In Vitro Availability of Iron in High-Tannin Sorghum. Effect of Enzymatic Oxidation of Phenolic Compounds

Doktorsavhandling, 2005

Iron deficiency anemia is the most common nutritional disorder in the world. In low income countries, it is to a large extent caused by low bioavailability of iron in the vegetable diet. Vegetable diets contain large amounts of compounds that inhibit iron absorption, e.g. phytate and polyphenols (tannins). Polyphenols inhibit iron absorption by forming insoluble complexes with iron in the gastrointestinal tract. The aim of the present thesis was to investigate how enzymatic oxidation affects phenolic compounds and subsequently the bioavailability of iron in high-tannin cereals. The bioavailability of iron was estimated with an in vitro digestion method that has been validated against absorption data and found to be a suitable method. Oxidation of model phenolic compounds, i.e. tannic acid and green tea extract, with commercial polyphenol oxidase (PPO) significantly increased in vitro iron availability in a model food system. These results imply that the oxidation products have a lower iron binding capacity than the original phenolic compounds. Incubation of phytate-reduced flour of sorghum or millet also resulted in lower amounts of phenolic compounds and a higher availability of iron. The phenolic content in sorghum decreased by more than 50% with PPO incubation, and in vitro available iron increased by almost 100%. Furthermore, combining the enzymatic treatment with traditional processing methods, such as cooking, soaking, germination and lactic acid fermentation, resulted in a further improvement in iron availability. Polyphenol oxidase can be found in several common fruits and vegetables, and incubation of dephytinized high-tannin sorghum with dialyzed extract of pear, banana or avocado effectively increased the in vitro iron availability. The effect was more pronounced than with commercial PPO, and incubation with avocado extract improved the availability more than 1.7-fold. It is probable that peroxidase activity and organic acids in the fruit extracts contributed to the improved iron availability. Investigations of the effect of oxidation on phenolic structures, both simple phenolic compounds and complex structures such as sorghum tannins, showed a decrease after incubation with PPO. The decrease in tannins was probably due to coupled oxidation reactions involving simple phenols. Catechol (ortho-dihydroxyl) and galloyl (trihydroxyl) groups are identified as iron binding phenolic groups in plant foods. Model phenols containing catechol groups were more easily oxidized than those containing galloyl groups, while the percentage decrease in catechol and galloyl groups in sorghum phenols were similar. In combination with methods that reduce phytate, enzymatic oxidation of phenolic compounds may be a promising way to increase the bioavailability of iron in high-tannin cereals at the household level.