Thiols & biotransformation (the science) also: 3SH, 3SHA, 4MSP, Bound thiol precursors
Why some hops + some yeasts produce passionfruit and guava aromas, even when the hops alone don't smell that way.
What it is, in plain English
Thiols are sulfur-containing aromatic compounds that smell intensely tropical at very low concentrations — passionfruit, guava, grapefruit, even the famous 'cat pee' note in Sauvignon Blanc. They show up in beer, wine, and some grains. Most hops contain bound thiol precursors — molecules that are flavor-inactive on their own but get cleaved during fermentation by certain yeasts, releasing the active aromatic thiol. This is biotransformation: yeast taking inactive precursor molecules and turning them into aromatic ones during fermentation. Phantasm powder works because Sauvignon Blanc grape skins are extraordinarily rich in these precursors. Specific yeast strains (Conan / London III, Berkeley Tropics, Omega Cosmic Punch) are 'thiol-positive' — they express the enzymes that release thiols efficiently. Other strains do this poorly. The combination of thiol-rich substrate + thiol-releasing yeast is what produces the most explosive hazy IPAs of the last 5 years.
The technical version
The three most-discussed thiols in beer are 3-sulfanylhexan-1-ol (3SH or 3MH, passionfruit / grapefruit), its acetate ester 3-sulfanylhexyl acetate (3SHA, more passionfruit), and 4-methyl-4-sulfanylpentan-2-one (4MSP or 4MMP, blackcurrant / cat pee). All three exist in bound forms (cysteine or glutathione conjugates) in hops, grapes, and grains. Yeast strains with active beta-lyase or carbon-sulfur lyase enzymes cleave the conjugates and release free thiols during fermentation. Detection thresholds are extraordinarily low — 3SH is detectable at low parts per trillion.
How brewers use it
Three levers: (1) substrate (hops with high bound-thiol levels — Citra, Mosaic, Nelson Sauvin, plus Phantasm powder), (2) yeast (thiol-positive strains), (3) fermentation conditions (active fermentation = active enzyme production). Some thiols can also be lost or oxidized post-release if the beer isn't packaged carefully. Mash hopping with Saaz or Cascade (both surprisingly high in bound thiols) can amplify the effect.
Who uses it
Every modern hazy IPA brewer who's optimizing for tropical character is now playing in this space. The 'next evolution' of hazy beer is largely being driven by thiol amplification.