Title: Bottle conditioning of beer Strategies to improve yeast refermentation performance
Other Titles: Hergisting van bier op fles Strategieën om de hergistingsperformantie van gist te verbeteren
Authors: Dekoninck, Tinne
Issue Date: 19-Oct-2012
Abstract: Bottle conditioning of beer, i.e. the additional fermentation of beer in the bottle, or simply ‘refermentation’, consists of the addition of small amounts of sugar and yeast to mature beerbefore bottling. Hereafter, the beer is incubated in heated chambers at ca. 25°C and refermented in preferably less than two weeks, ideally resulting in a fully carbon dioxidesaturated beer with an enriched flavour perception and prolonged flavour stability. In fact, the process seems rather simple, although many problems encountered in the industrial situationconfirm the opposite. The main problem resides in incomplete refermentations, in which the resulting beer contains a substandard carbon dioxide content, highly undesired by consumersall over the world.Despite the widespread use of bottle conditioning, in Belgium as well as the UK or USA, no extensive research on the process has yet been performed. Hence, the lack of in-depthknowledge hinders optimal process control. Therefore, the main objectives of this study consisted of elucidating the impact of both yeast and beer related parameters on therefermentation process, as well as the impact of various yeast propagation strategies in view of an improved refermentation performance of yeast. Firstly, a screening of industrial Saccharomyces cerevisiae strains was performed during refermentation of a high alcohol beer, to distinguish between ‘poor’ and ‘superior’refermenting strains. The ethanol tolerance of yeast was demonstrated to largely correlate with yeast refermentation performance. Furthermore, constant oxygen availability duringyeast cultivation was shown to result in reduced viability loss of the yeast culture in the bottle, although refermentation performance was the highest in case of oxygen-poor propagatedyeast. In a second experiment, the refermentability of several Belgian beers was evaluated, with special attention paid to the initial beer alcohol content and beer nutrient levels.Especially when dealing with a less ethanol tolerant yeast strain, high alcohol content of beer was demonstrated to severely hinder refermentation performance of yeast. Nevertheless, wheninitial alcohol levels of all tested beers were increased to 10 v/v%, a discrepancy regarding refermentation performance still existed between beers originally low, or high in alcoholcontent. Hence, ethanol is not the sole determining factor regarding the refermentability of a particular beer type. Furthermore, it was established that cell multiplication is no prerequisitefor a complete refermentation process, but could be beneficial due to an overall increased activity, as was observed when applying increased pitching rates. Since the employed beer type and yeast strain are usually ‘fixed parameters’ in the industrial situation, the feasibility of altering yeast’s refermentation characteristics through a variety ofpropagation strategies was evaluated in further experiments. It was demonstrated that the preconditioning of yeast toethanol (2.5; 5 or 7.5 w/v%) during propagation, prior to beerinoculation, greatly enhanced yeast refermentation performance. The adaptation process conferred increased ethanol tolerance to yeast cells, which was expressed through increasedaccumulation of trehalose, unsaturated fatty acids and ergosterol, likely positively contributing to the performance and viability of yeast in the bottled beer. To differentiatebetween exogenously added and endogenously produced alcohol, the switch was made from low density (LD; 5 w/v% glucose) to high density (HD; 15 w/v% glucose) growth media.Compared to non-adapted LD propagated yeast, propagation of yeast in HD media positively contributed to the refermentation performance of yeast. The increased ethanol toxicity, as wellas the increased osmotic stress encountered by yeast in HD media confer an increased general stress resistance to yeast, facilitating the refermentation process. Nevertheless, the beneficialeffect of adaptation to 5 or 7.5 w/v% ethanol could not be exceeded through the use of HD media. When switching from synthetic YPD to brewery wort medium, no improvementregarding refermentation performance of propagated yeast was observed, presumably due to decreased yeast activity during wort, compared to YPD propagation. The replacement ofglucose by maltose in the LD synthetic medium resulted in an improved refermentation performance of yeast, presumably due to the increased stress resistance of yeast cultivated inglucose-free medium. Altogether, the selection of an appropriate propagation strategy is a highly promising technique to improve the refermentation performance of a particular yeast strain, avoiding theneed to introduce an unfamiliar strain in the brewery with enhanced refermentation characteristics. More specifically, the pre-conditioning of yeast to ethanol prior to beerinoculation results, in less than ten days, in the complete refermentation of a very high alcohol beer, difficult to referment in the industrial brewery situation.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Food and Microbial Technology

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