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Yeast: Difference between revisions
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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://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> | ||