Microwave-Assisted Drying of Indigenous Nigerian Spices: Process Modelling and Quality Optimisation

This study investigates microwave-assisted drying of indigenous Nigerian spices including uziza leaf, uda seed, and utazi leaf, developing process models and optimising drying conditions for maximum retention of volatile aromatic compounds and bioactive constituents. Indigenous Nigerian spices command significant commercial value in domestic and international diaspora markets, but their post-harvest quality is severely compromised by traditional sun drying methods that cause volatile oil losses, colour degradation, and microbial contamination. Microwave drying offers rapid moisture removal with reduced thermal exposure that better preserves heat-sensitive aromatic compounds. This study uses a central composite rotatable design to investigate the effect of microwave power density, initial moisture content, and sample loading on drying time, essential oil retention, total phenolic content, and antioxidant activity. Experiments are conducted using a modified domestic microwave system instrumented for power control and temperature monitoring at the University of Port Harcourt food engineering laboratory. Page, modified Page, and Weibull thin-layer drying models are fitted to drying curve data. Findings reveal that microwave drying reduces drying time by 85 to 92 percent compared to solar drying across all three spices. Optimal microwave power density of 3 to 4 watts per gram achieves maximum essential oil retention of 82 to 91 percent compared to 34 to 47 percent for solar drying. The Page model provides the best kinetic fit for all three spice types. The study recommends microwave drying as the preferred technology for premium quality Nigerian spice production targeting export markets.

Keywords: microwave drying, Nigerian spices, essential oils, process modelling, quality preservation.