Bioethanol production from non-edible de-oiled Pongamia pinnata seed residue-optimization of acid hydrolysis followed by fermentation

De-oiled Pongamia pinnata seed cake has been gaining attention as a promising feed stock for ethanol production owing to the large amounts of carbohydrates (42% w/w) present in the seed biomass. This, coupled with the potential of seed productivity (>200,000 t annum-1) makes it suitable for the sustainable production of ethanol. The present research explores the application of glucose obtained from acid hydrolysis of the seed cake for ethanol production in a three step process: acid treatment, neutralization and fermentation. The Taguchi robust design of experiments was employed to study the effects of the parameters including acid type (H2SO4, HCl, H3PO4), acid concentration (2-6% w/w), and temperature (80-100 °C) on the formation of glucose. Among the reaction variables considered, acid concentration and temperature showed a positive effect on glucose release from the biomass with HCl the best catalyst compared to H2SO4 and H3PO4 showing highest glucose formation (173.4 g kg-1 seed residue) at 100 °C with 6% w/w HCl concentration. The energy required for this pretreatment was estimated to get an insight into the process energy demand (1080-1110 KJ kg-1 of seed cake). Downstream processing before fermentation included neutralization. Fermentation of hydrolysis product obtained from 2%, 4% and 6% HCl treatments (carried out using Saccharomyces cerevisiae) gave 67.52, 74.98 and 88.62 g ethanol kg-1 dry seed residue, respectively, corresponding to ∼31.45%, 34.92% and 41.28% of theoretical ethanol (214 g kg-1) formation, calculated based on ethanol produced per gram of carbohydrate in the seed residue.