3.2.4. Physiochemical Conversion Routes in CBP with Pretreatment Process

The scientific lead that summons the continuous supply of biomass as the natural way for biomass saccharification with high sugar productivity is still a challenge. The problem is not yet presented in research programs dealing with biomass processing. However, many scientists aspire to work on biomass optimization for several decades. The merged yield from biofuels and chemicals still requires boosting from both technical and economic perspectives. None of the current pretreatment processes has improved, since their results rely on the type of feedstock, downstream process configuration, and many other factors. Consolidated bioprocessing (CBP) associates an individual or consortium of microbes to deteriorate biomass. Physicochemical pretreatment and biological methods are considered a better approach to reduce the recalcitrant property, leading to high sugar yield.

The hydrothermal pretreatment, i.e., liquid hot water pretreatment, does not require fast decompression and does not need any chemicals or catalysts. The temperature lifted between 160–240 ◦C, which caused an increase in pressure that required preserving the water in the liquid state and activating the lignocellulose composition [85]. The main objective of the liquid hot water pretreatment is to dissociate and solubilize hemicellulose. It makes easy cellulose availability to cellulase and limits inhibitory compounds during the reaction. Nonetheless, for avoiding the generation of inhibitors, the response must be performed at a low pH, i.e., between 4 and 7, reducing monomers' production [85,86]. Scientists recently established that heating lignin and raising the temperature above glass transition temperature could split the bond's existence between carbohydrate lignin. It emerged into the migration of hydrophobic lignin present in liquid media from the cell walls and getting re-precipitated into small spherical droplets assembled on both sides (inside and outside) of fibers [87]. Almost all hemicellulose contents got detached, leaving cellulose fiber bundles behind with free lignin, because of the lignin's realignment on both fibers' sides. After hydrolysis, fast disintegration was reported by researchers, along with a higher degradation temperature [87]. In another study, material balance and multiscale characterization techniques utilized the systematic analysis of the effects of severity factor and pH on the lignin-carbohydrate complex (LCC). Researchers discovered that the severity factor affects xylan removal under a diverse range of temperature profiles, which means a high severity factor causes high xylan removal. The temperature does not affect xylan removal. The liquid phase (hydrolysate) causes pH to drop due to hemicellulose depolymerization and degradation. It results in the accumulation and production of the acetic acid, causing hydrothermal pretreatment, and hemicellulose sugar's high yielding capacity causes increased furfural productivity and xylose loss. Rare, increased hemicellulose sugar degradation causes reduced furfural production. In contrast, approximately 80% of the original xylan was lost [88].
