Transglutaminase mediated cross-linking of peanut and buckwheat proteins under dual-frequency ultrasound field: Functional attributes study

Abstract

Peanut protein (PP)/buckwheat protein (BWP) mixture crosslinked by transglutaminase (TGase) under dual-frequency ultrasound (DFU) field was developed and its structural and functional properties were characterized. The results showed that the optimal DFU parameters for PP-BWP crosslinking were 25 kHz/40 kHz dual-frequency ultrasound working time ratio of 10:10 (s/s), a temperature of 45°C, ultrasonic power density of 125 W/L and time duration of 30 min, resulting in the degree of crosslinking (DC) of 37.7 %, which is 125 % higher than that of the control. The solubility at pH 7.0, oil-holding capacity, and emulsifying capacity of the protein aggregate (peanut protein-buckwheat protein crosslinked by TGase under dual-frequency ultrasound, U-TG-PP-BWP) was significantly (P<0.05) improved compared with protein aggregates formed without ultrasound (TG-PP-BWP); however, the emulsion stability, foaming capacity and form stability decreased. Secondary structure analysis indicated no apparent difference between un-crosslinked protein and U-TG-PP-BWP. After ultrasound treatment, the α-helix decreased by 11.2 % and the β-sheet, β-turn and random coil increased by 7.7 %, 3.4 % and 4.7 %, respectively, in U-TG-PP-BWP compared to TG-PP-BWP. The microstructures showed that U-TG-PP-BWP formed a lamellar structure with many micropores while TG-PP-BWP aggregated into large and tight blocks. SDS-PAGE analysis showed that, compared to PP and BWP, the subunit bands of cross-linked TG-PP-BWP and U-TG-PP-BWP were altered at 14.4–116 kDa, yielding proteins larger than 116 kDa, suggesting covalent cross-linking of the two proteins. In conclusion, the enzymatic cross-linking of proteins can be promoted by DFU, providing a new approach to developing novel plant proteins.