Water activity reduction
as a strategy to improve microbial stability and enhance skin barrier function

When developing cosmetic products, manufacturers are faced with a number of challenges. A cosmetic formulation should not only be effective and well tolerated, but also remain stable over its entire period of use. Microbial stability in particular plays a major role here, as uncontrolled microbial growth can pose serious consequences for consumers’ health.

Causes and consequences of microbial contamination

Cosmetic formulations commonly contain enough water and nutrients to provide an excellent breeding ground for microorganisms such as bacteria, yeasts and molds. In addition, these are usually stored for months in warm, humid bathrooms, providing optimal growth conditions for germs. 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 color, odor or consistency of a formulation. On the other hand, they can also be pathogenic and lead to hazardous infections, especially among humans with a weakened or immature immune system.^1,2

Current criticism on 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 act very reliably, studies are accumulating with reports of commonly used preservatives causing skin irritation or allergic reactions.^3-5 As a result, consumers are increasingly questioning the safety of preservatives or even entirely rejecting their use in cosmetic products. In order to replace the criticized traditional preservatives, cosmetics manufacturers are increasingly looking for solutions and alternative preservation strategies.

Alternative preservation strategies

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 the importance for microorganisms

Water activity (a_w) is an important 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_w value is a measure of the “free”, unbound water in a product. This unbound water is freely available to microorganisms and can therefore be used for reproduction. A restriction of the accessibility of unbound water in turn leads to a deterioration of the initial conditions for microbial growth. In the literature, minimum a_w values are described for various microorganisms, below which their growth is supposed to be inhibited (see table).⁹ Some microbial species, such as Pseudomonas aeruginosa bacteria, prefer humid environments, which means that even a slight a_w reduction can restrict their growth. Aspergillus brasiliensis molds, in contrast, are less susceptible and show impaired growth at lower a_w values.

Minimum a_w values for the growth of pathogenic germs⁹

The minimum a_w values can be considered as a guideline and serve as orientation for the development of cosmetic formulations.

Water activity reduction

Pure water has an a_w of 1. By adding solutes (hygroscopic substances such as polyols, amino acids, sugars or salts), water is bound through intermolecular interactions and made inaccessible to germs. Accordingly, the water activity is being reduced. However, in order to achieve such a low a_w in 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_w, but also have a benefit for the skin as humectants. Furthermore, the maximum use concentrations of the respective substances recommended by the Cosmetic Ingredient Review (CIR) were taken into account.

Alternative preservation strategy simultaneously enhances the skin barrier function

Reducing the water activity of an O/W emulsion by adding a solute combination of Sodium Lactate, Cosphaderm® Propanediol natural, Erythritol, Betaine and Sodium PCA results in a considerable improvement of microbial stability. Challenge tests, which are used to assess antimicrobial efficacy, have shown a remarkable reduction in potentially pathogenic germs. In addition, the solute combination acts synergistically with our natural Multifunctionals Cosphaderm® GMCY and Cosphaderm® Magnolia Extract 98. Even a small amount of the Multifunctionals was sufficient to achieve a reliable antimicrobial protection of the formulation.¹⁰

To protect the skin from dehydration, we focused on endogenous components of the Natural Moisturizing Factor (NMF) and natural, harmless alternatives as a_w-lowering solutes. In our study¹⁰, the a_w-lowered 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 respectively on the treated forearm (2 times daily application of the aw-reduced formulation) and on the untreated side at two measurement time points: Beginning of the study (baseline; control values) and after 4 weeks. The * symbols indicate statistically significant results (p < 0.05).¹⁰

Thus, we have developed an alternative, natural and modern preservation strategy with a positive effect on the skin barrier, which offers the possibility of replacing traditional preservatives.

Do you have further questions about our developed water activity reduction strategy or would you like to receive an individual consultation?

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References

1. Wilson, L.A.; Ahearn, D.G. Pseudomonas-Induced Corneal Ulcers Associated with Contaminated Eye Mascaras. J. Ophthalmol. 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. Hosp. Infect. 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 Dermat. 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 Dermat. 2010, 63, 49–50.
5. Carvalho, R.; Maio, P.; Amaro, C.; Santos, R.; Cardoso, J. Hydrogel Allergic Contact Dermatitis and Imidazolidinyl Urea/Diazolidinyl Urea. Ocul. Toxicol. 2011, 30, 331–332.
6. Ballamwar, K.; Sahasrabuddhe, S.; Chafle, K. A Review: The Hurdle Technology- Self-Preservation Technology in Cosmetics. J. Sci. Res. Publ. (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. Optimization 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. Picture:https://de.freepik.com/vektoren/wasser-hintergrund”>Water background vector created by upklyak – en.freepik.com