Phenolic content in cereals and legumes. Influence of processing and effect on in vitro availability
Doctoral thesis, 2005
Iron deficiency anaemia is highly prevalent in Tanzania, and one of the major causes is the low bioavailability of iron in the plant food staple diet. Cereals and legumes contain large amounts of compounds that inhibit iron absorption, e.g. phytate and phenolic compounds including phenolic groups such as galloyls (trihydroxyl) and catechols (ortho-dihydroxyl). Several processing methods are also known to reduce the amount of phenolic compounds, but little is known about their effect on specific phenolic groups. Dephytinization has been shown to increase the bioavailability of iron from several cereals, but this treatment alone is not sufficient to increase the bioavailability in high-tannin grains. This study was conducted to determine and characterize the amount of phenolic compounds in some cereals and legumes commonly consumed in Tanzania and to study the effect of traditional processing methods in combination with polyphenol oxidase on phenolic groups and on in vitro available iron.
Total phenols, resorcinols and potential iron-binding phenolic groups (galloyls and catechols) were determined in the food grains using different colorimetric methods. Grains were subjected to community processing methods including dehulling, soaking, cooking, germination or fermentation. Phenolic compounds were oxidized by incubating phytate-reduced cereal slurries with polyphenol oxidase (PPO) or adding PPO during the fermentation processes. The in vitro available iron was measured as iron solubility at physiological conditions.
The methods used for the determination of phenolic compounds were reproducible and the sum of different phenolic groups correlated to the amount of total phenols (r2 = 0.97; p < 0.001). The high-tannin sorghum contained the highest amount of phenolic compounds (total phenols = 33.7 mg catechin equivalents (CE) g-1). Galloyls (tannic acid equivalents, TAE) were found in negligible amounts in most grains, however, appreciable amounts were found in the red sorghum variety called udo (~5.5 mg TAE g-1). Processing the grains resulted in variable effects on the phenolic content. Soaking in different solvents significantly (p<0.05) reduced the amount of total phenols in sorghum, finger millet and cow peas, with alkali and acidic media being more effective. Water soaking had no effect on total phenols in mung beans and none of the soaking media had any effect on phenols in brown kidney beans. Germination significantly reduced the amount of total phenols in all grains studied. Cooking the cereal grains had a better effect in reducing the amount of extractable phenols than germination and soaking in water, while dehulling was the most effective method (p<0.01) in legumes. Incubation of phytate-reduced high-tannin sorghum and finger millet slurries with PPO significantly reduced the amount of total phenols (p<0.05), and resulted in an improved in vitro iron availability, which was further enhanced by adding germinated flour prior to fermentation. Addition of phytase and PPO during the solid state fermentation of tempe from high-tannin sorghum significantly (p<0.05) decreased the amount of phenolics and, as a result, increased the in vitro available iron. Traditional community processing of high-tannin sorghum had only a minor effect on the in vitro available iron. Introducing enzyme treatment to the community processing methods may be a food-based strategy to avert iron-deficiency anaemia in developing countries that depend on high-tannin cereals and legumes as their staple foods.
iron binding phenols
in vitro iron availability