The Fermentation Equation

We all know what comes out of the bottle, but do we know or understand what goes into the bottle, and how it gets there in the first place? I knew grapes were fermented to make wine, but not any of the specifics. Having read up on this now, I have learned it’s not a one size fits all process (and if it were, how boring would wine be!), and it’s helped me appreciate the decisions these winemakers have to make in order to create their unique product, as well as learn how the various steps in the fermentation process lead to differences we can taste in the wines we drink. There are also steps that can be taken to correct some issues along the way. So let’s dive in!

Fermentation. Without it, we wouldn’t have wine. It’s a simple enough equation, but there are so many steps/decisions/details that affect the end result!

Sugar + Yeast = Alcohol + Carbon Dioxide

Simple, high-level steps for winemaking:

  1. harvest the grapes
  2. choose to destem or keep the stems and seeds
  3. choose to crush the grapes or keep whole
  4. alcoholic fermentation
  5. age the liquid
  6. bottle the liquid
  7. congratulations, you’ve got wine

Harvest. The process of picking the grapes is not as simple as picking a flower from the ground. There are so many factors that must be considered when it comes to the harvest. Depending on what the weather has been like, you may harvest earlier some years and later others. If there’s a huge storm expected, you may try to get the harvest done early (which may save the grapes from damage, and also reduce the risk that the grapes take on too much water). The harvest may be done at night or during the day, by machine or by hand. Check out “A Year In Burgundy,” a film which highlights these decisions (it’s on Netflix).

To destem or not to destem. The destemming process can be done by hand or machine. Nearly all white grapes are destemmed (which also includes removing seeds), as it is rare to want extra tannins or color added to these varietals. Reds may or may not be destemmed, but that may depend on the wine you’re ultimately producing; as we know, certain grape varietals automatically have higher tannins than others, so it may not be desirable to add even more tannins from another source.

Manual crushing/stomping, care of Lucille Ball! This is one of my bucket list items!!! Photo cred: grabbed this one off Pinterest.

Crush time. There are machines that will destem and crush the grapes, which releases the sugars from within the berries and mixes them with the yeast found on the outside of the skins. This creates your grape ‘must’, which is the term coined for the combined juice and skin of the grape. It’s at this step where reds and whites will veer off on different paths. Once crushed, it’s determined whether or not you will keep the skin/juice contact, or remove the skins altogether. Can you guess which color grape gets which?

REDS: Maceration. The skins are what give the wine its color (white zinfandel is made from the zinfandel grape, but the skins are removed for fermentation…it’s not a different grape, but it yields a rose because the skins aren’t left with the juices very long). Maceration is the process of soaking the skins and juice together, which builds a more robust aroma and creates a deeper weight/body and color for red wines.

WHITES: While some whites may be fermented with the skins a bit, it’s definitely not as long a process for reds. After the must is pressed and the skins removed, the whites are transferred to tanks or vessels to really settle into the alcoholic fermentation process (once sediment is removed).

BOTH: Sometimes yeast is added to the wines to boost the alcoholic fermentation process. Sometimes sugar is added (a process called “chapitalization” which is most frequently used when grapes are underripe). The more sugar and yeast, the more alcohol you’ll get in your wine. The fermentation process lasts until all the sugars have been converted to alcohol for dry wines, but if a sweeter wine is being produced, the process will be stopped early to retain some of the sugars. The winemakers will also choose whether or not to regulate the fermentation temperature, which affects the flavors and duration of the fermentation process.

Some countries, states, or viticultural areas have specific rules and laws about what processes you can use, and may also dictate the ABV allowed for specific types of wine.

A couple other types of fermentation: 

Carbonic maceration. This is the term used for a fermentation process which lets the grape begin to ferment as a whole – it’s not crushed – from the inside out, in a sealed container filled with carbon dioxide. It’s like the grape is going through a self-destruction phase.


Wines that undergo this process usually have a brighter color, lighter body, and greater fruitiness, but it’s not widely used. Beaujolais is one of the varietals that will typically go through this process.

Malolactic fermentation. This process is used for both reds and whites, though less frequently for the latter. It takes place after alcoholic fermentation is completed, and  converts tart malic acid into soft lactic acid. This creates more stability and a fuller mouthfeel in wine, while also eliminating some of that biting acidity (aging also helps reduce the amount of acidity). Chardonnay is the white varietal which this process is usually used for, and we can thank it for creating that signature buttery flavor and aroma which so many love.

While I’ve merely scratched the surface of the wine fermentation process, even this level of detail exponentially increased my understanding of the process and learn the variation among steps taken to produce red and white wines. Hopefully, you’ve gotten a better grasp of the processes, too!

If you want to know more, and see some more pictures to help you understand the process, here are a few other websites to check out:

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