Optimization of alkaline pretreatment and enzymatic hydrolysis of cocoa pod husk (Theobroma cacao L.) for ethanol production

Abstract

The present work shows the results of optimized alkaline pretreatment and enzymatic hydrolysis processes for the production of bioethanol from cocoa pod husk (CPH). Alkaline pretreatments were applied to evaluate the effect of NaOH concentration, residence time, and temperature using a Central Composite Design (CCD). SEM and XRD methodologies were used to study the structural characteristics of the solid fraction. Enzymatic hydrolysis assays were applied to optimize the enzyme (Cellic Ctec2, Novozyme) and solid loadings. The hydrolysate was subjected to fermentation using the yeast Saccharomyces cerevisiae Y2034. The results showed that the optimal condition for the alkaline pretreatment was 5% (w v−1) NaOH for 30 min at 120 °C, which increased the cellulose content of CPH from 27.68 ± 0.15 (untreated) to 57 ± 0.25%. SEM analysis showed changes in porosity and alterations in the biomass, while the XRD revealed an increase in crystallinity. The enzymatic hydrolysis was optimized to 10 FPU g−1 substrate and solid loading of 10% (w v−1), obtaining yields of 98.75%, equivalent to 66.80 g L−1 reducing sugars. During fermentation, 80.74% of reducing sugars were consumed, producing 18.06 g L−1 of ethanol in 24 h, corresponding to a theoretical yield of 51.45%. The results obtained show the potential of the use of cocoa pod husk as raw material for the production of bioethanol.