Topic of the Week 11/13/17 - Introductory and Certified

Thank you and for chiming in on the 1855 Classification last week!

This week: Sulfur

What purpose does sulfur serve during the winemaking process, how can it be added, and how can a winemaker try to control its effect?

  • Hope this will get us started!

    Sulfur, originally used as candles in the 1400s as a way to preserve Dutch and English barrels during shipping, is a crucial additive in the winemaking process.  Primarily, it acts as a preservative for the wine.  As an anti-bacterial agent, sulfur--most commonly used as SO2--prevents yeasts and bacteria from growing during the fermentation process.  This sulfur protects the wine from most fault-inducing microbes, but in lower amounts steers clear of the more-resistant fermentation yeasts.

    Sulfur is added during a couple of key moments in the vinification process: during the crushing of must and after malolactic fermentation.  Sulfur is usually added after the conclusion of the malo process because these bacteria are not as resistant to the compound.  These addition can be completed in a few ways, most notably in the form of a powder, sulfur-water, or with a gun which injects the SO2 gas into the wine directly.

    Despite the benefits, winemakers must use caution around the timing and amount of SO2 used, usually depending on the style.  White wines with excessive sulfur might develop the faulty aroma of a struck match.  Sulfur can also bleach some red wines by causing the anthocyanins, or "red" color, to drop out.  This usually does not affect wines meant for again, as the color will usually return after some time.  Understanding pH, potential alcohol, grape varietal, and aging goals will help a winemaker determine the appropriate time and amount of SO2 to include.

  • Sulfur in the production of wine and its constituent compounds in the wine itself, is a double-edged sword. The effects of attempts to control sulfur use can produce both good or bad results.

    How Sulfur Use In Wine Production Became Commonplace

    Sulfur, which means sulfur dioxide (SO2), has been used as a wine preservative for at least 600 or so years and probably much, much longer. We know that the ancient Greeks and Romans burned sulfur candles as a general fumigant and it’s generally assumed that they used it with wine, though the historical record hasn’t left us with many specific accounts.

    People often cite the fact that ancient Greek and Roman winemakers burned sulfur to help preserve wine although it is difficult to track down any real evidence. There are many mainstream articles about the history of sulfites in wine, including mentions on both the Guildsomm Website and Wikipedia, that attribute the practice of using sulfur in wine to the burning of Sulphur candles in barrels by the romans. The evidence is scant, but there is enough of it to suggest that the use of sulphur to preserve wine was known in the ancient world. The most easily found accounts of ancient sulfur usage techniques are:

    • In a chapter of Encyclopedia Naturalis Historia devoted to various methods used to preserve wine, Pliny speaks of Cato who “mentions sulphur.”
    • Horace alludes to this practice in a poem dedicated to the celebration of a glad anniversary: “This festal day, each time the year revolves, shall draw a well-pitched cork forth from a jar set to drink the smoke in Tullus’ consulship”.
    • John Kitto’s Cyclopedia of Biblical Literature says: “When the Mishna forbids smoked wines from being used in offerings, it has chiefly reference to the Roman practice of fumigating them with Sulphur, the vapor of which absorbed the oxygen, and thus arrested the fermentation. The Jews carefully eschewed the wines and vinegar of the Gentiles.”
    • The first solid mention of SO2in winemaking dates from a 1487 German text, still more than long enough ago to count for “centuries and centuries”.
    • Purified and Powdered potassium metabisulfite (the easiest way to dose wine) wasn’t available to winemakers until the twentieth century. Until then, the method of Candle Fumigation was the standard.

    Sulfur dioxide is, hands down, the most useful of all wine additives. Better still, it occurs naturally in every wine: all yeast make some naturally during fermentation, though rarely in quantities large enough to be of any real use. Added in larger doses it answers all sorts of winemaking wishes –protection against yeast and bacterial growth, protection against browning, preservation of aromatics by blocking the oxidation that destroys them– and it’s inexpensive and easy to use, too. Wine made without added SO2 almost always has a shorter shelf life, both because oxidation is likely to destroy fresh, fruity flavors and because it’s prone to microbial spoilage if the winemaking process wasn’t perfectly clean. And, in the quantities used in wine, it’s harmless to everyone save a minor fraction of asthmatics. SO2 is considered almost (almost) essential for good winemaking, SO2 is sunscreen for wine, protecting it against browning, lost aromatics, oxidized notes, and spoilage microorganisms. Nothing else is quite so effective; every other wine preservative adds some negative side effect in the way of flavor or fails to be as securely harmless.

    There are some sulfur compounds which positively affect the flavor profiles of the wines that they are contained in. Recent research has shown that they are important contributors to varietal character in many wines, so winemakers should be careful not to just eliminate them randomly.

    The Good - The Most Notable Volatile Sulfur Compounds with Positive Effects

    Thiols  - 4-mercapto-4-methylpentan-2-one (4MMP), 3-mercaptohexan-1-ol (3MH)and  3-mercaptohexyl acetate (3MHA). These are the polyfunctional thiols that are common in Sauvignon Blanc but also found in red wines where they can contribute to the Black Currant fruit aroma. All of these sulfur compounds can have a positive effect on flavor in the right environment.

    Benzenemethanethiol – Another Thiol that gives off aromas of Flint and Smoke. In the right conditions can also be positive.

    Dimethyl Sulfide -Adds Quince, Truffle and Black Currant flavors at lower levels.

    Carbon Disulfide – Adds Sweet, Ethereal and Slightly Green aromas that can be desirable in small amounts.


    The Bad - Undesirable Effects of Sulfur In Wine

    Volatile sulfur compounds, and specifically mercaptans, are the compounds largely responsible for the olfactory defect known as ‘reduction’, as well as a host of other maladies. Of the approximately 100 volatile Sulfur compounds that have been identified in wine, only a few are significant. Generally speaking, they are foul-smelling compounds that produce, even at low concentrations, a negative sensory impact on the wine.

    Another property of these compounds is that they are chemical chameleons, able to subtly change their form depending on the wine environment they are in: this is significant because if they are below detection threshold in one less smelly form, if conditions change, they may suddenly become noticeable in another more smelly form.

    Reduction is the term used to describe the presence of volatile sulfur compounds in wine. Reduction and oxidation are two different chemical processes that complement each other. In a chemical reaction, electrons change hands, and as one compound is oxidized another is reduced. If there is plenty of oxygen around, then chemical components in a wine will be gradually oxidized (the electrons are transferred from the chemical components in the wine to the oxygen). The end result is an oxidized wine. During fermentation the yeasts need oxygen, and in the early stages of red wine development a little oxygen is helpful because it allows the oxidation of some ethanol to acetaldeyde (also known as ethanal), which can then help with the development of tannins and pigmented polymers that are important in building structure and color (this is the theory behind microoxygenation). But after this, wine development is largely reductive: that is, it occurs best in the relative absence of oxygen.

    Redox potential is a measure of how oxidative or reductive a system, such as a wine in barrel or bottle is, and it is measured in millivolts (mV)—the higher the reading, the less reductive. Typically, an aerated red wine will have a redox potential of 400–450 mV, whereas storage in the absence of air for some time will reduce this to 200–250 mV. If levels get as low as 150 mV then there is a danger that reduction problems can occur. Exposure to oxygen through winemaking practices such as racking, topping up barrels and filtering, increases the level of dissolved oxygen in the wine and increases the redox potential, which will then return to 200–300 mV. In white wines, this redox level will change much more rapidly than red wines, because red wines have a higher concentration of phenolic compounds such as tannins which are able to interact with oxygen, and act as buffers. Another variable here is the level of free sulfur dioxide in the wine, which will act protectively by reacting with the products of oxidation. Yeast Lees also scavenge oxygen and protect the wine in a similar fashion, helping to lower the redox potential and create a more reductive environment. In modern winemaking, reductive conditions are encouraged: the protection of wines from oxygen by use of stainless steel tanks and inert gases helps to preserve fresh fruit characters.

    These reductive conditions—those in which oxygen is excluded—can also cause the development of the smelly forms of sulfur compounds. This is where the term ‘reduced’ comes from, and if this ‘reduction’ occurs before bottling, addition of oxygen may correct the fault. But it is important to note that these sulfur compounds can develop in wine even in non-reductive conditions, at which stage further oxygen exposure may result in turning a smelly wine into a smelly oxidized wine. Therefore, equating the term ‘reduced’ with the presence of volatile sulfur compounds is an oversimplification. 


    The Ugly - The Most Notable Undesirable Volatile Sulfur Compounds

    Hydrogen Sulfide – Smells like rotten eggs or sewage; This is the main offender, made by yeasts when they use one of the sulfur-containing amino acids as a nitrogen source. Stress also encourages its formation.

    Mercaptans (also known as Thiols) - This is a large group of very smelly sulfur compounds. Smells of cabbage, rubber, struck flint or burnt rubber. If hydrogen sulfide isn’t removed quickly, it can result in mercaptan production. This is a big worry for winemakers.

    Ethyl Mercaptan – Smells mostly like burnt match.

    Methyl Mercaptan (Methanethiol) – Attributed to producing odors of rotten cabbage, cooked cabbage, burnt rubber, stagnant water. One of the compounds implicated in screwcap reduction.

    Dimethyl Sulfide – Adds aromas of Cooked Vegetables, Cooked Corn and Canned Tomato at high levels.

    Diethyl Sulfide – Adds aromas of Rubber.

    Dimethyl Disulfide -  Produces Vegetal, Cabbage, Onion-like aromas at high levels.

    Diethyl Disulfide -  Produces aromas of Garlic and Burnt Rubber.

    These sulfur compounds, both good and bad, come mainly from yeasts. If yeasts are having a hard time finding enough nitrogen in the musts, they’ll make use of the amino acid Cysteine as a nitrogen source. Cysteine contains Sulfur, and this Sulfur is recombined chemically by the yeast metabolism to form the sulfur compounds that are the subject of this reply. It is of great importance, therefore, for winemakers to make sure that their yeasts are happy and have an adequate nitrogen supply. ‘Yeast Assimable Nitrogen’ is the technical term that’s used here. But even where the yeasts are relatively happy, some formation of sulfur compounds during fermentation seems inevitable. And these compounds need careful handling by winemakers if they aren’t to turn into problems.