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Reduction of vicinal diketones by yeast in dependence of carbon dioxide pressure
Gabriela Sepelov, Mariana Cvengroschov and Daniela Smogrovicov

One of the possibilities how to make beer production more effective is lowering the time of diacetyl and 2,3-pentanedione reduction. These compounds, collectively referred as vicinal diketones, are very important since their taste limits are very low and evolve undesirable change of beer flavour, which is known as buttery, cottage cheese or whey taste. The aim of our work was to study an influence of carbon dioxide pressure during the active carbohydrate fermentation and the end of fermentation on vicinal diketones reduction. We compared vicinal diketones reduction in young beers fermented under carbon dioxide pressure of 100kPa during the whole main fermentation and in beers fermented without carbon dioxide pressure until extract concentration decreased to 7.5P and then under carbon dioxide pressure of 50kPa. Experiments were done in the wort fermentation of gravity 12.5P and 13.5P. Lower carbon dioxide pressure accelerated vicinal diketones reduction and speeded up the end of fermentation. Wort concentration had no influence on vicinal diketones reduction in studied wort gravities (12.5P and 13.5P). Yeasts produce a-acetolactate and a-acetohydroxybuthyrate, precursors of vicinal diketones, as they synthesise the amino acids valine and leucine needed for protein synthesis. a-acetolactate and a-acetohydroxybuthyrate are excreted into the fermenting wort and converted nonenzymatically to 2,3-butanedione (diacetyl) and 2,3-pentanedione, collectively referred as vicinal diketones (Fig. 1). Towards the end of fermentation vicinal diketones are subsequently re-assimilated by the yeast and reduced enzymatically to butanediol and pentanediol [1,2,3,4]. Yeast strains vary in their capacity to excrete the a-acetolactate and certain fermentation conditions can enhance the production of this precursor [5]. Flavour limit of vicinal diketones was determined on 0.1mg/l and these compounds have big importance on taste. Above the flavour-threshold-level they cause an undesirable change of beer taste, which is known as a buttery, cottage cheese or whey flavour [6]. Butanediol and pentanediol, as well as acetoine are sensorically non-active. The breakdown of these vicinal diketones occurs parallel to other maturation reactions during the beer conditioning process and is therefore nowadays regarded as the essential criterion for the state of maturation of a beer [7]. Production of a-acetolactate is limited by lower pH-values of beer (under 4.6) but higher top-pressure during fermentation supports its production[8].

Descriptors: Bier, Vicinale Diketone, Diacetyl, 2,3-Pentandion, Kohlensuredruck

Monatsschrift fr Brauwissenschaft 56, Nr. 3/4, 44 - 47, 2003