# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
## 3 Biomass Treatments for Cellulose Recovery  
Due to lignocellulosic structure, the pretreatment is crucial for getting the fermentable sugars in the hydrolysis step. In fact, one of the main limiting factors to the microbial attack of lignocellulosic materials is their lignin content. Two main reasons have been proposed for the mechanism by which lignin interferes with the hydrolysis of cellulose:  
(a) lignin is a structural polymer confined into the plant cell walls and covalently linked to hemicellulose. This configuration prevents carbohydrate exposure to EH [51]; (b) the hydrolytic process of cellulose greatly depends upon the adsorption of enzyme onto the substrate: lignin could irreversibly adsorb enzymes leading to their inactivation. In this scenario, an effective biomass pretreatment is desirable to reach efficient enzymatic digestion and subsequent production of bioderived compounds [15; 52; 53; 54]. In fact, one of the major limiting factors to the microbial attack of lignocellulosic materials is their lignin content. A variety of pretreatment processes have been developed in recent decades to enhance the digestibility of lignocellulosic biomass, mainly:  
* Physical pretreatment (microwave [55; 56; 57], ultrasound [3], mechanical extrusion [58], pulsed electric fields [59]);
* Chemical-with alkali [60]; acid [61]; Oxidative delignification; Ozonolysis [62]; Organo-solsv process [63]; Ionic liquids (ILs) [64];
* Physicochemical-Hydrothermal pretreatment [65]; Steam explosion [66]; Ammonia fiber explosion (AFEX) [67]; Supercritical CO\({}_{2}\) (SC-CO\({}_{2}\)) [68] pretreatment;
* Biological (microorganisms, redox enzymes) pretreatment [69; 70]. Each cellulose isolation procedure has a distinct specificity on altering the chemical and physical structure of lignocellulosic materials [71]. The advantages and disadvantages of these pretreatment protocols are reported in Table 1. Among these processes, chemical and physicochemical approaches appear to be the most promising ones for industrial applications. Physical methods (such as chipping, grinding and milling) enable biomass size reduction with a surface area/volume ratio increase [72]. Moreover, thanks to shear forces most of them can decrease cellulose crystallinity. Among other physical methods, microwave (MW) irradiation is a broadly exploited biomass pretreatment [55; 56; 57]. It is fast and easy to operate, it requires less energy and it produces a small number of inhibitory compounds [73] even though its scalability is a challenging issue. Recently, pulsed electric fields (PEF) have been investigated in biomass pretreatment [59]. In particular, the high voltages applied on biomass by PEF (5-20 kv/cm for milliseconds) creates pores in cell membranes that enable to the appropriate agents to break down cellulose into free sugars suitable for further conversion. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Pretreatment for** & & & \\
**Cellulose** & **Processes** & **Possible Biomass Alterations** & **Remarks** \\
**Recovery** & & & \\ \hline \multirow{4}{*}{Physical} & \multirow{4}{*}{Irradiation} & \multirow{4}{*}{Decrease the polymerization degrees of cellulose} & \multirow{4}{*}{Cannot remove lignin and hemicelluloseulose} \\  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Pretreatment methods for lignocellulosic feedstocks. Multiple physicochemical and chemical approaches are widely used for biomass pretreatment to increase the accessibility of cellulose to various hydrolytic enzymes (cellularase and hemicellulose), for example, during steam explosion [66] in which biomass are subjected to saturated steam at high temperature (160-240 \({}^{\circ}\)C) and pressure (0.7-8.0 MPa) hemicellulose is solubilized while cellulose remains as a solid fraction, becoming more accessible for enzymatic hydrolysis.