Hop aroma in beer - changes in the flavour profile associated with different yeast strains and fermentation parameters

Abstract

Beer has a complex flavour profile made up of hundreds of compounds, derived from its ingredients and changes that occur during the production process. Although beer flavour has been extensively researched, some of the reactions underpinning flavour development and hop aroma in beer are still poorly understood. In particular, the impact of yeast, hops, and their interaction on the volatile organic compound (VOC) production is not well characterised. As such the focus of this research was to gain a better understanding of the impact of different yeast strains on hop flavour development in beer and in particular the interactions between yeast and hop compounds during fermentation. The thesis was structured into two steps; I) identifying and comparing suitable extraction and detection methods for the measurement of the hop related VOCs and their changes during fermentation, and II) evaluating the effect of different yeast strains and fermentation parameters on the VOC profile. As extraction methods usually have particular limitations and advantages towards specific volatile organic compounds (VOCs), four extraction methods were compared to determine the most suitable method for hop-derived VOCs in beer: namely headspace solid phase micro extraction (HS-SPME), stir bar sorptive extraction (SBSE, headspace sorptive extraction (HSSE), and solvent assisted flavour evaporation (SAFE). HSSE was found to be most suited due to its higher sensitivity towards hop-derived VOCs and ease of handling. To gain insights into the metabolic behaviour and biotransformation pathways, measurement systems such as GC-MS with lengthy extraction techniques are unable to provide sufficient time discrimination during fermentation. Proton transfer reaction – time of flight - mass spectroscopy (PTR-ToF-MS) is a direct measurement tool for volatiles in head space samples with high sensitivity. It was described for the first time in this thesis to successfully measure VOC changes during the fermentation of beer, and was able to differentiate different VOC profiles between yeast strains and hop cultivars. To identify VOCs of interest that were hop-derived or influenced through interactions of hops and yeast, hopped and unhopped beers brewed under the same conditions were compared with hot water hop extracts. Hop-derived terpenes were found to be significant but interestingly, some fermentation derived VOCs were also significantly affected by the presence of aroma hops, resulting in higher or lower concentrations compared to the unhopped control. Fifty VOC in beer were found to be significantly affected by the late hopping treatment and selected as compounds of interest. Different yeast species, but also genetic variations within a species, have been shown to produce different VOC profiles during fermentation. In this thesis, the effect of fifteen different yeast strains, comprised of four different yeast species (S. cerevisiae, S. pastorianus, S. bayanus, and Schizosaccharomyces pombe), on the levels of the selected hop-derived and hop influenced VOCs was analysed. The yeast strain used for the fermentation plays an important role in the generation and differentiation of hop-derived and hop influenced VOC, but it was not possible to make a generalisation on the effect of lager or ale yeast strains on VOC formation or biotransformation. It was found that individual differences between the yeast strains were more predominant. The effect of fermentation temperature and pitching rate on target VOCs was also investigated. Both parameters were found to exert differing effects on the various VOCs, with results dependent on the specific yeast strain used. Findings were supported through dynamic measurements of the fermentation with PTR-ToF-MS. The series of experiments demonstrated that yeast hop relations and changes during the fermentation were complex interactions as multiple effects were occurring simultaneously making an understanding challenging. However, the approach of incorporating temporal measurements with PTR-ToF-MS and sensitive GC-MS will facilitate the understanding of beer flavour.