Sodium Lactate is the salt of lactic acid and is often used in both cosmetics and food as a preservative (Huang and Huang). It’s generally considered safe as a food additive and has been shown to extend the shelf life of everything from breads to meats. This is because it has been found to have antimicrobial properties.
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It’s an antioxidant synergist and bodying agent, meaning that in a cosmetic product, it will work with antioxidants that are included as preservatives and that it can be used as a thickener in the formulation (FAO).
In addition to serving a functional antimicrobial purpose, sodium lactate can serve as a humectant moisturizer, as it’s naturally found in the skin. It’s also a pH adjuster, bacteriostatic agent, and a hardening agent that helps bar soaps retain moisture (Lotion Crafter).
The Material Safety Date Sheet ranks it as being potentially hazardous to eyes and skin (MSDS).