Identification of a new light-struck off-flavour in “light-stable” beers

For decades, MBT (3-methyl-2-buten-1-thiol) is known as the compound responsible for the lightstruck off-flavour in beer. This leads many brewers to adapt the procedures by using reduced hop extracts. Unfortunately, other off-flavours including onion-defects often characterize these “light stable” beers. In the present work, a commercial lager beer which did not contain isohumulones (blend of dihydroisoalpha acids; bottled in clear glass) was submitted to various natural aging. Whereas no MBT-defect (skunky-like) was detected by sensorial analyses, a strong “onion-like” off-flavour was evidenced in the samples exposed to light. GC-PFPD and GC-O analyses of global (XAD) and thiol specific (pHMB) extracts allowed us to identify 2-sulphanyl-3-methylbutanol (2S3MBol) as the key-off-flavour (AEDA Flavour Dilution = 32-1024 for 2S3MBol while only 8-64 for MBT). 2S3MBol revealed to be synthesized from 3-methyl-2-buten-1-ol (MBOH) found in hop extracts. The involved radicalar mechanism is strongly enhanced by light. Although reduced hop extracts improve light stability regarding MBT, aroma-extracts give rise to strong onion-like off-flavours in presence of light. The concentration of the hop allylic precursor should be monitored in commercial hop extracts.