Black soybean seed coat polyphenols and pinitol (Next generation of biologically active substances in soybean)

Abstract

In soybean, a number of biologically active, and thus beneficial, substances have been identified; including isoflavone, lecithin, and saponin. Some of these are currently being marketed because their health promoting functions have been publicly approved. Moreover, recent evidence has accumulated suggesting that soybeans could contain other unused compounds with valuable functions as food additives. In this review, we focus on black soybean seed coat polyphenols (BSCP) and pinitol (PI). BSCP mainly consists of procyanidin, cyanidin 3-glucoside, and epicatechin, which are all well known for their antioxidant properties. Procyanidin represents a group of oligomeric compounds formed from catechin and epicatechin molecules. BSCP is characterized by its specific composition of procyanidin, and is particularly rich in smaller oligomer forms. It is thought that procyanidin oligomer size is inversely correlated to bioavailability. PI (3-O-methyl-D-chiro-inositol) is an inositol derivative that chiefly exists in legumes and pines, and is characterized by its extremely high water-solubility. PI content in soybean seed was reported to be around 0.2% dry weight, while in soybean plant, it is one of the major low-molecular weight carbohydrates; moreover, it is very rich in soybean leaves (up to 2% of dry weight). Intriguingly, both BSCP and PI exert anti-diabetic and anti-obesity effects. However, they have been under-utilized mainly due to a lack of efficient methods for stable and effective handling. Here we discuss both plausible mechanisms that could enable the exertion of beneficial functions and recent developments in their preparation from raw materials.