Effects of Transglutaminase on Gel Properties of Buckwheat Protein Isolate

Buckwheat protein isolate (BPI) was extracted from buckwheat (Fagopyrum esculentum Moench.) by alkali solubilization and acidic precipitation, following by spray-drying or vacuum freeze-drying to prepare protein powder. The effects of transglutaminase (TGase) and drying methods on physicochemical and gelling properties of buckwheat protein isolate were investigated. The results showed that the protein components of freeze-dried buckwheat protein isolate (FBPI) were similar to BPI, whereas the degradation of 13S globulin was observed in spray-dried buckwheat protein isolate (SBPI). The α-helix contents of FBPI and SBPI were 22.10% and 20.00%, with corresponding β-sheet contents of 19.40% and 19.80% respectively, indicating the transformation of disordered structure in BPI after spray-drying or vacuum freeze-drying. As compared with BPI, SBPI possessed the higher surface hydrophobicity and more uniform particle with a small size, but lower solubility and water holding capacity, while FBPI exhibited similar physicochemical properties to BPI. The average particle sizes of FBPI and SBPI increased from 213.01 and 192.55 nm to 2289.01 and 1439.67 nm after TGase addition respectively, which due to the intermolecular crosslinking reaction. The potentials of BPI, FBPI and SBPI were −15.93, −29.43 and −29.35 mV, respectively, suggesting that drying treatment could further improve the stability of protein solution. Compared with SBPI, G' of BPI and FBPI decreased continuously during the heating process, implied that the gel structures of BPI and FBPI could be destroyed by heating. After pretreatment by heating and then cooling down, FBPI showed a significantly (P<0.05) higher storage module than that of SBPI and BPI, indicating the better gelation. The addition of TGase could significantly (P<0.05) improve the storage modulus and thermal stability of FBPI, but had no significant effect on SBPI. Based on the results of chemical force analysis, it was speculated that the formation of gelation for buckwheat protein isolate was mainly attributed to hydrophobic interaction and disulfide bond. The addition of TGase could promote the formations of hydrophobic interaction and disulfide bond, but led to the breakage of intermolecular hydrogen bond. In conclusion, buckwheat protein isolate dried by vacuum freeze drying could form the gel with good gelling properties and stability, incorporating with TGase. The results of this study could provide the theorical reference for the researches on gelation of buckwheat protein isolate, as well as the development and application of relevant gelling food.