Yeast is the catalyst for fermentation in wine production. A member of the fungi kingdom, this single-celled organism feeds off sugars, like glucose, that are present in grapes and produces alcohol and CO2 as by-products of this process. In addition to creating alcohol, yeast strains can influence the aroma and flavor of wine.
There are many strains of wild or “native” yeast present on the grapes as they come from the vineyard. Wild yeasts found on grape skins can produce unwelcome off odors when they ferment and can stop fermenting at 6-9% alcohol leaving lots of unfermented glucose in an unstable, easy to spoil wine. However, Pinot noir and Syrah grapes like to start fermenting with native yeasts and then the wine maker uses a large amount of cultured yeast near the end to ensure a complete fermentation.
Alternatively, the wine maker can choose to use commercial cultured yeast that has been proven to have specific results in the finished wine. Proponents of cultured yeast strains point to a stronger, steady and stable fermentation, control over the finished product and lower probability of stuck fermentations as advantages. There are different yeast strains for fermenting specific grape varieties.
The form of yeast typically used in winery production comes in a dried state, sold in vacuum packed containers, and must be hydrated and acclimatized to the juice. To do so, the yeast is mixed into warm water (104°F) then slowly added to a portion of the juice it will inoculate. This will provide it with sugar and begin to move the temperature of the yeast preparation towards that of the juice.
Glucose is heavier than ethyl alcohol so the wine maker will follow the progress of fermentation by measuring the drop in densitiy of the fermenting juice with a hydrometer,to estimate degrees Brix. At the beginning of fermentation (days 0-4) the degrees Brix will not drop because the yeast cells are metabolizing and increasing in numbers.
In the second stage, (days 5-11) the degrees Brix fall rapidly as the alcohol increases and the glucose drops. The density of the wine will eventually reach -1.0 to -2.0 degrees Brix and an increase in temperature of the juice, when fermentation is finished.
Yeast.... it's more than beer and bread.