Microbial-based separate and simultaneous saccharification and ethanol fermentation of poplar (Populus euramericana) substrate

The present study reports a simple all microbial-based consolidated bioprocess for the conversion of poplar parings into ethanol. The bacterium Bacillus cereus G9241 CP 026,376 and the yeast Candida tropicalis MF 289,181 were employed for this purpose. Potential of the cellulolytic B. cereus for saccharification of the substrate and bioethanol fermentation was unveiled by employing the separate hydrolysis and fermentation (SHF) as well as simultaneous saccharification and fermentation (SSF) processes. The bacterium as well as yeast grew well in the 2% substrate hydrolyzate and the cell counts reached up to 19,701 × 107 and 852 × 106/ml at 96 h, respectively. For the bacterially fermented saccharified substrate, the HMF content reduced from 489 ± 38.1 µg/L at 24 h to 232 µg/L (52.56%) at 96 h. In case of the yeast fermented poplar hydrolyzate, the HMF content dropped down to 351.3 ± 48.6 µg/L right at first sampling point and thereafter became un-detectable. In case of SSF, HMF could appear only at 72 and 96 h of fermentation with respective values of 260.3 ± 25.8 and 243 ± 8.66 µg/L. Acetic acid content among the differently fermented substrates ranged from 460 ± 230 to 5360 ± 503 mg/L. The bacterial cellulases yielded up to 6.742 and 8.561 mg/ml of glucose and xylose of 2% substrate, respectively. In case of yeast monoculture, the glucose and xylose contents reduced down to 34.17% and 85.28%, respectively, at 24 h post-inoculation with concomitant ethanol production of 634 ± 159 mg/L. Following 24 h of co-culturing of the microbes in the substrate hydrolyzate, the glucose and xylose reduced down to 39.69% and 82%, respectively, with accompanying ethanol level of 501.38 ± 46.7 mg/L. Glucose content of 24 h incubated SSF fluids was 1568 ± 226 mg/L, whereas the xylose remained non-detectable throughout the study period. Ethanol productions at 24, 48, 72 and 96 h of incubations for the SSF experiment were 140.43 ± 44.8, 60.18 ± 13.5, 177.78 ± 23.9 and 83.48 ± 10.3 mg/L, respectively. The simple experiments reported here provide a workable model to assess the potential of suitable microbes for bioethanol production from plants’ biowastes by a consolidated bioprocess without any drastic pretreatment.