Processing, Characterization, and Valorization of Soybean Hull

Abstract

This work aims to utilize soybean processing waste, soybean hull (SBH), as a low-cost, abundantly available biomass (annual global generation of ~23 million metric tons) for producing value-added products. Despite being generated in large amounts, the current application of SBH is mainly limited to animal feed, with a significant portion of it being landfilled or incinerated, leading to severe environmental issues. Although some successful demonstrations of SBH valorization have been reported in the literature, the focus was generally on isolating and fractionating a specific component, which requires costly downstream processing and generates significant amounts of effluents. In this study, we aim to develop scalable, cost- and energy-effective processing conditions for SBH valorization, emphasizing the full utilization or integrated multi-product approaches with the lowest emission of effluents. The physicochemical and morphological properties of the SBH after processing, the interactions between SBH components, structure-related functional properties, and application-specific characterization were evaluated to follow these approaches. A single-step aqueous/acid-catalyzed thermomechanical treatment was studied to evaluate the solubilization and activation of SBH's functional non-cellulosic components, e.g., pectin and protein, into the gel and liquid phases. The mass balance of the insoluble solids, gel, and liquid fractions showed that acidification (changing pH of the aqueous medium from neutral to 1.8) leads to a 10%, 20%, and 25% increase in the solubilization of total SBH, protein, and pectin to the liquid phase. Then, to further understand the effect of processing pH on SBH properties, the sequential isolation of protein- and pectin-rich fractions from liquid or/and gel fractions of hot water- and acid-treated SBH were done. The results of chemical composition analysis, functional groups, and thermal properties showed more exposure of the functional groups due to a decrease in particle size and conformational changes and better interpolymer interaction for the fractions isolated from the acid-treated SBH. Also, SBH fractions showed higher surface activity, enhanced gelling, and emulsifying ability than pure commercial protein and pectin, indicating their potential as gelling and emulsifying agents in the food, pharmaceutical, and cosmetics industries. Based on these results, acid treatment was used as the primary building block of the SBH valorization for the proposed applications throughout the dissertation. The whole acid-treated SBH was used as a binder (2%) in aquatic feed pellets (full utilization approach). The comparison of pellet strength and water stability by USDA-ARS revealed the superior performance of the SBH binder compared to the commonly used commercial ones. This superior performance can be attributed to the solubilization and activation of ~39% of SBH, rich in pectin, protein, and hemicellulose, and the increase in carboxylic acid and ethanol extractive contents (providing an increase in available hydrophilic and hydrophobic functional groups, respectively). The solids content of the produced binder can be increased from 10% to 55% by adding a pretreatment step to reduce particle size (grinding/PFI refining step), which decreases the shipping cost and extends the shelf life of the binder. An increase in solids content lowered pectin degradation compared to the low-solids binder, while comparable carboxylic acid and ethanol extractives contents were achieved, consistent with the higher dry and wet strength of the produced pellets (conducted by USDA-ARS). The SBH binder (10% solids) was also used as a coating binder, mixed with bentonite and a pH-sensitive bromocresol purple (BCP). A novel paper-based colorimetric detector was manufactured by applying the coating on filter paper after adjusting pH to 0.25-5.2 (a set of twelve detectors). The fabricated detectors were evaluated as a real-time freshness and spoilage indicator of fridge-stored catfish fillets (Ictalurus punctatus). The detection method was a visible color change from yellow to purple upon increased alkalinity in the atmosphere due to the emission of total volatile basic nitrogen (TVBN) compounds from spoiled meat. Freshness and spoilage of fish meat with varying weights and headspace were determined by correlating the detector's pH to the daily total viable count (TVC) on fish fillets. Elemental, structural, and functional analysis verified the formation of a packed SBH-bentonite matrix with enhanced gas adsorption capacity and effective BCP immobilization. Nanofibrilation of SBH binder (NFSBH) increased the overall visual color vibrancy and decreased the binder demand in the coating formulation. The activation time of the detectors remained unaffected by changing the headspace volume in the studied range (40 and 160 cm3). Nevertheless, as the weight of the fish increased, the detectors' optimal activation time and pH decreased. This study's results demonstrate that by adjusting the pH of the detector, it is possible to customize the detectors for various sample and packaging sizes. The multi-product integrated approach was taken to valorize the isolated gel fraction (a potential alternative to food and cosmetics emulsion gel) and the insoluble solids (as the precursor for producing high α-cellulose) from acid-treated SBH. The effect of processing conditions on the production yield and properties of a novel polysaccharide-protein enriched hydrogel (model responses) were studied using 3-factor 3-level central composite design (CCD) and response surface methodology (RSM) (pH: 1.8, 3.8, 5.8; time: 10, 35, 60 min; temperature: 65, 80, 95℃). Regression analysis revealed that all single factors were statistically significant, while time*temperature was the only significant interaction term. While the extraction of pectin and protein, and consequently, gel yield, was increased with elevating temperature and lowering pH, extended heating times resulted in depolymerization and a decrease in the gel yield. On the other hand, the pec/pro ratio was increased with increasing temperature and pH and decreasing the heating time. The hydrogels with the highest and lowest pec/pro ratio (H and L) were chosen to study their flow behavior. Yield strains of approximately 1.6% and 1.0% for hydrogel L and H (low mechanical strength), frequency-dependent storage and loss moduli, and shear thinning behavior would classify the SBH hydrogel as a weak physical gel. Hydrogel L has higher viscosity and strength (yield strain) but high heterogeneity and brittleness (two-step yielding in strain sweep and inconsistent flow behavior at low and high shear regions). In contrast, Hydrogel H showed relatively monotonous changes during strain- and shear-induced flow tests, indicating its homogenous structure and better hydrophobic/hydrophilic interpolymer interaction. These rheology results and the amphiphilic nature of the novel SBH hydrogels make this biomass a potential alternative source for emulsion gel production. The insoluble solids from acid-treated SBH were further processed by mild alkaline extraction, alkali treatment (pulping), and single-stage D-bleaching to produce dissolving pulp-grade high α-cellulose (SBH-DP). However, the calcium and iron contents (126.5 and 43 ppm) were significantly higher than the industry requirements (25 and 5 ppm). Higher levels of these metal ions decrease the dissolving pulp's processability (clogging the equipment) and quality (solvent and cellulose degradation). Despite testing several metal removal strategies, the calcium and iron contents were only reduced to 43 and 7.5 ppm, respectively. Incomplete metal removal was probably due to the high residual non-cellulosic polysaccharides (~6.5% hemicellulose and 1-2% pectin), which provide high cation exchange capability. Based on the results, bleached SBH's applicability was evaluated for an application with lower purity requirements, i.e., microcrystalline cellulose (MCC). Two sets of processing steps were employed to produce MCC from SBH. MCC-1 (the same as SBH-DP) had more processing steps and lower yield but higher purity of cellulose than MCC-2 (alkali treatment, acid treatment, and D-bleaching). However, the overall yield increases, and effluent discharge decreases with MCC-1 production by valorizing the acid and alkaline-extraction filtrates and incorporating the multi-product integrated approach. Also, since MCC-1 and MCC-2 exhibit similar powder and tablet properties, we can conclude that the MCC-1 production offers a superior pathway for the commercial production of MCC from SBH without sacrificing product quality. SBH MCCs exhibited larger particle size and broader particle size distribution, higher viscosity, crystallinity, and thermal stability compared to the commercial control (Avicel PH-101), leading to better flowability and compressibility of SBH MCCs. The MCCs were used as binders in a tableting blend (60% MCC). SBH MCCs had lower lubricant sensitivity and hardness compared to Avicel PH-101. All tableting formulations showed friability<0.1% and disintegration times within 90 seconds, making them well-suited for dry compression manufacturing of orally disintegrating tablets (ODTs). In summary, we developed cost-effective and scalable processing steps with lower chemical/energy consumption to produce value-added products from SBH using full utilization and multi-product integrated approaches.