Effects of polyphenol oxidation on in vitro iron availability in faba bean (Vicia faba L.) flour

Tannic acid (TA) and green tea extract (GTE) were added to faba bean flour, before and after incubation with polyphenol oxidase (mushroom tyrosinase), and the effect on in vitro iron availability was investigated. The inhibitory effect of TA and GTE was dose dependent, and the in vitro iron availability decreased from 64.5% in the pure faba bean flour to 9.8% (P<0.05) with the addition of 1mg of TA 1g1 to faba bean flour. The addition of 5mg of GTE resulted in low in vitro iron availability as after the addition of 10mg of TA (11.6% against 10.3% for 1mg TA). Incubation of the polyphenols with tyrosinase before addition to the faba bean flour significantly increased the in vitro iron availability. The increase in iron availability was significant (P<0.05) when amounts of 0.2, 0.5 or 1mgg1 TA or GTE added to faba bean flour, respectively, were oxidised, even with the lowest amount of tyrosinase (150u). Oxidation of 1mg TA or GTE with 150u tyrosinase increased the in vitro iron availability from 10.3% to 15.5% and from 19.2% to 26.1%, respectively. At the 300-u level, the addition of higher amounts of enzyme (from 600 to 900u) did not have any effect. The results from the study therefore suggest that the oxidation of polyphenols may be a promising way to increase the availability of iron in polyphenol-containing legume foods.