Sulphur, sulphites, or, to be precise, sulphur dioxide, is much maligned in the wine industry. This is somewhat understandable. Sulphur dioxide in low concentrations smells terrible and in high concentrations is quite toxic. And if that isn’t enough, sulphur, also known as brimstone, is strongly associated with hell. Those who did not heed the warnings of the Bible risked being “cast alive into a lake of fire burning with brimstone”. (Rev. 19:20) But, in fact, sulphur’s role in the context of wine is to play the hero in defending against an unlikely adversary, oxygen.
Oxygen and sulphur are, in many senses, conceptual opposites. Sulphur emanates a choking stench whereas oxygen seemingly cleanses, calms, and gives life. However, like sulphur, much of oxygen’s reputation is undeserved. Oxygen is an excitable gas, reacting with a variety of substances and breaking them down, a process known as oxidation. The browning of a cut apple, rotting leaves, rusting metal and even burning matches are all examples of oxidation. Wine, too, will naturally oxidize, unless the winemaker intervenes.
Wine is precariously balanced in a state of half-spoilage. When the grapes burst, yeast already on the skins metabolize the sugars, creating alcohol and a lot of carbon dioxide. That carbon dioxide is heavier than air and it blankets the tank, protecting the juice from oxygen. However, after the fermentation finishes, the carbon dioxide dissipates and oxygen comes into contact with the newly formed wine. Yeasts are not the only inhabitants of grape skins and the other hangers-on, oxidizing enzymes and a bacteria known as acetobacter, awaken in the presence of oxygen and go to work. The enzymes make the wine brown and break down its fruity flavor, and acetobacter quickly turns the wine into vinegar.
For wine to retain its pleasing taste, these processes must be prevented. When pure sulphur is burned, it oxidizes and forms sulphur dioxide. Throughout history, beginning with the Romans, burning a sulphur candle inside the barrel was a key step in the production of quality wine. Sulphur dioxide is now commonly added as a powder, to carefully control the quantity. Regardless of it’s form, sulphur dioxide has three very important functions: (1) it binds with oxygen to prevent it from interacting with the wine; (2) it breaks down the oxidizing enzymes; and (3) it kills the acetobacter. It is a serendipitous coincidence that all of these functions can be performed so safely and effectively by one readily available compound.
This isn’t to say that sulphur is beyond reproach. There have been times, especially in the middle of the twentieth century, when it was heavily used in excess of what was necessary. The result was wines that could smell of matches, or, under the wrong conditions, even rotten eggs. However, laws strictly limiting the amount of sulphur dioxide are in place, and consumers can be confident that their wines are safe for consumption. And, notwithstanding the urban legend to the contrary, even wines in Europe are made with sulphur because oxygen is just as damaging to wine on that continent as elsewhere in the world.
While sulphur may conjure associations with acrid smells, headaches, and even the Devil himself, when it comes to wine, think protective antioxidant instead.