to microbial contamination, either because the raw material contains a high initial bio-burden or because most natural materials are an excellent medium for microbial growth. Due to these obstacles, formulators are interested in finding novel ways to keep cosmetic products microbiologically stable.
Technologies in use
One formulation technique to achieve selfpreserving formulations or to reduce the need for preservatives is known as hurdle technology. This technique has been used in the food industry since the 1970s.9 Hurdle technology describes intelligent formulation using different preservation factors. Good manufacturing practices, appropriate packaging, careful choice of the form of emulsion, low water activity and low or high pH values can be used to control microbial growth in the absence of traditional preservatives.10 Additionally, chelating agents can be used to increase the efficacy of antimicrobial substances. Chelating agents remove metal ions from the cell membrane of the microbes forming complexes. The lack of metal ions weakens the cell membrane and allows a better penetration of the preservatives.11 A common chelating agent providing this effect is ethylenediaminetetraacetic acid (EDTA). Due to widespread use and poor biodegradability, EDTA has emerged as a persistent organic pollutant in the environment.12 Alternatives to EDTA are readily biodegradable chelating agents; e.g. the sodium salts of glutamate diacetate, iminodisuccinate, and ethylenediamine disuccinate. These complexing agents provide a comparable or potentially better boosting effect of the antimicrobial stabilisation system phenoxyethanol/ ethylhexylglycerin than does EDTA. Adjusting the pH of the formulation with citric acid instead of hydrochloric acid increases this boosting effect.13 However, formulators are often limited and cannot implement all factors of hurdle technology, driven by marketing requirements for the cosmetic product. Cosmetic additives providing antimicrobial capacity or a boosting effect can close the gap.
Multifunctional additives are molecules with more than one beneficial effect to the formulation or the skin; e.g. glycols, glycerol ethers, fragrance ingredients, and essential oils. Some of these widely-used multifunctional additives also display a certain antimicrobial efficacy. The synergistic combination of multifunctional additives can be used to achieve the antimicrobial stabilisation of cosmetic formulations without using traditional preservatives.
Glycols are broadly used humectants in cosmetic products. Their antimicrobial efficacy increases corresponding to their chain lengths, whereas the water solubility decreases with longer chain lengths. This leads to a lower antimicrobial effect in the formulation for linear glycols with more than eight C-atoms. A secondary effect is that glycols lower the water activity, which helps maintain the microbial stability of cosmetic preparations.
Phenethyl alcohol occurs widely in nature and can be found in many essential oils; including rose, carnation and hyacinth. It is used as a fragrance component and has a mild rose odour. It exhibits effective inhibitory action on microbes, especially Gram-negative bacteria. The mechanism of action of phenethyl alcohol is at the level of the cell membrane, causing a breakdown of the cellular permeability barriers. This alteration of membranes is assumed to lead to disruption of DNA and protein synthesis.14
Ethylhexylglycerin is a multifunctional cosmetic ingredient with excellent deodorising and skin care properties. It has a limited activity against spoilagecausing bacteria. However, if combined with alcohols, antimicrobial stabilisers like glycols or preservatives, ethylhexylglycerin acts as a booster, enhancing their antimicrobial activity.15 The chemical structure must be considered to explain the mode of action of this boosting effect. Due to its surfactantlike structure, ethylhexylglycerin has the capability to significantly reduce the surface tension of water. This improves the contact of the antimicrobial actives with the cell membranes resulting in better penetration and therefore a higher efficacy.15
Solution for antimicrobial stabilisation
The combination of ethylhexylglycerin with the fragrance ingredient phenethyl alcohol has been investigated with regard to its antimicrobial properties and to a possible boosting effect of ethylhexyglycerin on phenethyl alcohol.
All described tests have been done with nature-identical phenethyl alcohol. Natureidentical materials, synthetic versions of the same chemistries found in nature, offer a way to harness the best of nature without relying on the availability or quality of the materials that exist in nature. Sensiva SC 50 was used as single ingredient ethylhexylglycerin. The test combination sensiva PA 20 is a mixture of both ingredients in the ratio of 9:1.
Germ count reduction test The purpose of the germ count reduction test is to determine exposure times for antimicrobial substances. Oil-in-water emulsions are inoculated in the laboratory. The titre of the suspension is approx. 109 cfu/mL for bacteria, 108 cfu/mL for yeasts and 107 cfu/mL for moulds. Nutrient media are CSA (tryptone-soya-agar) for bacteria and SA (sabouraud-dextrose-agar) for yeast and mould. The plates are incubated for 48 hours at 37°C for all tested germs except the mould Aspergillus niger which is incubated for 48 hours at 25°C. Challenge test (Schülke Koko test) A mixture of bacteria, yeast and moulds are inoculated 6 times (once a week) into the test material, with the goal of keeping the test material germ free for this period. The inoculum contains pathogenic microorganisms as germs which are well known for product spoilage. All species have to be cultivated separately and mixed directly before the addition, to ensure a constant composition and germ count of the inoculum. Its germ count is approximately 108-9 cfu/mL, which means a germ count of about 106 cfu/mL in the sample. Nutrient media are CSA (tryptonesoya- agar) for bacteria and SA (sabourauddextrose- agar) for yeast and moulds.
Figures 3 and 4 show germ count reduction tests in oil-in-water emulsion for the bacteria Staphylococcus aureus and Escherichia coli, as well as the mould Aspergillus niger and the yeast Candida albicans. The tests have been conducted with phenethyl alcohol and ethylhexylglycerin individually, and with a mixture in the ratio of 9:1 (sensiva PA 20). Phenethyl alcohol achieved a reduction of the spoilage causing germs, whereas ethylhexylglycerin in this small amount had no effect when used alone. The killing rate of phenethyl alcohol was increased significantly for all germs by pairing it with ethylhexylglycerin at a ratio of 9:1. Pure phenethyl alcohol (0.45%/0.9%) reduced all germs to zero after 168 hours. The synergistic blend achieved the reduction to zero after 48 hours for C. albicans and S. aureus and after 6 hours for E. coli. A faster decrease was even observed in the persistent mould A. niger. The synergistic effect of the blend has also been shown in challenge tests (Schülke Koko test) with two formulations (Table 1-3). Based on experience, a cosmetic product without growth of microorganisms after six inoculation cycles (six weeks) can be considered microbiologically stable for 30 months, which is recommended for cosmetic products.16 The combination of phenethyl alcohol with ethylhexylglycerin (sensiva PA 20) stabilises the o/w cream well at a level of 1%. The w/o cream is well stabilised at a level of 0.75%. The single ingredients in the corresponding concentrations show no efficacy in the Koko test (Table 1). Ethylhexylglycerin was found to be an excellent booster for the antimicrobial capacity of phenethyl alcohol. The killing rate in the germ count reduction tests was accelerated. The boosting effect was confirmed by the results of the challenge tests. Depending on the composition of the formulations, 0.75% and 1% of a blend of these two components stabilised the emulsions effectively.
The contamination of cosmetic products is a high risk. The concept of hurdle technology is not always sufficient to achieve self-preserving systems or not all necessary factors can be implemented. The intelligent combination of multifunctional ingredients using synergism and boosting effects can be a solution to protect products from microbial contamination, are friendly to the skin and are accepted by the consumers. The blend of phenethyl alcohol and ethylhexylglycerin (sensiva PA 20) is an efficient stabilisation system, combining antimicrobial activity with skin care and deodorizing properties. The synergistic mixture is suitable for preservative-free cosmetics (according to the European interpretation), as well as sensitive applications; e.g. baby care, oral and lip care. Due to the high portion (90%) of nature-identical material in this blend, it can be an option to stabilise natureinspired cosmetics. PPCC
1 http://ec.europa.eu/consumers/sectors/ cosmetics/documents/directive/index_en.htm 2 http://www.mhlw.go.jp/english/dl/cosmetics.pdf 3 http://www.fda.gov/ 4 http://ec.europa.eu/health/ph_risk/committees/ 04_sccp/sccp_opinions_en.htm 5 Steinberg DC, Perservatives for cosmetics. Second Edition (2006). 6 Mintel GNPD (Global New Products Database); www.mintel.com 7 Kline & Company, Natural Personal Care 2010. Webinar, (March 2011). 8 Kline & Company, An overview of the global hair and skin care market. (April 2012). www.midwestscc.org/archives/Apr11Kline.pdf 9 Varvaresou A et al. Self-preserving cosmetics. Int J Cosmet Sci. 2009; 31 (3): 163-75. 10 Leistner L. Basic aspects of food preservation by hurdle technology. Int J Food Microbiol 2000; 55 (1): 181-6. 11 Kabara JJ, Orth DS. Preservative-free and selfpreserving cosmetics and drugs. Marcel Dekker, New York, 1997. 12 Yuan Z, VanBriesen JM. The formation of intermediates in EDTA and NTA biodegradation. Environ Eng Sci 2006; 23: 533-44. 13 Siegert W. Can biodegracable complexing agents replace tetrasodium EDTA to boost preservatives? SOFW Journal 2008; 134, 1⁄2. 14 Silver S, Wendt L. Mechanism of action of phenethyl alcohol: Breakdown of the cellular permeability barrier, J Bacteriol. 1967; 93 (2): 560-6. 15 Leschke M. A multifunctional ingredient for leave on cosmetics. Cosmetic Science Technology 2006. 16 Siegert W. Evaluation of the microbiological safety of finished cosmetic products. Euro Cosmetics 2010; 3: 16-9.
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