DisplayTitle

​S-4. The effects of insufficient nutrition on flavor compounds production, propagation and fermentation of yeast

ABSTRACT: In Japan, beer-flavored beverages brewed with no malt have been available since 2003. The raw materials used in beer-flavored beverages with no or less malt compared with regular beer generally contain fewer nutrients than malt, which sometimes leads to sluggish fermentation and the production of off-flavors. In this paper, we review our studies on the effects of insufficient nutrients on off-flavor compound production, propagation, and fermentation of yeast. Among the off-flavors, sulfur-containing compounds, such as hydrogen sulfide (H₂S), are of particular interest for many brewing scientists and brewers, because they have an unpleasant flavor and very low threshold. We have extensively studied the production of hydrogen sulfide (H₂S) and its precursor, sulfite (SO₂), during fermentation in lager yeast. Upon the addition trials of three concentrations of methionine, we found that five genes (MET3, MET5, MET10, MET6, and CYS4) were regulated by our microarray analysis system. Further gene expression analyses revealed that the gene expression balance of MET3 and MET10 led to the production of a higher level of sulfite (SO₂) in the lager yeast. We also found that metabolism from SO₂ to H₂S by yeast occurred depending on the pH value during the secondary fermentation. The amount of H₂S was higher at the lower pH value during secondary fermentation. Furthermore, in order to reduce the risks in brewing new products, we investigated the possible production of other off-flavors that might be important in brewing new raw materials. During the investigation, we detected several off-flavors that are different from sulfur-containing compounds. GC-MS-olfactometry analysis revealed that one of these flavors was indole. In brewing, indole has been recognized as an off-flavor and is thought to be produced by microorganism contamination during fermentation. It has not been reported that brewing yeasts produce indole during fermentation. We concluded that the lack of vitamin B₆ in wort led to the accumulation of indole in yeast due to the inhibition of tryptophan synthase reaction. Further study on the fermentation and propagation of yeast in the case of nutrient deficiency also shows that the lack of inositol induces a strange budding. In reviewing both of these findings, we discuss the essential combination of nutrients for brewing in order to reduce the risk of off-flavors and poor fermentation and propagation.