Methionine and sulphate as competing and complementary sources of sulphur for yeast during fermentation

Study of the uptake of sulphate and methionine by an ale yeast from a range of media showed that utilisation of sulphate was fairly strictly controlled but assimilation of methionine was not. Cells never took up more than about 0.3 mMol sulphate per litre whilst methionine, up to an initial concentration of 10 mMol per litre, was completely absorbed. Sulphate-grown cells had low intracellular pools of amino acids and methinonine was never detected. Methionine-grown cells contained methionine in both cytosol and vacuole and the concentration of several other amino acids also increased in such a way to suggest that methionine catabolism was occurring. With mixed sulphur sources methionine prevented uptake of sulphate when the concentration of sulphate was high but not when it was low suggesting the presence of two sulphate transporters with different control properties. Sulphate did not influence uptake of methionine. Addition of other amino acids to the medium did reduce the rate and extent of methionine uptake but not the intracellular pool sizes. Pilot plant studies suggested that SO₂ production in a brewery is more likely to be a reflection of the overall nutritive status of the wort rather than be connected to the initial methionine concentration.