# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
###### Abstract  
Residual biomass from agri-food production chain and forestry are available in huge amounts for further valorisation processes. Delignification is usually the crucial step in the production of biofuels by fermentation as well as in the conversion of cellulose into high added-value compounds. High-intensity ultrasound (US) and hydrodynamic cavitation (HC) have been widely exploited as effective pretreatment techniques for biomass conversion and in particular for cellulose recovery. Due to their peculiar mechanisms, cavitational treatments promote an effective lignocellulosic matrix dismantling with delignification at low temperature (35-50 \({}^{\circ}\)C). Cavitation also promotes cellulose recrystallization due to a partial depolymerization. The aim of this review is to highlight recent advances in US and HC-assisted delignification and further cellulose recovery and valorisation. biomass pretreatment; lignocellulosic valorisation; ultrasound; hydrodynamic cavitation; green solvents; enzymatic hydrolysis +
Footnote †: journal: Medical  
conversion of crop residues and agro-industrial by-products into value-added chemicals. Moreover, an integrated biorefinery process is more energy efficient in comparison with the current dispose of waste and, despite underutilised, agri-food waste could therefore represent a useful feedstock to establish a more sustainable global society through a biorefinery approach (Figure 1). Cellulose is the world's most abundant natural material that, like starch, is constituted of glucose. It contains hundreds to over ten thousand \(\beta\)-1,4 linked D-glucose monomer units in the form of unbranched straight chains, known as micro-fibrils (present in crystalline and amorphous form), interconnected with van der Waals forces and hydrogen bonds and covered by hemicellulose and lignin [14]. Nevertheless, unlike starch (or sugar), which nature has designed as quickly biodegradable and which has been employed by many pioneering biorefineries as staring material, cellulose as structural plant component is much more difficult to deconstruct (crystalline cellulose is 3-30 times less degradable as compared to amorphous structure). Indeed, its innate close polymeric alignment and its intimate relationship with other lignocellulosic components such as hemicellulose and lignin confer such recalcitrant properties. A delignification process with effective pretreatment to split cellulose and hemicelluloses from lignin is a crucial step and undoubtedly the most challenging [15]. The pretreatment aims to change the supramolecular structure of biomass in which the cellulose, lignin and hemicellulose are closely interconnected to finally release free sugars [16]. Although significant advances have been designed to improve the performances of the hydrolytic enzymes, pretreatment steps are unavoidable. It is common knowledge that hemicelluloses, owing to their branched and irregular structure, are more easily removed compared to other biopolymers forming lignocellulosic biomass and therefore relatively mild treatment such as extraction with hot water may allow their removal. Conversely, lignin stubbornly resists such treatments, as does cellulose; therefore, stronger pretreatments are required [17]. An optimized pretreatment process improves the enzyme accessibility to cellulose, promoting the recovery of the associated hemicellulose and lignin components, without degradation [18; 19; 20]. Meanwhile, the formation of inhibitory compounds that could negatively affect the subsequent fermentation step should be avoided. An effective pretreatment process affords: (i) enhanced free sugar production (with a minimum sugar loss and degradation), (ii) low detrimental by-products formation and (iii) sustainable operating cost. Figure 1: Biorefinery options for lignocellulosic biomass valorisation: a flowchart.