Editing Yeast
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The conversion of the fermentable carbohydrates (sugars) into ethanol and carbon dioxide gas is achieved by pitching yeast. However, other by-yeast metabolism products are also excreted into the fermenting wort and can affect the organoleptic properties (i.e., taste, color, odor and feel) of the beer. These by-products include esters, aldehydes, vicinal diketones, higher alcohols and acids, as well as sulfur compounds.<ref name="Ferreira">Ferreira, Inês M., and Guido, Luís F. [https://www.mdpi.com/2311-5637/4/2/23/pdf "Impact of Wort Amino Acids on Beer Flavour: A Review."] ''Fermentation.'' 2018, 4, 23.</ref> | The conversion of the fermentable carbohydrates (sugars) into ethanol and carbon dioxide gas is achieved by pitching yeast. However, other by-yeast metabolism products are also excreted into the fermenting wort and can affect the organoleptic properties (i.e., taste, color, odor and feel) of the beer. These by-products include esters, aldehydes, vicinal diketones, higher alcohols and acids, as well as sulfur compounds.<ref name="Ferreira">Ferreira, Inês M., and Guido, Luís F. [https://www.mdpi.com/2311-5637/4/2/23/pdf "Impact of Wort Amino Acids on Beer Flavour: A Review."] ''Fermentation.'' 2018, 4, 23.</ref> | ||
In beer, glucose, fructose, maltose, and maltotriose are | In beer, glucose, fructose, maltose, and maltotriose are consumed consecultively. The glucose, fructose and sucrose are depleted first because they are shorter chain sugars. The rate of maltotriose consumption is slowest, and so it is the last sugar to be depleted.<ref name=Vriesekoop>https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.2010.tb00425.x</ref> | ||
Boiled wort ferments more quickly than raw wort.<ref name=Vriesekoop/> ''Why?'' | Boiled wort ferments more quickly than raw wort.<ref name=Vriesekoop/> ''Why?'' | ||
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*https://www.sciencedirect.com/science/article/pii/B9780127999548000046 | *https://www.sciencedirect.com/science/article/pii/B9780127999548000046 | ||
A high level of oxygen addition into wort can have an impact on quality. It is intentionally added during wort cooling. Yeast requires oxygen to synthesize UFAs and sterols, which are critical cell membrane components that impact membrane fluidity. Depraetere et al. showed that under normal conditions, oxygenation of cold wort (8 ppm O 2 ) had no significant impact on the level of aging staling indicators for ale or lager beers over a 12 month ambient shelf-life test (14). At this stage of the process, oxidation of wort compounds appears to play little to no part compared with the change of aroma and flavor from yeast metabolism. It is important to note that the wort oxygenation rate can impact the levels of sulfites produced by lager yeast; increased oxygenation can cause a reduction in sulfite levels, leading to a loss of scavenging activity for carbonyl compounds, such as E2N (22).<ref name=golston>Golston AM. [https://www.mbaa.com/publications/tq/tqPastIssues/2021/Pages/TQ-58-1-0322-01.aspx The impact of barley lipids on the brewing process and final beer quality: A mini-review.] ''Tech Q Master Brew Assoc Am.'' 2021;58(1):43–51.</ref> | |||
*Depraetere, S., et al. 2008. The influence of wort aeration and yeast preoxygenation on beer staling processes. Food Chem. 107(1):242-249. | *Depraetere, S., et al. 2008. The influence of wort aeration and yeast preoxygenation on beer staling processes. Food Chem. 107(1):242-249. | ||
*https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5003802/ | *https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5003802/ | ||
The ventilation of wort before fermentation is a necessary step to ensure the full growth and better fermentation of yeast (Depraetere, De Schutter, Williams, & Delvaux, 2008). With further increase of dissolved oxygen in wort, the content of SO2 decreases obviously, which indicates that too much dissolved oxygen in wort inhibits the production of SO2 (Dufour, 1991). That is, controlling dissolved oxygen can improve generation of SO2 and antioxidative activity of beer. The pitching rate of yeast was found to be able to control the fermentation time and the peak number of yeast cells and also had an effect on the composition and flavors of beer (Verbelen, Saerens, Thevelein, & Delvaux, 2009). Low pitching rate affects the multiplication of yeast during the main fermentation stage, results in a smaller reduction in apparent extract and retards fermen tation. | The ventilation of wort before fermentation is a necessary step to ensure the full growth and better fermentation of yeast (Depraetere, De Schutter, Williams, & Delvaux, 2008). With further increase of dissolved oxygen in wort, the content of SO2 decreases obviously, which indicates that too much dissolved oxygen in wort inhibits the production of SO2 (Dufour, 1991). That is, controlling dissolved oxygen can improve generation of SO2 and antioxidative activity of beer. The pitching rate of yeast was found to be able to control the fermentation time and the peak number of yeast cells and also had an effect on the composition and flavors of beer (Verbelen, Saerens, Thevelein, & Delvaux, 2009). Low pitching rate affects the multiplication of yeast during the main fermentation stage, results in a smaller reduction in apparent extract and retards fermen tation. A higher pitching rate leads to vigorous growth of yeast and rapid decline of apparent extract during the main fermentation stage. An increase in pitching rate leads to a decrease in the quantity of SO2 produced by fermentation. This may be explained in that when the pitching rate is high, the yeast metabolism is exuberant, so that more sulfites are metabolized by the yeast to synthesize the amino acids needed for its own metabolism, resulting in less SO2 (Zhou, 2010). Therefore, the appropriate pitching rate is crucial for generating SO2 during beer fermentation (Zhao, 2012) Research progress on the antioxidant biological activity of beer and strategy for applications.] | ||
Higher wort gravity leads to greater SO2 production during fermentation (Zhou, 2010). Higher wort gravity increases the osmotic pressure of yeast cells and changes the metabolic pathway of yeast utilizing glucose, resulting in the production of more pyruvate, acetaldehyde, ethanol, etc., from glucose through glycolysis. Pyruvate and acetaldehyde are easily combined with sulfite, and the adduct products of sulfite and carbonyl compounds are secreted into the wort through cell membranes, increasing the content of SO2 in the wort (Gyllang, Winge, & Korch, 1989). However, when the wort gravity exceeds 12 ◦ P, the AOX of beer does not increase significantly as the wort gravity in creases, indicating that the antioxidant power of wort is affected not only by metabolic pathways but also by the reduction ability of yeast. In high gravity brewing, due to the inhibition of high osmotic pressure and high ethanol concentration, the metabolic reduction ability of yeast cells decreases, result in a sluggish generation of SO2 in wort, and the wort cannot reach the proper antioxidant level (Li, Sun, Zhao, & Zhao, 2012).<ref name=yangao/> | Higher wort gravity leads to greater SO2 production during fermentation (Zhou, 2010). Higher wort gravity increases the osmotic pressure of yeast cells and changes the metabolic pathway of yeast utilizing glucose, resulting in the production of more pyruvate, acetaldehyde, ethanol, etc., from glucose through glycolysis. Pyruvate and acetaldehyde are easily combined with sulfite, and the adduct products of sulfite and carbonyl compounds are secreted into the wort through cell membranes, increasing the content of SO2 in the wort (Gyllang, Winge, & Korch, 1989). However, when the wort gravity exceeds 12 ◦ P, the AOX of beer does not increase significantly as the wort gravity in creases, indicating that the antioxidant power of wort is affected not only by metabolic pathways but also by the reduction ability of yeast. In high gravity brewing, due to the inhibition of high osmotic pressure and high ethanol concentration, the metabolic reduction ability of yeast cells decreases, result in a sluggish generation of SO2 in wort, and the wort cannot reach the proper antioxidant level (Li, Sun, Zhao, & Zhao, 2012).<ref name=yangao/> | ||
Temperature is an important external factor affecting the | Temperature is an important external factor affecting the participation of yeast in biochemical reactions, directly influencing the compoSiri on and content of metabolites including antioxidants in beer (Yu, Chen, & Wang, 2006). The effect of the main fermentation temperature on the TBA value is significant, and a lower main fermentation temperature leads to a lower TBA value of the wort. This may be because higher main fermentation temperature promotes the metabolism of yeast and produces more higher alcohols. In the process of yeast fermentation, pathways of amino acid degradation and the synthesis and metabolism of carbohydrates form higher alcohols, which are easily oxidized by melanoidin catalysis, resulting in the formation of aldehyde carbonyl compounds and thus higher TBA value (Zhao, 2012). Theoretically, higher temperature leads to stronger reproduction and metabolism of yeast, stronger reduction of aging substances and higher AOX of green beer. However, this is not the case. The DPPH radical scavenging activity, oxygen free radical absorption capacity and reducing power of beer have been found to decrease with increasing fermentation temperature (Li, SunZhao, & Zhao, 2012). This may be because the fermentation temperature affects the proliferation and metabolic rate of yeast, as well as the AOX of green beer. Higher fermentation temperature leads to shortened fermentation period, lower concentrations of reduction agents and insufficient reduction of aging precursors, while low temperature prolongs fermentation and produces more reduction agents, so that the AOX of the beer is higher. In addition, lower temperature is also good for keeping SO2 in wort, improving the antioxidant capacity of wort. Hence, low-temperature fermentation is more conducive to the maintenance of higher AOX in beer.<ref name=yangao/> | ||
The beer filtration process reduces the contents of antioxidant phenolic compounds and melanoidins and the AOX of wort. During the cooling stage, the spontaneous adsorption of phenolic compounds and melanoidins on wort dregs and the polymerization and precipitation of catechins and epicatechins lead to the decrease of TPC in beer (Ruiz- Ruiz, Del Carmen Esapadas Aldana, Cruz, & Segura-Campos, 2020). With the increase of diatomite consumption, a large concentration of iron ions is introduced, which decreases the DPPH scavenging rate, because transition ions such as iron and copper play an important cat alytic role in the Fenton reaction, producing hydroxyl free radicals with high activity and reducing the oxidation resistance of beer (Jurková et al., 2012; Pascoe, Ames, & Chandra, 2003). The addition of tannins has an obvious effect on the rate of scavenging of DPPH free radicals, indicating that the addition of tannins will help to chelate iron ions and reduce the effect of iron ions in diatomite on beer. The reducing power of beer can be improved by maintaining pH within the range 4.3–4.4 (Han, 2016). After cooling and filtration, 6% of selenium is lost from the level in raw materials, and the total loss of selenium over the whole process of beer fermentation is 94% (Rodrigo et al., 2015). It can be seen that the percentage selenium loss is quite high, which deserves attention.<ref name=yangao/> | The beer filtration process reduces the contents of antioxidant phenolic compounds and melanoidins and the AOX of wort. During the cooling stage, the spontaneous adsorption of phenolic compounds and melanoidins on wort dregs and the polymerization and precipitation of catechins and epicatechins lead to the decrease of TPC in beer (Ruiz- Ruiz, Del Carmen Esapadas Aldana, Cruz, & Segura-Campos, 2020). With the increase of diatomite consumption, a large concentration of iron ions is introduced, which decreases the DPPH scavenging rate, because transition ions such as iron and copper play an important cat alytic role in the Fenton reaction, producing hydroxyl free radicals with high activity and reducing the oxidation resistance of beer (Jurková et al., 2012; Pascoe, Ames, & Chandra, 2003). The addition of tannins has an obvious effect on the rate of scavenging of DPPH free radicals, indicating that the addition of tannins will help to chelate iron ions and reduce the effect of iron ions in diatomite on beer. The reducing power of beer can be improved by maintaining pH within the range 4.3–4.4 (Han, 2016). After cooling and filtration, 6% of selenium is lost from the level in raw materials, and the total loss of selenium over the whole process of beer fermentation is 94% (Rodrigo et al., 2015). It can be seen that the percentage selenium loss is quite high, which deserves attention.<ref name=yangao/> | ||
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Yeast is regarded as the best "antioxidant" for brewing due to its strong ability to absorb dissolved oxygen.<ref name=niecon>Nielsen H. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1973.tb03517.x The control of oxygen in beer processing.] ''J Inst Brew.'' 1973;79(2):147–154.</ref> | Yeast is regarded as the best "antioxidant" for brewing due to its strong ability to absorb dissolved oxygen.<ref name=niecon>Nielsen H. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1973.tb03517.x The control of oxygen in beer processing.] ''J Inst Brew.'' 1973;79(2):147–154.</ref> | ||
Reducing Activity of Yeast. It is generally accepted that yeast metabolism can reduce aldehydes in the wort to their corresponding alcohols. The system responsible for this reduction has been found to be very complex and heterogeneous.149,154 Some aldehyde reductases regenerate NAD(P)+ from NAD(P)H and, therefore, maintain a suitable redox balance within the cell.154,193 Spiking of aldehydes to wort with subsequent laboratory-scale fermentation results in a lack of measurable aldehyde levels directly after fermentation and yeast removal. Moreover, the malt-like aroma disappears completely by this fermentation step. On the other hand, the corresponding alcohols and acetate esters showed to be present.153,165,193 Collin et al.155 suggested that the limiting step of carbonyl reduction is the uptake rate by the yeast, but this was countered by the findings of Debourg et al.,149 who worked with permeabilized yeast cells. Linear saturated aldehydes appear to be reduced more rapidly with increasing carbon number, and their reduction rate is higher than their corresponding branched or unsaturated aldehydes.149 Furfural and (E)-2-nonenal are reduced early in the fermentation process.63,115,194 Vesely et al.195 observed a clear decrease in, among others, 2-methylpropanal, 2-methylbutanal, 3-methylbutanal, furfural, and methional concentrations, at both 10 and 15 °C fermentations. Although the reduction rates were slightly higher at 15 °C, the resulting aldehyde concentrations were lower at 10 °C. Perpete et al. ̀ 182,193 reported an initially fast reduction of Strecker aldehydes in cold contact fermentation, which slowed and resulted after a few hours in a constant concentration. This end concentration is aldehyde-dependent, but can reach up to 40% of the initial concentration. Higher fermentation temperatures led to lower, but nonzero, end concentrations. Neither higher pitching rates nor different yeast strains or even a second pitching with fresh yeast affected the concentration of aldehydes at the end of fermentation. Similar results were obtained with laboratory-scale and industrial fermentation trials. This points to the interactions of the aldehydes with wort components rendering them nonreducible by the yeast, for example, imine formation and bisulfite adduct formation, but also, for example, weak binding to flavonoids at fermentation temperatures.153,182,193 As the free aldehydes are reduced by the yeast, the equilibrium between free and bound aldehydes restores the free form, yet this seems insufficient for complete aldehyde reduction.149 Aldehyde reduction by the yeast starts very early in the fermentation process, whereas sulfite production occurs at a later time.75 The protective effect of sulfite binding is, therefore, thought to be of rather limited importance.182 Yeast also reduces α-dicarbonyls, the intermediates of the Maillard reaction pathway and part of the Strecker degradation pathway. Addition of an isolated yeast reductase to beer with subsequent forced aging resulted in a lower concentration of dicarbonyl compounds.196 Overexpression of an involved reductase resulted in beers with 30−40% lower concentrations of Strecker aldehydes.197<ref name=baedec>Baert JJ, De Clippeleer J, Hughes PS, De Cooman L, Aerts G. [https:// | Reducing Activity of Yeast. It is generally accepted that yeast metabolism can reduce aldehydes in the wort to their corresponding alcohols. The system responsible for this reduction has been found to be very complex and heterogeneous.149,154 Some aldehyde reductases regenerate NAD(P)+ from NAD(P)H and, therefore, maintain a suitable redox balance within the cell.154,193 Spiking of aldehydes to wort with subsequent laboratory-scale fermentation results in a lack of measurable aldehyde levels directly after fermentation and yeast removal. Moreover, the malt-like aroma disappears completely by this fermentation step. On the other hand, the corresponding alcohols and acetate esters showed to be present.153,165,193 Collin et al.155 suggested that the limiting step of carbonyl reduction is the uptake rate by the yeast, but this was countered by the findings of Debourg et al.,149 who worked with permeabilized yeast cells. Linear saturated aldehydes appear to be reduced more rapidly with increasing carbon number, and their reduction rate is higher than their corresponding branched or unsaturated aldehydes.149 Furfural and (E)-2-nonenal are reduced early in the fermentation process.63,115,194 Vesely et al.195 observed a clear decrease in, among others, 2-methylpropanal, 2-methylbutanal, 3-methylbutanal, furfural, and methional concentrations, at both 10 and 15 °C fermentations. Although the reduction rates were slightly higher at 15 °C, the resulting aldehyde concentrations were lower at 10 °C. Perpete et al. ̀ 182,193 reported an initially fast reduction of Strecker aldehydes in cold contact fermentation, which slowed and resulted after a few hours in a constant concentration. This end concentration is aldehyde-dependent, but can reach up to 40% of the initial concentration. Higher fermentation temperatures led to lower, but nonzero, end concentrations. Neither higher pitching rates nor different yeast strains or even a second pitching with fresh yeast affected the concentration of aldehydes at the end of fermentation. Similar results were obtained with laboratory-scale and industrial fermentation trials. This points to the interactions of the aldehydes with wort components rendering them nonreducible by the yeast, for example, imine formation and bisulfite adduct formation, but also, for example, weak binding to flavonoids at fermentation temperatures.153,182,193 As the free aldehydes are reduced by the yeast, the equilibrium between free and bound aldehydes restores the free form, yet this seems insufficient for complete aldehyde reduction.149 Aldehyde reduction by the yeast starts very early in the fermentation process, whereas sulfite production occurs at a later time.75 The protective effect of sulfite binding is, therefore, thought to be of rather limited importance.182 Yeast also reduces α-dicarbonyls, the intermediates of the Maillard reaction pathway and part of the Strecker degradation pathway. Addition of an isolated yeast reductase to beer with subsequent forced aging resulted in a lower concentration of dicarbonyl compounds.196 Overexpression of an involved reductase resulted in beers with 30−40% lower concentrations of Strecker aldehydes.197<ref name=baedec>Baert JJ, De Clippeleer J, Hughes PS, De Cooman L, Aerts G. [https://www.themodernbrewhouse.com/wp-content/uploads/2017/03/Baert-Aldehyden.pdf On the origin of free and bound staling aldehydes in beer.] ''J Agric Food Chem.'' 2012;60(46):11449–11472.</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> | 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> | ||
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Starters for liquid yeast | 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. | 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> | ||
While oxidative stress is known to occur, is it significantly less that stress from carbon dioxide. High amounts of foam means that insufficient oxygen delivery is occurring.<ref>https://www.mbaa.com/publications/tq/tqPastIssues/2005/Abstracts/TQ-42-0128.htm</ref> | While oxidative stress is known to occur, is it significantly less that stress from carbon dioxide. High amounts of foam means that insufficient oxygen delivery is occurring.<ref>https://www.mbaa.com/publications/tq/tqPastIssues/2005/Abstracts/TQ-42-0128.htm</ref> | ||
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Stirring helps with oxygen diffusion<ref>https://www.cmbe.engr.uga.edu/bche4510/assign/Interlude.pdf</ref> | Stirring helps with oxygen diffusion<ref>https://www.cmbe.engr.uga.edu/bche4510/assign/Interlude.pdf</ref> | ||
*https://www.homebrewtalk.com/threads/the-ideal-starter-transcript-of-an-article-on-braumagazin-de.679661/ | *https://www.homebrewtalk.com/threads/the-ideal-starter-transcript-of-an-article-on-braumagazin-de.679661/ | ||
Biomass may actually be more important that cell count with regard to pitch rate.<ref>[https:// | Biomass may actually be more important that cell count with regard to pitch rate.<ref>[https://www.milkthefunk.live/podcast/2019/12/6/wiki-kwiki-005-lance-shaner-of-omega-yeast-labs "Wiki Kwiki #005 - Lance Shaner of Omega Yeast Labs"] (at ~30 minutes) Milk the Funk podcast, December 2019.</ref> However this isn't easy to measure at home. Pitching rate calculators are still useful for determining correct pitch rate. | ||
Yeast in worts rich in glucose may not be able to adapt to metabolize maltose and maltotriose, leading to slow or stuck fermentations.<ref name=bsp>Briggs DE, Boulton CA, Brookes PA, Stevens R. [[Library|''Brewing Science and Practice.'']] Woodhead Publishing Limited and CRC Press LLC; 2004.</ref> | Yeast in worts rich in glucose may not be able to adapt to metabolize maltose and maltotriose, leading to slow or stuck fermentations.<ref name=bsp>Briggs DE, Boulton CA, Brookes PA, Stevens R. [[Library|''Brewing Science and Practice.'']] Woodhead Publishing Limited and CRC Press LLC; 2004.</ref> | ||
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Nitrogen is generally plentiful in wort and typically does not require supplementation for beer production.<ref name="Ferreira"/><ref name=jonesbudde>Jones BL, Budde AD. [https://www.sciencedirect.com/science/article/abs/pii/S0733521004001067 How various malt endoproteinase classes affect wort soluble protein levels.] ''J Cereal Sci.'' 2005;41(1):95–106.</ref> The concentration of the amino acids isoleucine, valine, phenylalanine, glycine, alanine, tyrosine, lysine, histidine, arginine and leucine, are considered important, as these are an important part of the complex system regulating the biosynthesis of flavour-active compounds formed by yeast.<ref name="Ferreira"/> However, if supplementation is desired, a mixture of amino acids is more favorable to growth than when ammonium ions are the source of nitrogen.<ref name="Ferreira"/> Phenolic yeast may have a higher nitrogen requirement.<ref name="Ferreira"/> | Nitrogen is generally plentiful in wort and typically does not require supplementation for beer production.<ref name="Ferreira"/><ref name=jonesbudde>Jones BL, Budde AD. [https://www.sciencedirect.com/science/article/abs/pii/S0733521004001067 How various malt endoproteinase classes affect wort soluble protein levels.] ''J Cereal Sci.'' 2005;41(1):95–106.</ref> The concentration of the amino acids isoleucine, valine, phenylalanine, glycine, alanine, tyrosine, lysine, histidine, arginine and leucine, are considered important, as these are an important part of the complex system regulating the biosynthesis of flavour-active compounds formed by yeast.<ref name="Ferreira"/> However, if supplementation is desired, a mixture of amino acids is more favorable to growth than when ammonium ions are the source of nitrogen.<ref name="Ferreira"/> Phenolic yeast may have a higher nitrogen requirement.<ref name="Ferreira"/> | ||
Yeast consume at least 100-140ppm FAN in wort. Since proline cannot be utilized, wort has to contain 200-220ppm FAN. Inadequate nutrition can result in reduced yeast propagation and a delay in fermentation and maturation, and ultimately the retention of undesirable "young beer" off-flavors. Higher modified malts produce more FAN.<ref>Kunze, Wolfgang. "3.2 Mashing." ''Technology Brewing & Malting.'' Edited by Olaf Hendel, 6th English Edition ed., | Yeast consume at least 100-140ppm FAN in wort. Since proline cannot be utilized, wort has to contain 200-220ppm FAN. Inadequate nutrition can result in reduced yeast propagation and a delay in fermentation and maturation, and ultimately the retention of undesirable "young beer" off-flavors. Higher modified malts produce more FAN.<ref>Kunze, Wolfgang. "3.2 Mashing." ''Technology Brewing & Malting.'' Edited by Olaf Hendel, 6th English Edition ed., VBL Berlin, 2019, p. 230.</ref> If [[adjuncts]] are used, the brewer should consider using a protein rest (45-50°C) (see [[Mashing]]) or adding yeast nutrient. | ||
Worts that are prepared with reasonable percentages of malt tend to be rich in amino acids. Low FAN levels are undesirable in wort. The traditional rule is that serious problems (long lags, high diacetyl, etc) can result from FAN below 150-175ppm. A 12°P malt wort will typically have 225-275ppm FAN, which is ideal.<ref name=fix>Fix, George. ''Principles of Brewing Science.'' 2nd ed., Brewers Publications, 1999.</ref> As a general rule, it is usually desirable to keep FAN levels below 350ppm, something that can be achieved with a suitable [[mashing]] schedule. | Worts that are prepared with reasonable percentages of malt tend to be rich in amino acids. Low FAN levels are undesirable in wort. The traditional rule is that serious problems (long lags, high diacetyl, etc) can result from FAN below 150-175ppm. A 12°P malt wort will typically have 225-275ppm FAN, which is ideal.<ref name=fix>Fix, George. ''Principles of Brewing Science.'' 2nd ed., Brewers Publications, 1999.</ref> As a general rule, it is usually desirable to keep FAN levels below 350ppm, something that can be achieved with a suitable [[mashing]] schedule. | ||
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In many breweries producing South- | In many breweries producing South- | ||
ern German-style wheat beer, otherwise known as weissbier, after the installation of new cylindroconical fermentors, it is common for the beers to exhibit a noticeable decline in the bouquet characteristic of the style, which consists of primarily of compounds like isoamyl acetate (banana ester)[2]. The reason behind this somewhat diminished weissbier aroma is, among others, the high rate of yeast reproduction, which reduces the amount of the acetyl-coenzyme A available for ester formation. In addition, the high hydrostatic pressure in vertical vessels moderates the production of higher alcohols, thus reducing the numbers of reactants for the formation of esters. In short, the higher the liquid level is in a fermentation tank, the stronger the convection and homogenization, which results in a reduction in the formation of esters (fig. 4).<ref name=sacher2>Sacher B, Becker T, Narziss L. [http://www. | ern German-style wheat beer, otherwise known as weissbier, after the installation of new cylindroconical fermentors, it is common for the beers to exhibit a noticeable decline in the bouquet characteristic of the style, which consists of primarily of compounds like isoamyl acetate (banana ester)[2]. The reason behind this somewhat diminished weissbier aroma is, among others, the high rate of yeast reproduction, which reduces the amount of the acetyl-coenzyme A available for ester formation. In addition, the high hydrostatic pressure in vertical vessels moderates the production of higher alcohols, thus reducing the numbers of reactants for the formation of esters. In short, the higher the liquid level is in a fermentation tank, the stronger the convection and homogenization, which results in a reduction in the formation of esters (fig. 4).<ref name=sacher2>Sacher B, Becker T, Narziss L. [http://www.lowoxygenbrewing.com/wp-content/uploads/2017/04/pddvxvf.pdf Some reflections on mashing – Part 2.] ''Brauwelt International.'' 2016;6:392-397.</ref> | ||
The estery notes in beer have been observed to become more pronounced as the ratio of glucose to maltose tips in favor of glucose.<ref name=sacher2/> Alcoholic fermentation with yeast in the presence of high concentrations of glucose leads to a delay in the onset of maltose metabolism after an initial rapid decline in the extract content of the wort (similar to a "second lag phase"). This explains the plateau in the extract curve. During this time, the yeast are scarcely reproducing and are compensating with the synthesis of maltose permease and maltase. The diminished yeast reproduction results in overflow of the acetyl-CoA pool and thus greater ester formation and fruitier beers. | The estery notes in beer have been observed to become more pronounced as the ratio of glucose to maltose tips in favor of glucose.<ref name=sacher2/> Alcoholic fermentation with yeast in the presence of high concentrations of glucose leads to a delay in the onset of maltose metabolism after an initial rapid decline in the extract content of the wort (similar to a "second lag phase"). This explains the plateau in the extract curve. During this time, the yeast are scarcely reproducing and are compensating with the synthesis of maltose permease and maltase. The diminished yeast reproduction results in overflow of the acetyl-CoA pool and thus greater ester formation and fruitier beers. | ||
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This also comes into play when using [[adjuncts]] in brewing. | This also comes into play when using [[adjuncts]] in brewing. | ||
[[File:Flavor-compound-production.png | [[File:Flavor-compound-production.png]] | ||
In many Belgian-style specialty beers, POF+ S. cerevisiae strains are required to impart spice notes in the finished beer.<ref name=len>Lentz M. [https://www.mdpi.com/2311-5637/4/1/20 The impact of simple phenolic compounds on beer aroma and flavor.] ''Fermentation.'' 2018;4(1):20.</ref> There is a wide variety among these strains regarding POF activity. This at least partially explains the difference in volatile flavor compounds (phenolics and esters) produced by different strains such as those utilized for [[weissbier]] vs [[Belgian tripel]] styles for example. | In many Belgian-style specialty beers, POF+ S. cerevisiae strains are required to impart spice notes in the finished beer.<ref name=len>Lentz M. [https://www.mdpi.com/2311-5637/4/1/20 The impact of simple phenolic compounds on beer aroma and flavor.] ''Fermentation.'' 2018;4(1):20.</ref> There is a wide variety among these strains regarding POF activity. This at least partially explains the difference in volatile flavor compounds (phenolics and esters) produced by different strains such as those utilized for [[weissbier]] vs [[Belgian tripel]] styles for example. | ||
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Sterols and fatty acids are survival factors needed for the yeast cell membrane to function. As ethanol increases, hydrogen ions accumulate in cell requiring more energy to expel them. The pH decreases inside the cell causing cell death. Oxygen adds at end of growth phase increase sterol production. Therefore, microoxygenation and pump overs would be beneficial at 1/3 of the way through alcoholic fermentation (end of yeast growth phase).<ref name=kelly/> | Sterols and fatty acids are survival factors needed for the yeast cell membrane to function. As ethanol increases, hydrogen ions accumulate in cell requiring more energy to expel them. The pH decreases inside the cell causing cell death. Oxygen adds at end of growth phase increase sterol production. Therefore, microoxygenation and pump overs would be beneficial at 1/3 of the way through alcoholic fermentation (end of yeast growth phase).<ref name=kelly/> | ||
N assimilation: The manner in which N is assimilated by yeast depends on the source. Organic N (amino acids) is actively transported into the yeast cell. Through additional reactions N is incorporated into glutamine and glutamate and eventually used in the synthesis of other amino acids and nitrogenous compounds. This process is gradual and efficient compared to inorganic sources. Ammonium nitrogen (inorganic N) is consumed quickly and is less beneficial. Amino acid mixtures vs single N sources are more efficient because the yeast directly incorporates the amino acids into proteins rather than having to synthesize them. Ammonia, which exists as ammonium (NH4+) ions in must, is used by yeasts prior to amino acids. The presence of NH4+ delays timing and uptake of amino acids by yeast. The timing of N supplements and form of supplement will impact fermentation and volatiles. Types of N supplements include Diammonium phosphate (DAP), proprietary blends of DAP and amino acids (e.g. Superfood®, Fermaid K®, Actiferm) and balanced nutritional formulas containing inorganic N (e.g. Fermaid O®), organic N, sterols, yeast cell walls, fatty acids, yeast | N assimilation: The manner in which N is assimilated by yeast depends on the source. Organic N (amino acids) is actively transported into the yeast cell. Through additional reactions N is incorporated into glutamine and glutamate and eventually used in the synthesis of other amino acids and nitrogenous compounds. This process is gradual and efficient compared to inorganic sources. Ammonium nitrogen (inorganic N) is consumed quickly and is less beneficial. Amino acid mixtures vs single N sources are more efficient because the yeast directly incorporates the amino acids into proteins rather than having to synthesize them. Ammonia, which exists as ammonium (NH4+) ions in must, is used by yeasts prior to amino acids. The presence of NH4+ delays timing and uptake of amino acids by yeast. The timing of N supplements and form of supplement will impact fermentation and volatiles. Types of N supplements include Diammonium phosphate (DAP), proprietary blends of DAP and amino acids (e.g. Superfood®, Fermaid K®, Actiferm) and balanced nutritional formulas containing inorganic N (e.g. Fermaid O®), organic N, sterols, yeast cell walls, fatty acids, yeast autolysis products and others. DAP is best used with low N musts. Other balanced nutrients should be added as well. At a rate of 100 mg/L DAP, 20 mg/L YAN is added.<ref name=kelly/> | ||
Juice/must can be vitamin deficient as well as deficient in assimilable nitrogen when there is a high incidence of microorganisms (mold, yeast and/or bacteria). Growth of Kloeckera apiculata has been reported to rapidly reduce thiamine levels below those required by Saccharomyces sp. (18). Further, the use of SO2 may lead to additional reductions in levels of thiamine (15). Saccharomyces sp. has been shown to synthesize all required vitamins, with the exception of biotin. However, vitamin supplementation has been demonstrated to be stimulatory (19). Thus, it is usually desirable to add a mixed vitamin supplement with the nitrogen additions.<ref name=predic>Gump BH, Zoecklein BW, Fugelsang KC. [https://www.researchgate.net/profile/Bruce-Zoecklein/publication/226919430_Prediction_of_Prefermentation_Nutritional_Status_of_Grape_Juice/links/0fcfd508ea696b8c92000000/Prediction-of-Prefermentation-Nutritional-Status-of-Grape-Juice.pdf Prediction of prefermentation nutritional status of grape juice: The formol method.] Food microbiology protocols. 2001:283-96.</ref> | Juice/must can be vitamin deficient as well as deficient in assimilable nitrogen when there is a high incidence of microorganisms (mold, yeast and/or bacteria). Growth of Kloeckera apiculata has been reported to rapidly reduce thiamine levels below those required by Saccharomyces sp. (18). Further, the use of SO2 may lead to additional reductions in levels of thiamine (15). Saccharomyces sp. has been shown to synthesize all required vitamins, with the exception of biotin. However, vitamin supplementation has been demonstrated to be stimulatory (19). Thus, it is usually desirable to add a mixed vitamin supplement with the nitrogen additions.<ref name=predic>Gump BH, Zoecklein BW, Fugelsang KC. [https://www.researchgate.net/profile/Bruce-Zoecklein/publication/226919430_Prediction_of_Prefermentation_Nutritional_Status_of_Grape_Juice/links/0fcfd508ea696b8c92000000/Prediction-of-Prefermentation-Nutritional-Status-of-Grape-Juice.pdf Prediction of prefermentation nutritional status of grape juice: The formol method.] Food microbiology protocols. 2001:283-96.</ref> | ||
Pekur, et al. (24) reported that, at increased pressures, carbon dioxide reduces the yeast’s uptake of amino acids.<ref name=predic/> | Pekur, et al. (24) reported that, at increased pressures, carbon dioxide reduces the yeast’s uptake of amino acids.<ref name=predic/> | ||
== Articles to be reviewed == | == Articles to be reviewed == | ||
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*Verbelen, P. J., and Delvaux, F. R. Brewing yeast in action: Beer fermentation. In: Applied Microbiology. M. K. Rai and P. D. Bridge, Eds. CAB International, Oxon, UK. Pp. 110-135, 2009. | *Verbelen, P. J., and Delvaux, F. R. Brewing yeast in action: Beer fermentation. In: Applied Microbiology. M. K. Rai and P. D. Bridge, Eds. CAB International, Oxon, UK. Pp. 110-135, 2009. | ||
*[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5620630/ Microorganisms in Fermented Apple Beverages: Current Knowledge and Future Directions] | *[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5620630/ Microorganisms in Fermented Apple Beverages: Current Knowledge and Future Directions] | ||
*[http://www. | *[http://www.lowoxygenbrewing.com/wp-content/uploads/2017/04/fischer_0606.pdf Effects of hydrostatic high pressure on microbiological and technological characteristics of beer] | ||
*http://www. | *http://www.lowoxygenbrewing.com/wp-content/uploads/2017/04/poeschl_0807.pdf The Influence of Fermentation-Control on the Colloidal Stability and the Reducing Power of the Resulting Bottom Fermented Beers | ||
*Krogerus, K. and Gibson, B.: A re-evaluation of diastatic Saccharomyces cerevisiae strains and their role in brewing. Applied Microbiology and Biotechnology, 104 (2020), pp. 3745-3756. | *Krogerus, K. and Gibson, B.: A re-evaluation of diastatic Saccharomyces cerevisiae strains and their role in brewing. Applied Microbiology and Biotechnology, 104 (2020), pp. 3745-3756. | ||
*https://www.biorxiv.org/content/10.1101/2020.06.26.166157v1.full | *https://www.biorxiv.org/content/10.1101/2020.06.26.166157v1.full | ||
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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"] | ||
*[https://pubs.acs.org/doi/abs/10.1021/jf9037387 Decrease of Aged Beer Aroma by the Reducing Activity of Brewing Yeast] | *[https://pubs.acs.org/doi/abs/10.1021/jf9037387 Decrease of Aged Beer Aroma by the Reducing Activity of Brewing Yeast] | ||
== References == | == References == |