Optimization of thermochemical hydrolysis of potassium hydroxide-delignified Cocoa (Theobroma cacao L) pod husks under low combined severity factors (CSF) conditions

Abstract

This study identified the optimal conditions for acid hydrolysis of cocoa pod husks (CPH) pretreated and then hydrolyzed under combined low gravity conditions. Pre-treatment was carried out using 1% potassium hydroxide (KOH), using a biomass to solvent ratio of 9.143% (w/v), for 4 h at 30 °C, then the mixture is autoclaved for 15 min at 128 °C. Lignocellulosic analyzes were carried out on raw and pretreated husk. In order to determine the structural changes operated on the pretreated pod, Fourier transform infrared (FTIR) analyzes were also carried out. These analyzes confirmed a 93.87% removal of lignin after delignification. Furthermore, scanning electron microscopy/energy dispersive X‐ray spectrometry (SEM/EDX) revealed several changes within the pretreated matrix; showing a dominant presence of the elements carbon, oxygen and potassium, associated with an intensification of the peaks of magnesium and potassium. Acid hydrolysis on the pretreated pod was optimized under low combined severities factors (CSF) conditions between 1.11 and 2.45, using response surface methodology. A three-variable Box-Behnken design (autoclaving time (X1), biomass to solvent ratio (X2), and sulfuric acid (H2SO4) concentration (X3)) was used. The quadratic model with an R2 of 0.98 proved to be the most suitable, giving the following optimal hydrolysis conditions: 26.95 min of autoclave, biomass to solvent ratio of 9.53% (w/v) and 4.92% (w/v) H2SO4.18.77 g/L of reducing sugars was obtained for a CSF of 2.06. In addition, a marked decrease in the content of phenolic compounds was observed, shifting from 19.45 g/L in the raw delignified sample to 3.11 g/L in the delignified sample after reduction the extractable compounds. Thus, it is proved in this study that optimization of the acid hydrolysis process of CPH, after delignification with KOH, can improve the acid hydrolysis of polysaccharides, under conditions of low combined severity factors.