Valorization of coffee industry residues by subcritical water hydrolysis: Recovery of sugars and phenolic compounds

Two abundant coffee waste residues (powder and defatted cake) were treated using subcritical water (SubCW) for hydrolysis and extraction of reducing sugars (RS), total reducing sugars (TRS), and total phenolic compounds (TPC) under semi-continuous flow conditions. The flow-through process was carried out at 150, 175, 200 and 250 degrees C, with a water flow of 10 mL/min and reaction pressures of either 22.5 or 30 MPa. For treated coffee powder, the maximum observed sugar recovery was 6.3% for RS (150 degrees C and 30 MPa) and 9.0% for TRS (150 degrees C and 30 MPa). The maximum TPC recovery was 26.64 mg GAE (Gallic Acid Equivalent)/g powder coffee, observed at 200 degrees C and 22.5 MPa. For the defatted coffee cake, the maximum sugar yields were 8.79% and 17.23% for RS and TRS; both observed at a treatment temperature of 175 degrees C. The highest TPC yield was 55.31 mg TPC GAE/g defatted coffee cake, also at 175 degrees C. HPLC was used to quantify specific carbohydrates (arabinose, cellobiose, glucose, and xylose), 5-hydroxy-methyl-furfural (5-HMF) and furfural in both coffee waste hydrolyzates, providing evidence of thermal degradation of the coffee carbohydrates. Scanning electron microscopy of the treated samples revealed particles deposited on the surface and other signs of physical degradation of the biomass structure. Fourier Transform Infrared Spectroscopy of the residues revealed that the density of surface bound acid groups increased with increasing treatment temperature. The results presented here provide a basis for the use of subcritical water to obtain reducing sugars and phenolic compounds from coffee residue. (C) 2016 Elsevier B.V. All rights reserved.