ASSAY OF DEHYDROASCORBIC ACID IN BREAD AND DOUGH ADDED AS A CRYSTALLINE DIMER

The assay of dehydro-L-ascorbic acid (DHAA) in dough and bread was don e by reduction of DHAA to L-ascorbic acid (AA) in aqueous dithiothreit ol (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 eq uivalents of DTT, the conversion of DHAA to AA was stoichiometric afte r 5 min. In mixograms on flour-water doughs, DHAA added in dimeric for m 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 approximate to 120 min o f 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 o f DHAA produced no AA in the freshly mixed doughs, but the proofed dou ghs and fresh breads contained 4-10 ppm and 7-49 ppm of AA, respective ly. 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 20 0 ppm of DHAA retained 9.5% total vitamin C (13 ppm of AA + 6 ppm of D HAA. DHAA was 2-4 times more effective in improving loaf volume than a n 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 brea d flour, increasing concentrations of DHAA markedly improved bread up to approximate to 20 ppm, beyond which overoxidation occurred rapidly. In contrast, increasing concentrations of AA improved bread up to app roximate to 150 ppm with a broad tolerance up to 200 ppm. The improvin g action of DHAA was independent of the concentration of air in the mi xing bowl, and DHAA was much more heat-labile than AA.