Oxidation by potassium iodate of thiol groups in unleavened wheat flour doughs

When a flour was mixed at 25° into a dough in a nitrogen atmosphere with the addition of iodate, there was a rapid disappearance of thiol groups and of iodate in molar ratio 5:I. After that, the disappearance of thiol groups proceeded linearly with time and the disappearance of iodate followed first‐order kinetics, the mole ratio increasing from 0.25 to 2. If the mixing was interrupted after 5 min., the oxidation of thiol groups and the consumption of iodate in the resting dough both proceeded linearly with time (mole ratio I:I). Other conditions being constant, iodate additions up to a critical value are consumed quantitatively; a constant fraction of amounts beyond this value is consumed. The critical iodate addition is smaller for mixing in oxygen than in nitrogen, and larger for a reaction time of 45 min. between mixing and analysis than for zero reaction time. An extraction of the flour with light petroleum increases it, provided that mixing is done in nitrogen, while extraction with O.IM‐sodium chloride decreases it. The fraction of iodate consumed beyond the critical addition varies bewteen 17 and 38% depending on mixing, atmosphere, reaction time and extractions. The mole ratio thiol oxidised/iodate consumed varies widely with mixing and reaction time. Curves for iodate consumption against iodate addition show a break at the critical addition; this was not detected in curves for thiol oxidised against iodate added. After mixing dough for 5 min. in nitrogen a loss of 2–3 moles of thiol groups per mole of iodate consumed was found for normal and extracted flours and for the sodium chloride extract. The low values obtained for the mole ratio thiol oxidised/iodate consumed might be explained by a reaction between iodate and flour constituents other than thiol groups. It is shown that starch and (probably) the easily extractable lipid fraction are not responsible. The validity of the estimation of the amount of reactive thiol groups by critical iodate addition, as proposed by Bushuk,6 is contested.