Fermentation sounds simple enough, but when you break it down, it’s actually quite a tricky topic. There are a lot of variables at play. Even the slightest error can result in poor-tasting wine, and we can’t think of anything more tragic than that, can you? Luckily, the experts know what they’re doing, so you’ll only sample the best of the best. Whether heading to Napa to taste some Peju wines or simply eager to impress your foodie friends with some vino fun facts, this beginner’s guide to fermentation has all the answers.
What Is Fermentation?
Fermentation isn’t a new term for wine drinkers, but what does it mean, exactly? On the most basic level, wine fermentation is what happens when yeast consumes sugar and converts into alcohol and carbonation based on weight. The breakdown of wine to alcohol and carbon dioxide is about a 1:1 ratio. So, for example, if you have 5 gallons of a beverage and that contains 10 pounds of sugar, once it’s fermented, there will be roughly an equal balance of 5 pounds of alcohol and 5 pounds of sugar. The sugar that is not converted into alcohol goes into the air.
There are two main stages of fermentation: primary and secondary (also referred to as aerobic and anaerobic). The primary stage takes place for the first three to five days of fermentation. If you're not sure what this looks like, just look for foam. That's usually a pretty good indicator that things are on course. During this stage, the vessel is able to be in contact with air (hence the aerobic nickname). After about four to seven days of the primary stage, the secondary stage begins and lasts for a week or two. At this time, fermentation is slower and air exposure minimized.
Punch-Downs vs. Pump-Overs
During fermentation, all the solid parts of the grape (i.e., grape skins, stems, pulp and seeds) float toward the surface. Together, this lump of substance is called a "cap" and caps need to be dealt with one way or another. If it helps, think of the cap as a tea bag. For more flavor, you'd want to submerge the tea contents, right? The same theory applies to wine. If a vintner opts for a pump-over, that means the liquid is pumped from the bottom of the tank to the top, thus submerging the cap. The other option is the punch-down, which tends to be the more aggressive of the two methods because the cap is not only pushed down, but also broken up and submerged.
"Lees aging" is a term that describes a wine's mature phase. After fermentation, there are some leftover dead yeast cells and particles that are called lees. They're easy to spot because they often become visible sediments inside the barrel. When the lees eventually break down, they create a slew of other compounds: mannoproteins, polysaccharides and amino acids, to name a few. Since all of these compounds then interact with the wine, it yields a variety of aroma and flavors.
When it comes to wine, temperature is one factor that can’t be overlooked. This is definitely the case for fermentation, as a slight change in temperature can make or break your wine, so to speak. For example, if the temperature is too cold, the yeast might not be motivated to ferment.
On the flipside, if the temperature is too warm, the yeast will have no trouble fermenting but the flavor might taste less than ideal due to microorganism growth (they love warm temperatures!) or because there are more enzymes being produced. As a general rule, red wines require warmer fermentations, while white wines and rosés call for cold or cooler temperatures.
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