Combinatorial synthesis and sensorial properties of mercapto primary alcohols and analogues

Combinatorial chemistry was used to extend the knowledge of beer thiols by synthesizing 13 mercapto primary alcohols and 6 other compounds (mercapto ketones and corresponding alcohols). Their respective mass spectra, retention indices on two columns, odor descriptors, and best estimated-gas chromatography-lowest amounts detected sniffing (BE-GC-LoADS) are reported. The descriptor "cheese", commonly associated with mercapto aldehydes and thioesters, was rarely used for mercapto alcohols or mercapto ketones. Polyfunctional thiols containing at least six carbon atoms usually gave a delicate odor of rhubarb/carrot, whereas most of the mercapto primary alcohols of intermediate size were described as onion-like, plastic-like, or pungent. Sensorial analysis of beers spiked with copper(II) ions allowed confirmation of the participation of thiols in the overall odor of fresh beer. The finding of 4-mercapto-4-methyl-2-pentanone and 3-mercaptohexanol in an XAD-2 Amberlite resin beer extract indicates a requirement for a more efficient thiol extraction procedure allowing identification of relevant polyfunctional thiols for brewing scientists.