Concentration-Dependent Rheological and Sensory Effects of Walnut Leaf Extract in Cosmetic Emulsion Creams

Background/Objectives: Understanding how plant-derived extracts influence the rheological and sensory behavior of emulsions is crucial for developing stable and consumer-appealing formulations. Although walnut leaf extract (Juglans regia L.) is recognized for its bioactive properties, its structural impact on cosmetic emulsions has not been systematically characterized. This study aimed to investigate the effect of increasing walnut leaf extract concentration on the rheological profile, mechanical integrity during application, and sensory performance of oil-in-water creams. Methods: Four emulsion formulations (F1–F4) containing 0%, 1%, 3%, and 5% walnut leaf extract were prepared using Olivem 1000 and Olivem 300 as emulsifiers. Rheological measurements included amplitude sweep, flow curve, frequency sweep, and thixotropy tests to assess viscoelasticity, flow behavior, and recovery. A sensory evaluation was conducted by trained panelists to correlate rheological parameters with perceived product attributes. Results: All formulations exhibited pseudoplastic, shear-thinning behavior in well-structured cosmetic emulsions during application. The addition of walnut extract significantly modified rheological responses: at 1% concentration, an increase in storage modulus (G′) and shear-thinning ratio (η0/η∞) indicated structural reinforcement and improved spreadability, whereas higher concentrations (3–5%) led to structural softening and faster thixotropic recovery. The frequency sweep revealed a concentration-dependent shift from elastic- to viscous-dominant behavior. Sensory analysis confirmed these trends, with higher extract levels reducing stickiness and greasiness while enhancing absorption. Conclusions: Walnut leaf extract shows a concentration-dependent influence on the rheological behavior of the emulsions, strengthening the network structure at low levels while promoting softening and faster structural recovery at higher concentrations. The strong correlation between rheological and sensory parameters underscores the potential of walnut extract as a multifunctional ingredient for designing well-structured, non-greasy, and consumer-preferred cosmetic creams.