Yeast: Difference between revisions
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Although sufficient oxygen must be supplied to yeast to promote lipid synthesis and satisfactory fermentation, it has been demonstrated that oxygen can lower the viability of yeast, exerting its effect via superoxide or species derived from it (20). Upon exposure to oxygen, yeast responds by synthesizing SOD and catalase, enzymes that are suppressed under anaerobic conditions (20). As for all aerobic organisms, those enzymes are triggered to promote protection against radical damages. In the transition period necessary for the elaboration of these enzymes, yeast is susceptible to oxygen radicals, a problem that should be considered when designing systems for providing oxygen to yeast.<ref name=bammul>Bamforth CW, Muller RE, Walker MD. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-51-0079 Oxygen and oxygen radicals in malting and brewing: a review.] ''J Am Soc Brew Chem.'' 1993;51(3):79–88.</ref> | Although sufficient oxygen must be supplied to yeast to promote lipid synthesis and satisfactory fermentation, it has been demonstrated that oxygen can lower the viability of yeast, exerting its effect via superoxide or species derived from it (20). Upon exposure to oxygen, yeast responds by synthesizing SOD and catalase, enzymes that are suppressed under anaerobic conditions (20). As for all aerobic organisms, those enzymes are triggered to promote protection against radical damages. In the transition period necessary for the elaboration of these enzymes, yeast is susceptible to oxygen radicals, a problem that should be considered when designing systems for providing oxygen to yeast.<ref name=bammul>Bamforth CW, Muller RE, Walker MD. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-51-0079 Oxygen and oxygen radicals in malting and brewing: a review.] ''J Am Soc Brew Chem.'' 1993;51(3):79–88.</ref> | ||
Preparing yeast for fermentation | |||
Rehydrating dry yeast | |||
*https://www.brunwater.com/articles/water-for-yeast-rehydration | *https://www.brunwater.com/articles/water-for-yeast-rehydration | ||
*https://www.homebrewtalk.com/threads/dry-yeast-rehydration.681608/ | *https://www.homebrewtalk.com/threads/dry-yeast-rehydration.681608/ | ||
Starters for liquid yeast | |||
Yeast produce membrane lipids only when grown aerobically. In the initial growth phase, proper oxygen management leads to proper production and storage of sterols in the yeast cell, which can be shared with subsequent daughter cells. It is possible to increase yeast ethanol tolerance by promoting synthesis of sterols, by adding oxygen (air) in the starter and during fermentation. Yeast lees deplete the oxygen content and can impact the redox potential and formation of VSCs.<ref name="Zoecklein">Zoecklein B. [https://www.apps.fst.vt.edu/extension/enology/EN/133.html Enology notes #133.] Wine/Enology Grape Chemistry Group at Virginia Tech. Published 2007. Accessed 2020.</ref> | Yeast produce membrane lipids only when grown aerobically. In the initial growth phase, proper oxygen management leads to proper production and storage of sterols in the yeast cell, which can be shared with subsequent daughter cells. It is possible to increase yeast ethanol tolerance by promoting synthesis of sterols, by adding oxygen (air) in the starter and during fermentation. Yeast lees deplete the oxygen content and can impact the redox potential and formation of VSCs.<ref name="Zoecklein">Zoecklein B. [https://www.apps.fst.vt.edu/extension/enology/EN/133.html Enology notes #133.] Wine/Enology Grape Chemistry Group at Virginia Tech. Published 2007. Accessed 2020.</ref> | ||
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*https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-58-0014 | *https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-58-0014 | ||
slow/stuck fermentation | |||
*https://www.sciencedirect.com/science/article/abs/pii/S1389172301800633 | *https://www.sciencedirect.com/science/article/abs/pii/S1389172301800633 | ||
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[[Stuck fermentation]] | [[Stuck fermentation]] | ||
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. Increasing the FAN has minor and somewhat unpredictable effects on yeast growth and attenuation. | 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. Increasing the FAN has minor and somewhat unpredictable effects on yeast growth and attenuation. | ||
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*https://www.brewersjournal.info/free-amino-nitrogen/ | *https://www.brewersjournal.info/free-amino-nitrogen/ | ||
Flavor compounds | |||
During active biomass accumulation, H2S and ester production may occur. In this case, ester formation is stimulated by the presence of nitrogen, indicating that biosynthetic reactions are the source of these compounds. Once active growth has diminished and ethanol is accumulating, amino acid degradation occurs and, at this time, additional esters and fusel compounds may be produced. The fusel compounds come from the degradation of amino acids as nitrogen sources via the Ehrlich pathway.<ref name="Off">https://wineserver.ucdavis.edu/industry-info/enology/fermentation-management-guides/wine-fermentation/characters</ref> | During active biomass accumulation, H2S and ester production may occur. In this case, ester formation is stimulated by the presence of nitrogen, indicating that biosynthetic reactions are the source of these compounds. Once active growth has diminished and ethanol is accumulating, amino acid degradation occurs and, at this time, additional esters and fusel compounds may be produced. The fusel compounds come from the degradation of amino acids as nitrogen sources via the Ehrlich pathway.<ref name="Off">https://wineserver.ucdavis.edu/industry-info/enology/fermentation-management-guides/wine-fermentation/characters</ref> | ||
https://wineserver.ucdavis.edu/industry-info/enology/fermentation-management-guides/wine-fermentation/characters -- discussion of VSCs, esters, and aldehydes | https://wineserver.ucdavis.edu/industry-info/enology/fermentation-management-guides/wine-fermentation/characters -- discussion of VSCs, esters, and aldehydes | ||
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*https://www.homebrewtalk.com/threads/be-256-dry-fermetis-abbey-has-anyone-used-it-results.670423/ | *https://www.homebrewtalk.com/threads/be-256-dry-fermetis-abbey-has-anyone-used-it-results.670423/ | ||
*https://www.brunwater.com/articles/brewing-water-and-yeast | *https://www.brunwater.com/articles/brewing-water-and-yeast | ||
*[https://www.homebrewtalk.com/forum/threads/saving-yeast-at-the-starter-stage.676284/ Overbuild starters] | *[https://www.homebrewtalk.com/forum/threads/saving-yeast-at-the-starter-stage.676284/ Overbuild starters] | ||
*Drying kveik | *Drying kveik | ||
*Under beer, jars vs vials | *Under beer, jars vs vials | ||
*Isotonic saline | *Isotonic saline | ||
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*https://www.themodernbrewhouse.com//forum/viewtopic.php?f=9&t=2074 | *https://www.themodernbrewhouse.com//forum/viewtopic.php?f=9&t=2074 | ||
==Articles to be reviewed== | Molecular oxygen is taken up by yeast at the start of the fermentation and is used by the cell to synthesise sterols and unsaturated fatty acids which are essential components of the yeast’s membrane. The need for oxygen can be removed if sterols (e.g. ergosterol) and unsaturated fatty acids (e.g. oleic acid) are added directly to the wort. In terms of releasing energy, aerobic respiration is more efficient than anaerobic respiration. However in yeast the temptation to use the available oxygen for aerobic respiration is suppressed through a mechanism described as the Crabtree effect. In the presence of glucose sugars (above 1% by weight) yeast (Saccharomyces spp) uses glucose to produce alcohol and uses the oxygen to produce the necessary lipid compounds. The presence of insufficient lipid compounds will lead to a defective fermentation due to inadequate yeast cell reproduction, which in turn will lead to:<ref name=oro>O'Rourke T. [https://www.themodernbrewhouse.com/wp-content/uploads/2016/11/The-role-of-oxygen-in-brewing.pdf The role of oxygen in brewing.] ''Brewer International.'' 2002;2(3):45–47.</ref> | ||
• Slow and sticking fermentations | |||
• Off flavours – e.g. poor removal of diacetyl and acetaldehyde | |||
• Poor yeast crop in terms of quantity and vitality | |||
• Low ester formation | |||
Excess oxygen will lead to: | |||
• Rapid fermentations | |||
• Excessive yeast growth and hence beer losses | |||
• Higher ester production – giving fruitier flavoured beers | |||
== Articles to be reviewed == | |||
*https://onlinelibrary.wiley.com/doi/pdf/10.1002/jib.242 | *https://onlinelibrary.wiley.com/doi/pdf/10.1002/jib.242 | ||
*https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.2007.tb00259.x | *https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.2007.tb00259.x | ||
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*[https://www.sciencedirect.com/science/article/abs/pii/0922338X9290137J "Effect of pitching yeast and wort preparation on flavor stability of beer"] | *[https://www.sciencedirect.com/science/article/abs/pii/0922338X9290137J "Effect of pitching yeast and wort preparation on flavor stability of beer"] | ||
==References== | == References == | ||