Yeast: Difference between revisions

==Nutrition==

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.

Deficits in FAN directly can lead to an insufficient and slow start of the fermentation, insufficient fermentation performance, and stuck fermentations.<ref name=pahl>Pahl R, Meyer B, Biurrun R. Wort and Wort Quality Parameters. In: Bamforth CW, ed. [[Library|''Brewing Materials and Processes: A Practical Approach to Beer Excellence.'']] Academic Press; 2016.</ref> Low molecular weight nitrogen compounds, especially amino acids, influence the metabolism of the yeast. In particular, they impact the production of higher alcohols and vicinal diketones.

The minimal wort FAN level to achieve satisfactory yeast growth and fermentation performance in normal gravity wort fermentations (12°P) is 130 mg/L but, for rapid attenuation resulting in higher ethanol production, increased levels of wort FAN are required (170–190 mg/L). Meilgaard suggested that, during normal wort gravity fermentations, a minimum FAN level of 150 mg/L is required to permit rapid and complete attenuation. However, optimal wort FAN levels differ from fermentation to fermentation and from yeast strain to yeast strain; thus, they are considered controversial and unverified and are just guidelines. Furthermore, the optimum FAN values change with different wort sugar levels.<ref name=lekkas>Lekkas C, Hill AE, Stewart GG. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-2014-0113-01 Extraction of FAN from malting barley during malting and mashing.] ''J Am Soc Brew Chem.'' 2014;72(1):6–11.</ref> Wort FAN  is affected by the malt/adjunct ratio, the mashing schedule, barley variety, barley growth conditions, and various malting parameters. Under high-gravity brewing conditions, brewing yeast strains need extra FAN to cope with increased osmotic stress, other stress conditions, and the additional yeast growth required for efficient wort fermentation. This means that, as wort gravity increases, the levels of assimilable nitrogen should also increase in order that a certain rate of glycolytic flux and high cell viability and vitality is maintained. This avoids incomplete fermentations. Finally, FAN is used to provide not only nitrogen to the yeast cells for growth but also the wort nitrogen content or its metabolic products which affect beer flavor compounds and overall stability.

See also: [[Protein]]