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Title: Transcriptional and metabolic analysis of flavour development during brewing fermentation
Author: Ashraf, Nadim
ISNI:       0000 0004 2742 6212
Awarding Body: University of Nottingham
Current Institution: University of Nottingham
Date of Award: 2012
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Esters and higher alcohols (fusel alcohols) are important group of metabolites that make a substantial contribution to the development of the flavour profile of the final product. The aim of this research was to develop tools that would permit analysis of flavour volatiles formed during fermentation and use them to determine the impact of initial dissolved oxygen (DO) levels on key fermentation performance indicators. Analysis of solvent extracts and headspace of fermented samples by gas chromatography-mass spectrometry (GC MS) was optimised to permit the quantification of higher alcohols and esters and compared to the data generated by a novel direct mass spectroscopy technique known as atmospheric pressure chemical ionisation-mass spectrometry (APCI-MS). Using these analytical techniques the impact of key process conditions, particularly the effect of initial dissolved oxygen, on fermentation performance and formation of flavour compounds were investigated. It was observed that reductions in the initial DO, increases cell growth rates and utilisation of wort metabolites during the first few hours fermentation. However, as the fermentation continued the final yeast biomass yield was adversely affected. The flavour metabolite concentrations during fermentation were not found to be significantly affected, except in a limited number of scenarios where the changes observed would be unlikely to be perceived as they were well below the sensory threshold. Although reducing initial DO yielded lower peak values of VDK, this apparent benefit was mitigated by the observation that diacetyl uptake was slower in fermentation, where reduced oxygen had been made available to the yeast. The net impact of reducing initial DO was therefore limited in the scenarios examined, however it is anticipated that some negative impacts may be observed if cropped yeast from reduced DO fermentations were repeatedly recycled. Thus future work would involve a serial re-pitching experiment, preferably in a pilot scale, to ascertain the impact of reduced DO using serially re-pitched yeast cells. 2
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available