Walnut Protein Gels Induced By Ultrasound-transglutaminase And Its Controlled Release Properties

Walnut meal is by-products of the production of walnut oil,which is rich in protein and is usually used as animal feed or discarded,resulting in a waste of walnut protein resources.At present,the research on walnut protein is mostly concentrated on the modification,and there are few reports on the gel properties of walnut protein.Therefore,in this paper,walnut protein was extracted from walnut meal,and the ultrasound-transglutaminase(TGase)combination technology was used to construct the walnut protein gel system to explore the effect of ultrasound-TGase treatment on the structure and functional properties of walnut protein gels.The walnut protein gels was used as the carrier to load tea polyphenols,and its slow-release effect on tea polyphenols under simulated human gastrointestinal environment was analyzed.The main conclusions are as follows:(1)The effects of ultrasonic power,ultrasonic time and protein concentration on the texture properties of walnut protein gels were explored.Based on the single factor experiments and response surface model,the process parameters of ultrasound modified walnut protein were optimized,and the best conditions for ultrasound-TGase cross-linking to prepare walnut protein gels were obtained: ultrasound power 500 W,ultrasound time 23 min,protein concentration 20%,under these conditions,the hardness of the gels was(125 ±3.26)g.(2)The effect of ultrasound-TGase treatment on the functional properties of walnut protein gels was studied.The results showed that the solubility of walnut protein gels after TGase treatment was lower than that of protein gels without TGase treatment,and showed a "U" curve with p H.When p H was 5,the solubility was the lowest.With the increase of the amount of TGase,the water holding capacity(WHC)and oil holding capacity(OHC),emulsifying activity index(EAI)and end system identifier(ESI),foaming property(FC)and foam stability(FS)of the gels increased at first and then decreased,but they were higher than those of the samples before cross-linking.The results of in vitro digestibility showed that TGase cross-linking decreased the digestibility of protein gels.The results of the lowest gelation concentration showed that the self-sustaining gels began to form when the protein concentration was 18%,and the gel texture improved with the increase of protein concentration.(3)The effect of ultrasonic-TGase treatment on the structure properties of walnut protein gels was studied.The results showed that the addition of TGase improved the texture properties of walnut protein gels,the maximum fluorescence intensity of the protein gels reduced,and a red shift occurred.With the increase of the amount of TGase,the content of free sulfhydryl group decreased from 8.91 ?mol/g to 6.14 ?mol/g,and the surface hydrophobicity decreased from 548 to 89.Scanning electron microscopy results showed that after ultrasonic-TGase treatment,a good three-dimensional network structure was formed inside the walnut protein gels,which showed uniform density and uniform pore size.Hydrophobic interaction was the main force to maintain this structure.Fourier transform infrared spectroscopy showed that walnut protein gels were mainly composed of ?-sheet and the secondary structure of the walnut protein gels changed after cross-linking.The content of ?-sheet reduced from 53.71% to 51.97%,and ?-turn increased from 15.45% to 16.51%.With the increased of cross-linking degree,the viscosity of protein gels increased,G'>G",and the system exhibited solid-like properties.Differential Scanning Calorimetry(DSC)results showed that the ordered structure and thermal stability of the protein gels decreased after ultrasonic-TGase treatment.(4)The effect of ultrasonic-TGase on the release control of tea polyphenols in walnut protein gels was studied.The results showed that TGase cross-linking could improve the encapsulation efficiency and loading capacity of walnut protein gels for tea polyphenols,and the encapsulation efficiency reached the maximum of 93.41% when the amount of TGase was 9%.The texture and rheological properties of the drug-loaded gels were improved with the increase of the amount of TGase.In addition,with the increase of the amount of TGase,the sustained release effect of protein gels on tea polyphenols gradually improved.