Contribution of wort valine to diacetyl levels in wort during brewery fermentation

Abstract

Diacetyl and 2,3-pentanedione are butter-tasting vicinal diketones produced as by-products of valine and isoleucine biosynthesis during wort fermentation, and their presence in beer is usually unwanted. Yeast are able to reduce diacetyl and 2,3-pentanedione into compounds with higher flavour thresholds, and this is usually achieved during a resource-and time-consuming maturation phase after the primary fermentation. One promising method of decreasing diacetyl production during fermentation is through the control of the valine content of the wort, since valine is involved with the feedback inhibition of enzymes controlling the valine biosynthesis pathway and indirectly the formation of diacetyl precursors. The objective of this thesis was to investigate the influence of valine supplementation, wort amino acid profile and free amino nitrogen content on diacetyl formation during wort fermentation with the lager yeast Saccharomyces pastorianus, by carrying out a series of small-scale (2-litre) and medium-scale (10-litre) fermentations, which were sampled and analysed for diacetyl and amino acid concentrations, along with standard beer parameters.

Supplementation of valine to the wort resulted in decreased maximum (up to 37% lower) and final (up to 33% lower) diacetyl concentrations in all trials, and the composition of the amino acid spectrum of the wort also had an impact on diacetyl and 2,3-pentanedione production during fermentation. The results suggest that there is no correlation between the concentrations of wort amino acids and diacetyl production per se, but rather a negative correlation between the uptake rate of valine (and also other branched-chain amino acids) and diacetyl production was found. The results from all four experiments agree that fermentation performance and yeast growth was not affected by the studied levels of amino acid supplementations, implying that supplementation of valine or other amino acids will not affect the fermentation time nor the attenuation level achieved. Supplementation of amino acids did however have a minor effect on the concentrations of higher alcohols and esters linked to the amino acids via the Ehrlich pathway, suggesting that large levels of supplementation could affect the flavour profile of the beer.

It can be concluded that modifying the amino acid profile of wort, especially in respect to valine and the other branched-chain amino acids, is an effective way of decreasing the amount of diacetyl formed during fermentation. This knowledge can be utilized for decreasing the maturation time of beer, however further research is required before it becomes feasible at an industrial scale.