Microwave-assisted Hydrothermal Hydrolysis of Maltose with Addition of Microwave Absorbing Agents

Hydrolysis of polysaccharides is one of the important steps for biorefinery. This paper demonstrates effects of microwave heating on hydrothermal hydrolysis of carbohydrate by comparing microwave and induction ovens. Identical thermal history could be obtained by high heating rate of both methods. To have higher sensitivity for detection of microwave effects, maltose was used as the simplest substrate. Hydrothermal hydrolysis was conducted at heating temperatures at 180 °C to 220 °C for 0 - 30 min of heating time with 4 min of come-up time. These heating profiles were precisely traced by using PID-aided temperature control. Hydrothermal hydrolysis of maltose started above 180 °C. Higher saccharification rate was achieved at higher temperature, however, degradation of glucose became predominant at 220 °C. Highest saccharification rates were almost the same by both heating methods, however, glucose yield was relatively higher at severer condition in microwave heating showing higher stability of glucose under electromagnetic field. To improve the microwave-assisted hydrothermal hydrolysis, several kinds of microwave absorbing agents including salt, metal oxide and graphite were tested. From the screening, halide salts (LiCl, LiBr, NaCl, NaBr, KCl, KBr) improved the reaction and the highest saccharification rate attained around 70% at lower temperature (200 °C) with salt concentration of 1% w/w. There was no significant difference among kinds of halide salts with different alkali metals. Sulfate, metal oxides and graphite did not show good effect. By monitoring the microwave outputs, the additions of halide salts were also found effective for reducing the total energy consumption. Increase in heating temperature required higher energy consumption when the water was used for medium, however, addition of sodium chloride at 1% w/w decreased the total energy consumption down to 60 - 70%. The results suggested that assistance of salts may be effective to improve hydrothermal hydrolysis of biomass under electromagnetic field. This method may be usable for salt containing biomass such as marine biomass.