Q. With many people allergic to sulfur dioxide, I did an experiment using no sulfur dioxide in one batch and proper sulfur dioxide in another. I used RC212 yeast in both. What surprised me is that the sulfited batch had much more lees than the unsulfited one. Just curious what chemically might be going on? For what it’s worth, the one with the sulfur dioxide had a much better outcome.
Todd Giese
Bull Valley, Illinois

A. I love a good cellar experiment, especially one that pits conventional wisdom against curiosity. Your trial gets right at a topic that comes up often in the wine world: Sulfur dioxide, the misunderstood workhorse of winemaking.

Before diving into the lees question, it’s worth clearing up a persistent myth about sulfites and wine headaches. Many people believe they’re “allergic” to sulfites, but true sulfite allergy is actually quite rare. The U.S. Food and Drug Administration estimates that only about 1% of the population, and a somewhat higher percentage of people with severe asthma, have a genuine sulfite sensitivity. For everyone else, sulfites are a normal and ubiquitous part of both fermentation and the modern food supply.

Sulfur dioxide occurs naturally in wine because yeast produce it during fermentation, typically in the range of 10–20 ppm. That means there’s no such thing as a truly sulfite-free wine. Even wines labeled “no sulfites added” still contain the small amount generated by the yeast.

Sulfites are also present in many everyday foods. If someone can comfortably enjoy dried apricots, beer, bottled lemon juice, or most deli meats, they’re already consuming sulfites regularly. The amount found in many dried fruits is often significantly higher than what you’d find in a typical glass of wine. In other words, sulfites tend to get blamed for headaches far more often than they deserve.

Back to your experiment.

The fact that the sulfited batch produced more lees makes sense when you think about what sulfur dioxide does. When added at crush or shortly afterwards, sulfur dioxide acts as a microbial traffic cop, suppressing unwanted bacteria and wild yeasts while allowing your selected yeast strain, in this case RC212, to take the lead.

RC212 is a robust fermenter with good tolerance to sulfur dioxide, so the addition you made likely didn’t bother it. Instead, it likely reduced the number of competing organisms and spoilage bad guys (other yeast, fungi, and bacteria) present in the must. With fewer microbes fighting for nutrients or producing inhibitory compounds, your cultured yeast could carry out a healthy, efficient fermentation.

A strong fermentation usually means more yeast growth, followed by more yeast settling out afterward. That yeast biomass becomes part of the lees layer. In other words, your sulfited batch may simply have produced more yeast cells and more orderly sedimentation, leading to the thicker lees deposit you observed.

In the unsulfited batch, things were probably a bit more chaotic on the microbial front. Without sulfur dioxide holding the line, naturally occurring yeasts and bacteria can remain active in the must. Some may ferment sugars slowly, some may produce off-aromas, and others may simply hang around in suspension longer. The result can be a wine that looks cloudier and forms a less compact sediment layer.

Active microbial populations can also interfere with how well solids settle. Instead of forming a tidy layer at the bottom of the container, particles may remain suspended, producing a thinner or more diffuse lees deposit. 

The better outcome in the sulfited batch isn’t surprising either. Sulfur dioxide serves several important roles beyond microbial control. It also helps protect juice and wine from oxidation and can suppress the formation of undesirable compounds produced by spoilage organisms. Without it, a wine is more vulnerable to browning reactions, oxidative aromas, and a whole host of microbial byproducts that can produce everything from vinegary notes to sulfurous “funk.”

Used thoughtfully and in appropriate amounts, sulfur dioxide helps guide fermentation toward the desired yeast strain, protects the wine during its vulnerable early stages, and ultimately leads to a cleaner, more stable finished product. Your experiment nicely illustrates that point. By giving RC212 a little help from sulfur dioxide, you created conditions where it could do its job effectively, resulting in a healthier fermentation, a well-settled lees layer, and, not surprisingly, a better wine.

More on Sulfur Dioxide

• Sulfur Dioxide: Fact and Myth
• Sanitizing With Sulfur Dioxide and Citric Acid