DRYING KINETICS AND BIOACTIVE COMPOUND RECOVERY FROM COFFEE GROUNDS USING INFRARED RADIATION: A PATHWAY TOWARD CIRCULAR BIOECONOMY

Coffee grounds, generated in large quantities by industry and domestic consumption, represent an environmental liability when disposed of improperly. However, they are a promising source of bioactive compounds, such as phenolics and flavonoids, whose recovery through drying and extraction processes contributes to the circular economy and reduction of environmental impacts. This study investigated the drying kinetics of coffee grounds (DKG) subjected to drying with and without infrared (IR) radiation, using inlet air temperatures of 313, 333, and 353 K. Experimental drying data were used to construct moisture loss curves and fitted to the Page and Fick models using Statistica software (version 9.0). The effective moisture diffusivity (Deff) was estimated using the one-dimensional flat-plate diffusion model, with an average sample thickness of 0.51 ± 0.11 cm. Deff values ranged from 1.6× 10⁻¹¹ to 3.4 × 10⁻1¹ m²/s, increasing with temperature and with the application of IR radiation. The highest diffusivity was observed for conduction drying with IR at 353 K, confirming the enhancement of internal mass transfer by radiative heating. The Page model adequately described the drying behavior under all tested conditions. Dried DKG samples were used for the extraction of phenolic compounds, flavonoids, and pigments, as well as for antioxidant activity assays. Extractions were performed in a shaking bed (297 K, 150 rpm, 60 min), applying the same procedure and the same solvent mixtures for all extracts. In each extraction, 50 mL of methanol:water (50:50, v/v) and 50 mL of acetone:water (70:30, v/v) were used. After centrifugation at 5000 rpm for 15 minutes, the supernatants were combined and completed with distilled water to a final volume of 100 mL. The highest total phenolic content was obtained in convection drying without IR at 313 K (263.94 ± 9.20 mg GAE/g dry basis), while drying with IR at 313 K showed the highest flavonoid content (171.34 ± 6.92 mg CE/g dry basis). These results demonstrate that the drying behavior of coffee grounds can be effectively described by a simple mathematical model and highlight the potential of infrared-assisted drying as a promising approach for the sustainable valorization of this agroindustrial waste.