Wort Nitrogenous Sources—Their Use by Brewing Yeasts: A Review

Abstract

The sources of wort nitrogen that are assimilable by yeast are mainly amino acids, ammonium ion, and to a lesser extent, di- and tripeptides. These substances are partly formed during malting and partly during mashing, due to the action of barley proteases on hordein. The level of wort amino acids varies depending on the barley variety, malting conditions, and mashing parameters, but the overall spectrum of amino acids is similar among worts. Brewing adjuncts dilute nitrogenous substances while increasing the wort fermentable extract. Consequently, high-adjunct worts are nitrogen deficient. Yeast food can be used to increase assimilable nitrogen levels; however, amounts added may be insufficient for full stimulation of fermentation. Wort amino acids are taken up by yeast in a characteristic sequence during fermentation, although environmental changes can alter this pattern. Absolute yeast nitrogen requirements in the literature differ, and such levels have never been ascertained for high gravity or very high gravity fermentations. Because of the preferred preparation methods for these worts, they can be expected to be nitrogen-limited. The role of free amino nitrogen (FAN) in the brewing fermentation is mainly incorporation into new yeast protein; however, nitrogen levels profoundly affect almost every aspect of fermentation performance. Of critical import are the levels of higher alcohols, vicinal diketones, and esters, all of which are intimately related to wort nitrogen profiles. This review covers the factors affecting the wort nitrogen content, the assimilable nitrogen levels and their profile in wort and beer, and the role of usable nitrogen in the fermentation. In addition, the effects of limiting and excess quantities of assimilable nitrogen are reviewed in light of current research findings.