Learn About Wine Fermentation

Learn About Wine Fermentation

To learn about wine you need to know the basics of how wine is made. We’ve covered a lot of territory over the past few months and with the new year upon us we delve even further into our wine knowledge. Now that you know what wine is, where wine grapes are grown, and are familiar with different wine grape varieties, let’s get into some wine making information. I find the most important topic of winemaking is how to turn grape juice into alcohol. If not the most important, then certainly the most rewarding. I like to drink wine. I like when wine makes me dizzy. I like to get really dizzy from drinking wine. And if you want to learn about wine, you have to know at least how wine does that. So, let’s get dizzy with some winemaking basics.

While wine has been around for more than 5,000 years, the process through which wine is made has only been understood for a little over 160 years, since Louis Pasteur in 1850 discovered that yeasts convert sugar to alcohol.

What Is Wine Fermentation?This process of converting sugar to alcohol is a wholly natural and organic process called fermentation. One thing I should mention about fermentation is that while wine yeasts ingest sugars and metabolize them as alcohol, sulfites are naturally produced. And I think I’ve mentioned already that sulfites act as a preservative to keep the wine from spoiling, browning and oxidizing quickly. If I haven’t, now you know. As yeast metabolize the sugars heat and CO2 are produced. It’s in the heat that the sulfites are made.

Yeasts exist wherever there are sugar solutions in nature. It’s used to make bread. It is used to make beer. And, of course, there is wine. There are countless of species of yeasts found in vineyards all over the world and each vineyard has its own composition of yeasts as a result of climate, soil and other aspects of terroir.

Still, naturally present yeasts are not responsible for fermentation. That’s because the waxy skin of a wine grape acts as a shield against yeast, keeping the yeast from its food source within the grape: sugar. But even if that were not the case, not all grape sugars are fermentable. Yeast can only metabolize two of four sugars found in wine grapes: glucose and fructose; the others, ribose and xylose, are not vulnerable to fermentation. Because of this, no wine is ever completely dry.

Once the grapes are ready for fermentation the wine grower either waits for fermentation to begin on its own from the indigenous yeasts on the grapes or a rehydrated yeast powder (cultured yeast) is be added to start the process.

Let’s look at the two different types of wine yeasts, cultured and indigenous, and the values of each.

Cultured Wine Yeasts

  • The yeast will initiate fermentation quickly
  • Alcohol production is easily calculated
  • Have higher alcohol production due to a lab-developed tolerance to alcohol
  • Can ferment at lower temperatures due to a lab-developed tolerance to cooler temperatures
  • Produce fewer by-products
  • Produce a wine that is clean and varietally true with few off-odors

Indigenous Wine Yeasts

  • Produce more acetic acid in wines, which adds a rustic note
  • Produce more ethyl acetate in wines, which, in small amounts, adds complexity
  • Produce more glycerol, an odorless, sweet-tasting alcohol
  • Produce more higher alcohols, both pungent and fusel
  • Produce more phenyl ethanol which imparts rose, floral and honey aromas
  • Produce more SO2
  • Produce more acetaldehyde which imparts maderized, sherried notes in wine
  • Have less predictable alcohol production
  • Produce a wine that is more complex, softer, rounder and fuller

Let me back up a minute and talk briefly about calculating alcohol production.

The amount of sugar present in the wine grape is a function of ripeness. Grape growers use this measure as one of the key criteria to determine the date of harvest. As the sugar level is directly related to the possible finished alcohol level in a wine…which, in turn, reflects the potential stability of the finished product…accurate sugar measurements are paramount.

In the U.S., grape sugars are measured in degrees Brix. In order to estimate the final ethanol level of the finished wine, the simple rule of thumb is to divide the Brix reading by two. For example, grapes at 24 degree Brix equates to wine with 12% alcohol. Really only 90% of the sugars are converted into ethanol and CO2. The other 10% is diverted into the production of other by-products like glycerol, acetaldehyde, succinic acid, ethyl acetate, acetic acid, higher alcohols and lactic acid in wines.

A truer conversion is closer to 55%, so 24° Brix would actually be about 13% alcohol.

Baumé is the unit of measure in France gauging alcohol levels in milliliters per 100 ml of wine, so 15 Baumé will produce a wine with 15% alcohol. Oechsle is the measurement of Germany that measures the density of the must (the juice, skin, seeds and stems of crushed wine grapes), subtracts 1.0 and multiplies the result by 1,000. Why does it need to be so complicated? I don’t know, I’m not German. Even Austria has its own measurement, the Klosterneuberger Mostwaage (KMW). It also measures the density of the must. Take the degrees Oechsle, divide by 5 and round up. So a wine with a must measuring 69° Oechsle will measure 14° KMW.

I’m not a winemaker so this kind of math is over my head for the most part. But my goal was to present to you everything you need to learn about wine fermentation and this was in the mix, so there ya have it.

Until next time.

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    1. GnJ says:

      Great site for the beginner and beyond. I have enjoyed wines for a very long time and find your site very interesting and informative. As a new novice blogger I’m always looking for great links to add to my own blog. Yours is definitely worth adding. Enjoyed your site and will return again and again!! My own advice to those just getting into wine is “experiment and have fun!”

    2. Phil says:

      Hey thanks for the nice comment GnJ. Check your email.


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