Microwave-Assisted Extraction of Bioactive Compounds and Chemical Characterisation of Coffee By-Products and Coffee

This study reports the effective reuse of coffee residues from roasting and brewing processes using microwave-assisted extraction (MAE) of bioactive compounds. Principal Component Analysis (PCA) showed a significant impact (p < 0.001) on phenolic acid and alkaloid amounts among coffee (green and roasted) and its by-products (silver skin and spent grounds). Furthermore, a linear mixed-effects model identified that, apart from sample type, extraction temperature (50 and 70 °C) and ethanol volume fractions (50 and 70%, v/v) considerably influenced the quantities of bioactive compounds. The best conditions were achieved using 70% (v/v) ethanol at 70 °C. In addition to caffeine and chlorogenic acid, which were found in the highest amounts, the coffee and by-product samples contained high levels of total fibre (20.7–27.8%) and total fat (1.5–12.4%). Fatty acid analysis showed a dominance of oleic acid (7.55–12.7%), palmitic acid (19.3–40.3%), and linoleic acid (15.6–41.8%), with their saturated and unsaturated nature confirmed by Fourier Transform Infrared (FTIR) Spectroscopy. The reported data highlight the potential of coffee by-products as high-value sources for bioactive compound recovery.