As the production of soybeans has increased, the resulting byproducts of soybeans generated during its processing have also increased. In general, 1 ton of soybean residues (at 10% moisture content) are generated with 820 kg of soybean (30 bushels) production, which is lower than corn or grain sorghum (40 bushels, 1,090 kg) but higher than wheat (20 bushels, 545 kg) (Wortmann et al., 2012). Some soybean straw is potentially utilized as agricultural fertilizer, animal feedstock, and energy resource for rural energy; however high lignin (>20% w/w in straw) and anti-nutritional components in waste restrict their direct use. For better utilization of soybean wastes, an efficient pretreatment step is required to alleviate biomass recalcitrance because of the complex crystalline structure between cellulose-hemicellulose-lignin in the lignocellulosic biomass, including soybean straw (Kim et al., 2022, 2018). It is well-established that a pretreatment process can facilitate biomass saccharification by solubilizing hemicellulose and/or lignin and disrupting the crystalline structure of cellulose (Cao et al., 2015; Kim et al., 2016). Previous studies with different pretreatments such as supercritical water (Vedovatto et al., 2021), ball milling (Liu et al., 2016), and alkaline and acid pretreatments (Qing et al., 2017) were shown to disrupt the crystalline structure of soybean straw and enhance its enzymatic hydrolysis. Unutilized soybean residuals are often discarded/burned onto land or water which causes environmental issues with their high biochemical oxygen demand and toxic compounds; therefore, effective utilization of these residues is required (Liu et al., 2015).

: This project investigates the feasibility of using an agricultural feedstock, soybean waste, to produce lactic acid. The pretreatment of soybean straw affects solubilizing hemicellulose and lignin, disrupting cellulose structure that increases sugar yields by allowing more enzyme accessibility. The following enzymatic hydrolysis of oligosaccharides in the pretreated solids into monomeric sugars with co-fermentation to lactic acid will be explored. A genetically engineered strain of Lactobacillus pentosus will be employed to improve the yield of soluble 5-carbon and 6-carbon sugars.

Screening enzyme candidates and determination of the best enzyme mixture: It has been shown that low enzyme loadings are available to release monomeric sugars from low lignin agricultural biomass such as corn pericarp (Kim et al., 2017) or pretreated lignocellulosic materials using liquid hot water (Cao et al., 2015; Kim, 2018; Kim et al., 2016). The physical properties of the pretreated cellulose or low lignin sample were shown to be altered and mechanisms were proposed that led to a leveling in particle size during enzyme hydrolysis of lignin-free, microcrystalline cellulose. This approach utilized enzyme mixtures that carried out hydrolysis for hours and suggested that enzyme penetration of the cellulose or polysaccharide
was the likely mechanism. The mixture of cellulase and hemicellulase preparation is effective in cleaving the linkage of polysaccharide and cellulose at low enzyme loadings. The highest yield of sugars is the objective of this research and will be recovered and fermented into lactic acid. The PI, Dr. Kim, and his lab have significant experience with enzyme activity measurement and formulation for different applications. Cellulase and ß-glucosidase activities will be measured using procedures from the International Union of Pure and Applied Chemistry (IUPAC). All other enzyme activities will be measured following previous reports in the literature (Dien et al., 2008; Kim et al., 2017).

Optimization of lactic acid production: Over the years, the PIs’ group has developed several methods to enhance enzymatic saccharification yields and subsequent microbial fermentations for lactic acid production. In addition to measuring the properties of soybean straw hydrolysate and increasing the lactic acid yield, we are interested in understanding the co-fermentation of C5 and C6 monomeric sugars using the co-fermenting L. pentose strain. We intend to use the conditions identified for the raw material preparation described above. We will optimize the enzyme saccharification and microbial fermentation in the flask levels. The microbial fermentation of the C5 and C6 sugars will be accomplished for fermentation processing. The pre-cultured L. pentose strain will be added before fermentation; the consumption and production of sugars and lactic acid will be monitored and determined using HPLC promptly.

Updated September 18, 2024:

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Our interests are complementary to those of the soybean grower and industries closely related to soybean processing and production. We try to convert the lignocellulosic materials into simple sugars for subsequent fermentation or chemical conversion to biochemicals and bioproducts (e.g., lactic acid). We believe that soybean biorefineries with diversified product portfolios offer great potential for Maryland soybean farmers and sugar producers to capture the added value and obtain a higher return on investment while achieving energy and economic goals simultaneously.