Part 1 of “Vine to Wine” covered the making of wine from after the grapes have been picked until the juice is ready to be fermented. Here we will cover the fermentation process in a winery.
Once you have the juice (called must at this point) it gets put into the fermentation vessel. Fermentation vessel is just a fancy name for whatever container they are going to use to ferment the wine. There is a huge variation in the size of the fermentation vessels used, from just a few gallons for home wine makers or experimental batches to thousands of liters for big industrial wineries. There is a range of materials used to make the vessels too. The traditional material used is wood. Concrete, glass, fiberglass, and stainless steel are also used. In ancient times, wine was fermented in ceramic amphorae and some wine makers are experimenting with doing that again.
A note on temperature control: The temperature a wine is fermented at makes a big difference in how it tastes. Fermentation itself generates heat. Wine makers deal with this a few different ways. Some use smaller vessels and/or cool fermentation rooms that allow the heat from fermentation to dissipate more quickly. Some use vessels that have cooling systems built into them. Some do nothing and let nature take its course.
The wine maker may add yeast or he or she may count on the naturally occurring yeast found on grape skin to do the job. The benefits of each approach are endlessly debated by wine geeks. In brief (and ducking to avoid as many as possible of the bombs that will be lobbed by wine geeks) here are some of the benefits of each approach. Adding yeast can make for a more consistent product. It ensures that you know what the dominant strain of yeast in the wine is, how much yeast you are starting with, and when fermentation will start. Using wild yeast is a traditional method (tradition is important in the wine world) and can result in more complex flavors.
Once the wine is in the fermentation vessel and you might think the wine maker would get a bit of a break but not so. For most wines, the winemaker wants to make sure that as much as possible of the sugar in the grape juice gets fermented. Yeast has a tendency to settle to the bottom of a fermentation vessel. If you have a small vessel this can happen pretty quickly. If you have a large vessel you can have part of the vessel with plenty of yeast while the top has very little. Bad things can happen to musts that have sugar but not enough yeast or alcohol. One of the worst is they can turn to vinegar instead of wine. This happens when acetobacter bacteria ferments the must instead of yeast. Just as yeast makes alcohol, acetobacter makes vinegar.
To prevent this, many winemakers use a technique called pumping over. It is pretty much what it sounds like. They use a pump to move wine from the bottom of the vessel to the top. Sometimes it is just a circulation thing and sometimes the wine is sprayed over the top to increase the level of oxygen in the wine. Usually pumping over is only done in the early stages of fermentation. For some styles, such as Sherry, it is done periodically through the process.
Some wines are quickly fermented (for a matter of several days to a couple of weeks) and then moved on to the storage or bottling processes. Some wines are allowed to ferment for much longer. If so, the wine is frequently moved from one fermentation vessel to a second one. The purpose is to remove the wine from the various solids such as yeast and bits of grape skin, etc. that have settled to the bottom of the vessel. These solids are called lees. Some styles of wine are left in contact with the lees for extended periods of time. Some wines are removed from the lees quickly and may be moved from vessels more than once. The process of moving the wine from one vessel to another is called racking.
There is one more process to talk about before we let our wine go off to the aging cellars or the bottling line. This has the technical name of malolactic fermentation. Malolactic fermentation is a pretty fancy name for a pretty simple and common process. If you want to sound like a wine geek you can always ask “Has this wine undergone malolactic fermentation?”
There is a fair amount of acid in wine and that is a good thing. One kind of acid in wine is malic acid. If some of the fermentation takes place at warmer temperatures, there is a type of bacteria that “eats” malic acid the way yeast “eats” sugar. With yeast you get alcohol and carbon dioxide. With the malic acid eating bacteria you get lactic acid. Lactic acid is still an acid but it is less strong of an acid than malic acid. It is the acid in milk.
Red wines, which are typically fermented at warmer temperatures than whites or roses, almost always undergo malolactic fermentation. You won’t come across like a wine geek if you ask about it for a red wine.
With white wines that have been fermented at cooler temperatures, the winemaker has to decide if she or he wants to let the wine warm up and undergo malolactic fermentation. One way is not better than the other. They are just different. Wines that have gone through the process tend to be less acidic and taste “bigger” or “rounder”. They tend to feel smoother in the mouth. If they haven’t been through the process they tend to taste crisper and be more refreshing.
Part 3 will go through aging and bottling.