Concentration variation of sodium hydroxide (NaOH) pretreatment and its effect in the lignocellulosic compositions of dried ripe Carabao mango peels

Abstract

Alkaline pretreatment refers to the application of alkaline solutions such as NaOH, Ca(OH)₂, or NH₃ to remove lignin and a part of the hemicellulose, and efficiently increase the accessibility of enzymes to the cellulose. Mango peels are one of lignocellulosic waste materials containing a greater amount of cellulosic properties. In this study, the effect of varying the alkaline concentration in the lignocellulosic compositions of dried ripe Carabao mango peels was determined at varying NaOH pretreatment concentrations: 0.6 M, 0.8 M and 1.0 M. The compositions before and after the pretreatment were compared to determine the best condition to obtain the maximum delignification. Results showed that among the three concentrations, 1.0 M NaOH pretreatment gave the most favorable result. It decreased the amounts of acid soluble lignin (from 7.79% to 2.14%) and extractives (from 41.51% to 7.77%) and increased the amounts of holocellulose (from 32.10% to 72.11%), alpha -cellulose (from 26.27% to 50.47%) and hemicelluloses (from 6.16% to 21.62%). Further evidence showed that the least amount of reducing sugar present in the filtrate which is 3.85 mg glucose/mL was found at 1.0 M NaOH indicating that more cellulosic compositions in the substrate were converted to reducing sugar after pretreatment. The results suggest that at 1.0 M NaOH, mango peels would be best utilized by fermentative microorganisms in any downstream process.