• Ultrasound pre-treatment

Ultrasound technology, which produces cavitation and acoustic streaming, has also been regarded as a promising pre-treatment for the degradation of lignin. The particles nearby can be crumbled because the powerful hydro-mechanical shear forces in the bulk liquid is generated. Ultrasound irradiation disrupted lignocellulose structure and breakdown the crystalline nature of cellulose structure, apart from provoking cellulose depolymerisation and solubilisation, lysis of cell walls and membranes as well as improvement of the solubilisation of organic matter [36]. It was demonstrated that a hydrodynamic shear force in aqueous phase was generated from ultrasound, which increased the disintegration of coarse particles and so promoted the surface area for enzyme activity [37]. As the cavitation can generate high temperatures and high pressures (often referred as "hot spots"), the reaction times in heterogeneous solid-liquid systems can be reduced and the chemical degradation could be accelerated via the production of oxidative species. Combined with other biomass pre-treatments such as ozone or alkaline pre-treatment, a decrease of hemicellulose and lignin content was observed when sugarcane bagasse was pre-treated by ultrasound [38].

Notwithstanding, there is a lack of papers concerning the application of ultrasound to OP as a pre-treatment in the bioethanol production scheme. Nevertheless, some works can be found in literature. Thus, The application of ultrasound to OP to extract antioxidant compounds has been assessed [39]. Different ethanol/water ratios (20%, 50% and 80% of ethanol concentration), amplitude percentages (30%, 50% and 70%) and process times (5 min, 10 min and 15 min) were assessed. From the results on total phenolic compounds, total flavonoid content and antioxidant activities, these authors concluded that ultrasound pre-treatment could be a first step of the process within a biorefinery context and OP a potential source of natural antioxidants [39]. In the available literature it can be also found the effects of ultrasonic treatments on organosolv black liquor from olive tree pruning residues [40]. Up to 20% monosaccharides were degraded when 15 min of ultrasound treatment was applied. The monomeric sugars were increased from 3% to 16% as result of the lignin-carbohydrate complex rupture that was caused by ultrasonic irradiation [40].
