PRESERVATIVES
100 90 80 70 60 50 40 30 20 10 0
n Staphylococcus aureus n Pseudomonas aeruginosa n Escherichia coli n Candida albicans n Aspergillus brasiliensis
12 9 8 7 6 5 4 3 0 2 7 14 Days Figure 5: Challenge test results against Aspergillus brasiliensis
1000 cfu/g (log cfu/g=3) represent 50% relative logarithmic reduction.
Results and discussion The result of the challenge tests against the five strains of microorganisms are shown in Figures 1-5. The antimicrobial performance of the tested formulations against the bacteria Staphylococcus aureus (Fig. 1), Pseudomonas aeruginosa (Fig. 2) and Escherichia coli (Fig. 3) was too strong to reveal detailed differences between most of the emulsions under the given test conditions, even though differences may exist for the kill rates between the time of inoculation and the first evaluation of plate counts after two days. Kill rate kinetics against fungi are generally slower, allowing a better comparison of the individual emulsions regarding their performance against the yeast Candida albicans (Fig. 4) and the mold Aspergillus brasiliensis (Fig. 5). In Figure 6 the emulsions are compared with respect to their overall antimicrobial performance. The larger the bar the less favourable is the influence of the emulsifier on the performance of the preservation system. Emulsifiers are classified into three
groups: non-ionic, anionic and cationic. Cationic emulsifiers have not been part of this investigation. However, their potential antimicrobial activity is well established.3 In the tested series of emulsifiers and emulsifier blends no strict rule can be established that an anionic emulsifier is more favourable than a non-ionic emulsifier with respect to the positive influence on the preservation, but a certain tendency to support this theory can be observed. Despite the fact that surface active compounds made from hydrocarbon
62 PERSONAL CARE February 2016 21 1 28 2 8 3 9 6 4 1 7 Emulsifier (see Table 2)
Figure 6: Relative influence of the emulsifier system on the antimicrobial performance.
building blocks are known for their antimicrobial properties,4
the comparison of
the results of emulsion 11 and 12 clearly reveals the negative influence of the hydrocarbon related structural feature in this test. Because the level of cetearyl alcohol has been standardised in all formulations, the only difference between emulsion 11 and 12 is the level of cetearyl glucoside. The higher dosage of cetearyl glucoside in emulsion 12 compared to emulsion 11 is further decreasing the antimicrobial performance. Emulsion 10 is the third emulsion in the panel containing hydrocarbon building blocks. Emulsions 10, 11 and 12 are the least preserved emulsions in the test. The relatively bad test result for
emulsion 5 comes as a surprise. It was expected that the component sodium lauroyl glutamate would have a beneficial effect on the preservation performance as a surfactant based on a medium chain fatty acid.5
The result for emulsion 7
can be explained by the presence of hydrogenated lecithin. Lecithin and polyethoxylated surfactants are used in microbiological laboratories as neutralisers to inactivate the preservatives according to the commonly used test protocols.6 Even though the droplet size can be considered as a possible influencing factor due to the potential correlation between the overall surface area of the emulsion droplets and the interaction with antimicrobial ingredients, the challenge test results do not reveal such a correlation.
Conclusions The emulsifier system showed a pronounced influence on the antimicrobial performance of a surfactant and organic
acids based preservation system. Emulsifier blends comprising hydrocarbon moieties showed a negative influence on the antimicrobial performance of the tested preservation system. Anionic emulsifiers have a tendency to support the preservation activity better than non-ionic emulsifiers. It is important to understand that most ingredients have an influence on the product preservation. This influence can be favourable or detrimental. It cannot be assumed that even a slight change of a formulation or the variation of the dosage of a component does not alter the result of the preservation performance. Challenge tests need to be performed in order to proof sufficient preservation after each change in a formulation composition.
PC
References 1 Council Directive 82/368/EEC of 17 May 1982, Article 6.
2 Petersen W. Antimicrobial ingredients for self- preserving cosmetics. Euro Cosmetics 1999; 7 (2): 28-36.
3 Cozzoli O. The role of surfactants in self- preserving cosmetic formulas. In: Kabara JJ, Orth DS eds. Preservative-free and self-preserving cosmetics and drugs. New York: Marcel Dekker, 1997: 75-118.
4 Matsumura S, Imai K, Yoshikawa S, Kawada K, Uchibori T. Surface activities, biodegradability and antimicrobial properties of n-alkyl glucosides, mannosides and galactosides. J Am Oil Chem Soc 1990; 67 (12): 996-1001.
5 Kabara JJ. Structure-function relationships of surfactants as antimicrobial agents. J Soc Cosmet Chem 1978; 29: 733-41.
6 ISO 11930:2012. Cosmetics – Microbiology – Evaluation of the antimicrobial protection of a cosmetic product, Annex C.
5 10 11 12 10 11
Remaining log cfu/g (%)
Emulsifier (see Table 2)
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