Wine Basics – Vine to Wine (Part 2)

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.

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7 Responses to Wine Basics – Vine to Wine (Part 2)

  1. The Whiner says:

    How does adding yeast (as opposed to relying solely on natural yeast) let the winemaker know how much yeast the wine starts with? I would think that the starting amount of yeast is a bit of a mystery either way. Whether the winemaker adds yeast or not, there is the yeast on the grape skin and the winemaker doesn’t know how much of that there is.

    • admin says:

      Well you are right. I should have said it lets the wine maker know there is enough yeast. It is hard to have too much yeast when you start. Although in the right conditions yeast multiplies geometrically, so do other competing microorganisms. If you don’t have enough yeast there is a high risk of having a bad batch of wine. If you add yeast you cut the risk of a bad batch.

      That said some winemakers go over the top and treat the grapes or juice to remove or kill wild yeast before adding the yeast they want. Its not common though.

  2. Alex N. says:


    This post has brought me some nice memories of my childhood back in Romania. My grandparents on my mother’s side have a house in the Dobrogea region, by the Black Sea, a region well known in Romania for its wines (the Murfatlar wines being the most popular). Just like everyone else, they have grapes planted in their front yard, and grandpa used to make his own wine every year. Rarely did it ever last until the next vintage 😉 . There was no meal without a couple of glasses of spritz (wine cut with sparkling water). We have a name for young wines like that that have not settled, it’s “tulburel” (little cloudy one).

    But what really made me melancholy was the must. At harvest time, one of the more delightful pleasures is to sample the year’s must and make right old feast of it, serving it with grilled mutton pastrami, polenta, and garlic sauce…

    Keep the good work coming!

    • Steve M says:

      Hey Alex-what a great post-are any of these Romanian wines available comercially in the U.S?

      • Alex N. says:


        I know I am terribly behind on replying to you, my apologies. Unfortunately, many, if not most of the good wines are not available in the US. And conversely, most of the Romanian wines that you find, I would never call good. Let’s hope that will change, but knowing Romanians, I would not hold my breath.


  3. Hillary Murtha says:

    What’s must? Is that what the grape juice is called as it’s in the initial stages of fermentation? As a wine-process ignoramus, I think of “must” as a moldy smell – ie, this kitchen sponge smells musty.

    I must know …

    • admin says:

      Yep “must” is the juice before it is fermented and in the early stages of fermentation. The whole point of the “Wine Basics” posts is to be a resource for people. Hope it helps.