To make white wine is quite simple in concept. A winemaker gets some freshly harvested grapes, presses the juice out of them, ferments that juice using yeast, lets it mature and then bottles the wine.
In reality, the process takes twists and turns at each stage, even though grape juice and yeast are the only necessary ingredients.
How white wine grapes are harvested
Freshness is vital to make quality white wine. As soon as a crew plucks the grapes from the vines, the rush is on.
Typically, harvest takes place early in the morning when the grapes are cool from the night air. In some cases, mobile lighting rigs illuminate the vines so workers can do their jobs even before sunrise.
The grapes are delivered quickly to the winery in bins, trailers or on truck beds. They’re pressed in a matter of hours to get the juice and pulp out of the skins. Grapes harvested by hand are in clusters or bunches. Those harvested by machines have already been removed from their bunches.
Whole bunches go through a destemming machine usually to separate the grapes from the stems. It also serves to crack the grapes open gently before pressing. Any juice that is created at stages prior to the press is known as free run. Winemakers that seek a more classic, and usually higher-priced, white wine deposit bunches or clusters into the press whole.
How do they press grapes?
Wine presses come in many shapes and sizes. The classic wooden (or steel) basket press pushes down on the grapes to squeeze the juice out of the skins, which are left behind to be composted. A bladder press works like a balloon that’s inflated inside a tank. Pressure forces the grapes to the sides, where the juice is pushed through screens.
At this stage, many winemakers add sulfur dioxide gas or potassium metabisulfite to neutralize any spoilage microbes and native yeast on the grapes. It also prevents the juice from absorbing too much oxygen. Other winemakers refrain from this until the juice is fermented.
Once the juice is free of the skins, the winemaker pumps it into a chilled tank to let it settle for at least a few hours. Bits of skins, stems and other debris drop to the bottom so the partially clarified juice on top can be removed, or “racked,” to another tank or to barrels. It’s now ready for fermentation.
White wine fermentation(s)
Fermentation is not magic, but it can seem like it. After yeast is added, the juice rests for a day or more. It will begin to foam, warm in temperature and exhale potent fruit vapors and dizzying carbon dioxide. It appears to be alchemy, but it’s really biochemistry.
When yeast hooks up with sweet grape juice, it converts sugar to alcohol, consumes oxygen and produces carbon dioxide and heat. The winemaker guides the process by heating, cooling, stirring, aerating and sometimes feeding the yeast until most or all of the sugar has converted to alcohol.
Commercial laboratories produce dozens of yeasts for winemakers to choose, depending on the grape variety and type of wine they want to produce.
It’s not always necessary to add yeast, however. Native yeast is already present in microscopic form in virtually every vineyard and winery. These tiny yeast cells will wake up when exposed to the juice, begin to feed on the sugar and multiply.
The vast majority of the world’s white wine ferments in stainless steel tanks. Some, notably Chardonnay, may ferment in oak barrels. Fermentation in new barrels adds significant flavor and texture to white wine. Barrel fermentation in used or neutral barrels contributes mostly a smooth texture.
What is malolactic fermentation?
During yeast fermentation or the maturation period, winemakers can allow or prevent malolactic fermentation (ML). Instead of yeast, it’s bacteria that conducts this conversion in the new wine. It transforms malic acid, which has a green apple-like tartness, into more buttery-tasting lactic acid.
This second fermentation is different than the one that occurs with sparkling wines.
The maturation process can be as little as four months for a fresh, light white, or two years and more for some reserve-style white Burgundies or other ageworthy whites.
Another stylistic choice is deciding whether to keep white wine on its lees, a silty layer of dead yeast that forms in the bottom of the tank or barrel. Lees can add a fresh-bread aroma to the wine and protect it from oxidation. When stirred periodically, lees can also contribute a richer mouthfeel.
Are white wines filtered?
During this period, the winemaker clarifies the wine by a variety of methods. The simplest is racking, or siphoning the wine from one barrel to another while leaving behind sediment. Another filtration process is called fining, which uses the addition of egg whites (albumen), isinglass or bentonite to clear up a wine that looks hazy.
Most commercial winemakers also filter their white wines through membranes with micron-sized pores to complete the clarification process and remove any microbes that could spoil the wine in the bottle. Typically, the winemaker makes a final adjustment to the sulfur dioxide level in the wine, which ranges from less than 10 parts per million (ppm) to a legal limit of 250 ppm in U.S. wines and 200 ppm in European wines.
Bottling white wine
These finishing touches must be done with great care to maintain quality. That’s because the wine is vulnerable when it makes the journey from a tank to its ultimate destination of a bottle, can or pouch. All this movement can expose it to oxygen, which can diminish its ageability and rob it of fruitiness.
In the highly automated bottling process of most large wineries, bottles are filled by one machine, and then proceed on a conveyor to the next machine, where they’re sealed with a cork or synthetic closure. That’s followed by a foil capsule or topped with a screwcap. The next machine affixes the front and back labels before yet another places the bottles in boxes, ready for shipping and sale.
The white wine has been made. The job is done. Until, that is, the next harvest season rolls around.