If you’ve ever smelled a New Zealand Sauvignon Blanc, you’ve smelled thiols (“thigh-all”, rhymes with “style”). These are the aroma compounds that smell like grapefruit, guava, passion fruit, and yes, sometimes cat pee.
What are thiols?
Thiols are odorous compounds that contain sulfur. Our understanding of thiols is a recent development in the wine world. In 1995, some researchers in Bordeaux identified a volatile thiol, called 4MMP for short, which is a dominant aroma in Sauvignon Blanc. They found this compound after they realized that adding copper to the wine removed tropical fruit smells. Since they knew that copper reacts with sulfur to form an odorless compound, they worked backward and concluded that the tropical fruit aromas must be coming from a sulfur-containing volatile thiol.
Since then, many different volatile thiols have been found. The most common include:
4MMP (4‐mercapto‐4‐methylpentan‐2‐one): Boxtree, passion fruit, broom, black current
3MH (3-Mercaptohexan-1-ol): Passion fruit, grapefruit, gooseberry, guava
3MHA (3-Mercaptohexyl acetate): Passion fruit, grapefruit, box tree, gooseberry, guava
You can see that one aroma compound can produce a range of different smells depending on its concentration. In the right amount, volatile thiols will smell like tropical fruits. In the wrong amount, they can smell like cat pee.
Which wines have thiol aromas?
Sauvignon Blanc is the thiol king, but thiols are also present in Semillon, Gros and Petit Manseng, Merlot, Grenache, Cabernet Sauvignon, and rosé wines from Provence.
How do thiols form?
Thiols start as aroma precursors in the grape berries in the vineyard. These precursors are called cysteine S-conjugates or glutathione S-conjugates, which simply means that cysteine (an amino acid) or glutathione (a compound that contains cysteine) is bound to a sulfurous compound. For example, a precursor for 3MH is Cysteine-3MH.
Much like terpene floral aromas, thiols do not become odorous until fermentation when yeasts break down the precursors and release volatile thiols.
One classic thiol-dominant wine is Sauternes, a sweet wine from the Bordeaux region that is made from Semillon grapes attacked by the fungus Botrytis cinerea. Thiol precursors in the grapes develop after the fungus sets in. To fight the attack of Botrytis, the grapes will defend themselves by producing a toxic aldehyde, hexanal. The problem is that hexanal is not only toxic to Botrytis, but also to the grape. So to lower the toxicity, glutathione will bind to the hexanal. Enzymes break this glutathione-hexanal compound down into a cysteine S-conjugate. This cysteine-3SH is the precursor for 3SH, which smells like grapefruit. In short: thiols can develop as a natural, biochemical response of the grape berry to ward off an attacking pathogen!
Do thiol aromas change with age?
Once these thiols have formed, they are not totally stable. In the presence of oxygen, they can bind with quinones, the compounds that turn wine brown. This “traps” the thiols and they become odorless. To prevent this aroma loss, sulfites, ascorbic acid, or glutathione can be added to grape juice before fermentation. Instead of binding to the thiol, the quinone will bind to the sulfite/ ascorbic acid/ glutathione. This is why it’s important to limit oxygen exposure at bottling, and/or to add an antioxidant. Wines that are aged on the lees (the dead yeast cells) are also more protected from aroma loss. The reason is that hydrogen sulfide (H2S) is released from the lees, and H2S will bind to quinones faster than thiols, preserving the thiol aroma.
- Thiols generally smell like tropical fruits. At high concentrations, they can smell like cat pee.
- Thiols are the dominant aroma in Sauvignon Blanc wines, especially from New Zealand. But other white grapes (Semillon, Gros/Petit Manseng) and red grapes (Grenache, Merlot, Cabernet Sauvignon) can also have thiol aromas.
- Thiols start out as precursors in berries in the vineyard. They do not become odorous until fermentation when the yeasts degrade the precursor and release the odorous component.
- Botrytized wines like Sauternes have very high thiol aromas since thiol precursors are built up in the berry as a means of self-protection after the fungal attack.
- Oxidized wines can lose their thiol aromas. This is why adding an antioxidant like sulfur, glutathione, or ascorbic acid is important in white wines. Aging on the lees can also reduce oxidation and aroma loss.