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Assay of Dehydroascorbic Acid in Bread and Dough Added as a Crystalline Dimer¹

¹Contribution 97-58-J. Kansas Agricultural Experiment Station, Manhattan, KS. ²Graduate research assistant and professor, respectively, Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66506. ³Corresponding author. E-mail: xzlu@ksu.ksu.edu

The assay of dehydro-L-ascorbic acid (DHAA) in dough and bread was done by reduction of DHAA to L-ascorbic acid (AA) in aqueous dithiothreitol (DTT) at pH 6–7 followed by quantitation of the AA using HPLC with electrochemical detection. At room temperature and pH 6.6, with 4.0 equivalents of DTT, the conversion of DHAA to AA was stoichiometric after 5 min. In mixograms on flour-water doughs, DHAA added in dimeric form at 200 ppm had no effect on absorption but increased mixing time by 9–19% with the same effects occurring in full-formula doughs. AA added to doughs did not affect mixing peak time or absorption. Mixing bread doughs with an initial level of 25–200 ppm of AA based on flour (14% mb) produced DHAA in the freshly mixed doughs at concentrations of 20–51 ppm, or from 80 to 26% of AA added. During ≈120 min of fermentation and proofing, the levels of AA in the doughs increased by 4–10 ppm. Mixing bread doughs with an initial level of 25–200 ppm of DHAA produced no AA in the freshly mixed doughs, but the proofed doughs and fresh breads contained 4–10 ppm and 7–49 ppm of AA, respectively. Fresh bread made from dough with 200 ppm of AA retained 66% total vitamin C (110 ppm of AA + 21 ppm of DHAA), whereas bread made with 200 ppm of DHAA retained 9.5% total vitamin C (13 ppm of AA + 6 ppm of DHAA). DHAA was 2–4 times more effective in improving loaf volume than an equal weight of AA in no-time dough, and 1.5–2 times more effective in straight-dough. In straight-dough bread made with a commercial bread flour, increasing concentrations of DHAA markedly improved bread up to ≈20 ppm, beyond which overoxidation occurred rapidly. In contrast, increasing concentrations of AA improved bread up to ≈150 ppm with a broad tolerance up to 200 ppm. The improving action of DHAA was independent of the concentration of air in the mixing bowl, and DHAA was much more heat-labile than AA.