Sustainable Conversion of Waste Robusta Coffee Husk into Tea: Optimization, Properties and Diffusion Kinetics

Abstract

This study presents a sustainable approach to upcycle Robusta coffee husk (RCH), a major agro-industrial residue into a functional instant tea product. By combining optimized roasting and aqueous extraction, the natural polyphenols, organic acids, and aromatic precursors of RCH were effectively recovered and stabilized. Response surface methodology guided the selection of mild processing conditions that enhanced antioxidant potential while maintaining sensory quality. Microscopy, FTIR and headspace GC-MS analyses revealed that roasting transformed the dense lignocellulosic matrix into a porous, extractable structure and generated desirable aroma compounds through Maillard and caramelization reactions. The resulting extract was formulated into an instant tea powder with balanced flavor, high solubility, and microbial safety, demonstrating its applicability as a natural beverage or functional ingredient. Diffusion-kinetic modeling of TPC release fitted Fick’s second law at 35–45 °C (R² > 0.96), with diffusion coefficients ranging from 1.73 × 10⁻⁹ to 3.19 × 10⁻⁹ cm² s⁻¹. This work highlights the underexplored potential of coffee husk as a sustainable bioresource for food applications, contributing to waste reduction and circular bioeconomy development.