Characterization of Black Lime (Citrus × aurantiifolia) Powder and Its Effects on the Physicochemical, Antioxidant, and Sensory Properties of Shortbread Cookies

Black lime (Citrus × aurantiifolia) is a dried key lime product traditionally used in Middle Eastern cuisine, characterized by an intense sour taste, distinctive citrus aroma, and dark color. Owing to its content of dietary fiber and bioactive compounds, particularly phenolics, black lime powder (BLP) may represent a promising ingredient for food enrichment. This study characterized BLP and evaluated the effects of replacing 3–15% of wheat flour with BLP on the physicochemical, antioxidant, spectroscopic, and sensory properties of shortbread cookies. Compared with wheat flour, BLP contained higher levels of dietary fiber, ash, and phenolic compounds, with protocatechuic, p-coumaric, vanillic, and ferulic acids, as well as isovitexin and vitexin, among the predominant compounds. Increasing BLP addition increased ash, soluble and insoluble dietary fiber, total phenolic content (TPC), and radical-scavenging activity. At 15% BLP, total dietary fiber reached 10.53% of cookie dry mass, while TPC and ABTS and DPPH radical-scavenging activities were approximately 5.5-, 11.1-, and 28.9-fold higher than in the control, respectively. These compositional enhancements were accompanied by technological changes, including increased moisture content and water activity, reduced cutting force, darker and less yellow cookies, and changes in starch organization and gluten structure indicated by FTIR analysis. Overall acceptability remained comparable with the control at 3–12% BLP but decreased at 15%, with 6–9% providing the most favorable balance between sensory quality and compositional enrichment. These findings indicate that BLP can be used as a fiber- and phenolic-rich ingredient for shortbread cookie enrichment.