## 3 Results and Discussion
### Analysis of VOCs in Base Wines
The wines aged on sonicated lees at the amplitudes of 60% and 90% (samples 60 and 90, respectively) showed a high concentration of C6-alcohols, compounds responsible for the herbaceous and vegetal notes of wines (Figure 2 and Table 2) [26]. Regarding fused alcohols, the treatment of the lees with ultrasound at the highest amplitude (90%) increased the concentration of some alcohols, such as 2-methylpropan-1-ol, 3-methylbutanan-1-ol, 3-methylpentan-1-ol and octan-1-ol in wine (sample 90). These compounds at concentrations below 300 mg/L contribute to the aromatic complexity of wines [26]. Similar results were found for 2-phenylethanol, an alcohol with a rose-like aroma [26], showing a remarkable increase in this compound in the wines aged on ultrasound-treated lees, especially in samples 60 and 90, compared with the wine aged on ultrasound-untreated lees (sample L) and the control wine (sample C). Ultrasound treatment of the lees also had an impact on the concentration of some acids, noting a higher concentration of 2-methylpropanoic, butanoic, 3-methylbutanoic, octanoic and hexanoic acids in the wines aged on ultrasound-treated lees at an amplitude of 60% and 90% (samples 60 and 90, respectively) than in the wine with ultrasound-untreated lees (sample L). A similar trend was found for phenylbenzene (1,1\({}^{\prime}\)-biphenyl) (floral note) and octan-2-one ketone (floral and fruity note). A detailed review of the literature on this subject has highlighted the limited knowledge that exists on the impact of ageing on ultrasound-treated lees on the volatile composition of the wine. Similar to our results, [PERSON] et al. [9] found that the concentrations of total esters, 2-phenyl ethanol and acetaldehyde were higher in wines aged on sonicated lees compared to wines aged on ultrasound-untreated lees after 30 days of ageing. It is recognised that glycoproteins and polysaccharides from yeast autolysis can interact with the volatile compounds of wine, modifying their volatility and perception [3; 28] and modulating the aromatic wine profile [3]. However, this interaction depends on the structure and the composition of these biomolecules, as well as the type and the concentration of the volatile compounds [2; 29]. As discussed above, the concentration of each volatile compound (Table 2) was not lower in the wine aged on ultrasound-untreated lees (sample L) than in the control wine without lees (sample C) despite the reported capacity of the lees to interact with aroma compounds [29]. Therefore, it could not be inferred whether ultrasound treatment increased the release of volatile compounds from the lees. It has been reported that yeast lees can consume significant amounts of oxygen after alcoholic fermentation [30]. Therefore, the increased content of the volatile compounds in the wine aged on lees treated at the amplitude of 90% (sample 90) could be due to a great reducing power of these lees that could prevent the oxidation of the volatile compounds. Ultrasound may have caused a chemical and structural change in the lees improving their ability to consume oxygen. [PERSON] et al. [9] suggest that the ultrasound treatment of lees increases the antioxidant capacity of the wine aged on lees due to an enhanced release of proteins and glucans with antioxidant activity from the sonicated lees. Moreover, some enzymes may be released from yeast cells during autolysis, such as esterases involved in acetate and ester synthesis [26; 31], increasing the concentration of these compounds in the wines aged on ultrasound-treated lees. Concerning fused alcohols, they can be synthesised from their corresponding amino acids [26]. It is plausible that ultrasound enhanced the reported release of amino acids during yeast autolysis [32], raising the concentration of fused alcohols in the wines aged on ultrasound-treated lees.