50 PRESERVATIVES A ■ B ■ Failed ■


60 50 40 30 20 10 0


S. aureus P. aeruginosa E.coli Figure 7: Surfactant-based rinse-off products


cosmetic products on the market, more fails are recorded for each specific microorganism. When moving to the next higher pH range, 6.0-7.0, more fails were found for E.coli, S.aureus and C.albicans. However, again the data input must be


kept in mind. The preservation systems in these tests may have been chosen with the pH already factored in, meaning a higher percentage of antimicrobials were added. In addition, the preservative group of organic acids are only active at a pH<6.0. When they are employed at higher pH, more fails have to be expected. In a different approach, we focused the


analysis not on pH or preservative, but on the formulation type. It is well known among experts in the field that a rinse-off system can have different preservation requirements compared to an emulsion or a water-based system. This is probably due to the different environments and water activities. A further important factor is the nature of


other ingredients, since preservation efficiency is always based on the interplay between all substances present in a formulation. For example, the presence of solvents such as propanediol or butylene glycol can facilitate the contact between preservative and water phase, thereby improving the preservation efficacy.


30 25 20 15 10 5 0


S. aureus P. aeruginosa Figure 9: W/O emulsions PERSONAL CARE July 2022 E.coli C. albicans A. brasiliensis C. albicans A. brasiliensis


180 160 140 120 100 80 60 40 20 0


S. aureus P. aeruginosa Figure 8: O/W emulsions Figures 7-10 show challenge tests in


each of the four formulation concepts: a) Surfactant-based rinse-off products, b) O/W emulsions, c) W/O emulsions and d) Aqueous systems. As a general trend it is noteworthy that


P.aeruginosa produces less ‘B’ results, but more ‘Fails’ than the other two bacterial strains. One could conclude that usually a preservative either works well against P.aeruginosa, or not at all, and that there is little in-between. Furthermore, in aqueous systems this


organism rarely poses a problem. However, it is noteworthy that, even though these graphs show a correlation, they do not necessarily show the causality. Although it appears that P.aeruginosa


is more easily combated in water-based systems, this should not suggest that this species can dwell less efficiently there. An alternative explanation could be that contact between the microorganism and the preservatives is more efficient in such aqueous systems. It is important to recognise the


complexities and pitfalls of cosmetic preservation, and not fall into the trap of over-analysing your data. It is, however, useful to combine this information with the insights gathered on preservation systems: If


A ■ B ■ Failed ■


70 60 50 40 30 20 10 0


S. aureus P. aeruginosa Figure 10: Aqueous systems www.personalcaremagazine.com E.coli C. albicans A. brasiliensis


a preservative has a known weakness against P.aeruginosa, it may still be suitable if the pH is below 5.0 or the formulation base is water.


Conclusion The data presented in the figures above leads to many interesting questions and conclusions, and its analysis is not exhausted in this article. Here, we merely wanted to introduce the methodology and show some interesting examples. As we can see, meta analysis is a powerful tool in understanding and predicting challenge test results. The downside of this approach however


is clear: large amounts of data are needed to allow for a meaningful bundling and interpretation of results. At evident ingredients, we are constantly working on expanding our PET database to broaden our knowledge on cosmetic preservation with all its complexities and intricacies. We believe in sharing this knowledge, be


it in the present article, or with clients and likeminded companies or institutes in our scientific community. Microbiologically stable and safe cosmetic products for everybody is a realistic and reachable goal – if we work together and not lose ourselves in petty disagreements over who’s preservative blend works best.


The data is here. Let us put it to good use. A ■ B ■ Failed ■ PC E.coli C. albicans A. brasiliensis A ■ B ■ Failed ■


Number of Challenge Tests


Number of Challenge Tests


Number of Challenge Tests


Number of Challenge Tests


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