Yeast: Difference between revisions

Brewer's yeast strains cannot assimilate proteins and longer chain peptides due to the fact that cells hardly secrete proteases during brewing. The assimilable nitrogenous compounds for brewer's yeast are known as free amino nitrogen (FAN) which can be defined as the sum of FAA, ammonium ions, and to a lesser extent, di- and tripeptides.<ref name=lei>Lei H, Zheng L, Wang C, Zhao H, Zhao M. [https://www.sciencedirect.com/science/article/abs/pii/S0168160512006150 Effects of worts treated with proteases on the assimilation of free amino acids and fermentation performance of lager yeast.] ''Int J Food Microbiol.'' 2013;161(2):76–83.</ref> The transport of FAA across the cell membrane is active, driven by the proton gradient via specific and general amino acid permeases. FAA have been categorized into four groups in ale yeast on the basis of their assimilation patterns (Jones and Pierce, 1964). In this model, group A is reported to be assimilated immediately after the yeast cells come into contact with wort and almost totally consumed after a few hours of fermentation. Group B is taken up more slowly, but assimilated gradually throughout fermentation. Group C is not utilized until group A have disappeared from the wort. Pro is the sole member of group D and is also the least preferred amino acid by brewer's yeast, because its dissimilation requires the presence of a mitochondrial oxidase which is inactive under anaerobic conditions. However, it has been proven that this assimilation pattern is often specific to the conditions employed and among them the yeast strain's nutritional preferences is perhaps more significant.