Yeast: Difference between revisions

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Pitching rates also had an effect on the EA (endogenous antioxidant) value of finished beer (Table IV). The EA value of beer increased with pitching rates, while the fermentation time significantly decreased. In the case of 54 × 106 cells/ml, the EA value of beer significantly increased, but the profile of volatile compounds in beer was rather different from the others (data not shown). In the range of pitching rates conventionally used, the effect of pitching rates on the EA value of beer was considered not great. The sulfite content in beer slightly increased with pitching rates. The effect of pitching rates on the EA value might be caused partly by the differences in the sulfite content of beer, although the EA value was not necessarily in proportion to the sulfite contents. Fermentation temperature had an effect on the EA value of finished beer (Table V). Fermentation temperatures of 9, 12, and 15°C were tested. The EA value of these beers was almost the same, although the fermentation time was significantly decreased with the rise in temperature. Although the data is not shown here, in the case of beer brewed with another yeast strain, a different result was observed: The EA value of beer brewed at a low fermentation temperature had the tendency of having higher sulfite content and higher EA value than those of the beer brewed at a high temperature. The reports on temperature dependency of sulfite production during fermentation were different among some researchers (2,15,19). It seemed that the effect of fermentation temperature on sulfite production might be different based on the physiology of the yeast strains used. Thus, the effect of fermentation temperature on beer flavor stability seemed to be different among the yeast strains used. Lustig et al, on the other hand, reported that higher temperature during the primary fermentation might be harmful from the view of residual concentration of some aging aldehydes (11). These results suggested that the selection of fermentation temperature for improving beer flavor stability must also be made after careful consideration. To clarify where there is a simple relationship between EA value and sulfite concentration of beer, or not, the plot of EA values against sulfite concentration is shown using the data from these various fermentation experiments (Fig. 4). These results show that sulfite is one of the essential and important determinants (antioxidants) of EA value, but other factors may also influence EA value.<ref name=uchono/>

YAN has the most impact on fermentation speed compared to other compounds. It impacts yeast biomass at the beginning of fermentation and sugar transport during fermentation. At the end of growth phase, N is depleted resulting in decreased protein synthesis and sugar transport. A YAN addition at this point reactivates protein synthesis and sugar transport increasing the fermentation rate. Oxygen is rapidly consumed in the beginning of fermentation. Decreased oxygen inhibits sterols and fatty acid synthesis by yeast. This causes decreased yeast growth and viability at the end of fermentation.<ref name=kelly>Kelly M. [https://psuwineandgrapes.wordpress.com/2020/07/28/why-when-and-how-to-measure-yan/ Why, when, and how to measure YAN.] Penn State Extension Wine & Grapes University website. 2020. Accessed online March 2024.</ref>

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 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/>

== Articles to be reviewed ==