Stainless steel fermentation tanks at Caliterra Winery in Chile.
For white wines, the two most common fermentation vessels are stainless steel and oak. Stainless steel tanks retain fresh fruit flavors in two ways. First, they have cooling jackets that prevent the wine from overheating during fermentation. Second, since stainless steel is inert, they do not add any flavors that may mask the grapes' flavors.
Oak barrels, especially new oak barrels, can add flavors like vanilla, spice and oak. If the inside walls of the barrels were charred over a fire as they were constructed, the wines can also acquire smoky, toasty flavors. Barrels allow small amounts of wine to evaporate, concentrating the flavors and making the wine seem richer. With successive vintages, barrels become relatively neutral and impart fewer flavors to the wine. The drawback with barrels is that they are very expensive and require more care than steel tanks.
Malolactic Fermentation: Malolactic fermentation, or ML, is a secondary fermentation that softens the sharp, tart acidity found in some wines, both white and red. Chardonnay is frequently put through ML to make the wine taste smoother, rounder and richer. ML also adds buttery flavors, increasing the wine's complexity. ML is avoided with varieties like Riesling, with its hallmarks of primary fruit flavors and sharp acidity, and in wines with very low levels of acidity.
ML changes the acid balance of wine in two ways. First, it reduces the total level of acidity, softening the wine. Second, it converts the malic acid found in wine into lactic acid. Malic acid is sharper, like the tart acid found in green apples. Lactic acid is the smoother acid found in milk.
Sometimes partial ML is used to soften the wine slightly. In this case, wine that hasn't seen ML is blended with a portion that has. The winery's tasting notes or the label on the bottle may specify the final blend, using the phrase "50 percent ML" or "75 percent ML," for example.
Sur lie aging: Sur lie means "on the sediment." The sediment is from the expired yeast cells and the grape particulates remaining in the barrel or tank after fermentation. With sur lie aging, most commonly used for Chardonnay and Champagne, the wine develops a rich, creamy mouthfeel.