Biomass Pretreatment and Saccharification

Structural Characteristics of Lignocellulosic Biomass

  Lignocellulosic biomass is basically composed of cellulose, hemicellulose, and lignin. Cellulose is a highly stable polysaccharide with molecules consisting of more than 10,000 glucose units linked by β(1-4) glycosidic bonds. Hemicellulose is an amorphous, branched heteropolysaccharide composed of hexoses (glucose, mannose, galactose), pentoses (xylose, arabinose), uronic acid, and deoxyhexoses (rhamnose, fucose), and has a short chain length compared to cellulose. Lignin is a hydrophobic polymer containing cellulose and hemicellulose chains, which contribute to the stiffness and hardness of plant structures. Lignin is an amorphous matrix and is the second most abundant natural polymer after cellulose. The main components (cellulose, hemicellulose, and lignin) of lignocellulosic biomass form a very dense, stable, and resistant structure that requires significant mechanical, chemical, or biological forces to destroy.

Alkali Pretreatment & Enzymatic Saccharification

  Pretreatment to remove lignin and hemicellulose from the cell wall is necessary to increase the efficiency of saccharification of lignocellulosic biomass. Alkaline pretreatment causes saponification of the lignin-carbohydrate complex, which solubilizes and removes lignin and hemicellulose from the cell wall. Consequently, the availability of enzymes to the cellulose in the pretreated biomass is improved, which allows for the biological conversion of cellulose to glucose. This means that saccharification efficiency can be further improved by optimizing the process variables of the alkaline pretreatment. Our research group has researched to optimize an alkali pretreatment process and an enzymatic saccharification process to recover glucose from various lignocellulosic biomass (e.g., spent coffee ground, canola straw, and chestnut shell).

  Chestnut shells (CSs) are a high-carbohydrate waste generated by food industries. Our research group optimized the potassium hydroxide pretreatment conditions of CSs using response surface methodology to efficiently glucose recover via enzymatic saccharification. Here, biomass-to-glucose conversion (BtGC) was set as the response value, and the optimal conditions for maximum BtGC were derived. After optimal KOH pretreatment, the BtGC of pretreated CSs was improved 2.5-fold compared to the control (non-pretreatment). Furthermore, the enzymatic digestibility of pretreated CS was improved to 75.7% through enzyme reaction profiling experiments. Finally, bioethanol fermentation was performed by Saccharomyces cerevisiae using CS hydrolysates and the bioethanol yield was estimated to be 150 g-bioethanol/kg-biomass, which increased 4.7-fold compared to the control (32 g-bioethanol/kg-biomass).

Dilute Acid Pretreatment & Enzymatic Saccharification

  Acid pretreatment is one of the methods that break the lignocellulosic matrix by cleaving the glucosidic bonds. This process primarily solubilizes the hemicellulose portion of the biomass but also solubilizes a portion of the lignin, which makes the cellulose more accessible to further enzymatic attack. In other words, the liquid fraction generated by acid pretreatment consists mainly of hydrolyzed hemicellulose sugars (mainly xylose), whereas the solid fraction contains some lignin and cellulose. Dilute acid pretreatment, in particular, is less toxic, corrosive, and hazardous and thus can be expanded to industrial scale. Our research group has researched the design of dilute acid pretreatment processes and enzymatic saccharification processes to recover glucose from various lignocellulosic biomasses.

  Miscanthus straw is a notable feedstock for its high biomass yield per unit planted area, the ability to be grown without pesticides or fertilizers, low maintenance costs, and long life span. Our research group confirmed that miscanthus straw contained 43.4% glucose, 25.6% xylose, 1.4% galactose, 1.7% arabinose, 20.2% lignin, 2.4% ash, and 5.3% other. Hence, we designed a process to efficiently remove xylan (hemicellulose fraction) from miscanthus straw by dilute sulfuric acid pretreatment for glucose production via enzymatic saccharification. After pretreatment, the conversion of solubilized xylose in the liquid fraction was 71.2%. Following enzymatic hydrolysis, the glucose conversion was approximately 86.4%, more than 4-fold higher than the control (untreated).

Acid Hydrolysis

  Acid hydrolysis can achieve high sugar recovery yields from lignocellulosic biomass by using concentrated acid solvents. This process has the advantage of not requiring an enzymatic hydrolysis step. Using an acidic solvent, hemicellulose is first hydrolyzed into monomeric sugars, and then the cellulose is depolymerized into cellulose oligosaccharides, releasing additional monomeric sugars. However, during this process, monosaccharides can be excessively decomposed and converted into fermentation inhibitors such as acetic acid, formic acid, furfural, and 5-(hydroxymethyl)furfural. Utilization of biomass hydrolysates containing these compounds as fermentation substrates can inhibit microbial growth. These substances can be removed through a detoxification process, but this may increase overall operating costs.