Plant-Derived By-Products as Multifunctional Ingredients in Sustainable Cosmetic Emulsions: Concentration-Dependent Effects of Orange Peel Microparticles

The introduction of regulations restricting the use of microplastics in cosmetics, combined with the growing demand for sustainable cosmetic formulations and the continuing consumer interest in naturally derived ingredients, increases the need to develop alternative functional materials. The aim of this study was to evaluate the effect of plant-derived microparticles obtained from agro-industrial fruit by-products (orange peel) on the properties of cosmetic emulsions as a function of their concentration. Model emulsions containing 0–5% orange peel microparticles were prepared and analyzed in terms of physicochemical properties (viscosity, pH, color, stability), sensory characteristics, antioxidant activity (DPPH), and their impact on skin biophysical and optical parameters. Results showed that increasing microparticle concentration significantly enhanced emulsion viscosity (from 17,000 to 36,000 mPa·s) and antioxidant activity (from 13% to 77% DPPH inhibition). Application studies revealed a 54% increase in skin hydration and a fourfold reduction in transepidermal water loss (from 24 to 6 g/m2/h) for formulations with the highest concentration. However, concentrations containing 3-5% led to reduced emulsion stability. Although higher concentrations (4–5%) provided stronger antioxidant and skin-related effects, the 2% formulation provided the most favorable balance between physical stability, functional performance, sensory acceptance, and formulation appearance. These findings indicate that natural microparticles derived from orange peel can serve as multifunctional, sustainable ingredients in cosmetic emulsions, providing structuring, antioxidant, and skin-barrier-supporting effects.