Ethanol production from poplar wood through enzymatic saccharification and fermentation by dilute acid and SPORL pretreatments

Dilute acid (DA) and Sulfite Pretreatment to Overcome Recalcitrance of Lignocelluloses (SPORL) pretreatments were directly applied to wood chips of four poplar wood samples of different genotypes (hereafter referred to as poplars; Populus tremuloides Michx. ‘native aspen collection’; Populus deltoides Bartr. ex Marsh × Populus nigra L. ‘NE222’ and ‘DN5’; P. nigra × Populus maximowiczii A. Henry ‘NM6’) to evaluate their bioconversion potential. Plant biomass recalcitrance (PBR) was defined to quantitatively determine the recalcitrance of the poplars. Using DA pretreatment, NM6 produced the lowest bioconversion efficiency with a total monomeric sugar yield of 18% theoretical and an ethanol yield of 0.07 L kg−1 of wood compared with an aspen sugar yield of 47% theoretical and an ethanol yield of 0.17 L kg−1 of wood. Similar comparisons following SPORL pretreatment were 43% versus 55% and 0.11 versus 0.20 L kg−1 of wood for NM6 and aspen, respectively. Bioconversion performance of NE222 and DN5 fell between that of aspen and NM6. While substrate lignin content and lignin removal by pretreatments did not affect substrate enzymatic digestibility, the wood lignin content was found to negatively affect xylan or hemicellulose removal using both DA and SPORL pretreatments. The ability of lignin protecting hemicellulose removal dictates PBR through affecting disk milling energy for size reduction of pretreated wood chips, substrate enzymatic digestibility. The SPORL pretreatment not only improved sugar and ethanol yields over DA for all four poplars, but also better dealt with the differences among them, suggesting better tolerance to feedstock variability.