The Hidden Double Life of Yeast

By kourofskywine on August 4, 2025

By Kevin Kourofsky

The Italians call mushrooms fungi. That sounds like a nicer way of saying fungus, but fungi is also what we call this kingdom of living things. It includes every form of fungus from mushrooms to the stuff between your toes. Yuck! But without this life-form, we would not exist as humans. Fungus includes the strands of fungal matter that live beneath tree roots, trading nutrients to the trees in exchange for carbohydrates produced by the trees. No fungus, no trees. It includes the wonderful truffles that spice our food, as well as those Shitake mushrooms that I love on pizza. It saves lives as penicillin. It helps to rot down dead growth to return the building blocks of nature to the soil. And for us winemakers, yeast. Yeast is a member of the fungus kingdom, but a very special member. It turns mere grape juice into nectar, by simply changing sugar to alcohol and carbon dioxide. Not bad for a single cell microorganism.

There are many genuses of yeast, but for the most part winemakers use the genus saccharomyces cerevisiae. These yeasts are generally strong fermenters and are used by bakers, beer brewers and winemakers. But there is nothing simple about a choice of yeast. Each yeast has its strengths and weaknesses. And this can be crucial to a successful fermentation. It’s not as simple as focusing on the potential flavors and aromas the choice of yeast can impart. A good winemaker must match the yeast strain to the particular fermentation, the type of sugars to be fermented, the strength of those sugars, the temperature of the fermentation, as well as potential aromatic challenges. This article will not discuss the basics of a good fermentation, it will talk about accomplishing more than basic fermentation with the same yeast.

The Problem of Sugar

Fructose, glucose, dextrose, sucrose and sorbitol. The mind boggles. We usually don’t think much about the type of sugar in our juice or must. I certainly didn’t, until I hit a stuck fermentation. It turns out that sugars are chemically different and that you must match your yeast strain to the type and strength of the sugar content of the fermentation. (It is also necessary to consider the overall amount of sugar in the fermentation, but that will be discussed below). Failure to do this can lead to a struggling or stuck fermentation. When yeast struggles, it often produces hydrogen sulfide (HS2), with its rotten egg aromas. So you have to understand what types of sugar you’re faced with.

Grapes generally produce about the same percentage of glucose and fructose, though often a bit more glucose. Fructose is much sweeter than glucose and harder for the yeast to break down and convert to alcohol. Saccharomyces cerevisiae, our standard wine yeast, is more glucophilic than fructophilic, which is a fancy way of saying that our usual yeasts will struggle to ferment fructose.

We can learn a lot from fruit winemakers about this problem. Apples do not have a 1 to 1 ratio, like grapes. Apples are closer to ¾ fructose to ¼ glucose. So, for apples you need to choose a strong fermenter, like Lalvin EC-1116, EC1118, DV10, or 71B.  So too, grapes that are left longer to ripen, called “hang time,” may develop more fructose sugars, causing a disruption to the 1 to 1 ratio of grape juice. Yeast may struggle to complete the fermentation. This may leave some fructose in the wine. As fructose is sweeter than glucose, the wine may have a noticeable sweet aftertaste.

Juice that has high levels of sugar, such as late harvest, will also cause yeast to struggle regardless of the nature of the sugars in the juice. Though yeast lives on sugar, too much sugar can overwhelm the yeast’s ability to digest the sugars and can impede fermentation. Add to that obstacle increased alcohol produced by the yeast, and this “double whammy” may lead to a stuck fermentation.

I have also found that Juice that has been concentrated for shipment, then re-constituted to a normal level, often produces a problem with the yeast bringing the juice to dryness. The fermentation stops at around 1 to 2 brix. I found this also with table sugar added to the juice. But my research of the science in this area doesn’t seem to support my theory that the sugars are changed in the process. And normal table sugar, sucrose, is mostly glucose and fructose in a 1 to 1 ratio, like grape juice. But this seems to happen to other winemakers so I think it’s a “real thing,” if unexplained. So, the choice is to use a stronger fermenting yeast in these circumstances, and perhaps adding extra nitrogen toward the end of fermentation, may give the yeast a final boost to get the fermentation to dry.

The Problem of Malic Acid

Malic acid is not always a winemaking problem. Some aromatic whites, such as Riesling, need malic acid’s raciness. For fruit winemakers, malic acid is the primary acid in the juice. But, many red wine makers tend to want to remove all the malic acid from the wine, to keep an acid-tannin balance that doesn’t irritate the palate. So, for them malic acid is wholly unwanted. Malic acid bacteria (malolactic fermentation) will usually solve that problem.

This style of malolactic reduction is usually an all or nothing prospect. Once the wine is inoculated with the bacteria, it must reach its completion, or the winemaker risks a restart of the fermentation in bottle. The result is a fizzy wine with a slight off odor. High SO2 can help prevent the re-fermentation, but the strength of the sulfate will lessen over time.

It is true that with the right (and expensive) equipment, a professional winemaker can play around with the extent, or length of a malolactic fermentation. This usually involves a sterile filtering, which removes the malolactic bacteria, before bottling. Most amateur winemaker’s equipment does not quite reach the sterile point, and usually a series of filtering from lower to higher grades of filter pads is required. That process can be extremely harsh, stripping the wine of much of its flavor.

Yeasts can provide an answer for the winemaker who wants to lessen, but not remove all the malic acid in the juice or must. We can again look to fruit wine makers for a solution. Fruits, other than grapes juice, don’t contain tartaric acid. So, fruit wine and cider makers tend to use malic-reducing yeasts that can make the final wine rounder, without removing the essence of the fruit. Grape winemakers can also use this natural process, converting some of the malic acid, perhaps up to 30%, in the process of the initial fermentation.

For instance, Chardonnay can be made in a racy style or a softer buttery style. The buttery style usually undergoes a full malic acid fermentation. Chardonnay has a light aroma and lacks aromatic phenols found in other whites, such as Sauvignon Blanc and Riesling. The lactic acid produced by the malolactic fermentation creates “buttery” flavors and aromas found in this West Coast style. On the other hand, the more acidic, “racy,” style of Chardonnay usually doesn’t have a malolactic fermentation. Think Chablis, from the northern Burgundy region. So, what if you want a less jarring, but still food friendly Chardonnay?

By using a malo-eating yeast, some of the raciness can be lessened without creating a “butter-bomb.”  Such yeasts can create a happy medium, without worrying that any residual malolactic bacteria will re-start in the bottled wine.

What are these yeasts? There are many, some with greater and some with lesser malo-erasing effects. A popular one among hard cider makers (where too much malo is a recurring problem) is Lalvin 71-B.  Others include Lalvin-C, Lalvin-ICV, Lalvin Opale, Lallemand Uvaferm VRB, and Anchor Exotics. Though they may have different brand names, many are produced by the parent company Scott Labs. These are just some of these yeasts and they are readily obtainable.

The Problem of Vegetative and Herbaceous Odors and Flavors

In warm climate areas, protecting a grape’s varietal expression can be a challenge as heat may produce an exorbitant amount of sugar before flavor compounds can fully form. These wines can be shallow in flavor. In cool climate areas, getting reds to fully ripen with enough sugar and with mature varietal flavors is the problem. Warm climate areas use methods to slow down the sugar production to allow the fruit to have phenolic ripeness. In cool climate regions, there are some methods to assist ripening, such as leaf pulling, but there is little that can be done to ripen the fruit in a poor growing year. Some of these wines can have an undesirable “woody” or “bell pepper” quality. These are called “herbaceous” or “vegetative” notes in the juice which transfers into the wine.

This is not only a seasonal problem; it can be a varietal problem. For instance, Cabernet Franc is notorious for developing these herbaceous qualities in off years. What do you do if that vegetal fruit is the crop in your cellar? No yeast can turn an under-ripe grape into ripe wine. Yet, certain yeasts can assist in reducing these unwanted aromas during fermentation process.

For Cabernet Franc, ICV-GRE is known to help emphasize its spicy and violet notes, while reducing some of the vegetal and herbal notes. Similarly, this is true of the Bordeaux varietals Cabernet Sauvignon and Merlot. Lalvin ICV-D21 is said to help reduce herbaceous qualities in Cabernet Sauvignon. Enoferm-CSM is said to help maintain varietal expression while de-emphasizing under ripe fruit flavors in grape wine. Likewise Lalvin BM-45 yeast.   

Be careful to avoid an aroma enhancing yeast, such as Lallemand BDX, that emphasizes all aromas, as that yeast might emphasize the bad along with the good. It’s not just choosing a yeast to help, but avoiding, those that work against your desired outcome.

The Problem of High Acid Wines

Yeast functions best at a PH between 3.00 and 4.00. At the extremes of these two measurements yeast can struggle. Like the problem of too much sugar, yeast can fail to thrive in wines that are too high in acid. These are generally white wines that are naturally high in acid, like some hybrids and aromatic whites like Riesling. Wines that are a base for sparkling wine are picked early and high acid is integral to the process. For wines such as these, PHs of 2.80 or 2.90 are not uncommon.  

Rather than focusing on the probable aromatic profile of the yeast, it’s the yeast’s muscle power and ability to get the job done that is the most important feature. Wine yeasts like Lalvin EC-1116, EC-1118. 71-B, or DV-10 are powerful fermenters and great choices.

If you are making the second fermentation in-the-bottle of a hoped to be sparkling wine, a very strong fermenter is needed. I recommend EC-1118 or DV-10 only.

The Art of Winemaking

Making wine can be as easy as ignoring the juice or must and waiting to see what nature does with it. That is the theory of “Natural” winemaking. That philosophy is a little too scary for me. There are also partial interventionists, or full-on Interventionists. I’m more in the full-on category. I like to help my grapes make the best wine possible: clean with varietal flavors. So it’s engrossing to read the description of the marvels that a particular yeast can accomplish. Such as enhancing cherry flavors in Pinot Noir. Or giving a wine more a fullness in the mouth. But that is only half of the story. Choosing the yeast for your fermentation by simply reading a description on the back of the package can lead to sub-optimal results.

Winemaking is a combination of science and art. A good winemaker knows what the positives are in the grapes as well as the negatives. The “art side” of winemaking allows the maker to picture what is possible with this year’s harvest. The “science side” allows the maker to formulate the series of interventions, a route if you will, to obtain that result. Sometimes you need to get to where you are going in the fastest but dullest, way, forgoing the adventure in favor or a sure result. Like choosing US 90 versus Route 66. Your yeast choice can be like that. Though sometimes, when the stars are right, you can get to where you are going and still get your kicks on Route 66.