Freddy Delvaux

Parameters Affecting Ethyl Ester Production by Saccharomyces cerevisiae during Fermentation

ABSTRACT Volatile esters are responsible for the fruity character of fermented beverages and thus constitute a vital group of aromatic compounds in beer and wine. Many fermentation parameters are known to affect volatile ester production. In order to obtain insight into the production of ethyl esters during fermentation, we investigated the influence of several fermentation variables. A higher level of unsaturated fatty acids in the fermentation medium resulted in a general decrease in ethyl ester production. On the other hand, a higher fermentation temperature resulted in greater ethyl octanoate and decanoate production, while a higher carbon or nitrogen content of the fermentation medium resulted in only moderate changes in ethyl ester production. Analysis of the expression of the ethyl ester biosynthesis genes EEB1 and EHT1 after addition of medium-chain fatty acid precursors suggested that the expression level is not the limiting factor for ethyl ester production, as opposed to acetate ester production. Together with the previous demonstration that provision of medium-chain fatty acids, which are the substrates for ethyl ester formation, to the fermentation medium causes a strong increase in the formation of the corresponding ethyl esters, this result further supports the hypothesis that precursor availability has an important role in ethyl ester production. We concluded that, at least in our fermentation conditions and with our yeast strain, the fatty acid precursor level rather than the activity of the biosynthetic enzymes is the major limiting factor for ethyl ester production. The expression level and activity of the fatty acid biosynthetic enzymes therefore appear to be prime targets for flavor modification by alteration of process parameters or through strain selection.

Bioprocess Intensification of Beer Fermentation Using Immobilised Cells

Beer production with immobilised yeast has been the subject of research for approximately 30 years but has so far found limited application in the brewing industry, due to engineering problems, unrealised cost advantages, microbial contaminations and an unbalanced beer flavor (Linko et al. 1998; Branyik et al. 2005; Willaert and Nedovic 2006). The ultimate aim of this research is the production of beer of desired quality within 1–3 days. Traditional beer fermentation systems use freely suspended yeast cells to ferment wort in an unstirred batch reactor. The primary fermentation takes approximately 7 days with a subsequent secondary fermentation (maturation) of several weeks. A batch culture system employing immobilization could benefit from an increased rate of fermentation. However, it appears that in terms of increasing productivity, a continuous fermentation system with immobilization would be the best method (Verbelen et al. 2006). An important issue of the research area is whether beer can be produced by immobilised yeast in continuous culture with the same characteristic as the traditional method.

The Chemistry of Aging Beer

Packaged beer is a closed system that continuously will be submitted to alterations by chemical reactions. These alterations will lead to perceivable changes in flavor, foam and colloidal stabilities. In this chapter, the underlying mechanisms of the reactions leading to aging of beer are discussed. Radical reactions, often initiated by reactive oxygen species, have an important contribution to beer staling and form a continuous thread to the stability of beer. Oxidation of unsaturated fatty acids and Maillard reactions leads to hundreds of different compounds. Furthermore degradation of hop bitter acids and carotenoids, oxidation of polyphenols, hydrolysis of glycosides and esters or synthesis of esters, acetalization of aldehydes will all contribute to the aging of beer. When using antioxidants, attention has to be paid to the slower reacting oxygen species such as H2O2, rather than the most reactive species as the hydroxyl radical.