Study of subcritical water scale-up from laboratory to pilot system for brewer’s spent grain valorization

Abstract

The feasibility of an industrial-scale subcritical water (SW) system by scaling up from laboratory to pilot scale in a discontinuous operation mode has been investigated regarding the valorization of brewer´s spent grain (BSG). The subcritical water treatment (SWT) of BSG at the pilot scale in a discontinuous mode at 170 °C and 22 min, resulted in the release of 56% of the total carbohydrates present in the BSG. A total pentose yield of 78% was achieved (18% as monomer and 82% as oligomer). The concentration of inhibitors in the hydrolysate was relatively low, 0.22 g/L, 0.31 g/L, and 0.13 g/L of furfural and acetic and formic acids respectively. Other high-value compounds were obtained, such as 6.5 g peptides/L (64% protein yield), 21 mg free amino acids/g protein (2.17% aa yield), and a total phenolic content (TPC) of 17.84 mg GAE/g dry BSG. In general, the results showed good reproducibility when scaling up from laboratory to pilot SW process. Good reproducibility of the scale-up was found for the release yield of arabinoxylo-oligomers, and gluco-oligomers from BSG, also for the protein yield and the release of free amino acids. However, the xylo-oligomers yield was 13% higher at the lab scale than at the pilot scale and higher concentrations of monomers and phenolic compounds were found in the SW hydrolysates obtained at the lab scale. Differences in the preheating time in both systems seem to have an effect on the hydrolysis yield of some the biopolymers. SWT modified the composition of the residual solid, as a decrease in the concentration of hemicellulose and an increase in the glucan content was observed, which may enhance the digestibility of the solid improving a further enzymatic release of glucose from the remaining solid. • Subcritical water treatment allows an integral valorization of brewer´s spent grain. • Structural components were recovered as peptides and oligomers by subcritical water. • The protein and amino acids yields were similar at lab- and pilot- scales. • The sum of carbohydrates released as oligomers were similar at both scales. • Heating rate greatly affected the monosaccharides yield and TPC in the hydrolysates.