Optimization of Microwave-Assisted Inorganic Salt Pretreatment for Production of Fermentable Sugars from Spent Coffee Ground

Spent coffee ground (SCG) is a solid waste that is generated in the coffee brewing process for coffee beverage production. SCG hold a great potential in reducing sugar production as it consists of high amount of carbohydrates. However, SCG is a lignocellulosic biomass (LCB) which requires pretreatment to degrade the lignocellulosic structure and enhance enzyme accessibility during saccharification process. Hence, this research aims to study the performance of microwave-assisted inorganic salt pretreatment for generation of reducing sugars. Different concentrations of NaCl was applied to determine their effects on reducing sugar produced as well as pH, and solid recovery. Pretreatment with 3% NaCl was found to yield the highest reducing sugar concentration of 43.56 mg/mL, hence it was selected for the subsequent optimization study. Process optimization was designed by using the Response Surface Methodology approach (RSM) with Central Composite Design (CCD), based on three pretreatment parameters; solid to liquid ratio (1:50 - 8:50), microwave power (300W – 800W) and irradiation time (1-10 minutes). The optimized conditions were achieved at solid to liquid of 8:50, microwave power of 800 watt and irradiation time of 10 minutes for a maximum response of 40.89 mg/mL. Moreover, it was observed that microwave-assisted NaCl pretreatment had significantly caused surface morphological changes of the SCG and the removal of functional groups in lignin, resulted in increment of the crystallinity value. In conclusion, microwave pretreatment is a promising green technology for fermentable sugar production from SCG.