Yeast: Difference between revisions

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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. In contrast, 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).<ref name=yangao>Yang D, Gao X. [https://www.sciencedirect.com/science/article/abs/pii/S0924224421001552 Research progress on the antioxidant biological activity of beer and strategy for applications.] ''Trends Food Sci Technol.'' 2021;110:754-764.</ref>

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 participa tion of yeast in biochemical reactions, directly influencing the compo sition 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/>

==Preparing yeast for fermentation==