This week the flavour compound of the week is taking a turn down dark and filthy alley to be confronted by the twisted mutant dog that is the lower mercapatan or thiol. Lower mercaptans smell horrendous, horrific and disgusting. I was in a chemistry lab when a student accidentally made some in a fume cupboard and even under extraction the stench was staggering and resulted in widespread emesis. The definition next to lower mercaptan in my copy Malting and Brewing Science by Briggs et al. is simply “stench”. Mercaptans are otherwise described as drains and putrefaction. They are produced during the decomposition of sulphur-containing amino acids or pepetides. Humans abhor of mercaptans because they are poisonous to us. According to the Guinness Book of Records 2000, ethyl mercaptan (ethanthiol) is the smelliest substance known to man. The concentration of mercaptans in beer is never much more than a trace but this is sufficient to give a rank note to the beer but not to poison you. Mercaptans in beer are evidence of infection with bacteria or a serious metabolic problem with a yeast! I have detected it in a few cask ales in my time, only fleetingly but long enough to put me off drinking the stuff.