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

• Published in: The Journal of supercritical fluids Vol. 120; pp. 75 - 85
• Main Authors: Mayanga-Torres, P.C., Lachos-Perez, Daniel, Rezende, C.A., Prado, J.M., Ma, Z., Tompsett, G.T., Timko, M.T., Forster-Carneiro, T.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.02.2017
• Subjects: Coffee residues; Fermentable sugars; FESEM; FTIR; Phenolic compounds; Subcritical water hydrolysis; Waste-to-energy; Fermentable sugars; Subcritical water hydrolysis; FTIR; FESEM; Coffee residues; Waste-to-energy; Phenolic compounds
• ISSN: 0896-8446; 1872-8162
• DOI: 10.1016/j.supflu.2016.10.015

[Display omitted] •The subcritical water hydrolysis of two coffee residues in subcritical water were studied.•The operational conditions were different temperatures, two supercritical pressures at water flow rate.•Higher yields of reducing sugars and phenolic compounds recovered from residues coffee in subcritical water.•The concentrations of sugars were identified (arabinose, cellobiose, glucose and xylose), 5-hydroxy-methyl-furfural (5-HMF) and furfural in the hydrolysates.•The residual solid was analyzed by scanning electron microscopy. 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°C, with a water flow of 10mL/min and reaction pressures of either 22.5 or 30MPa. For treated coffee powder, the maximum observed sugar recovery was 6.3% for RS (150°C and 30MPa) and 9.0% for TRS (150°C and 30MPa). The maximum TPC recovery was 26.64mg GAE (Gallic Acid Equivalent)/g powder coffee, observed at 200°C and 22.5MPa. 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°C. The highest TPC yield was 55.31mg TPC GAE/g defatted coffee cake, also at 175°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.