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

•Pongamia pinnata seed residue represents a promising source for ethanol production.•Effect of H2SO4, HCl and H3PO4 on P. pinnata seed residue hydrolysis was studied.•Acid treatment for 60 min was used for hydrolysis of P. pinnata seed residue.•41% of theoretical maximum ethanol was formed from the seed residue conversion.

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.