## 2 Secondary Fermentation of Sparkling Wines
### Yeast Strain Selection  
The sensory profile of sparkling wines made by the traditional method is strongly influenced by yeast selection for the secondary fermentation, and research suggests that different strains can significantly impact the aging process and final chemical composition [11]. Advances in yeast selection have facilitated the identification of strains specifically suited to winemaking, enhancing fermentation efficiency, stress resistance, and sensory quality [9; 12]. The selection of yeast strains for experimentation is primarily guided by essential technological characteristics, such as tolerance to high pressure and alcohol content, as well as the ability to thrive at low temperatures with minimal production of SO\({}_{2}\) and off-flavors. Additionally, the yeast strain should exhibit optimal flocculation and autolysis capacity during aging, thereby extending the interaction between the base wine and lees [6; 13]. For  
Figure 2: Schematic representation of traditional sparkling wine production. this reason, a wide variety of _Saccharomyces cerevisiae_ strains are commercialized and used for the first and second fermentation of grape munets and base wines, respectively [14]. [PERSON], [PERSON] et al. [15] tested six flocculent _Saccharomyces cerevisiae_ wine strains with different flocculation degrees and autolytic activity and two commercial strains in traditional sparkling winemaking. The results showed a considerable diversification quantitatively and qualitatively in the production of aroma molecules (namely ethyl octanoate, responsible for sour and apple aroma, 3-methyl-1-butanol, and 2-phenylethanol, which contribute to herbaceous, rose, and sweet aroma), and sparkling wines obtained with autochthonous flocculent strains presented a higher number of alcohols and esters, already after 3 months, highlighting the possibility of exploring them as starter cultures to produce differentiated traditional sparkling wines [15]. A comprehensive review regarding biotechnologies in sparkling wine was published in 2011 by [PERSON] et al. [9] and it summarizes some _Saccharomyces cerevisiae_ strains that produce good quality sparkling wines although with different characteristics, such as alcohol tolerance (_S. cerevisiae (ex hyanus)_ BCS103), good flocculation capacity (_S. cerevisiae ph.r. buyanus_ PB2019), the enhancement of sensory characteristics (_S. cerevisiae_ (hybrid)), and autolytic capacity (_S. cerevisiae_ r.f. _bayanus_) [9]. The use of commercial active dry yeast (ADY) is essential in the fermentation process of the traditional method for their ability to withstand high acidity, ethanol concentration, and low pH, and for their flocculation capacity under carbon dioxide pressure [8; 16]. A study performed by [PERSON] et al. [16] on yeast dynamics during starter culture preparation (also known as _pied de cucve_) demonstrated that the kinetics of sugar consumption are initially rapid during acclimatization to the alcoholic medium but slow during active growth, regardless of the base wine tested. While initial differences in viability and population dynamics among ADY strains tend to diminish over time, specific strains exhibit distinct fermentative capacities. For instance, _Saccharomyces cerevisiae bayanus_ R _Vitileve_ DV10 showed the highest sugar consumption rate during _pied de cucve_, whereas _S. cerevisiae bayanus Lalvin_ EC-1118 achieved the greatest pressure increase during the second fermentation [16]. Furthermore, [PERSON], [PERSON] et al. [17] reported that Cava sparkling wines fermented with ADY exhibited significantly higher aroma scores, underscoring the impact of yeast selection on the sensory profile of the final product. These outcomes emphasize the role of commercial ADY, ensuring an efficient second fermentation and enhancing the organoleptic quality of sparkling wines [17]. In traditional sparkling wine production, riddling and disgorgement are demanding and time-intensive processes.