Abstract
The main objective of this thesis was to determine the effect of yeast strain and fermentation on the sensory perception of hop flavour in beer. This project evaluated the differences between 12 different yeast strains on the flavour perception of a standardized wort made using bittering hops (Simcoe) and late-hopped with one aroma hop (Motueka). The yeast selection was a range of ale (S. cerevisiae), wheat beer (S. cerevisiae), lager (S. pastorianus), wine (S. cerevisiae wine yeast hybrid), Chardonnay (S. cerevisiae) and Champagne (S. bayanus) yeast strains.
The beer was then evaluated by sensory analysis using a free sorting task methodology. A panel of 14 subjects including trained, untrained and beer experts were recruited for the study. The 12 beer samples were evaluated in triplicate over three separate sessions to form groups according to the similarity of their sensory attributes, with descriptors allocated to each group. The beer sorting task data was analysed using multiple correspondence analysis (MCA) and a frequency table (contingency table) to understand the sensory attributes driving the differences between the beer samples. The top five most frequently used attributes were hoppy, fruity, sulphury, bitter and citrusy. Based on a textual analysis of the sensory attributes generated per beer sample, the significant sensory attributes associated with specific beer samples were determined. The beer fermented with 34-70 yeast was associated with a hoppy flavour, whereas both OTA 79 and Wy-1272 beers were significantly related to sulphury. Spicy flavour notes were significantly associated with WB-06 and WLP 730 beers, and estery was also significantly associated with the WLP 730 beer. VIN 13 beer was significantly related to sour and metallic, whereas WLP 001 beer was significantly related to astringent. The projections illustrated all beer samples were considerably different and this was due to differences in yeast strain used to ferment the 12 samples.
Volatile analysis in beers was carried out using headspace solid-phase microextraction (SPME) and gas chromatography mass spectrometry. From the volatile analysis a total of 102 volatile organic compounds (VOCs) were detected, including terpenes, esters, alcohols and phenols, of which 77 VOCs were significantly different between beer samples (p-value <0.05). All the selected VOCs clearly demonstrated that the selected yeast types (ale, lagers, wine yeast) behave differently under a constant temperature to generate distinct VOC profiles, thus influencing perceived beer flavour. Wine and lager yeast strains expressed different outcomes compared to what was expected according to existing literature and industry information, which was considered to be due to the fermentation temperature being different to the recommended fermentation temperature. Additionally, the process of biotransformation showed significant difference in terpene esters and terpenoid compounds regulated by yeast strain, which also contributed to the different sensory perception in the beer samples.
The integration of sensory and volatile analysis supported the association of flavour terms given to the beers and the volatiles detected in the beers. Beer made with OTA 29 yeast (Champagne yeast strain, S. bayanus) had the highest concentration of esters, mainly ethyl and acetate esters, specifically propyl acetate (celery and raspberry). The production of these volatiles supports the sensory analysis, which described beer OTA 29 as having a fruity flavour profile in beer. Beer made with WLP730 (Chardonnay yeast), Exotic (wine yeast) and WB-06 (wheat yeast) contributed to high 4-ethenyl-2-methoxyphenol levels, which corresponded to the sensory analysis, which described these beers as spicy. Beer made with 34/70 (lager yeast, S. pastorianus) was described as very hoppy and this outcome was supported by volatile analysis which presented to high levels of nerol and geraniol.
Yeast selection is an important element in brewing, as yeast interact with hop compounds to determine the final flavour in beer as well as the type of beer produced. It is reported that yeast strains interact with hops differently to produce a variety of flavour profiles. The current study successfully demonstrated that all beers expressed different chemical composition and perceived flavour profiles based on sensory perception and volatile analysis. This research will provide scientific understanding on how different yeast strains affect hop flavour in beer. This project will also provide information to breweries on how to optimise hop flavours in beer in order to meet consumer demands.