## 3 Results and Discussion
### Fermentations
#### 3.2.2 Fermentation Stability  
Thermally-dried free or immobilized kefir cells were further evaluated regarding their fermentation efficiency in simultaneous alcoholic and ML single batch fermentations at 30 \({}^{\circ}\)C, after storage at various temperatures (\(-\)18, 5, and 20 \({}^{\circ}\)C) for a period of 1, 3, and 6 months, respectively (Table 3). According to the results, fermentation time, ethanol content, ethanol production yield, glycerol and citric acid concentrations, and total volatile values were significantly (\(p<0.05\)) affected only by the nature of thermally-dried kefir cells. Malic acid concentration and malic acid conversion, ethanol productivity values, residual sugars concentrations and sugar conversion values were also significantly (\(p<0.05\)) affected by the storage temperature, while strong interactions (\(p<0.05\)) were noted in some cases. On the contrary, no significant differences were observed in pH and volatile acidity values, and in acetic acid and lactic acid content. Fermentation times were significantly (\(p<0.05\)) increased after storage, for all thermally-dried kefir cells [53], most likely due to decrease in cell viability or inactivation of yeasts [25], but were still significantly (\(p<0.05\)) lower when immobilized cells were used. As a result, daily ethanol productivities were reduced during storage, but not significantly in all cases, and remained several folds higher than those encountered in the traditional practice [35; 54]. Interestingly, ethanol concentrations up to 10.1% (\(v/v\)) were recorded, while high sugar conversion values and low residual sugars concentrations were documented, in all cases. Adequate ethanol yield values (up to 0.47 g/g) were recorded for both thermally-dried free and immobilized kefir cells after 1 month of storage [37], but increased storage duration (regardless the conditions) resulted in yield losses, as previously reported [55]. Consequently, ethanol concentrations were reduced (although not significantly) compared to those derived after 1 month of storage, but still remained in ranges suitable for low alcohol winemaking [2]. However, sugar conversion \(\sim\)99% was documented and residual sugars \(<2.0\) g/L were detected in all cases, indicating high rates of biomass recovery [39]. The use of aeration would potentially enable faster growth of kefir biomass (by employing sugar respiration) prior to alcoholic fermentation, but adverse effects may occur (e.g., excessive production of acetic acid and high volatile acidity), that are unacceptable in wine [56]. Moreover, lack of agitation or aeration control results in a decrease of the operational costs [18]. Malic acid conversion values (up to 34.8%) were recorded in fermentations with thermally-dried immobilized culture after storage, and an increase in malic acid degradation was observed in lower storage temperatures, as previously shown in winemaking using wet or freeze-dried kefir cells [14; 17], but not significantly in all cases. Similarly, glycerol content, acetic acid and lactic acid concentrations as well as total acidity, volatile acidity and pH values remained in levels usual for wines [17; 32; 54], indicating the metabolic stability of the thermally-dried kefir cells during storage [53].