What are the primary metabolic pathways of non-Saccharomyces yeasts during wine fermentation, and how do they influence the final wine's sensory profile?

Non-Saccharomyces yeasts, present naturally on grape skins and in wineries, play a role in the early stages of wine fermentation before Saccharomyces cerevisiae becomes dominant. These yeasts have distinct metabolic pathways that significantly influence the final wine's sensory profile. One key pathway is the production of glycerol. Many non-Saccharomyces yeasts, such as Metschnikowia and Candida species, produce higher levels of glycerol than S. cerevisiae. Glycerol contributes to the wine's mouthfeel, adding a perception of sweetness and viscosity. Another important metabolic pathway is the production of volatile aroma compounds. Non-Saccharomyces yeasts can produce a variety of esters, terpenes, and other aroma compounds that contribute to the wine's complexity. For example, Torulaspora delbrueckii can produce esters that impart fruity and floral aromas, while Hanseniaspora species can produce terpenes that contribute citrus and tropical fruit notes. However, some non-Saccharomyces yeasts can also produce undesirable aroma compounds. For instance, Brettanomyces produces volatile phenols, which can contribute to barnyard, medicinal, or smoky aromas that are considered spoilage characteristics at high concentrations. Another relevant pathway is the production of organic acids. Some non-Saccharomyces yeasts, such as Schizosaccharomyces pombe, can metabolize malic acid, reducing the wine's acidity. This can be beneficial in wines with high acidity, but excessive malic acid degradation can lead to unbalanced wines. Furthermore, non-Saccharomyces yeasts can influence the nitrogen metabolism during fermentation. They consume different nitrogen sources compared to S. cerevisiae, potentially impacting the availability of nitrogen for S. cerevisiae and influencing the production of volatile aroma compounds. The impact of non-Saccharomyces yeasts depends on the specific species, their relative abundance, and the fermentation conditions. While they can contribute to wine complexity and enhance certain aroma characteristics, their activity must be carefully managed to prevent the development of off-flavors or spoilage.