## 2 Secondary Fermentation of Sparkling Wines
### Yeast Strain Selection
To enhance efficiency and accelerate this step, the use of immobilized yeasts has also been investigated. Although their application in enological practices remains relatively limited, a study by [PERSON], [PERSON] et al. [18] on the influence of immobilized yeast strains on the aromatic profile of long-aged Spanish sparkling wines established that immobilized systems could serve as an effective alternative to free yeast cells, potentially improving aromatic quality [18]. On the other hand, [PERSON], [PERSON] et al. [19] found no significant differences in the aromatic profile between Verdejo wines fermented with free and immobilized yeast strains [19]. Despite this inconsistency, the use of immobilized yeasts in the second fermentation of sparkling wines may simplify riddling and disgorging processes, without negatively impacting key quality parameters. More recently, the use of indigenous yeast strains as starter cultures, particularly for secondary fermentation, has gained attention as a strategy to enhance the distinctive characteristics of regional wines, as these strains are adapted to specific environmental conditions, contribute to organoleptic differentiation, and help mitigate fermentative issues [20; 21; 22]. [PERSON], [PERSON] et al. [23] screened 133 _S. cerevisiae_ strains based on technological (fermentativepower and vigor, SO\({}_{2}\) tolerance, alcohol tolerance, and flocculence) and qualitative criteria (acetic acid, glycerol, and H\({}_{2}\)S production), finding that indigenous strains performed comparably to commercial starters. Their use in refermentation offers a viable approach to enhance product differentiation while preserving traditional winemaking techniques [23]. In 2018, [PERSON], [PERSON] et al. [24] confirmed the suitability of autochthonous \(S\). _cerevisiae_ to improve the quality of regional Apulia sparkling wines [24]. In agreement with this investigation, [PERSON], [PERSON] et al. [25], using Bombino Bianco and Nero grapes, showed that, regarding aroma and a metabolomic approach, autochthonous yeast strains can be an influential tool for innovation and market differentiation [25]. The growing economic interest in the sparkling wine sector has led to renewed focus on microbial resource management, particularly the use of non-_Saccharomyces_ yeast, creating opportunities to produce innovative wines with unique characteristics to meet the demands of the competitive international market [12]. The key oenological properties of non-_Saccharomyces_ yeasts have been extensively reviewed [26], along with their potential to address various technological and safety challenges, including the regulation of volatile acidity, alcohol reduction, increased glycerol production, enhancement of varietal aroma expression, and mitigation of contaminants. The current knowledge of non-_Saccharomyces_ yeasts in starter cultures for secondary fermentation has been reported, highlighting the impact on chemical composition and sensory attributes of traditional sparkling wine. The review was focused on _Torulaspora delbrueckii_, _Metschnikowia pulcherrina_, _Schizosaccharomyces pombe_, and _Saccharomyces ludwigii_, with variations in amino acids, biogenic amines, volatile organic compounds (VOCs), aroma compounds, glycerol, and proteins described, which influence flavor and foaming properties. Due to limited studies on sparkling wines, findings from still wine have been used to assume potential effects, particularly concerning nitrogenous compounds, VOCs, proteins, organic acids, and sensory characteristics [12; 27]. [PERSON], [PERSON] et al. [27] explored making natural sparkling wines using non-_Saccharomyces_ yeasts. Spanish base wine made from Airen (white) and Tempranillo (red) grapes was used in this experiment. Even though the total amount of volatile compounds was similar between yeast strains, _Schizosaccharomyces pombe_ showed aromatic differences compared with the traditional _S. cerevisiae_ and produced red sparkling wines with higher pyranoanthocyanin content and color intensity [27].