Effects of mineral ions on yeast performance under very high gravity beer fermentation

Abstract

The effect of three divalent cationic concentrations of Zn2+, Mg2+, and Ba2+ on yeast (Saccharomyces pastorianus) fermentative performance was investigated at independent and four variable combinations in 24 experimental runs in accordance with the experimental design. Very high gravity wort of 21°P was prepared from barley malt, hops and water, to which the metal and their combinations were supplemented and subsequently pitched using lager brewing strain S. pastorianus and allowed for 96 h fermentation. After 96 h of fermentation, highest wort fermentability of %F = 30.16 and %F = 29.53 which were not significantly different (p � 0.05) were obtained for wort medium supplemented with 12, 900, and 4 ppm (Zn2+, Mg2+, and Ba2+) and 900 and 4 ppm (Mg2+ and Ba2+) respectively. Maximum ethanol yield of 8.4347% (v/v) was obtained in wort medium supplemented with the metal combination ratio of 12:900:4 (Zn2+, Mg2+, and Ba2+). For the individual metal concentration levels tested, high wort fermentability of %F = 27.94 and %F = 26.03 which were not significantly different (p � 0.05) was obtained for wort media supplemented with 700 ppm Mg2+and 2 ppm Ba2+. High ethanol yield of 7.8844% (v/v) and 7.6245% (v/v) was recorded for Mg2+and Ba2+ at concentration of 700 ppm and 2 ppm, respectively. Wort pH of 4.75 was found optima! for the yeast activity. Concentrations of 10 ppm Zn2+ and 4 ppm Ba2+, were found to reduce yeast performance. Thus, the study established relationships between the metal ions on yeast fermentative performance and the impact of Ba2+ on yeast fermentation process - with concentrations of 1 and 2 ppm Ba+found significant (p :S 0.05).