- This dissertation has focused on lignin byproducts that result from the alkali processing of wheat straw. The first study evaluated the application of “standard” lignin quantification methods for summative analyses of alkali-based processes. Mass closure during NaOH processing at 25, 50, and 75°C of wheat straw was evaluated using the acid insoluble lignin (AIL) and acid soluble lignin (ASL) assays (per U.S. National Renewable Energy Laboratory guidance). In general, a fraction of the water and ethanol soluble extractives of straw measure as lignin, hereafter termed artifactual lignin. Actual AIL recovered from the alkali insoluble fraction resulting from alkali processing (AP) of extractive free wheat straw (EFS) at 25, 50 and 75°C was 64, 48, and 34% of the AIL in the original wheat straw, respectively. The percent overestimation of AIL in the insoluble phase from AP of native wheat straw (NS) decreased with increasing pretreatment severity. The trend was that the higher the severity of AP the lower the artifactual AIL in the insoluble phase. Summation of the AIL and ASL data indicated alkali processing resulted in a loss of both actual and artifactual AIL in both the soluble and insoluble phases. The amount of lignin loss in such processes was a function of the severity of the process.
The second study focused on the recovery of wheat straw lignin via precipitation from alkali extract liquor (AEL). Lignin recovery is important due to lignin’s potential for subsequent value-added processing. In this work, wheat straw was treated with 5% NaOH at 50°C for 5 h to produce AEL. Lignin recovery from AEL, quantified as precipitated solids, acid insoluble lignin (AIL), and acid soluble lignin (ASL), was evaluated over a range of pH (1.5 to 7), temperature (4° to 70 °C), supplemental salts (0 to 2.5M) and presence/absence of hemicellulose. Lignin recovery, measured as ash-adjusted solids, generally increases as pH decreases. Temperature does not appear to significantly affect rates or extents of solids recovery at low pH; at neutral pH, increased temperature increased rates of precipitation but had minimal to no effect on extent of lignin recovery following a one hour precipitation period. At neutral pH and moderate temperature, ionic strength significantly affects solids recovery. Increased ionic strength increases rates and extents of lignin precipitation. Hemicellulose interferes with rates of lignin precipitation and appears to co-precipitate. Structural implications of these results are considered.
The aim of the third study was to determine the feasibility of using lignin-rich fractions of alkali processed wheat straw as “functional dietary fibers” based on their in vitro bile acid binding capacity. Bile acid binding capacity is important in this context because of the documented relationship between fecal excretion of bile acids and serum cholesterol levels. In this study, wheat straw (WS), alkali extracted solids (AES), enzyme saccharification residues (ESR), and alkali lignin (AL) were used to determine bile acids (cholic acid, glycocholic acid, and taurocholic acid) binding capacities. Bile acid adsorption was determined by measuring the partitioning of bile acids between the solid phase (fiber) and the liquid phase (phosphate buffer, pH 6.5) at 37°C. In general, bile acid associated with the different wheat straw fractions but the extent of association was not proportional to lignin content as hypothesized. Among the bile acids tested, cholic acid showed the best correlation between amount adsorbed and lignin content. AES, ESR, and AL all had higher bile acid binding capacities than WS. Bile acid adsorption to AL ranged from 2 to 29 μmol bile acid per gram substrate, with cholic acid and taurocholic acid showing the highest and lowest adsorption capacities, respectively. A contributing factor limiting alkali lignin-bile acid association was determined to be electrostatic repulsion based on experiments of evaluation the importance of ionic strength and cation addition. Chemical modification of lignin-associated carboxyl groups of AL using a carbodiimide reagent was attempted to address the electrostatic repulsion issue. Chemical modification of the alkali lignin (MAL) significantly increased its bile acid binding capacity. Bile acid binding by the MAL was in the range of 48 to 54 μmol bile acid per gram substrate. The combined results from this study demonstrate the potential of using a major food processing/production byproduct, i.e. wheat straw AL, as a “functional dietary fiber’ for bile acid sequestration.