Thiols in flour and baking quality

Abstract

Baking quality (LV) was defined as loaf volume per gram of loaf protein. Further attempts to find negative correlation between LV and small thiols bound by disulphide bonds (SS) to gluten failed, but this was almost certainly because the method was flawed. A new approach was devised to measure on the same sample: free sulphydryl groups (SH) in glutenin, diffusible thiols in flour, and thiols bound to protein by SS. Both methods showed the existence of bound thiols; corrected values in the old method had a mean of 1·5 μmol per gram of gluten; in the new they ranged from ∼ 0·9 to 2·3 μmol per gram of flour protein with a mean of 1·6. Altogether three ways were tried of measuring diffusible thiols in flour (possibly the rheologically active ones). The most specific reagent measured 14–16% of accessible thiol as diffusible. A second method gave slightly lower values but this could be explained by side reactions. High values, with a third reagent, were almost certainly due to incomplete reaction. Probably for the first time, it was shown that untreated glutenin molecules in flour carry free SH. The use of phenyl mercuric acetate suggested ∼ 1/8 SH a glutenin chain (of average mol. wt 50 000). Higher results with iodoacetamide ranged from 0·17 to 1·1, but these probably included extra SH, from reaction of glutenin with active thiols before they could be blocked, and/or from attack on labile SS in glutenin. The amount of free SH on glutenin was negatively correlated with LV in the Chorleywood Bread Process (CBP), but not significantly in the 3 h long fermentation process (LFP). The results suggest that an active thiol attacks a glutenin SS when flour is wetted to add both SH and bound thiol; it can also form an SS with an SH on glutenin by oxidation. The hypothesis that the mol. wt of glutenin controls about half the variation in baking quality is compatible with recent findings that baking quality depends on the type of high molecular weight subunit in glutenin. Payne found one-third to two-thirds the variation in baking quality was linked with variation of subunit composition, so both the mol.wt of glutenin, and its high mol.wt subunits, affect baking quality. Further studies of the effect of thiols on the mol.wt of glutenin (and therefore on baking quality) could well be done on wheats of constant subunit composition. With a commercial flour the CBP was less tolerant of added thiols than the LFP. Cysteine is only about a tenth as active as the others in the LFP. Its effect (when large amounts were used) did not seem to involve an enzyme since the d- and l-forms were equally active. In a trial with 24 flours, protease activity was uncorrelated with LV. A further measurement was made of SH in three stored flours.