Tracking of hop-derived compounds in beer during fermentation with PTR-TOF-MS

The craft beer market is experiencing a rapid increase in growth. To help brewers optimise hop character, and to make beer with distinctive hop profiles, a better understanding of the role that yeast play in the development of hop character is required. Beyond anecdotal evidence, a paucity of published data exists on the interactions between hop varieties and different yeasts and the resulting effect on beer flavour. In the current study, proton transfer reaction-time of flight-mass spectrometry (PTR-TOF-MS) was used as a novel, direct and real-time analytical method to monitor small-scale fermentations carried out in 20 mL vials (3 mL sample volume) at 20 ◦C with repeated measurements of the headspace VOCs every six hours for four days. A design matrix of two yeast strains (California Ale and Edinburgh Scottish Ale) and two New Zealand aroma hop varieties (Motueka and Nelson Sauvin), together with their respective no addition controls, were used to investigate yeasthop interactions. VOCs that showed evidence of hop-yeast interactions were identified. Differentiation between isomeric compounds was achieved through separation with fastGC and identification of compounds was supported by GC-MS and scientific literature. The results highlighted the advantages of using online analytical measurements, such as PTR-TOF-MS, to understand temporal changes that occurred in VOCs during fermentation. For example masses such as ms145.121 (2-nonanol, or ethyl hexanoate), ms105.068 (pentanethiol, or 3-methyl-1-butanethiol) and ms173.153 (isoamyl isovalerate, or octyl acetate) initially increased during the fermentation process and then decreased towards its end due to competing reactions. Distinct differences were observed in the VOCs profile of the different beers based on combinations of yeast strain and hop type; e.g. samples with Motueka and California Ale were associated with higher levels of ms75.077 (2-methyl- 1-propanol), while samples with Motueka and Scottish Ale had higher concentrations of ms137.132 (pinene, or myrcene) and ms89.057 (3-methyl-1-butanol). A better understanding of how hop-derived compounds in beer are influenced during fermentation by yeast will improve our understanding of the generation of the hop aroma of beer and will give insight on how to achieve a desired hop character by selecting yeast strains and modifying fermentation parameters