## 3 Results and Discussion  
Commercial- and Wild-Type Yeast Strains Performences in Synthetic Medium/Aerobic Experiments and Yeast Selection  
Ethanol production in wine is based on the ability of yeast strains to catabolize six-carbon molecules present in must into ethanol, a two-carbon compound [11]. The bioprocesses where yeasts convert glucose to ethanol are known as the glycolytic pathway followed by ethanol fermentation [38]. Fermentation (_viz._ \"anaerobic\"-type transformation of glucose into ethanol) may occur despite the presence of O\({}_{2}\) in the culture medium in significant concentrations when the initial concentration of the employed carbohydrate (i.e., glucose and/or fructose and/or sucrose) is higher than a \"critical\" value [39]. In fact, for a remarkable number of yeast species (the so-called \"conventional\" yeasts), even with the significant presence of oxygen in the fermentation medium (i.e., DOC values \(\geq\) 20% \(v/v\) and in some cases >50% \(v/v\)), if sugar concentration is higher than a critical (and in many instances not very high) concentration (e.g., \(\alpha\) 9 g/L or even lower), respiration is impossible; furthermore, despite the oxygen saturation conditions imposed, the microorganism shifts its metabolism completely towards the fermentative pathway and the subsequent biosynthesis and accumulation of ethanol into the medium. This phenomenon is known as the \"[PERSON] effect\" (named after the English biochemist [PERSON]), Pasteur contrary effect, or as catabolic repression by glucose [12; 40]. Specifically, for the mentioned type of yeasts (the \"Crabtree\"-positive ones), in somehow elevated sugar concentrations imposed in the culture medium, the mitochondria degenerate, the proportion of cellular sterols and fatty acids decrease and both the enzymes involved in the oxidative part of the metabolism (namely the Krebs cycle and the oxidative phosphorylation chain) and the constituents of respiratory chains are subjected to catabolite repression, leading to the elaboration of the ethanol fermentation despite oxygen-sufficient culture conditions [12; 40; 41]. All yeast strains tested in shake-flask trials converted glucose into ethanol (EtOH) and dry yeast biomass (X), despite aerobic conditions imposed into the medium (in all trials and irrespective of the strain, the initial glucose concentration and the culture time, the DOC was always \(\geq\)20% \(v/v\), indicating sufficient aerobic conditions into the shake-flaskenvironment [7; 29; 41], and the obtained results are depicted in Table 1(a) (Glc\({}_{0}\)\(\approx\) 100 g/L) and Table 1(b) (Glc\({}_{0}\)\(\approx\) 200 g/L). In all trials performed, non-negligible quantities of glucose were relatively rapidly consumed and converted into DCW and (mostly) ethanol; therefore, all selected yeast strains were referenced as positive to \"Crabtree effect\" in accordance with the literature [12], that by far considers the _S. cerevisiae_ species as the most \"classic\" yeast strain in which this phenomenon occurs [40; 41; 42; 43; 12; 44]. It is interesting to indicate that in the trials with Glc\({}_{0}\)\(\approx\) 100 g/L, with the exception of the wild-type non-commercial LMBF Y-54 strain that consumed the majority of glucose quantity that was found in the growth medium, all other LMBF Y- strains did not consume all available sugar quantity of the medium, whereas further incubation did not lead to increased glucose assimilation, but to degradation (oxidation) of ethanol, that in most cases was not accompanied by a DCW concentration increase (thus no diauxic growth occurred). Concerning X production of the wild-type non-commercial yeasts growing on glucose at Glc\({}_{0}\)\(\approx\)100 g/L, by far the highest biomass producer was the strain LMBF Y-54 (X = 7.1 g/L; see Table 1). The X\({}_{\rm max}\) value of this strain was recorded at t = 121 h, being =10.5 g/L (kinetics non shown; this value was obtained after ethanol oxidation that occurred when glucose had been depleted from the medium).