Germinated mung bean protein hydrolysate antioxidant activities and prevention of lipid oxidation

Abstract

Mung bean seeds were germinated, at 30±2OC, in the dark for 0, 6, 12, 18 and 24 h. Crude protein content of germinated mung bean significantly (p˂0.05) increased during germination period. The 24 h-germinated mung bean (24 H-GMB) had the highest protein content but not significantly different compared to that of 18 h-germinated mung bean (18 H-GMB). However, this germination period did not affect the ratio of each amino acid to total amino acids in terms of content. Total phenolic content (TPC) of germinated mung bean significantly increased during germination period (p˂0.05). The 24 H-GMB showed the highest TPC, amounting to 5.78 mg gallic acid g-1 db, the value doubled increased compared to 0 h-germinated mung bean (0 H-GMB), amounting to 2.81 mg gallic acid g-1 db. The 18 H-GMB was chosen as a raw material for protein hydrolysate production and 0 H-GMB was used as the control. These samples were hydrolyzed at 50OC, pH 6.0. Four concentrations of Flavourzyme® (0, 1, 3, and 5% by dried weight of the germinated mung bean) were useds and hydrolysis periods were 0, 1, 2, 3, 4 and 5 h. The results showed that degree of hydrolysis (DH) increased with increasing in both Flavourzyme® concentration and hydrolysis period in both 0 H-GMB and 18 H-GMB. The DH significantly affected the amount of free amino group (FAG) and TPC in all hydrolysate samples obtained from both 0 H-GMB and 18 H-GMB (p˂0.05). Hydrolysis of 0 H-GMB and 18 H-GMB with 5% Flavourzyme® for 5 h gave the highest FAG and TPC compared to other conditions in its group. There was a positive linear correlation between DH and FAG (R2 = 0.920 for 0 H-GMB protein hydrolysate and R2 = 0.997 for 18 H-GMB protein hydrolysate) and between DH and TPC (R2 = 0.971 for 0 H-GMB protein hydrolysate and R2 = 0.962 for 18 H-GMB protein hydrolysate). Antioxidant activities of the protein hydrolysate were measured using chemical methods including 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity, ferric reducing antioxidant power (FRAP), and metal chelating activity, and using electron spin resonance (ESR) method for DPPH radical scavenging activity, hydroxyl radical scavenging activity and carbon-centered radical scavenging activity. It was found that antioxidant activities of the germinated mung bean protein hydrolysates, measured using both methods, increased when DH increased. This trend was observed in both of the 0 H-GMB protein hydrolysate and 18 H-GMB protein hydrolysate. Higher antioxidant activity was found in the 18 H-GMB protein hydrolysate compared to that obtained from 0 H-GMB protein hydrolysate. Phenolic acid in the freeze dried germinated mung bean hydrolysate was determined by HPLC with an online DPPH radical scavenging activity system. The results showed that antioxidative phenolic acid found in the 0 H-GMB protein hydrolysate and 18 H-GMB protein hydrolysate was gallic acid and certain unidentified compounds. The above results showed that FAG and TPC contribute antioxidant activities of germinated mung bean protein hydrolysates.