## 1 Introduction  
The industrial production of sparkling wines involves two sequential stages. The first consists of fermenting the sugars in grape must, producing a base wine. The second stage consists of fermenting the base wine in a stopped bottle and aging (traditional method). The traditional method of sparkling wine production presents a unique biochemical challenge for _Saccharomyces cerevisiae_, requiring yeast to conduct alcoholic fermentation in extreme conditions of high ethanol (10-12% \(v/v\)), elevated CO\({}_{2}\) pressure (5-6 bar), and nitrogen limitation in sealed bottles. Therefore, nitrogen metabolism plays a crucial role in sparkling wine production during the second fermentation in the bottle. Amino acids, key components of this process, significantly influence the composition, quality, and sensory characteristics of sparkling wines [1]. These organic compounds impact various aspects of wine production and aging, from fermentation to the development of complex flavor profiles [2]. Sparkling wines contain various amino acids, with L-alanine, L-glutamic acid, L-lysine, L-arginine, L-phenylalanine, L-valine, and glycine being particularly prevalent [3]. The protein content and specific amino acids (L-arginine, L-asparagine, L-histidine, and L-tyrosine) are significantly correlated with foam stability [4; 5]. The amino acid composition of sparkling wines is influenced by factors such as grape variety, maturity, winemaking techniques, and aging processes [6]. During fermentation, amino acids serve as essential nutrients for yeast, contributing to the production of volatile compounds that shape the wine's aroma profile [7; 8]. As sparkling wines age, particularly those produced using the traditional method, amino acids participate in various chemical reactions, including the Maillard reaction, which generates compounds associated with desirable sensory attributes such as bready, roasted, and caramel notes [2; 9]. Other important compounds associated with nitrogen metabolism are biogenic amines. Biogenic amines are detrimental to health and originate in food, mainly from the decarboxylation of the corresponding amino acid and the transamination of aldehydes and ketones [10]. Biogenic amines play a complex role in the flavor profile of sparkling wines as a flavor influence, aroma influence, concentration variation, and due to their aging effects. During aging, the concentration of some biogenic amines may increase, which can influence the evolution of flavors over time. Several enzymes are involved in the catabolism and synthesis of amino acids in wine: decarboxylases (catalyze the decarboxylation of amino acids, leading to the formation of biogenic amines), transaminases (play a role in the interconversion of amino acids involved in reamination reactions), proteases (break down proteins into peptides and amino acids through enzymatic hydrolysis), Ehrlich pathway enzymes (crucial for the catabolism of aromatic amino acids, involving transaminases, decarboxyllases, and dehydrogenases), glutamate dehydrogenase (involved in the incorporation of ammonia into glutamate), glutamine synthetase (catalyzes the formation of glutamine from glutamate and ammonia, playing a role in nitrogen assimilation), and sulfur amino acid metabolism enzymes (active during the second fermentation, involving the metabolism of methionine and cysteine) [1]. The activity of these enzymes can be influenced by various factors, including yeast strain, fermentation conditions, and nitrogen availability. Understanding and manipulating these enzymatic processes can help winemakers to modulate the amino acid profile and, consequently, the sensory qualities of the wine [7]. The aim of this work is to characterize strain-specific nitrogen metabolism strategies during second fermentation and to establish the relationships between nitrogen utilization, protein expression, and fermentation performance in the two tested flor yeast strains.