Influence of Cultivation Temperature on Oligosaccharides and Isoflavones in Soybean Sprouts

Abstract

Soybean is one of the small sized food legumes that contains high amount of nutritional compounds such as isoflavones, amino acids, vitamins, soyasaponin (Kim et al., 2013). As a food source, soybean can be consumed as it is or processed into other products such as tofu, soy sauce, soy paste and soybean sprouts. The variety of soybased food is produced to enhance the flavour and digestibility for increasing customer acceptance (Liu, 2008). Soybean sprout is one of the soy-based foods that produced by cultivating soybean seedlings for certain period to get certain length. It has been reported that most of the nutritional compounds such as amino acids, myo-inositol metabolites, isoflavone glycosides, soyasaponins, phytosterol (Gu et al., 2017), tocopherol (Vasantharuba et al., 2007) and dietary fibre (Tiansawang et al., 2016) increase during cultivation process of soybean sprouts, while some compounds such as phytic acid, trypsin inhibitors, compounds with lipoxygenase activity and oligosaccharides decrease (Labaneiah and Luh, 1981; Mostafa et al., 1987; Bau et al., 1997). Previously, it has been reported that the soy oligosaccharides, which is classified as raffinose family oligosaccharides (RFOs), causes flatulence and is recognized as one of the undesirable substances for human diet (Rackis, 1981). However, the recent research reported that RFOs could provide good impact to human health especially in the absorption process of isoflavones into the body (Tamura et al., 2003). Soybean sprout contains higher amount of isoflavones compared to soybean seed as raw material. Twelve distinct forms of isoflavone are found in soybean sprout; aglycones (daidzein, genistein and glycitein), -glycosides (daidzin, genistin and glycitin), malonyl glycosides (6 -Omalonyldaidzin, 6 -O-malonylgenistin and 6 -Omalonylglycitin) and acetylglycosides (6 -O-acetyldaidzin, 6 -O-acetylgenistin and 6 -O-acetylglycitin). Among them, malonyl glycosides are considered as main isoflavone form in soybean sprouts, especially 6 -Omalonylgenistin is the major compound (Quinhone Júnior and Ida, 2015). Isoflavones glycosides have a poor absorbance in human body (Izumi et al., 2000; Setchell et al., 2002) and poor bioactivity compared to their aglycones due to the presence of sugar moiety by -glycosidic linkage. In human digestion process, the beneficial isoflavone aglycones can be produced by hydrolysing -glycosidic bond of isoflavone glycosides including malonyl glycosides and acetyl glycosides. Moreover, the production of equol, which has much higher bioactivity than isoflavone glycoside is also related to the hydrolysis of daidzin (Setchell et al., 2002; Zubik and Meydani, 2003). Enteric bacteria in human intestine play main role on the hydrolysing process of -glycosidic bond by producing -glycosidase (Ki Suk et al., 2002). RFOs are the staple food for the growth of human enteric bacteria population (Saito et al., 1992). The abundance of enteric bacteria in human intestine will enhance the -glycosidase activity to hydrolyse the isoflavones glycosides for producing more active isoflavones compounds. Therefore, producing soybean