Notes on winemaking

While updating my note about food fermentation, I realized how much wine-specific information it contained. As I am really interested in this topic, here is a living note about winemaking specifically.

Winemaking key steps

Most of the time, the process of winemaking can be summarized in a few key steps.

1. Growing and harvesting the grapes
2. Crushing and destemming the grapes
3. Pressing (can occur before or after all fermentation)
4. Primary ethanol fermentation
5. Secondary malolactic fermentation
6. Clarification (filtering and/or fining)
7. Aging
8. Bottling

Those steps are not used for all wines. For instance, traditional Georgian wines usually need the grapes to be crushed, but not destemmed. Many white wines will also not go through a malolactic fermentation.

Crushing, destemming and pressing the grapes

Crushing means gently squeezing the grapes to break the skins and let the juice out. Destemming is taking the grapes off the stems. In traditional winemaking, people sometimes crush grapes by stomping on them barefoot. Pressing means squeezing grapes to separate the juice from the solids.

A very tannic wine will create a dry, puckering sensation in your mouth. These tannins are originally present in the grapes seeds, skin, and stems. The less contact there is between these three elements and your final grape must, the less tannic your wine will be.

To avoid tannins in white wines, winemakers usually skip the crushing phase entirely: the grapes are directly pressed, their juice is extracted with minimal contact with skins and stems.^[1]

The opposite is true for red wines, winemakers want to incorporate some tannins (and skin color) into the final result. The juice is allowed to stay in contact with the dark skins for any duration of time. As soon as the fruit skin is pierced, the first ethanol fermentation starts (see fermentation process explained below). However, the skin contact period doesn't have any impact on that fermentation process ­— it only affects the wine color and tannin level.

All in all, it is possible to mix-and-match those maceration methods. Here is a table that summarizes them (from Wikipedia):

First ethanol fermentation

Ethanol fermentation or alcoholic fermentation is the process where yeasts — a.k.a. mushrooms — convert sugars into energy in an anaerobic environment^[2]. The by-products of this transformation are ethanol and carbon dioxide — in other words, alcohol and bubbles. The "bubbles" part is not so important, as the winemaker will usually let that carbon dioxide escape.

With ethanol fermentation, the yeasts die after a while: either when all the sugars have been converted into ethanol or when the alcohol content has reached about 15% alcohol per unit volume — strong enough to kill all types of yeasts.^[3]

Winemakers use this yeast property to make fortified wines (e.g. Porto wine). They can stop the fermenting process by adding some extra alcohol in the wine. This usually results in a sweeter and stronger wine. The fermenting process can also be stopped by simply dropping the temperature of the tank.

Carbonic maceration is specific ethanol fermentation technique. It is usually used for wines like Beaujolais, and results in wines with low tannins that should be drunk young.

Instead of fermenting from crushed grapes, most of the fruits are kept whole in a sealed container. The fruits at the bottom are crushed­ — because of gravity — and undergo conventional ethanol fermentation. The carbon dioxide produced saturates the air, creating an anaerobic environment.^[4] The carbon dioxide gas stimulates the fermentation process, which occurs inside each berry.

Second malolactic fermentation

In winemaking, malolactic fermentation is usually done after the first ethanol fermentation, by introducing lactic acid bacteria (LAB) into the must. Those bacteria will consume the tart-tasting malic acid, naturally present in grape must, and produce softer-tasting lactic acid and carbon dioxide.

The final aromas are said to be less acid, apple-like and more buttery, milky, oily or vanilla-like. Most red wines undergo malolactic fermentation, as well as some fuller white wines like the stereotypical Chardonnay.^[5] On the contrary, winemakers prevent wines such as Riesling, Gewürztraminer, and Sauvignon Blanc to go through malolactic fermentation to maintain a more tart or acidic profile.

One of the main reasons to undergo malolactic fermentation is to stabilize the wine and avoid having the fermentation start uncontrolled later on. Wild LAB native to the grapes (or present in the cellar) can initiate a spontaneous malolactic fermentation that might give an off-taste to some natural wines. That's why a specific desirable bacterium is often consciously added to the wine for this second fermentation.

There are a few ways to prevent malolactic fermentation. The first method is to use sulfites (sulfur dioxide), as LAB can be very sensitive to it. Sterile filtration is also an option, as it filters out almost all bacteria and yeast, ensuring the wine's microbiological stability.

A winemaker has to make sure the malolactic fermentation has run its course before bottling their wine. If not, the fermentation might continue in the bottle, adding carbon dioxide to the drink and making it slightly sparkling.^[6]

Aging

Aging a wine before bottling is optional. Depending on the recipient, the wine will have more or less oxygen exposure, and be impacted differently in terms of flavor and texture. Some aging recipients include:

• Oak barrel
• Clay amphora (Qvevri)
• Stainless steel tank
• Concrete tank
• In-bottle

The "sur lie" method is a winemaking technique where the wine is aged on its lees, which are the dead yeast cells and other particles that remain after fermentation. Instead of removing the lees right away, the winemaker leaves the wine in contact with them for weeks to several months, sometimes longer.

The breakdown of yeast cells (called autolysis) releases compounds that change aroma and taste. Some of these compounds give the wine aromas of butter, nut, and yeast. Proteins also result from this breakdown: they bind to the tannins and create a smoother mouthfeel.

This is my living note on winemaking — so it might appear a bit dry and incomplete. I will update it as I learn more about this topic! ⊙