White wines are typically not fermented with their skins and seeds attached. Most white wine grapes go directly into a pneumatic wine press which gently squeezes the grapes with an elastic membrane. This is how it works:
Some white wines soak with the skins and seeds for a short period of time. This adds phenolics (like tannin) but overall, it increases the richness of white wines. (BTW, this is how orange wine is made!)
Juice and grape must is now transferred to fermentation vessels.
There are many different kinds of fermentation tanks. The three most popular types are wood, stainless steel, and concrete. Each has their own unique traits that affect how the wine ferments.
Next comes the most important part: the yeast.
Many winemakers opt to use commercial yeasts to better control the outcome of the fermentation.
Other winemakers develop their own local yeast strains or let nature take its course and allow “wild” yeasts ferment the wine naturally.
Either way, here’s essentially how it works:
Yeast consumes the sugar in the grape must and then poops out ethanol.
Grape must sweetness is measured in Brix and very basically, 1 Brix results in 0.6% of alcohol by volume.
For example, if you pick grapes at 24º Brix, you’ll get a wine with 14.5% alcohol by volume. (The actual concept is a bit more complicated, but this dirty fast version works!)
Red wines ferment a bit hotter than whites, usually between 80º – 90º F (27º – 32º C). Some winemakers allow fermentations to rise even higher to tweak the flavor.
White wines, on the other hand, need to preserve the delicate floral and fruit aromas, so they’re often fermented a lot cooler, around 50º F (10º C) and up.
While the wine is fermenting, carbon dioxide is released, which causes grape seeds and skins to rise to the surface.
Some winemakers control this by punching down the “cap” three times a day.
Other winemakers prefer to use “pump overs,” where juice from the bottom is gently poured over the top of the skins and seeds.
The choice of “punch down” vs “pump over” really depends on the type of wine grape and desired taste profile. Generally speaking, lighter wines use punch downs and bolder wines use pump overs. But, as with all things wine, exceptions abound!
When the fermentation is done, it’s time to rack the wine out of the fermentation vessel.
The juice that runs free (without being pressed) is generally considered the purest, highest quality wine. It’s called “free run” wine and is kind of like the “extra virgin” wine.
The rest of the wine is “press wine” and is generally slightly more rustic, with harsher-tasting phenolics.
Press wine is typically blended back into the free run wine. (Remember: the less waste, the better!)
Finally, the wine moves into what the French call “élevage.” Élevage is like a fancy way of saying, “waiting around.”
That said, a lot happens in the winery while we wait for wine to cure into something great.
Wines go into barrels, bottles, or storage tanks. Some wines will wait for five years before being released; others, just a few weeks.