Wine science and sulfurs – the good and the bad

We all know different wines can smell and taste extraordinarily different. Chardonnays are typically buttery, peachy and oaky and rosés reminds us of strawberries and cream, but why is this so? It is remarkable that we can start with the same species of fruit, the grape (Vitis vinifera), crush the berries, take the fruit through the fermentation process, and the end product is dramatically different depending on the grape variety and the way it has been made.

The fermentation process occurs thanks to our tiny single-celled friends, the wine yeast, particularly the species known as Saccharomyces cerevisiae. In fact, the cerevisiae part of the name means “derived from beer” so we are dependent on yeast to not only provide us with delicious wines, but also beer, sake, bread, olives, chocolate and fermented meats.

As these busy yeasts ferment the juice from grape berries, they consume the sugars and make carbon dioxide and alcohol. While yeasts are fermenting they produce aroma compounds as by-products of their metabolism. These by-products are the molecules that give wines their characteristic bouquet and make them so attractive and delicious.

Although wine is comprised of mostly water, alcohol, acids, and 1 percent aroma compounds, only very small concentrations are required for the nose to detect them. Wine contains thousands of different aroma molecules and our noses contain an equally high number of olfactory receptors which can tell the differences between these compounds with great accuracy. Like a lock and key, each receptor fits a specific molecule to send a cascade of signals to the brain to identify the aroma.

Approximately 10 percent of aroma compounds contain sulfur atoms and are known as volatile sulfur compounds (VSCs). Some day-to-day foods we eat which contain sulfur compounds include onions, garlic and leeks, and the VSCs in these vegetables are the same as those found in wine. Unsurprisingly, VSCs in wine tend to be classified as off-flavours and are generally formed as a result of yeasts scavenging sulfur, an essential nutrient, from the grape juice to make sulfur-containing amino acids for growth and survival. This process of utilising sulfur happens in every fermentation, so a wine will always contain VSCs. At low concentrations these tend to not be detectable or they provide positive characteristics. But when yeasts become stressed while doing their job, particularly due to a lack of nutrients, sulfur compounds can get out of hand and cause problems in our wine.

When yeast are stressed and lack nutrients they first make the small sulfur compound hydrogen sulfide (H2S), which smells like rotten eggs. Winemakers can easily reduce H2S if detected early enough by adding nutrients. However, if this problem is not controlled, larger and more stinky sulfur compounds can be formed. These compounds are very reactive and can interact with one another to form further pungent species as well as break down chemically in wines. Winemakers can try to remove these compounds by using copper sulfate fining but recent research has shown that if removal isn’t completely effective, sulfur compounds can be re-released when wine is stored.

It is sounding like VSCs are the bad guys but in fact, they represent the good, the bad and the ugly of the world of aroma compounds. I have given many examples of the bad and the ugly, but there are certain sulfur compounds which give positive characters at certain concentrations. The key aromas in New Zealand sauvignon blanc, other than the fresh green grassy notes, are fruity passionfruit and grapefruit notes. These tropical aromas are also derived from the action of yeast on odourless precursors during fermentation and are a type of sulfur compound known as a volatile thiol.

Another positive aroma that comes from the presence of sulfur compounds in wines is flintiness (gunflint, gunpowder and struck match). These aromas are usually described in certain styles of barrel-fermented chardonnay. This style of wine can be polarising because at high concentrations the wines smell very strongly of buttered popcorn and gunpowder, which consumers either love or hate. The source of this flinty note is thought to be derived from a VSC called benzyl mercaptan, but other sulfur compounds, including the tropical thiols, may enhance flinty characters in wines. As for how benzyl mercaptan is formed, stay tuned – this is something that we are currently working on at the University of Auckland.

In white wines and at high concentrations in red wines, a small sulfur compound called dimethyl sulfide (or DMS for short) smells unpleasantly like vegetables such as asparagus, cooked cabbage and creamed corn. But when present at lower concentrations in red wines, DMS can enhance fruity aromas and contribute lovely blackcurrant notes. In certain red wines the presence of DMS can result in an increase in aromas such as quince, olive and truffle notes. In pinot noir, sulfur compounds can provide a forest floor note and in rich reds like syrah, some of the meaty characteristics are thought to be derived from volatile sulfur compounds.

So as you can see, although sulfur compounds can provide unpleasant notes, they also provide the pleasant funkiness and punchy pungency that gives wine interest and character. Cheers to sulfur compounds in wine!