High Solid Loading in Dilute Acid Hydrolysis of Orange Peel Waste Improves Ethanol Production

Abstract

Orange peel waste was converted into ethanol by consecutive acid-catalyzed steam-explosion (ACSE), enzymatic saccharification, and fermentation with Saccharomyces cerevisiae F15. With the aim of increasing the concentration of fermentable sugars in the hydrolysate and to fully recycle the spent acid liquor as the suspending medium for saccharification, the technical feasibility of increasing the solid loading in the ACSE step from 160 to 480 g L⁻¹ was assessed. At high solid loading in the ACSE pretreatment (HSLAP), the solubilization degrees of polysaccharides were lower while those of potential inhibitors (e.g., acetic and formic acids and phenols) were generally higher than those found at low solid loading (LSLAP). However, residual solids from both solid loadings showed similar susceptibility to enzymatic saccharification (ES) in a 7-L stirred-tank reactor (STR) (cellulase, 12 FPU g⁻¹ cellulose; pectinase, 25 IU g⁻¹ dry matter; 72 h incubation at 50 °C). Fermentation was performed in a 1-L STR along five repeated batches with the first one being used to enable yeast proliferation. By using the hydrolysates arising from the HSLAP-ES combination, it was possible to rely on a fermentation medium with a 2.5-fold higher concentration of simple sugars and to double the ethanol concentration in the final beer to be distilled of. However, the higher content of inhibitory compounds in hydrolysates from HSLAP-ES than in the LSLAP-ES ones led to a reduction in the ethanol yield per unit substrate consumed (0.49 vs. 0.41 g g⁻¹, respectively) and overall productivity (3.4 vs. 2.7 g h⁻¹, respectively).