Germinated rice bean Vigna umbellata hydrolysate and effect of foam-mat drying on its antioxidant activity

Abstract

The aims of this research were to study the effect of germination, enzymatic hydrolysis, and foam-mat drying temperature on physico-chemical properties and antioxidant activities of foam-mat dried rice bean protein hydrolysate. Firstly, the effects of germination period and pretreatment with 1% (w v-1) citric acid on chemical composition and antioxidant activities were studied. Rice beans were soaked in water and 1% (w v-1) citric acid solution for 6 h before germination for 0-24 h. Moisture content, total phenolic content (TPC) and the reducing sugar content of beans from both treatments increased with increasing germination time (p≤0.05). Pretreatment showed strong impact on an increase in TPC. The 18 hour-germinated rice beans (GRB) of both treatments showed the highest crude protein content. 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity and ferric reducing antioxidant power (FRAP) of the water and ethanolic extracts obtained from both treatments increased as germination period increased (p≤0.05). Secondly, the GRB and non-germinated rice bean (NGRB) were then hydrolysed using 5% (by dry weight of rice bean) Flavourzyme®. The reaction was conducted at 50°C for 24 h. The results showed that free amino group, degree of hydrolysis, TPC, and antioxidant activities (DPPH radical scavenging activity and FRAP) of hydrolysate of GRB significantly increased with increasing hydrolysis period (p≤0.05). All parameters tested from hydrolysate of GRB were higher than those of NGRB over 0-24 h of hydrolysis. The hydrolysis period for 21 h was chosen for preparation of protein hydrolysates in this study. Thirdly, drying kinetic and mathematical model of foam-mat dried protein hydrolysate were studied. Foamed protein hydrolysate was dried at two different drying temperatures (60 and 70 °C). Drying rate of protein hydrolysate of GRB was slightly lower than that of NGRB protein hydrolysate at both drying temperatures. Weibull distribution and Midilli model could suitably describe the drying characteristic of both foam-mat dried rice bean protein hydrolysate. The samples were dried until reaching a moisture content of about 8.5% (dry basis). Physico-chemical properties of foam-mat dried protein hydrolysate were investigated. TPC and total flavonoid content of both GRB and NGRB foam-mat dried at 60 °C were higher than those of 70 °C (p≤0.05). Gallic acid, catechol and epicatechin were major phenolic compounds in both foam-mat dried samples. DPPH radical scavenging activity and FRAP significantly decreased with increasing drying temperatures (p≤0.05). It was found that %DPPH radical scavenging activity measured using electron spin resonance (ESR) was 1.20-1.32 times higher than using colorimetric method, at the same concentration of protein hydrolysate. The higher TPC, total flavonoid content, and antioxidant activities of foam-mat dried protein hydrolysate of GRB compared to NGRB were observed. The low molecular weight peptide (lower than 3 kDa) showed the higher antioxidant activities than high molecular weight peptide (higher than 3 kDa). The main components responsible for the antioxidant activity of foam-mat dried rice bean protein hydrolysates was phenolics.