Wine fermentation happens when the sugars of a grape is converted into alcohol by extremely active cells of yeast. Fermentation is the ancient process that chemically transforms grapes into complex aromas and flavors found in wine.
Where Does Wine Fermentation Happen?
After the grapes are picked in the vineyard, they are typically sorted into “good fruit” or “bad fruit” categories and the “good” fruit (free from shriveling, mold, etc.) is destemmed and crushed before the juice is fermented.
Wine fermentation typically takes place in stainless steel or concrete tanks as well as in neutral oak barrels.
Wine Fermentation and Yeast
The yeast used in fermentation may be naturally occurring, as those commonly found on a grape’s skin in the vineyard or they may be a specific strain of cultured yeast that the winemaker introduces to keep fermentation consistent, while producing desirable flavor and aromatic compounds. The role of the yeast is to consume the grapes’ sugar and break it down into alcohol and carbon dioxide while generating complex flavor and aromatic compounds in the process. Fermentation is the reason that wine smells and tastes like way more than mere grape juice. The process of fermentation creates thousands of stereoisomers that allow wine, made only from grapes, to smell like violets, cherry and cocoa and taste like strawberries, fig and blackberry jam.
Factors Affecting Wine Fermentation
.The higher the sugar content in the grape the higher the alcohol content in the wine, if there is not vintner intervention.
The common form of sugars that reside in a grape’s juice are the fairly familiar glucose and fructose.
Sweet Wines – The primary role of grape sugar is to be converted into alcohol. Sweet wines may be made by either interrupting the fermentation process by lowering the fermentation temperature (and killing the yeast) to halt the conversion of sugar to alcohol or by adding a distilled spirit, also known as fortification, which also kills the yeast and stops the conversion process. Both methods result in higher levels of residual sugar and sweeter wines. Key examples of lowering fermentation temps include Riesling and Moscato. While Port wines would be the most famous example of adding a distilled spirit to fortify the wine.
- White Wine Fermentation – White wines are fermented with just the juice, sans skins, seeds and pulp. They are typically fermented at chilly temperatures to slow the conversion of sugar to alcohol and retain the freshness factor that is so desirable in today’s white wine. Fermentation for white wines usually lasts between 2 to 6 weeks.
- Red Wine Fermentation – Red wines are typically fermented with their skins, seeds and pulp to add color, tannin, as well as aromatic and flavor compounds. Fermentation temperatures are warmer than white wines and usually the process takes less time, typically 1 to 3 weeks in the tank. After fermentation, the grape solids and “floaters” like seeds, pulp and skins are removed and the wine goes through a second fermentation called malolactic fermentation. This second fermentation, dubbed “malo” for short, takes lactic bacteria and converts the harsh, tart malic acids (think green apple) into softer, silkier lactic acids (think milk), reducing overall acidity and improving textures and overall palate impressions.
Wine Fermentation 101
Technically speaking in alcohol fermentation, sugar + yeast = alcohol, CO2, heat and flavor compounds
Fermentation is where the magic happens. After harvest the grapes are sorted and separated, slightly crushed to split the grape’s skin and allow the juice to flow. The grape juice, skin, and seed are collectively called the “must.” In the case of red wines these essential components will all be fermented together to extract the color and tannin from the grape skins. With white wines, fermentation generally takes place with only the grape juice, skins are removed prior to the startup of the fermentation process. Yeast, either naturally occurring or more typically added by the winemaking team, jump starts the metabolic process of converting the grape’s innate sugars into alcohol.
This chemical conversion results in the formation of thousands of chemical compounds. Many of them are highly aromatic chemical compounds ranging from fruit to floral, and veggie to earthy. It’s these unique chemical compounds that give wine, made only from grapes, a spectrum of smells ranging from fruits, flowers, earth and a variety of other familiar scents, not just grape juice. Typically wine fermentation takes place in stainless steel tanks or neutral oak barrels. The primary fermentation process typically takes anywhere from one to six weeks depending on the wine. With all red wines and some white wines (like Chardonnay) going through malolactic fermentation.
Sparkling wine and Champagne undergo a unique secondary fermentation designed to capture the CO2 bubbles. This bubble-making process takes place either in a tank (the Charmat method) or in the bottle (the traditional Champagne method). In both cases of secondary wine fermentation, additional yeast and sugar is added to the base wine to initiate another round of fermentation, but this time in a closed or capped environment to trap the carbon dioxide bubbles, resulting in the famous bubbles of sparkling wine.
Fermentation is mission critical to all winemaking endeavors. While it has been utilized for centuries in the making of wine, cheese, beer, and flour, today’s modern winemaking systems and temperature controlled technology allow vintners to fine tune the fermentation process to deliver wines of exceptional quality and freshness vintage in and vintage out.
By Stacy Slinkard, About Food