Optimization of Drying Temperature and Time for Enhancing Phytochemical Composition, Antioxidant Activity, and Sensory Properties of Selected Indigenous Spices in Nigeria

Drying is a critical postharvest operation that influences the composition and functional activity of bioactive compounds in spices. This study investigated the effects of drying temperature and time on phytochemical composition, antioxidant activity, and sensory quality of three indigenous spices (Tetrapleura tetraptera, Xylopia aethiopica, and Monodora myristica) using Response Surface Methodology (RSM). The study adopted a comparative multi-response optimization approach to identify species-specific drying conditions that simultaneously maximize phytochemical retention, antioxidant activity, and sensory quality. A Box–Behnken Design generated seventeen experimental runs, with drying temperature (50–70 °C) and time (24–36 h) as independent variables. Total phenolic, flavonoid, and terpenoid contents were quantified using validated spectrophotometric methods, while antioxidant activity was assessed through 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging and ferric reducing antioxidant power (FRAP) assays. Drying conditions significantly (p < 0.05) influenced all responses, with pronounced species-dependent effects. Xylopia aethiopica dried at 70 °C for 30 h exhibited the highest total phenolic (13.12 mg GAE/100 g) and flavonoid contents (9.26 mg QE/100 g), while Monodora myristica recorded the highest terpenoid content (27.48 mg BSE/100 g) under the same conditions. The highest DPPH radical scavenging activity was also observed in X. aethiopica (16.24 µmol TE/g DW), whereas Tetrapleura tetraptera dried at 70 °C for 36 h exhibited the highest FRAP value (11.67 mg GAE/100 g). Sensory evaluation indicated that T. tetraptera dried at 70 °C for 36 h achieved the highest overall acceptability. The response surface models showed good fit and predictive reliability, with high coefficients of determination and non-significant lack of fit (p > 0.05). Drying temperature and spice type were identified as the major factors influencing the measured responses. These findings demonstrate that optimal drying conditions are species-specific and highlight the value of integrated multi-response optimization for enhancing the functional and sensory quality of indigenous spice powders.