The lees in grape juice are essentially dead yeast cells that remain after the fermentation process. When making wine, liquid fermentation is employed, where active yeast is added to grape juice. This yeast consumes the sugar present in the juice and converts it into two main byproducts – alcohol and carbon dioxide.
During the fermentation process, the yeast cells multiply rapidly and consume the sugar in the grape juice. As the yeast metabolizes the sugar, it produces alcohol as a result. The carbon dioxide is also released as a byproduct, creating bubbles or fizziness in sparkling wines.
Once the fermentation is complete, the yeast cells that have consumed all the available sugar start to die off and settle at the bottom of the fermentation vessel. These dead yeast cells, along with other sediment and grape solids, form what is known as the lees.
The presence of lees can have both positive and negative effects on the final product. On one hand, lees can contribute to the flavor and complexity of the wine. The dead yeast cells release compounds during their decomposition process, which can add unique aromas and flavors to the wine. These compounds, such as amino acids and polysaccharides, can enhance the mouthfeel and overall character of the wine.
On the other hand, excessive amounts of lees can also have negative effects. If the wine remains in contact with the lees for too long, it can develop off-flavors and a “yeasty” or “bread-like” taste. This is particularly true for wines that undergo extended lees aging or are left on the lees for an extended period after fermentation.
Winemakers have different techniques for managing the lees in grape juice. One common practice is called “racking,” where the wine is transferred from one vessel to another, leaving the sediment behind. This helps to clarify the wine and remove any unwanted flavors that may have developed from prolonged contact with the lees.
In some cases, winemakers intentionally leave the wine in contact with the lees for a certain period to enhance its flavor and texture. This technique, known as “sur lie aging,” is commonly used in the production of certain white wines, especially those from regions like Burgundy in France. Sur lie aging can impart a creamy and complex character to the wine, adding depth and richness to the final product.
Lees in grape juice are the dead yeast cells that remain after the fermentation process. They can contribute to the flavor and complexity of the wine, but prolonged contact with the lees can also result in off-flavors. Winemakers employ various techniques to manage the lees and achieve the desired characteristics in the final product.