(167c) Evaluation of Hydrothermal Pretreatment and Fermentation Processes to Improve Lipid and Ethanol Production from Corn Germ Meal, a Model for Lipid Producing Energy Crops

Biodiesel, an energy-dense biofuel produced from vegetable oils, supports comparable engine performance while emitting less particulates, hydrocarbons and carbon monoxide (Graboski & McCormick, 1998). The high costs and low yields of oil feedstocks have been slowing down the pace to commercially produce biodiesel at a large scale. The Center for Advanced Bioenergy & Bioproducts Innovation (CABBI) has been developing high yielding energy crops with the capability to accumulate lipids, thus providing an opportunity for high vegetable oil production per unit land. However, these crops are still under development. Bioethanol and biodiesel can be produced simultaneously from lipid producing energy crops: bioethanol is derived from cell wall saccharides and biodiesel is derived from vegetable lipids. These lignocellulosic feedstocks are highly recalcitrant due to its complex interconnected structure of cellulose, hemicellulose and lignin; thus, the efficient production depends on recovery of lipids and hydrolysis of cell wall saccharides. A high lipid concentration also economically and technically benefits the oil extraction process. The objective of this work was to use the hydrothermal pretreatment on biomass for lipid enrichment and to investigate how lipid profile changed during simultaneous saccharification and co-fermentation (SSCF). Hydrothermal pretreatment, also known as liquid hot water pretreatment (LHW), treats biomass with liquid hot water at high temperatures (160 to 240 °C) without an added catalyst, generating fewer inhibitors compared to other pretreatment methods (Mosier, 2013). LHW solubilize hemicellulose and partially remove lignin, increasing the oil concentrations and improving the saccharification of pretreated solids. SSCF, a common method to produce ethanol from lignocellulosic feedstocks, is a process where enzymatic hydrolysis and fermentation are combined to co-ferment xylose together with glucose. This work investigated the effect of SSCF on lipids recovery and lipids composition from biomass.

The lipid producing energy crops are still being developed and availability of these crops is limited. Corn germ meal, the solid residues from corn germ after oil extraction in a wet milling process, was used as a model feedstock in this work. It contained 31.0 % cellulose, 22.4% hemicellulose, 13.5% lignin, and 2.3% oil. To investigate the optimal condition for liquid hot water pretreatment (LHW), the germ meal was pretreated with hot water at 20% solid loading at 160 and 180°C for 10 and 15 minutes. Composition analysis of raw and pretreated germ meal was performed using standard National Renewable Energy Laboratory (NREL) protocols. The oil concentrations in raw germ meal and pretreated solids were determined using solvent extraction techniques with hexane, isopropanol and sodium sulfate solution (6.7%, w/v) (Huang et al., 2017). The extracted oil was analyzed for measurement of polar and non-polar lipids and their compositions. Raw germ meal and pretreated solids were hydrolyzed with cellulase and hemicellulase enzyme to determine the effect of pretreatment on cellulose and hemicellulose conversion. Through pretreatment, the oil concentrations in pretreated solids increased by 2.2 to 4.2 fold and 63 to 100% of hemicellulose were hydrolyzed and removed from the pretreated solids. Lipid yield (mg of lipid/ g of raw biomass) was increased by 1.1 to 1.5 fold. The most severe pretreatment condition of LHW, at 180°C for 15 min, gave the maximum oil concentration (9.7%, w/w), the maximum lipid yield (34.0 mg of lipid/g of raw biomass) and the highest conversions of glucose and xylose (99.0% and 32.8%, respectively). Lipid content of pretreated corn germ meal increased with more severe pretreatment, implying a similar trend for lipid producing energy crops.

To investigate how SSCF impacts oil content, the raw and pretreated samples were simultaneously hydrolyzed with a mixture of cellulase and hemicellulase, and co-fermented with a C5/C6 fermenting yeast. Each sample was co-fermented for 96 h and at various yeast concentrations to study the effect of residence time and yeast concentration on oil recoveries and ethanol yields. Ethanol yields were computed. After evaporating ethanol from the fermented broth, the lipids were extracted using organic solvents extraction (Huang et al., 2017). Lipid recovery efficiency was calculated. The extracted oil was analyzed for the composition of polar and non-polar lipids.

LITERATURE CITED

Graboski, M. S., & McCormick, R. L. (1998). Combustion of fat and vegetable oil derived fuels in diesel engines. Science, 24(97), 125–164.

Huang, H., Moreau, R. A., Powell, M. J., Wang, Z., Kannan, B., Altpeter, F., ... Singh, V. (2017). Evaluation of the quantity and composition of sugars and lipid in the juice and bagasse of lipid producing sugarcane. Biocatalysis and Agricultural Biotechnology, 10(March), 148–155. https://doi.org/10.1016/j.bcab.2017.03.003

Mosier, N. S. (2013). Fundamentals of Aqueous Pretreatment of Biomass. Aqueous Pretreatment of Plant Biomass for Biological and Chemical Conversion to Fuels and Chemicals, 129–143. https://doi.org/10.1002/9780470975831.ch7