## 3 Results and Discussion
### Wine Flavour Profiling
As evidence, the sensory evaluations reported in the literature revealed a stronger fruity character in wines fermented with mixed cultures than in control wines [42]. For example, 1-butanol, 3-methyl-, acetate (compound **6**), which was also identified in the wine sample that had been obtained by the co-inoculation of immobilized cells of _H. osmophila_ and _S. cerevisiae_ (W2) and in the control (C), was found to confer the typical banana flavour. This is characteristic of the _S. cerevisiae_ metabolism; however, when present in excess, it could mask key varietal characters of wines [43]. Among the ester aromatic compounds detected in W1 sample, the most important was acetic acid, 2-phenylethylly ester (compound **11**). As described in previous studies, its concentration was higher in wines obtained with mixed cultures than in wines produced by _S. cerevisiae_ pure cultures and it was modulated by changing the initial yeast ratio [42]. Acetic acid, 2-phenylethylly ester imparts \"raspberry\"- and 'honey'-like aromas to wine [44]. Ethly esters of fatty acids also contribute with pleasant fruity and floral odours to wine aroma. Seven ethyl esters of straight-chain fatty acids with an even number of carbon atoms (C\({}_{8}\)-C\({}_{18}\)) were identified in sample W1, i.e., octanoic acid, ethyl ester (compound **10**), decanoic acid, ethyl ester (compound **12**), dodecanoic acid, ethyl ester (compound **16**), tetradecanoic acid, ethyl ester (compound **18**), ethyl 9-hexadecenoate (compound **19**), hexadecanoic acid, ethyl ester (compound **20**) and linoleic acid ethyl ester (compound **21**). The concentration of fatty acids ethyl esters is dependent on wine aging: it generally decreases as the storage time gets longer, especially at low pH and high temperature values [45; 46]. Although their production has been shown to be lower when apiculate yeasts are inoculated, like those belonging to the genus _Hansenispora_[47], a greater presence of these compounds was found in this study in W1 wine compared to the control, in which only octanoic acid, ethyl ester and dodecanoic acid, ethyl ester occurred. As regards to W2 wine, octanoic acid, ethyl ester; acetic acid, 2-phenylethylly ester; tetradecanoic acid, ethyl ester; and hexadecanoic acid, ethyl ester were identified as representative compounds belonging to the ester class (Table 3). All the volatile fatty acids detected in the three samples occurred in their esterified form. The exception was acetic acid, which was also detected in its free form (compound 7). It should be noted that a decrease in the retention time of acetic acid was observed for the W2 wine, which may have been due to the hydrolysis of ethyl acetate. As representative compounds belonging to the class of aldehydes and terpen, acetaldehyde (compound **1**) and nerolidol (compound **15**) were, respectively, identified in the W1 wine. Moreover, the first one was also detected in the W2 wine sample which had been fermented with immobilized cells of _H. osmophila_ in co-cultures with _S. cerevisiae_ and in the control. Acetaldehyde is derived from alcohol fermentation by yeasts. In this study, no intense chromatographic peaks were observed which related to other aldehydes in the total ion current chromatograms of the three analyzed samples. This was most likely because they were involved in several reactions with wine phenolics, whose products impacted wine colour, flavour and astringency [48]. Nerolidol (compound **15**), a compound with a characteristic floral odour similar to that of rose [49], was not detected in the W2 sample, nor was it found in the control. This occurred because, in general, the presence of flavour compounds belonging to the terpen class is due to some specific enzymatic activities of the yeasts, usually those activities linked to non-_Saccharomyces_ species which can contribute to increasing the sensory profile of the wine [50]. Instead, compound **14**, i.e., phenol, 2,5 bis (1,1-dimethylethyl) was the only volatile phenol detected in the co-fermented wine W1.