Malt thiol precursors: which grains carry them

What the literature measured

• Pale barley malts contain glutathionylated 3-mercaptohexanol precursor in the low hundreds of µg/kg range, and some special malts carry comparable or higher amounts; wheat is a poor carrier Chenot et al. 2023.
• Thiol groups in malt protein fall as colour rises and are undetectable in the darkest roasted malts, which is why roast and heavily caramelised malts rate negligible here Fleischer & Hellwig 2022.
• A genetically modified yeast fermenting an all-barley wort with only a light Cascade addition produced free 3MH well above its sensory threshold, whereas wheat, rice, and sorghum worts carried far less precursor Molitor et al. 2022.
• Malt and hops both supply thiol S-conjugates, and a lager yeast can release thiols from them during fermentation; disulfide-bonded thiols in malt and hops are a further precursor pool Chenot et al. 2022 Kishimoto et al. 2022.

The levels below are a relative ranking, not a molar yield. Hoparoma uses them as one input to its thiol model; the resulting aroma is a research-based estimate, and the perceived effect of malt precursors in a heavily hopped beer is still an open question in the literature.

The scale

• High Reserved for specialty base malts documented as unusually precursor-rich. None of the library's standard malts qualify yet.
• Medium Standard pale barley base malts and lightly kilned specialty malts.
• Low Light roast or light caramelisation; precursors reduced.
• Negligible Fully roasted or heavily caramelised; precursors effectively gone.
• Low (non-barley) Wheat, rye, and oat products; barley-type precursors are scarce.

Base malts

Crystal and caramel malts

Roasted malts

Adjunct grains

Other