Enhancement of Sugar Release from Taro Waste Using Ohmic Heating and Microwave Heating Techniques

Abstract

The enhancement of sugar release from taro waste slurries pretreated by ohmic heating and microwave heating techniques was evaluated, and the potential mechanism of ohmic heating associated with the breakdown of cell structure was investigated. After ohmic and microwave pretreatments, images of the taro tissues were taken using a fluorescent microscope and a field emission scanning electron microscope (FESEM). The glucose yields from the samples pretreated by ohmic and microwave heating were 28.63 ±4.82 and 27.99 ±6.65 g per 100 g dry base weight of taro waste, respectively. Although there was no significant difference between the two glucose yields, ohmic pretreatment needed only 28% of the energy input of microwave pretreatment to produce the equivalent glucose yield. For validation of electrophoretic cell destruction by ohmic heating, slices of taro skin tissues were exposed to the electric field for 10, 20, and 30 s and then stained with fluorescent dye. In the dye diffusion experiment, microscopic images of taro tissues treated by ohmic heating indicated that the dye diffused well into most portions of the taro skin cells. FESEM images also showed that the cell walls of samples after ohmic treatment were completely disintegrated and damaged, and starch granules in the vacuolar interior became exposed to the surroundings for enhanced enzymatic reactions. Results showed that ohmic heating contributed to (1) the release of starch granules from taro waste, leading to higher sugar yields, and (2) enhancement of enzymatic activities for starch hydrolysis within a short incubation period. It can be concluded that the developed ohmic heating pretreatment technique prior to alcohol fermentation could be applicable to other feedstocks, and the oscillating electric field caused the breakdown of the recalcitrant taro cellular structure.