Enhanced Ethanol Production from Pomelo Peel Waste by Integrated Hydrothermal Treatment, Multienzyme Formulation, and Fed-Batch Operation

Pomelo peel is an abundant pectin-rich biomass waste in China and has the potential to serve as a source of fuels and chemicals. This study reports a promising way to deal with pomelo peel waste and to utilize it as raw material for ethanol production via simultaneous saccharification and fermentation (SSF). An integrated strategy, incorporating hydrothermal treatment, multienzyme formulation, and fed-batch operation, was further developed to enhance the ethanol production. The results show that hydrothermal treatment (120 degrees C, 15 min) could significantly reduce the use of cellulase (from 7 to 3.8 FPU g(-1)) and pectinase (from 20 to 10 U g(-1)). A multienzyme complex, which consists of cellulase, pectinase, beta-glucosidase, and xylanase, was also proven to be effective to improve the hydrolysis of pretreated pomelo peel, leading to higher concentrations of fermentative sugars (36 vs 14 g L-1) and galacturonic acid (23 vs 9 g L-1) than those with the use of a single enzyme. Furthermore, to increase the final ethanol concentration, fed-batch operation by adding fresh substrate was employed in the SSF process. A final solid loading of 25% (w/v), which is achieved by adding 15% fresh substrate to the SSF system at an initial solid loading of 10%, produced 36 g L-1 ethanol product in good yield (73.5%). The ethanol concentration is about 1.73-fold that at the maximum solid loading of 14% for batch operation, whereas both of them have a closed ethanol yield. The results indicate that the use of the fed-batch mode could alleviate the decrease in ethanol yield at high solid loading, which is caused by significant mass transfer limitation and increased inhibition of toxic compounds in the SSF process. The integrated strategy demonstrated in this work could open a new avenue for dealing with pectin-rich biomass wastes and utilization of the wastes to produce ethanol.