Talk Fermentation Like a Wine Pro

A few weekends ago, my girlfriends and I went wine tasting in a touristy town outside of Seattle. As I was going over the tech sheet, the description of a wine piqued my interest. According to the write-up, this wine was fermented with two different yeast strains. Curious, I asked the tasting room manager for more information.

Wine tasting

“Well, I am not a winemaker,” he prefaced and then proceeded to describe what essentially was a case of stuck fermentation.

If you get the sense that the term “stuck fermentation” sounds more dire than what is presented in the tech sheet, you are right! For this month’s post, we will go over some wine fermentation terms so that you can talk like a wine pro in a tasting room.

Alcoholic Fermentation

All wines go through alcoholic fermentation. This is often referred to as primary fermentation. Wine grapes are typically harvested at a sugar level of 20-25 Brix. During alcoholic fermentation, yeast converts the sugar into alcohol and carbon dioxide. To make a dry (as opposed to a sweet) wine, the fermentation will typically go for 2-3 weeks till the sugar level drops to 0 Brix. At which point, the yeast will run out of sugar to consume and become dormant. With a starting Brix of 20-25, the resulting wine will be at 11.5-15% of alcohol by volume (ABV).

Spontaneous vs. Inoculated Fermentation

Spontaneous fermentation is how wine and other fermented goodness were discovered. It refers to the fermentation caused by ambient or natural yeasts. However, not all yeast strains are capable of fermenting to dryness. Nor do they always produce the flavors you want in a wine. Except for very established wine regions where the natural yeast strains have proven success in fermenting and making good wine, one would be relying on chance to make wine using spontaneous fermentation.

Pitching yeast in inoculated fermentation

With modern winemaking, yeast strains have been commercially cultivated to reliably ferment and to produce certain characteristics in wine. In inoculated fermentation, wineries will first treat the must (fancy term for crushed grapes or juice to be fermented) with sulfite to prevent spoilage from wild yeasts and bacteria. After a couple of days, the selected yeast strain will then be pitched into the must to start the fermentation process. 

Stuck Fermentation

Sometimes alcoholic fermentation gets sluggish over time. A fermentation is considered stuck when Brix is stagnant for over 48 hours. Stuck fermentation is a symptom of stressed yeast and is a winemaker’s nightmare. Some of the stressors include:

• Inadequate yeast nutrition - Beside sugar, yeast needs nutrients to properly propagate and complete the fermentation process. There are established nutrition protocols for different yeast strains to ensure successful fermentation.
• Hostile must temperature - If the must is too cool, the yeast will become dormant, and fermentation will halt. On the converse, an excessively hot must may kill the yeast. Keeping the must at 75-85 degrees Fahrenheit is a safe guardrail.
• High alcohol must - This is often the result of fermenting grapes with a high starting Brix. The must then reaches an alcohol level that is toxic to the yeast before all the sugars are fermented. Hence, the Brix level stays stagnant and does not fall to 0.

Measuring Brix using a hydrometer

The fix for stuck fermentation is to restart it. This can be tricky and often requires a different yeast strain that can tolerate the specific must environment presented by the stuck fermentation. The resulting wine is often of a lower quality or exhibits less desirable characteristics than intended.

Primary vs. Second vs. Secondary Fermentation

This is a surprisingly confusing topic, and I have seen the terms used differently. But this is how I understand the difference:

• Primary fermentation refers to fermentation prior to racking. Racking is the process of transferring wine from one vessel to another to remove sediments and dead yeasts. Some winemakers rack in the middle of alcoholic fermentation while others do it after.

• Second fermentation refers to a new alcoholic fermentation due to the presence of sugar. This may be accidental if there is sugar left from a prior fermentation. Or it may be intentional where more yeast and sugar are added to a still wine to trigger a second fermentation and subsequent carbonation. That is how a sparkling wine is made.

• Secondary fermentation refers to fermentation after racking. If racking occurs in the middle of alcoholic fermentation, then secondary fermentation is the continuation of that. If racking occurs after alcoholic fermentation is complete, then secondary fermentation may refer to malolactic fermentation if used.

Racking from barrel to carboy

Malolactic Fermentation

Often known as malo or MLF, malolactic fermentation is the process of converting tart malic acid (think green apple) in wine to creamy lactic acid (think milk) using a bacteria called Oenococcus oeni. MLF is common in making red wine to create a velvety round texture. It is rarely used in making white wine except to create a buttery Chardonnay. MLF is sometimes known as secondary fermentation.

That concludes the primer on fermentation terms. Go forth into that tasting room and talk fermentation like a pro. Or at least spot a marketing spin. Now you know.