Stripping of aroma compounds during beer fermentation monitored in real-time using an automatic cryotrapping sampling system and fast gas chromatography/mass spectrometry
Improvements are reported for a recently described instrument made up of an innovative headspace sampling and injection system built using a longitudinally modulating cryogenic system mounted around the sampling loop of a two-position loop injector, which had been hyphenated to a low thermal mass gas chromatograph. Replacing the bare capillary used previously for cryotrapping by a coated capillary opened additional potential fields of application for the instrument since more volatile analytes, with boiling points above 140 °C instead of 190 °C, could now also be quantitatively cryotrapped. Detection using a mass spectrometer instead of a flame ionization detector enabled the investigation of dynamic processes that release complex mixtures of volatile compounds in widely varying amounts. The performance of the improved system was illustrated by monitoring in real time, with 5 min resolution, of the stripping of volatile aroma compounds by fermentative carbon dioxide during beer fermentation. Compounds derived from hops, including monoterpenes, sesquiterpenes and some esters, which are present in the initial wort but are not formed subsequently, were observed to be stripped off early in the fermentation process. Fusel alcohols are the earliest volatile compounds derived from yeast metabolism, which are found to be released in significant amounts from the wort. More than ten fermentation-derived esters were observed to be stripped from the wort later into the fermentation. Although similar, their release kinetics exhibited clear differences which were the consequences of their different biosynthetic production rates and physical–chemical properties. The release curves of further compounds and how they can lead to the elucidation of their origin are also discussed.