Microbial spoilage of cosmetic formulations has always been of special concern for the industry, since it can lead to product degradation or, in the case of pathogens, constitute a threat to consumer safety. Chemical preservatives are added to cosmetics, pharmaceuticals, and foods in order to protect them against microbial contamination. The growing skepticism of consumers regarding the safety of chemical preservatives in combination with the fact that long-lasting skin health is often associated with the use of natural ingredients has led the cosmetics industry to seek alternative approaches for cosmetics preservation. A recent trend in cosmetics preservation is the replacement of traditional chemical preservatives by antimicrobial agents that are not legislated as preservatives according to Annex VI of Commission Directive 1976/768/EEC3-5 but that are safe and effective as preservatives. An approach to achieve preservative-free cosmetics is the selection of natural compounds that have been characterized as safe and effective against microorganisms, in order to decrease or eliminate the use of traditional chemical preservatives and to formulate cosmetics with improved dermo-cosmetic properties, i.e., lower skin irritation and/or contact sensitization. A number of well-known plant-derived essential oils and extracts have exhibited excellent antimicrobial properties; thus, they have been used for the effective preservation of cosmetic formulations. Among others, derivatives of Rosmarinus officinalis, Lavandula officinalis, Pteronia incana, Artemisia afra, Thymus vulgaris, Eucalyptus globulus,12 Laurus nobilis,12 Salvia officinalis,12 and Melaleuca alternifolia13,14 have been reported to be effective natural preservatives. Medium-polar substances also belong to the class of alternative antimicrobial agents. Examples of such agents as caprylyl glycol and monoglycerides of capric acid and caprylic acid, i.e., glyceryl caprate and glyceryl caprylate, besides being moisturizing agents, exert antimicrobial activities. Due to their emulsifier-like structure, with a hydrophilic and lipophilic part, they interfere with the cellular structures of microorganisms and disintegrate cell membranes. Many studies have been reported concerning the use of glyceryl caprylate as an antimicrobial substance alone or in combination with other antimicrobial compounds for cosmetics preservation. We have to keep in mind that the chemical composition of fragrances plays a key role concerning the antimicrobial activity of essential oils and extracts obtained from natural sources. Various aldehydes and alcohols, i.e., aromatic and aliphatic compounds,  terpenes and organic acids, are among the most active compounds. In the past, a fragrance mixture that mainly consisted of benzyl acetate, phenethyl alcohol, and linalool had been proposed as an alternative preservative in order to reduce the number of parabens used in cosmetic formulations. Today, several antimicrobial fragrance ingredients are commercially available, such as levulinic acid (4-oxo-pentanoic acid), which were found in cellulose.

INCI: Levulinic acid