# Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments
## 3 Biomass Treatments for Cellulose Recovery
### Acoustic Cavitation (AC) as Effective Biomass Pretreatment
In addition, the concentration of fermentation inhibitors (such as furfural, 5-hydroxymethylfurral (HMF), ferulic and \(p\)-coumaric acid), was monitored for each US-treated CSS sample after EH step, showing always negligible amounts (less than 1 mg/L). As a consequence, the hydrolysate of US assisted pretreated CSS in presence of diluted acid may be directly fermented without a time-consuming detoxification step. The same authors also optimized a combined process based on US and mild alkaline conditions for suitable CSS pretreatment (2020) [93]. A central composite design was exploited to identify the best operating conditions for the alkaline UAT tests on CSS aiming at the maximization of reducing sugar yields (YS). The authors stated that the highest sugar yield (0.6 g/g) was obtained starting from the US pretreated CSS under 5-min sonication at 11% \(w/v\) biomass loading, and 75-min autoclave in presence of 5% \(w/v\) of NaOH. Moreover, the alkaline pretreatment alone was exploited as a benchmark, providing a hydrolysis yield higher than that measured for US alone. On the other hand, alkaline UAT produced a slight increase of sugars outcome compared to the conventional protocol. Time screening confirmed that longer sonication treatment (from 5 to 30 min) did not offer any improvement. All the delgification processes allowed the exposure of cellulose microfibrils resulting in an increased crystallinity index, but the maximum value has been obtained with the alkaline UAT. According to [PERSON] et al., some phenolic compounds could be recovered from the pretreated liquor. The alkaline and the alkaline UAT increased phenolic compound release of about 6.5- and 7-fold, respectively, if compared to the raw CSS. In particular, the phenolic content of the recovered liquid after alkaline UAT hydrolysis of CSS was 25 mg GAE g/raw CSS, larger than that reported by [PERSON] et al. (2016) [94] for CSS. Another effective alkali UAT was reported by [PERSON] et al. (2017) [95] starting from chili post-harvest residue. The biomass UAT (80 kHz, 0-60 min) in presence of NaOH, KOH or Ca(OH)\({}_{2}\) was followed by a hydrothermal treatment in autoclave for 60 min at 121 \({}^{\circ}\)C before EH. [PERSON] et al. explored the interaction effects between biomass loading and sonication time on EH. Maximum reducing sugar yield (0.44 g/g) was described for longer sonication time (50-60 min) and middle levels of biomass loading (11.0-15.0% \(w/w\)). Alkali medium plays a crucial role in delgification, providing reactive hydroxyl ions, acting by means of three major reactions on lignin: (i) fragmentation; (ii) degradation and dissolution; (iii) condensation. The UAT described by [PERSON] et al. (2017) could effectively remove lignin and hemicelluloses from chili post-harvest residue, enhancing the cellulose recovery and the final sugar yield. Inhibitor analysis performed on the hydrolysates showed that furfurral, HMF as well as organic acids like citric acid, succinic acid and propionic acid were not present. Hence, fermentation can be carried out without prior detoxification step. Fermentation of the non detoxified hydrolysate yielded 1.9% of ethanol with a fermentation efficiency of 70%. Acoustic Cavitation Coupled with Alternative Solvents  
Considering the hindering effects of lignin on cellulose recovery and conversion, together with the biomass recalcitrance induced by vegetal textures, several studies were conducted investigating new and alternative solvents able to disrupt the matrix network and/or solubilize lignin. In this context, a novel cellulose isolation process based on the use of bio derived \(\gamma\)-valerolactone (GVL) was developed by [PERSON] et al. (2017) [96] with the aim to valorise bamboo components into digestible cellulose, fermentable sugars and lignin. Bamboo, as a grassy plant with rigid structure, was pretreated using a GVL/water mixture (60:40) containing 0.05 mol/L of acid (H\({}_{2}\)SO\({}_{4}\)), resulting in a liquid fraction enriched with cellulose.