THE PRESENCE OF TWO DIMETHYL SULPHIDE PRECURSORS IN MALT, THEIR CONTROL BY MALT KILNING CONDITIONS, AND THEIR EFFECT ON BEER DMS LEVELS

Abstract

The precursor of dimethyl sulphide (DMS) which is formed during barley germination and is present in green malt differs from that present in malt kilned at high (> ∼75°C) temperatures in that it cannot be metabolized to DMS by yeast. It is therefore termed ‘inactive’ precursor to distinguish it from the ‘active’ precursor present in kilned malt, which is metabolized to DMS by yeast. During malt kilning, inactive precursor is changed into the active form, but only at temperatures at which destruction of both precursors is also taking place. Therefore, for any kiln there is an optimum temperature range over which maximum conversion of inactive to active precursor can be combined with minimum destruction of both precursors. The DMS in beers brewed from green malt or malts kilned only at low (< ∼70°C) temperatures is almost entirely the result of precursor destruction during wort boiling, final levels being governed not only by the conditions of boiling, but also by the extent of losses during fermentation and subsequent processing. In contrast, the DMS in beers brewed from malts kilned at higher temperatures is mostly formed during fermentation, and since beer DMS levels can be related to wort and beer DMS precursor levels, they can be better predicted and controlled. The method used for the measurement of total DMS precursor levels in malts is described in detail.