Compositional and functional study of antioxidant polyphenols in edible beans and their related products

Abstract

Edible beans are natural sources of antioxidant polyphenols, but their polyphenol composition and bioactivities have been little investigated compared to many fruits and vegetables. In this study, phenolic composition and bioactivities, mainly antioxidant capacity, were systematically investigated in edible beans and their related products.

A large scale screening was performed to investigate antioxidant capacity and total phenolic content in 42 edible bean genotypes. Edible beans had a wide range of antioxidant polyphenols and their bound phenolics were generally higher than in common fruits and vegetables. The red sword bean (Canavalia gladiata) was found to have the highest level of antioxidant polyphenols among the genotypes tested. Further study showed that the sword bean coat was rich in antioxidant polyphenols, mainly gallic acid and its derivatives. In addition, pigmented edible beans had higher levels of antioxidant polyphenols than non-pigmented ones due to their pigmented bean coats, which were rich in proanthocyanidins and flavonoids, and exhibited antioxidant and antibacterial effects.

Next, antioxidant polyphenols were investigated in edible bean sprouts. Germination was found to cause accumulation of antioxidant polyphenols in 12 edible bean sprouts, and the mung bean (Vigna radiata) sprout had the highest level of antioxidant polyphenols among them. Further study found that antioxidant polyphenols in green and black mung bean sprouts increased in a time-dependent manner during a 5-day germination period, accompanied with agradual increase of several phenolic compounds, such as caffeic acid, gallic acid, p-coumaric acid and rutin.

Subsequently, the influences of thermal treatments on antioxidant polyphenols of bean products were investigated. Hot air drying at 70 and 80 °C for 24 hr was found to notably increase antioxidant polyphenols in mung bean sprouts, simultaneously with evident browning and increased content of caffeic acid. On the other hand, steaming was better than boiling to increase antioxidant polyphenols in five edible bean milks. In particular, steaming substantially increased isoflavone glucosides but slightly reduced isoflavone aglycones in soybean milk. Overall, thermal treatments could induce interconversion between soluble polyphenols, and release of bound polyphenols and the products of Maillard reaction, all of which could contribute to increased antioxidant polyphenol contentin tested bean products.

Finally, the effects of fermentation on antioxidant polyphenols in edible legumes and bean milks were investigated. Fermentation had varying effects on their antioxidant capacity, while in general increasingtheir total phenolic content. Fermented mottled cowpea had the highest antioxidant polyphenol levelamong eight fermented edible legumes, with substantially increased catechin content. Additionally, the lipophilic fraction had much higher free radical scavenging capacity than the commonly investigated hydrophilic fraction in fermented and non-fermented mung bean and soybean milks. Fermentation did not significantly change two main flavones vitexin and isovitexin in mung bean milk. On the whole, fermentation could improve the bioavailability of legume polyphenols.

In conclusion, edible beans and their processed products can be good sources of natural antioxidant polyphenols. They can be confidently recommended for consumption in human diets to help prevent oxidative-stress related chronic diseases, and can be developed into functional foods with potential health benefits.