Thiols and Hop Burst
If one class of molecules explains the explosive tropical intensity of the modern hazy IPA, it is the polyfunctional thiols. These sulfur-containing compounds are perceptible at almost unimaginably low concentrations and have become the central obsession of cutting-edge IPA brewing.
#What a Thiol Is
A thiol is an organic compound containing a sulfurβhydrogen (βSH) group. The relevant ones in beer are called polyfunctional thiols because they carry additional chemical groups. They smell of grapefruit, passionfruit, guava, and β in excess β "catty" or blackcurrant.
| Thiol | Shorthand | Aroma |
|---|---|---|
| 3-Mercaptohexan-1-ol | 3MH (3SH) | Grapefruit, passionfruit |
| 3-Mercaptohexyl acetate | 3MHA | Tropical, intense passionfruit |
| 4-Mercapto-4-methylpentan-2-one | 4MMP | Blackcurrant, "catty," box tree |
Some thiols are perceptible at nanograms per liter. A near-undetectable trace can dominate the aroma of a beer β which is what makes them so powerful and so finicky.
#"Hop Burst" β Where Thiols Come From
The term hop burst describes a brewing approach that loads almost all hops into the late-boil, whirlpool, and dry hop stages rather than the bittering addition. This both preserves volatile oils and maximizes the delivery of thiol precursors.
Crucially, most thiols do not arrive in the beer ready to smell. They come bound and odorless β locked to a cysteine or glutathione molecule. They must be released, and that is a job for fermentation.
#Thiolized Yeast β The Modern Breakthrough
Ordinary brewing yeast releases some bound thiols using an enzyme called a Ξ²-lyase, but most strains are weak at it. The breakthrough of the last decade has been "thiolized" yeast β strains (some bred, some bioengineered) with high Ξ²-lyase activity that aggressively cleave precursors and free the thiols.
The same hops and same wort, fermented with a thiolized strain instead of a standard one, can produce a dramatically more tropical, juicy beer β the aroma was always present, just locked away.
Hop variety matters too: Sabro, Citra, and Mosaic are notably thiol-rich, and specially processed "thiol-boosted" hop products are now sold (see Hop Products and Formats).
#Why Hazy IPAs Lean on Thiols
The New England IPA is the natural showcase: low bittering, big late and dry charges, active-fermentation hopping, and increasingly a thiolized strain from the IPA yeast lineup. Together these make the most of a fragile, spectacular aroma.
Because they are reactive sulfur compounds, thiols are highly vulnerable to oxidation. A thiol-driven IPA is at its peak within weeks β see Hop Fade and Oxidation and IPA Freshness and Shelf Life.
#Continue Reading
- Biotransformation β how bound thiols get released
- Hop Aroma Compounds β the wider aroma picture
- IPA Yeast Strains β including thiolized strains
- New England IPA β the style built around thiols