Water activity
reduction
to improve microbial stability and strengthen the skin barrier

Cosmetics manufacturers face a number of challenges when developing cosmetic products. A cosmetic formulation should not only be effective and well tolerated, but also remain stable throughout its entire shelf life. Microbial stability plays a particularly important role here, as uncontrolled microbial growth can have serious consequences for consumer health.

Causes and consequences of microbial contamination

Cosmetic formulations often contain sufficient water and nutrients to provide an excellent breeding ground for microorganisms such as bacteria, yeasts and moulds. In addition, they are usually stored for months in warm, humid conditions that are ideal for germ growth. The proliferation of germs is ultimately also favored by the consumer himself, since the cosmetic product is readily taken out of the packaging with fingers and is thus constantly re-contaminated.

On the one hand, microorganisms can cause changes in the colour, odour or consistency of a formulation. On the other hand, they can also be pathogenic and lead to dangerous infections, especially in people with weakened or immature immune systems.^1,2

Current criticism of traditional preservatives

To ensure the quality of a product and its safety for consumers, chemical preservatives have been added to personal care products for decades. Although these are very effective, there are an increasing number of studies reporting that common preservatives cause skin irritation or allergic reactions.^3-5 As a result, consumers are increasingly questioning the safety of preservatives or even rejecting their use in cosmetic products altogether. In order to replace the criticised traditional preservatives, cosmetics manufacturers are increasingly looking for solutions and alternative preservation strategies.

Alternative preservation approaches

A possible alternative is provided by self-preserving products based on the principles of “hurdle technology”. The concept was first developed in the 1970s for the food industry to control microbial growth in food, but can also be transferred to the cosmetics industry.^6,7 It involves an intelligent combination of factors that act as hurdles to the survival of germs. For example, microbial growth can be inhibited by adjusting certain parameters, such as the pH or the water activity of a formulation.⁸

Water activity and its significance for microorganisms

Water activity (a_w) represents a significant hurdle for the survival of microorganisms. In contrast to the water content of a formulation, which indicates the total amount of water including molecularly bound water, the a_wvalue is a measure of the „free“, unbound water in a product. This unbound water is freely accessible to microorganisms and can therefore be used for reproduction. Restricting the availability of unbound water in turn leads to a deterioration in the conditions for microbial growth. In the literature, a_w-Limit values described below which microbial growth should be inhibited (see table).⁹Some microbial species, such as, Pseudomonas aeruginosa prefer humid environments, which means that even a slight aw reduction can restrict their growth._w-value reduction can limit bacterial growth. Aspergillus brasiliensis moulds, on the other hand, are less susceptible and only become active at lower a_w-values in growth.

a_wLimits for the growth of pathogenic germs⁹

Water activity reduction

Pure water has an a_wvalue of 1. By adding solutes (hygroscopic substances such as polyols, amino acids, sugars or salts), water is bound due to intermolecular interactions and made inaccessible to germs. Accordingly, the a_wthe water activity is being reduced. However, in order to achieve such a low a_win a cosmetic formulation that microorganisms are hindered in their growth, high amounts of water-binding substances are required. For this reason, in our approach (link to article: https://doi.org/10.3390/cosmetics9030053) we have combined various hygroscopic substances that are not only suitable for lowering the a_wbut also have a benefit for the skin as humectants. Furthermore, the Review of Cosmetic Ingredients (CIR) recommended maximum concentrations of the respective substances are taken into account.

Alternative preservation strategy simultaneously strengthens the skin barrier

The reduction of the a_wvalue of an O/W emulsion by adding a solute combination of sodium lactate, Cosphaderm® Propanediol natural, Erythritol, betaine and sodium PCA lead to a significant improvement in microbial stability. In preservation stress tests, which are used to assess antimicrobial effectiveness, a significant reduction in potentially pathogenic germs was observed. In addition, the solute combination acts synergistically with our natural Multifunctionals Cosphaderm® GMCY and Cosphaderm® Magnolia Extract 98. Even a small amount of multifunctionals is sufficient to achieve reliable antimicrobial protection of the formulation.¹⁰

To protect the skin from dehydration, we have used a_w-reducing solutes on endogenous components of the Natural Moisturising Factor (NMF) and natural, harmless alternatives. In our study¹⁰ the a_wlowered formulation, containing the solute combination, was found to enhance the skin barrier function: the water content of the stratum corneum (SC hydration) significantly increased and transepidermal water loss (TEWL) significantly decreased (see figure).

Study results of TEWL and SC hydration measurements on the treated forearm (twice-daily application of the a_w-reduced formulation) and on the untreated side at two measurement points: start of the study (baseline; control values) and after 4 weeks. The * symbols indicate statistically significant results p < 0.05.¹⁰.

Our approach of reducing water activity using humectants has thus resulted in the development of an alternative, natural and modern preservation strategy with a positive effect on the skin barrier, offering the possibility of replacing traditional preservatives, which are increasingly coming under criticism.

Do you have further questions about our water activity reduction strategy or would you like individual advice?

Then we look forward to your call or message by email.

Sources:

1. Wilson, L.A.; Ahearn, D.G. Pseudomonas-induced corneal ulcers associated with contaminated eye mascaras. J. Ophthalmology. 1977, 84, 112–119.
2. Madani, T.A.; Alsaedi, S.; James, L.; Eldeek, B.S.; Jiman-Fatani, A.A.; Alawi, M.M.; Marwan, D.; Cudal, M.; Macapagal, M.; Bahlas, R.; et al. Serratia marcescens-contaminated baby shampoo causing an outbreak among newborns at King Abdulaziz University Hospital, Jeddah, Saudi Arabia. Hospital Infections. 2011, 78, 16–19.
3. de Groot, A.C.; White, I.R.; Flyvholm, M.-A.; Lensen, G.; Coenraads, P.-J. Formaldehyde releasers in cosmetics: relationship to formaldehyde contact allergy. Contact Dermatitis. 2010, 62, 2–17.
4. García-Gavín, J.; González-Vilas, D.; Fernández-Redondo, V.; Toribo, J. Allergic contact dermatitis in a girl due to several cosmetics containing diazolidinyl urea or imidazolidinyl urea. Contact Dermatitis. 2010, 63, 49–50.
5. Carvalho, R.; Maio, P.; Amaro, C.; Santos, R.; Cardoso, J. Hydrogel Allergic Contact Dermatitis and Imidazolidinyl Urea/Diazolidinyl Urea. Ocular Toxicology. 2011, 30, 331–332.
6. Ballamwar, K.; Sahasrabuddhe, S.; Chafle, K. A Review: The Hurdle Technology—Self-Preservation Technology in Cosmetics. International Journal of Scientific Research Publications (IJSRP) 2020, 10, 820–827.
7. Varvaresou, A.; Papageorgiou, S.; Tsirivas, E.; Protopapa, E.; Kintziou, H.; Kefala, V.; Demetzos, C. Self-Preserving Cosmetics. J. Cosmet. Sci. 2009, 31, 163–175.
8. Leistner, L. Basic Aspects of Food Preservation by Hurdle Technology. J. Food Microbiol. 2000, 55, 181–186.
9. Kerdudo, A.; Fontaine-Vive, F.; Dingas, A.; Faure, C.; Fernandez, X. Optimisation of Cosmetic Preservation: Water Activity Reduction. J. Cosmet. Sci. 2015, 37, 31–40.
10. Nadarzynski, A.; Scholz, J.; Schröder, M.S. Skin Barrier Enhancing Alternative Preservation Strategy of O/W Emulsions by Water Activity Reduction with Natural Multifunctional Ingredients. Cosmetics 2022, 9, 53. https://doi.org/10.3390/cosmetics9030053
11. Image: https://de.freepik.com/vektoren/wasser-hintergrund“>Water background vector created by upklyak – en.freepik.com