Guaiacol and 4-Methylphenol as Specific Markers of Torrefied Malts. Fate of Volatile Phenols in Special Beers through Aging

Phenol-specific extracts of 12 Belgian special beers were analyzed by gas chromatography hyphenated to olfactometry (AEDA procedure) and mass spectrometry (single ion monitoring mode). As guaiacol and 4-methylphenol were revealed to be more concentrated in brown beers (>3.5 and >1.1 mu g/L, respectively), they are proposed as specific markers of the utilization of dark malts. Analysis of five differently colored malts (5, 50, 500, 900, and 1500 degrees EBC) allowed confirmation of high levels of guaiacol (>180 mu g/L, values given in wort, for 100% specialty malt) and 4-methylphenol (>7 mu g/L) for chocolate and black malts only (versus respectively <3 mu g/L and undetected in all other worts). Monitoring of beer aging highlighted major differences between phenols. Guaiacol and 4-methylphenol appeared even more concentrated in dark beers after 14 months of aging, reaching levels not far from their sensory thresholds. 4-Vinylphenols and 4-ethylphenols, on the contrary, proved to be gradually degraded in POF(+)-yeast-derived beers. Vanillin exhibited an interesting pattern: in beers initially containing <25 mu g/L, the vanillin concentration increased over a 14 month aging period to levels exceeding its sensory threshold (up to 160 mu g/L). Beers initially showing an above-threshold level of vanillin displayed a decrease during aging