| The potential of yeast preoxygenation for application in the brewing industry | |
Abstract/OtherAbstract
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The fermentation process is of major importance in beer production. During fermentation, yeast cells use the available nutrients in wort for growth and maintenance metabolism. In order to grow during the first fermentation phase, yeast cells need to synthesize appropriate levels of essential membrane compounds. Hence, one major role of dissolved oxygen is to promote the biosynthesis of ergosterol and unsaturated fatty acids in brewery fermentations. Traditionally, oxygen is provided through wort oxygenation but this technique has several disadvantages. In order to avoid those, yeast cells can be directly exposed to oxygen before fermentation in a controlled process known as preoxygenation, thereby removing the requirement for subsequent wort oxygenation. However, industrial applications of this technique are scarce, due to the inconsistent and limited scientific studies and the lack of knowledge regarding the biochemical background of the preoxygenation of yeast cells. Therefore, the main objectives of this study were to examine the influence of preoxygenation on beer production parameters and to gain better insight in the yeast’s metabolism during both processes.<br/> <br/>First of all, the preoxygenation ability of different ale and lager yeast strains was studied. Despite the different response of the yeast strains tested, successful results have been achieved. Large variations in sterol and UFA synthesis during preoxygenation were observed. Though, the fermentation performance of preoxygenated yeast cells in de-aerated wort was a good indication for the success of the treatment.<br/> <br/>In a next series of experiments, the possibility to use yeast preoxygenation as an alternative to the conventional wort aeration process in normal brewing practice has been studied on pilote scale. This demonstrated that the use of the preoxygenation technique enables us to produce beers with similar characteristics as the traditionally brewed beers. Yeast fermentation performance, yeast growth and aroma compound formation were comparable. Profiles of sterols and unsaturated fatty acid levels during preoxygenation and subsequent fermentation or during the traditional fermentation also seemed to be very similar. Moreover, cropping of the yeast cells and reuse of them after preoxygenation has proven to be possible without loss of quality. It could also be concluded that preoxygenation can be used successfully for refermentation processes. Though, since preoxygenated yeast cells seem to be more sensitive to the storage conditions than untreated yeast cells, it is recommended to use fresh yeast cells, as stored cells are generally of inferior physiological condition. In general, it is suggested to minimize the yeast storage time at temperatures as low as possible.<br/> <br/>We also performed a comparative genome-wide expression analysis on yeast samples harvested during the exponential phase of the fermentation in order to compare yeast behavior during fermentation after preoxygenation or wort aeration. No major differences in gene expression halfway fermentation were obtained, proving once again that preoxygenation and wort aeration lead to similar results. However, aeration of both yeast and wort resulted in a slightly shorter exponential phase, bringing the yeast faster into stationary phase. This resulted in biosynthetically more active yeast cells, with an enhanced yeast growth and an overall increased fermentation performance (14%). Microarray analyses demonstrated that the expression of yeast genes involved in amino acid metabolism, reserve carbohydrate metabolism and stress resistance were accordingly affected.<br/> <br/>Next, a first attempt was made to better characterize the oxygen need of yeast cells. Yeast cells must be capable of responding rapidly at the physiological level to changes in their oxygen environment, for example to protect themselves against potential oxidative stress. Using real-time PCR, genes involved in transcriptional control, respiration, detoxification, stress response and lipid metabolism were examined during preoxygenation. This experiment showed that, during preoxygenation, the yeast cells seem to acquire stress resistance as they accumulate the stress-protectant trehalose and induce the expression of genes involved in detoxification processes and of stress-responsive genes. <br/> <br/>Finally, the general belief is that wort aeration has a negative impact on wort quality, which could result in more rapid beer staling. Since this study showed that almost identical beers could be produced with the preoxygenation as with the wort-aeration technique, preoxygenation proved to be the ideal tool to investigate the effect of wort aeration on beer flavor stability. GC-MS analyses and sensory analysis did not reveal significant differences in beer staling between the wort aeration and the preoxygenation treatment, for pilsner as well as for ale type beers. Therefore it can be stated that the classical wort aeration process does not have a negative impact on beer ageing.<br/> <br/>It can be concluded that yeast preoxygenation is a worthy alternative to wort aeration. Moreover, our study has provided several new insights into the behavior of yeast cells during the preoxygenation and fermentation process in normal brewing practice. |
Authors
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Depraetere, Sofie |
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Contributors
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Vanderleyden, Jozef, Winderickx, Joris |
Publication Detail
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Publisher : K.U.Leuven Type : Electronic thesis or dissertation, Elektronische thesis of dissertatie Format : 1253314 bytes, application/pdf |
Date Detail
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2007-07-10 |
Subject
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Copyright Information
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Only internally accessible from the K.U.Leuven intranet, Enkel intern toegankelijk via het K.U.Leuven-intranet |
Other Details
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Languages : en |
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