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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 volatile organic compounds (VOCs) for four days. A design matrix of two yeast biotypes (California Ale and Edinburgh Scottish Ale) and two New Zealand aroma hop cultivars (Motueka and Nelson Sauvin), together with their respective no addition controls, were used to investigate yeast-hop interactions. The results highlighted the advantages of using online analytical measurements, such as PTR-ToF-MS, to understand temporal changes that occur in VOCs during fermentation. Distinct differences were observed in the VOCs profile of the different beers based on combinations of yeast biotype and hop cultivar; e.g. samples with Motueka and Scottish Ale had higher concentrations of m/z 89.057 (3-methyl-1-butanol). Complex dynamics were observed for VOC development during the fermentation; e.g. production maxima for masses such as m/z 145.121 (2-nonanol or ethyl hexanoate) and m/z 173.153 (isoamyl isovalerate or ethyl octanoate). Introduction 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. Anecdotally, brewers report that some yeasts “promote” hop flavour or accentuate one hop character over another, while other yeast biotypes are considered to be “hop neutral” or known to reduce hop intensity. However, to date very little scientific research has been published on the impact of different yeast biotypes and fermentation parameters on hop flavour in beer or on the mechanisms responsible for differing aroma and flavour development [1]. Traditionally, hop flavour development has been assessed using GC-MS based approaches, where hop derived compounds in the final beer are identified and measured. The drawback with this approach is that it is time consuming and provides little information on changes that occur during brewing and fermentation. Furthermore, the sensitivity of GC-MS to detect volatile organic compounds (VOCs) is dependent on the extraction method employed and the volatility and polarity of the target analytes and their affinities towards the chosen solvent or solid phase material [2,3]. In this paper PTR-ToF-MS was used to measure and compare the dynamic changes in VOCs during the production of beer containing one of two aroma hops (Motueka and Nelson Sauvin) in combination with one of two yeast biotypes (California Ale and Scottish Ale) and their respective no addition controls. Tobias M. Richter et al. 172

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