by Tamara Scully
Sulfur is an element indicated on the periodic table as “S.” In its many different forms, it is found in a wide range of foods. Many amino acids contain sulfur, and it is present in leafy greens and cruciferous vegetables. Yeast produce sulfur in the form of sulfur dioxide (SO2), so it occurs naturally in alcoholic beverages.
If sulfur dioxide is combined with water, a dissociation occurs, forming a positively charged hydrogen atom and a negatively charged bisulfite (HSO3) ion, which can further destabilize into two hydrogen atoms and a sulfite. A sulfite is a sulfur-containing molecule commonly found in processed foods, dried fruit and wines.
Yet sulfur – or more particularly, sulfites – have taken the spotlight as controversial additives in food and libations. What role does sulfur play in winemaking? What is the difference between naturally occurring sulfur and added sulfur? Should winemakers be adding sulfur to their products, and if so, when and how much?
The term “natural wines” has come to refer to any wine made with fruit grown organically or biodynamically which has not had nothing added to it (such as sulfur) or processed out of it, allowing fermentation to proceed naturally, relying on the local microbiome to craft the final product.
But even natural wines aren’t sulfur-free, and in fact can contain high levels of sulfur, as Demi Perry of the Mansfield Lab at Cornell University said in a 2018 Northern Grapes webinar. “Not all sulfites that are in wine have been added,” she said.
Yeast can be high or low sulfite producers. A low producer creates no more than 20 mg/liter of sulfur dioxide, while a high producer creates as much as 80 mg/liter. No wine, therefore, is technically “sulfite-free.” In the United States, however, wines with less than 10mg/liter of total sulfur dioxide can obtain a waiver from having to state “contains sulfites” on the label.
Researchers have found the amount of sulfur dioxide produced by natural yeasts during wine fermentation shows “significant variation from year-to-year and grape-to-grape,” Perry said.
Sulfur is beneficial to winemakers as it acts as a natural microbial, preventing spoilage. It is also antioxidasic, preventing browning. Sulfur scavenges hydrogen peroxide, inhibiting undesirable reactions which can produce unwanted esters. While sulfur does occur naturally during the winemaking process, adding additional sulfur can ensure that levels are high enough for the positive benefits to occur.
But sulfur isn’t all good. And all forms of sulfur really aren’t equal, although they are often referred to in confusing terms. The chemical reactions which occur when sulfur is suspended in water, to form sulfur dioxide, bisulfite and sulfite, is pH-dependent. At a typically wine pH level (about 2.5 – 3.5), bisulfites are primarily occurring, with a small amount of sulfur dioxide.
Sulfur dioxide binds to sugars or to phenolic compounds and is the piece of the puzzle which protects from oxidative browning. Acetaldehyde, primarily formed during fermentation, is also inhibited when it binds to sulfur dioxide. Because high acetaldehyde content causes a bruised apple and other negative sensory characteristics, limiting its presence via bonding it with sulfur dioxide is needed.
Molecular sulfur dioxide levels include both free sulfur dioxide molecules as well as those that have already been bonded. It is the level of free sulfur dioxide that is available which is most important to winemakers. In order to ensure its appropriate availability, sulfur can be added to wine at various steps along the way.
But too much sulfur dioxide can cause unpleasant tastes, a burnt metallic aroma, sneezing and a burning sensation. High levels also will discolor red wines, bleaching them.
In the U.S., the total sulfur dioxide levels in wine cannot exceed 350 ppm, whether from yeast or from added sources of sulfur. In the European Union and Australia, requirements are based on the type of wine and are generally more stringent.
Perry explained that the more sugar there is, the more bound sulfur dioxide there will be. So sweet wines, with higher sugar contents, are often allowed higher total sulfur levels to allow for that additional bonding and leave enough sulfur dioxide to bind to the acetaldehydes and to control spoilage microbes.
Getting It Right
As pH levels increase, more free sulfur dioxide is needed in order to reach the desired levels of molecular sulfur dioxide. For red wines, 0.6 mg/liter is the target. For white wines, aim for 0.8 mg/liter, Perry said. These are the ranges which are useful to inhibit microbial spoilage.
“Don’t add too much sulfur dioxide to begin with,” Perry cautioned. “How acidic does it need to be to dissociate into the bisufite ion?” is the question winemakers need to be concerned with. Wine pH has a direct input on your sulfur dioxide content, according to Perry.
Free sulfur dioxide is a function of pH. A red wine at a pH of 3.2 requires 15 ppm of free sulfur dioxide added to achieve its targeted 0.6mg/liter of molecular sulfur. The same red wine at a pH of 3.5 requires 30 ppm of free sulfur dioxide to reach the target.
Adding sulfur is typically done using potassium metabisulfite (KMBS). This granular salt is not all available as sulfur dioxide when added to the wine, so a quick calculation is needed to determine how much of it to add to reach your desired molecular sulfur dioxide levels. Using KMBS, rather than potassium, needlessly adds salt to the product and isn’t recommended.
The inadvertent addition of sulfur is often overlooked. When cleaning barrels with elemental sulfur, residual levels can add as much as 10 ppm of total sulfur dioxide to a 225-liter barrel. And unburned sulfur forms hydrogen sulfide gas, complete with its rotten egg smell.
The intentional addition of sulfur dioxide can happen after harvest, post-fermentation or pre-bottling. For natural yeast to do its job, don’t add too much sulfur dioxide at the post-harvest stage. If the fruit is dirty, adding sulfur dioxide now can help prevent spoilage. Malolactic fermentation (MLF), if desired, can be inhibited by sulfur, so this will influence whether or not (or when) to make additions. At crush, levels of sulfur dioxide for red wine should be between 25 – 50 ppm, and at least 30 ppm and up to 50 ppm for white wines.
After fermentation, white wines may need protection against oxidative browning and protection from spoilage microbes. Adding sulfur dioxide here would be warranted. Reds need less added protection.
Pre-bottling, sulfur dioxide can be added if the wine will be exposed to high levels of oxygen, as with hand bottling. The level of residual sugars, the anticipated shelf life pre-release and the wine pH level will also help to determine whether or not adding sulfur dioxide at this stage is beneficial. The rule of thumb is to aim for 30 ppm of free sulfur dioxide at pre-bottling.
Measuring the amount of free sulfur dioxide can be accomplished via several methods of iodine titration, in which the color indicates the amount of sulfur dioxide available. While subjective, these methods are much less expensive than the most sensitive tests, economically only practical for large-scale producers, and offer small producers an option.
“I’m just here to tell you that if you do add sulfur, you’re going to get some antimicrobial effects as well as anti-oxidative effects, and other benefits as well,” Perry concluded.