Use of 5,5′-Dithiobis (2-Nitrobenzoic Acid) as a Measure of Oxidation During Mashing

Ellman's reagent (5,5′-dithiobis [2-nitrobenzoic acid]), DTNB, has been used to demonstrate a loss of free thiol groups during mashing. Such oxidative loss can be minimized by the use of a nitrogen blanket and deaerated liquor and by sparging the grist with nitrogen immediately before use. With these precautions, storage of milled grist in air does not influence the yield of thiols and, by implication, milling under nitrogen is not necessary. Stirring mashes, either under air or nitrogen, results in greater loss of thiols, and the introduction of air at any stage in the mash is damaging. The rate of thiol oxidation in mashes of malt is considerably faster than in solutions of thiol compounds. This suggests that thiol oxidation may be catalyzed by mash constituents. Such oxidation may lead to the creation of hydrogen peroxide rather than water as a by-product.