underground winery

After the last winter frost has passed and the grape vines lay dormant, viticulturists prepare for the coming spring while the air is still cold. At this time of year, they have two jobs. The first is pruning, removing dead and unhealthy stems to leave only the most promising buds. The second is training wire maintenance, repairing and lifting the wires so that when the buds and shoots awaken in the warming weather, their leaves enjoy the greatest possible sunlight exposure.

Both are done in strategic preparation for the fall when ripe, sweet grapes are harvested, extracted, and fermented. Fermentation has been used to make alcoholic drinks for millennia, and wine fermentation depends on an ages-old combination of agriculture, biology, and chemistry.

Ripening the Grapes

The journey from vine to wine begins in the vineyard, where grapes are carefully tended and monitored as they ripen. As harvest approaches, winemakers closely track the grapes' sugar content, acidity, and flavor development to determine the optimal time to pick.

During ripening, the grapes undergo a process called veraison, where they change color and begin to accumulate sugars. Sunlight is essential for this process, as the grapevine leaves use photosynthesis to produce glucose and fructose. The sugars are then transported to the grapes, increasing their sweetness as they mature.

The amount of sugar in the grapes at harvest is critical, as it determines the potential alcohol content of the finished wine. Winemakers measure the sugar content in degrees Brix, with one degree roughly equating to 1 gram of sugar per 100 grams of grape juice. Most wines are harvested between 20-26 degrees Brix, which can yield alcohol levels of 11-15% after fermentation.

Harvesting and Juice Extraction

Once the grapes have reached the desired level of ripeness, they are harvested by hand or machine and transported to the winery to prevent spoilage. Damaged or underripe fruit are removed before the high-quality grapes are crushed or pressed to extract the juice.

For red wines, the grapes are typically crushed and fermented with the skins, seeds, and stems, all contributing to color, tannin, and flavor. For white wines, the grapes are pressed to immediately separate the skins and stems from the juice, resulting in a clearer, lighter-colored wine.

The freshly extracted grape juice, called must, is then ready for fermentation. The must may be left to settle for a short period to allow any solid particles to sink to the bottom before the clear juice is racked off and transferred to fermentation vessels.

Yeast Fermentation

The heart of the winemaking process is fermentation, where yeast consumes grape juice sugars, converting them into alcohol and carbon dioxide. Depending on the style of wine being made, this process typically occurs in stainless steel tanks or oak barrels.

Yeast is naturally present on grape skins, but many winemakers choose to inoculate the must with a specific strain of cultured yeast to ensure a consistent and predictable fermentation. The yeast is added to the must and begins to feed on the sugars, producing alcohol and CO2 as byproducts.

As the yeast population grows and the fermentation progresses, the temperature of the must increases. Winemakers carefully monitor and control the temperature to prevent the yeast from becoming stressed or producing off-flavors. Red wines are typically fermented at higher temperatures (75-85°F) to extract more color and tannin, while white wines are fermented at cooler temperatures (45-60°F) to preserve their delicate aromas and flavors.

Throughout fermentation, the yeast consumes the available sugars until they are depleted, at which point the yeast cells begin to die off and settle to the bottom of the fermentation vessel.

How Long Does Wine Take to Ferment?

On average, the primary wine fermentation process takes around two weeks, although some wines take longer. Red wines tend to have longer fermentation times as the grapes contain more sugar, and the skin and seeds require more time to break down.

After the primary fermentation, some wines undergo a different type of fermentation known as malolactic fermentation (MLF). Bacteria convert the tart malic acid into softer lactic acid. MLF takes from a few weeks to a few months, depending on the wine and the winemaker's goals.

Most red wines, including Cabernet Sauvignon, Merlot, and Shiraz undergo malolactic fermentation. It is less often used in white wine production, but there are exceptions, including Chardonnay, some Viognier, and other full-bodied whites.

Following fermentation, wines are typically aged for a period of time before bottling. While the primary fermentation of wine generally takes 5-14 days, the entire process from grape to bottle can span several months to years, depending on the specific wine and the techniques employed by the winemaker.

Secondary Alcohol Fermentation in Sparkling Wine

For sparkling wines like Champagne and Cava, a secondary fermentation creates the signature bubbles and effervescence. After the base wine has completed its primary fermentation, it is bottled with a sugar and yeast mix called the liqueur de tirage.

The bottles are then sealed and stored on their side. As the yeast consumes the added sugar, it produces alcohol and CO2 that are trapped in the bottle and dissolved into the wine, creating the bubbles.

After several months or years of aging on the lees (dead yeast cells), the bottles are riddled (gradually turned and tilted) to collect the sediment in the neck of the bottle. The necks are then frozen, and the sediment is disgorged, leaving a clear, sparkling wine ready for dosage, a small addition of sugar to balance the acidity, and final corking.

Wine fermentation is a complex and fascinating process that transforms simple grape juice into a beloved beverage enjoyed around the world. From the careful tending of the vines to the final bottling, every step of the winemaking process requires skill, knowledge, and a deep understanding of the science behind fermentation.