Addition of Yeast
Some wineries rely on ambient yeast cells to come in contact with the grape must naturally. Most modern wineries, however, choose to control the environment more closely, and add their own strains of yeast in an enclosed tank.
When yeast comes in contact with grape sugars, it converts them into alcohol, heat, and carbon dioxide. The riper and more sugary the grapes, the stronger the final wine can potentially become.
Because this reaction produces heat, it is important for winemakers to keep the temperature low to prevent the wine’s flavor from boiling off. Up to that point, though, the warmer the temperature, the stronger the flavor. With a long, cool fermentation, a winemaker can produce a light, fruity wine.White wines are fermented in a closed tank to prevent oxidation and to avoid the must from browning. Red wines have their own protection as they are vinified with the skins. The skins rise to the top of the tank and form a cap. This cap is usually physically persuaded to interact with the must by several different methods. Winemakers can either pump the must over the cap, which is commonly referred to as “pumping over,” or physically punch down the cap into the must, “punching the cap.” Some wineries rely on machines to do this, which ensures a very exact process that tends to help soften tannins.Many wines are fermented in small oak barrels, especially serious white wines. The exception here is light bodied, aromatic white wines. These are usually fermented in steel tanks to preserve the grapes’ natural fruit character.
Carbonic Maceration is a type of fermentation that is extensively but not exclusively practiced in the Beaujolais region of France. Whole bunches of uncrushed grapes are placed in a closed tank. The weight of the grapes on top crushes those on bottom, releasing juice that ferments naturally. The juice in the uncrushed grapes ferments within the grape. This process can take from one to three weeks depending on temperature and is usually done in a controlled, oxygen-free environment. The result is an extremely bright, fresh and juicy style of wine.
During fermentation, a winemaker makes the decision whether or not to add acid (acidification) or sugar, for the purpose of raising the alcohol content, not sweetness. This process of adding sugar is called chaptalization. In many areas, these practices are not allowed under the regions’ appellation laws. In many areas of France, however, chaptalization is common in years that are too cool for grapes to ripen fully and develop the necessary levels of sugar.
Next: Malolactic Fermentation