The effect of raw materials and yeast quality on beer stability
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Beer stability is a critical quality parameter and a large amount of research has attempted to understand and optimise it. The present study focused on the flavour and foam aspects of beer stability and novel methods of analysis were employed to investigate the influence of yeast physiological state, pitching rates, pasteurisation methods and raw materials on these important parameters. Laser scanning confocal microscopy and flow cytometry were used to gain an in-depth understanding of physiological state and protein dynamics of brewing yeast. Yeast cell components such as glycogen, neutral lipids, trehalose, bud scars, DNA and intracellular proteinases were successfully visualised and quantified. Co-localisation studies with a yeast GFP clone expressing proteinase A (Pr A) were conducted to explore expression and excretion of Pr A in a variety of stress situations encountered during wort fermentations. Pr A plays a key role in the degradation of the hydrophobic polypeptides responsible for the beer foam stability. The impact of flash pasteurisation compared to sterile filtration on Pr A levels, hydrophobic polypeptide content and foam stability has been investigated. Another focus of this study was to elucidate the origin and fate of polypeptides, throughout wort production and fermentation. Special emphasis was placed on the changes in size and/or hydrophobicity of foam active polypeptides throughout the brewing process in order to identify stages in the process where retention of foam active polypeptides and consequently beer foam stability could be increased. It was attempted to determine what proportion of polypeptides found in beer originate from the cereals and the hops and how these polypeptides evolve throughout the brewing process. High and lower gravity beers were compared with regard to their polypeptide distribution and degree of polypeptide glycosylation. Understanding the mechanisms of beer staling is a longstanding research priority of brewers in order to extend beer shelf life. It was attempted to shed light on the mechanisms involved in beer staling, the role of Reactive Oxygen Species and their interaction with thiol groups in beer proteins. Several approaches for inhibiting or retarding oxidation of the beer matrix were explored. Novel methods for the assessment of oxidative stability and content of thiol and carbonyl groups in beer proteins were developed.