An investigation on citrus peel as the lignocellulosic feedstock for optimal reducing sugar synthesis with an additional scope for the production of hydrolytic enzymes from the aqueous extract waste

Abstract

Abstract In the current study, the use citrus peels as the lignocellulosic feedstock for the production of reducing sugars and hydrolytic enzymes was evaluated. Citrus peels were subjected to a two-step pre-treatment procedure to obtain two major fractions-the water soluble pectin rich fraction and the water insoluble solid fraction enriched with cellulose and hemicellulose. Three parameters (solid loading, acid concentration and time) that were found to be critically influencing the acid hydrolysis process were optimized by using Response surface methodology (RSM) - central composite design (CCD) for the improved synthesis of reducing sugars. A maximal total reducing sugar concentration of 13.34 g/L was predicted for the optimal processing conditions of solid loading = 3.87% w/w, acid concentration = 1% w/w and time = 48.4 min and at the processing temperature of 121 °C in a steam autoclave. Experimental validation studies carried out at these optimal conditions showed a slightly higher reducing sugar concentration of 13.65 ± 0.3 g/L than the predicted value. Further studies demonstrating the use of pectin-rich liquid fraction as the nutrient medium for hydrolytic enzymes production resulted in enzyme activities of 5.38 ± 0.2 IU/mL for pectinase and 1.17 ± 0.1 FPU (Filter Paper Units) for cellulase, without supplementation of any salts to the medium. These results suggest that the pre-treated citrus peels can serve as a potent feedstock for bioethanol production, while by-products resulting from pre-treatment can be employed to produce hydrolytic enzymes such as pectinases and cellulases as demonstrated through this work.