May 7, 2020

Fermentation is our Friend

We have looked at how the grapes go from fruit to juice before.  But what makes it wine?  Simply put, yeast added to sugary grape juice starts a metabolic process called fermentation that converts the sugar to ethyl alcohol and carbon dioxide.  There are some other things produced during the process, like heat and some other compounds such as acetates, esters, and fusel oils. All of these can affect the final flavors and mouth feel of the finished product.  It’s the wine maker’s job to know about and manage all of these reactions and factors. Like I said, simple stuff right?

Various wine vats

This is a very old and complex process that sometimes just happens to sugary liquids when exposed to naturally occurring yeasts in the environment.  Early civilizations used fermented liquids to quench thirst because the chemical reactions in a sense purified the liquids and made it safer to drink than a lot of the natural water sources they had available at the time.  Even Timothy in the Bible was urged to take a little wine to cure what ailed him.  Franciscan monks also became experts in beer brewing to harness the power of fermentation for good in the middle ages. The word fermentation comes from the Latin word “fervere” which means “to boil”.  If you think about a batch of fermenting liquid, it does produce a lot of bubbles that imitate a good steady boil.

It is a simple concept, but devilishly complicated in its execution.  Decisions for the winemaker include what type of wine they want to make, the type of yeast to use (which may change due to the condition of the grapes they just turned into juice), the amount of time you want to let the process go one (which determines the sweetness of the wine), when to start and what type of vessel to complete the fermentation process in and so on. For the science geeks in the audience, the change of one sugar (glucose) molecule into two alcohol and two carbon dioxide molecules look like this:  C6H12O6 -> 2C2H5OH + 2CO2.

You can ferment grape juice in any number of containers from plastic buckets in your basement to large stainless steel, temperature-controlled tanks.  I’ve seen large underground concrete tanks in Spain (with wood fires set underneath to help them maintain a productive temperature) and mound-shaped tanks in Portugal. Here in Idaho folks use steel tanks, plastic totes, clay amphorae, concrete eggs, huge wooden tanks (aka tuns), square concrete tanks, wooden barrels, and even big terra cotta vats. They all have a subtle influence on the end product in your glass.

Punch Down

Red wines are usually fermented while still on their skins so there is a “crust“ of stuff that will rise to the top of the container while fermentation is going on. Every few hours that material needs to be turned over back into the container to make sure fermentation is progressing uniformly. It’s either punched down with a paddle-like tool or juice from below is pumped over it. This can be a dangerous time for the winemaker and their assistants as the process is giving off CO2 and they have their faces right down in that gas. Fatalities and near misses have happened throughout the wine industry during punch-downs as folks lose consciousness and fall into the vats. Next time you take a winery tour look for the CO2 detectors! White wines are usually separated from their skins before fermentation so they ferment away without mechanical intervention.

When do you stop fermenting? If you want a dry (not sweet) wine you wait until the yeast digests all the sugars and there you go.  But what if there is some sugar left in the bin? It’s generally a bad thing to have fermentation happen twice (except for Sparkling wines) so lowering the temperature of the liquid can kill off the yeast and stop the process too.  That’s why most stainless tanks have cooling jackets around them, so the winemaker can control the residual sugar in their final product and in your glass. On the other end of the spectrum if you want to make a sweeter red wine you can add more alcohol to the container and kill the yeast that way as well.  Port-style wines have used this process and hence are called “Fortified” wines as they have fortified the alcohol content (usually with a distilled wine product like brandy) to arrest the fermentation process.


There is one secondary fermentation process that does happen near the end of the red winemaking that is a good thing.  Malolactic fermentation converts the tart malic acid into mellow lactic acid to give the wine a beautiful creamy feel in your mouth.

With all these chemical and microbial reactions going on in the winery, there is one more really important step that has to happen to ensure consistent quality in wine.  CLEANING!  Winemakers have to be assiduous in the hygiene routine they keep in the winery.  Even something as innocuous as using chlorine bleach in the winery can introduce a problem substance called TCA (Cork Taint) into a winery that may never go away.  Steam cleaning is a great way to kill off unwanted chemicals and microbes in the winery so notice how clean your favorite wineries are and the great amount of care they take to keep the facility spotless.  It makes a difference!

Hopefully, this has given you a bit of appreciation for the complexity and knowledge that goes into making the great wine we are lucky enough to have here in Idaho.  Chat with a friendly Idaho winemaker and get their take on the fermentation stage of winemaking.  They will fill in a lot of details that I left out and may even break out a bottle of their favorite bottle to share!

Cheers to 2020 and all the wines of the 2019 vintage that are patiently waiting and getting better just for our enjoyment!


The Idaho Wine Ambassador

© 2020 Jim Thomssen

About Jim Thomssen, the Idaho Wine Ambassador:  Jim grew up in Minnesota but moved west to get away from the snow. He landed in Washington state with a degree in Economics.  He discovered the wines of Washington in the 1980s as the region emerged, and when his banking career brought him to the Treasure Valley in 1993 he saw the wine region in Idaho had the same potential.  Jim has worked with and volunteered for the Caldwell Economic Development, The Idaho Wine Commission, The University of Idaho, Great Northwest Wine, and the Sunnyslope Wine Trail over the last ten years to help develop the Idaho wine industry and promote Idaho wines.  Jim is an avid wine traveler and has visited Napa Valley, the Alsace, and Portugal.  He earned the title of Ambassador after arranging a trip to the Rioja in Spain with an Idaho Winemaker to explore the differences and similarities between the Snake River AVA and the Rioja Alta.

Sunnyslope Wine Trail


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