Optimization of cascade low-pressure steam heating and organic acid pre-treatment on pineapple wastes for fermentable sugars production

Abstract

Sugar synthesized from lignocellulosic biomass can potentially supplement the increasing demand of various applications in industries. Pre-treatment of lignocellulosic biomass is challenging due to the need to break the rigid and compact structure to produce sugar. In this study, pineapple waste (PW) were subjected to a cascade pre-treatment of (i) low pressure steam heating (LPSH) and (ii) maleic acid (MA), which aims to improve delignification, increase enzyme accessibility to cellulose and hemicellulose in the PW while reducing the production of inhibitors. The best conditions for pressure (kPa), solid loading (% w/v) and time (min) of LPSH were screened. The resulting solid biomass from LPSH were proceeded to subsequent pre-treatment of MA and optimized by response surface methodology-based Box-Behnken design, where the influence of pre-treatment temperature, acid concentration and time were studied. A total of 68 % w/w delignification with high hemicellulose removal (79.5 %) were achieved while 77.6 % cellulose was retained in the solid residue after cascading both pre-treatments. No 5-hydroxymethyl furfural (5-HMF) and acceptable amount of furfural (1.8 g/L) were detected in the hydrolysate by High Performance Liquid Chromatography (HPLC) analysis, with negligible amount of phenolic content (0.01 g/L) was observed. In comparison, the PW pre-treatment with combined LPSH and conventional sulphuric acid (H2SO4) produced 3.6 g/L of furfural and 0.4 g/L of 5-HMF, at similar optimized conditions. The pre-treated PW were further characterized by scanning electron microscopy and Fourier transform infrared spectroscopy to elucidate structural morphology and functional group changes. The optimized cascade pre-treatments can provide up to 54.79 % of glucose yield and 69.23 % of xylose yield. Furthermore, 67.87 % reduction of lignin content from cascade pre-treatment can substantially enhance the glucose yield up to 95.76 % and xylose yield up to 99.07 % during enzymatic hydrolysis using the mixture of cellulase and hemicellulose. This study shows that the cascade pre-treatment of LPSH and MA can decompose the lignin structure of the biomass with a negligible amount of inhibitors and enhance the effectiveness of enzymatic treatment.