The role of mannoproteins in wine quality

Mannoproteins are among the most critical yet less understood components that contribute to the quality of wines. These molecules, belonging to the class of glycoproteins, are proteins attached to sugar molecules, specifically mannose residues. Found predominantly (25-50%) in the cell walls of Saccharomyces cerevisiae yeast, mannoproteins play a pivotal role in providing structure and rigidity to the cell through proteoglycans containing 5-20% peptide portion and 80-95% mannose sugar chains.

The release of mannoproteins occurs primarily after the yeast cells' death, during a phase known as autolysis. In this process, β-glucanases are released into the periplasmic space, leading to the breakdown of the cell wall and the liberation of high-molecular-weight mannoproteins, usually bound to the cell wall's glucans. Further enzymatic hydrolysis enriches wine with mannoproteins and peptidomannans, the latter generated from mannoproteins through the action of α-mannosidases and proteases.

Mannoproteins significantly impact the wine's quality through various mechanisms. They enhance the wine's stability—tartaric, by preventing crystallization reactions; protein and color stability, by interacting with wine tannins and proteins. They also improve organoleptic perceptions, adding to the sensation of body and volume. Furthermore, mannoproteins stabilize the aromatic fraction, delaying its perception and extending the aftertaste. Their role in aggregating thermolabile proteins reduces the need for bentonite during clarification and supports the growth of lactic acid bacteria, aiding malolactic fermentation. In red wines, they reduce astringency and bitterness by interacting with phenolic compounds.

The association of mannoproteins with lees aging is particularly noteworthy. Lees, the dead yeast cells and other microorganisms left after fermentation, undergo autolysis, releasing mannoproteins that modify the wine's physicochemical properties and organoleptic profile, often in desirable ways. Aging on lees provides wines with more volume, unctuousness, and varietal character, contributing to integration and rounding of flavors while preserving aroma persistence. Additionally, the natural protective function of mannoproteins reduces the need for sulfites. However, lees aging requires careful monitoring to prevent reduction issues and microbial deviations, with enzymatic preparations or yeast derivatives serving as alternatives to manage these risks.

Mannoproteins play an essential role in enhancing wine stability (protein, tartaric, and color) and improving its sensory attributes, including aroma persistence and mouthfeel volume. Various methods, such as using manoprotein-rich preparations or selecting manoprotein-producing yeast strains, can increase their concentration in wine. This article aimed to shed light on the importance of mannoproteins in both the sensory aspects and stability of wine, enhancing our understanding and appreciation of this beloved beverage.