Metal Ion Concentration and Release by a Brewing Yeast: Characterization and Implications

The absorption and excretion of various metal ions by a brewing yeast strain are continuous processes throughout fermentation. The K+ ion is loosely bound to the cell as evidenced by its occurrence in high concentrations in the fractions obtained by washing cells in Ringer's lactate solution. As much as 3 mg/g dry weight yeast of the ion was measured in these fractions. The highest concentrations were, however, measured in the mannoprotein fractions (20 mg/g dry weight yeast). The Ca++ ion also occurred in high concentrations in the cell wall where it was released by washing cells in Ringer's lactate solution with EDTA. The highest concentrations, however, were contained in the mannoprotein layer (3 mg/g dry weight yeast). Zn++ and Mg++ were concentrated in the mannoprotein fraction of the cell wall at 0.13 and 2.9 mg/g dry weight, respectively, as well as in the cytosol (0.9 and 4 mg/g, respectively). The wort ion concentrations fluctuated throughout fermentation, but the final beer levels were considerably lower than the initial wort levels. Yeast physiological condition drastically affected the extents of uptake and excretion. The measurement of these phenomena may provide the basis for simple vitality assays. In general, less vital cells were ineffective at concentrating metals, with the exception of calcium, which was present in elevated concentrations in poor-quality yeast.