| Rice bran protein?RBP?has a wide range of sources,it is cost-effective and highly nutritional.Therefore,it has been extensively concerned by researchers worldwide.Different processing methods have been applied in research to modify the functional properties of RBP to promote its application in the preparation of edible films and other food products.Heating is one of the common processing methods used to modify the properties of RBP.The heat treatment process usually results in aggregation of proteins depending on treatment conditions such as temperature,duration,and p H value.Under different p H values,the protein structure is changed to different extents.The changes in the protein structure during heating under different p H values reflect in changes in the functional properties of protein,which may promote its application in a wider range of products.In this study,different forms of protein aggregates were prepared by heating of RBP solution under different p H values.The prepared RBP aggregates were characterized using different modern analysis techniques such as rheological analyzer,transmission electron microscope?TEM?,Fourier infrared spectroscope?FTIR?,and differential scanning calorimeter?DSC?.Additionally,the RBP aggregates were used to prepare edible films,and the effects of different aggregates with different morphological structures on mechanical and functional properties of films were investigated.Moreover,Layer by layer?LBL?technology was used to prepare RBP aggregates-chitosan LBL films,and the influence of RBP aggregate and chitosan concentration on the properties of LBL films was investigated.Furthermore,antioxidant and antibacterial properties,as well as degradation characteristics of RBP aggregates and LBL films,were measured which play a theoretical basis for subsequent application research of the film.Research contents and results can be concluded as follows:?1?Physical properties and microstructure of RBP aggregates obtained after heating at p H 2-12 revealed that the turbidity of aggregates was negatively correlated with solubility,and the aggregates showed the highest solubility under alkaline conditions.RBP soluble aggregates showed fibrous forms at p H 2 and 3 with an average particle size of 177.6 nm.However,spherical soluble aggregates with an average particle size of 98.47 nm were formed under neutral conditions.On the other hand,at p H 10-12 larger amorphous aggregate particles were produced.Stability of the aggregates enhanced?G'>G"?as p H increased,and the best gel network structure was formed at p H 11 and 12 with higher thermal stability,secondly was fibrous aggregate,but the spherical aggregate was less stable.FTIR spectra showed that p H value significantly influenced characteristic groups of the amide II band of RBP aggregates,also the alkaline conditions affected the intensity of the amide III band at 1210 cm-1.Meanwhile,generating fibrous aggregates was accompanied by a transition of ?-helix to the ?-sheet structure.?2?The RBP aggregates,prepared at p H 2,7,or 11,were used for preparing films and compared untreated RBP films.The results showed that RBP aggregates with different morphological structures significantly affected the physical properties and internal structure of the films.The RBP aggregation films showed higher transparency,water resistance,and thermal stability than those of the untreated RBP films.Also,films made from amorphous aggregates showed good physical properties,and the elongation at break increased from 5.72 % to 31.87 %.The water vapor permeability?WVP?was only 1.25×10-13g/m·s·Pa,which is excellent water blocking performance compared to those reported in previous research.Additionally,the RBP aggregation films showed a more uniform,smooth,and dense internal structure under the scanning electron microscope?SEM?compared to the untreated RBP films.Furthermore,the changes in p H and formation of aggregates influenced the secondary structure of films.RBP aggregation films showed lower ?-helix,?-turn,and random curl content,and greater ?-sheet content and this trend is consistent with the effect of p H on the secondary structure of RBP aggregates.These results indicated that the preparation of films using protein aggregates results in films with enhanced physical properties and improved internal structure.?3?The RBP aggregate-chitosan LBL films were prepared by LBL technology.The results showed that chitosan concentration has no significant effect on the physical properties and thermal characteristics of films,but has a significant effect on mechanical properties.As chitosan concentration increased,the tensile strength of the films gradually improved,and the elongation at break reached a maximum value of 57.75 % at a chitosan concentration of 3 %.However,the physical properties and structure of LBL films prepared by different protein aggregates show large differences.The structure of LBL films prepared by fibrous RBP aggregates showed certain voids and visible grooves on the surface,which reflected in greater WVP value(3.52×10-13g/m·s·Pa),low water resistance,and poor thermal stability.On the other hand,the globular protein aggregate LBL films showed a tight internal structure with small voids,which reflected in good water resistance and thermal characteristics,but weaker mechanical properties than that fibrous aggregates.Moreover,the amorphous protein aggregates resulted in a film with a denser network structure and strong electrostatic adsorption between layers,which reflect in a good WVP value(1.02 ×10-13g/m·s·Pa),greater thermal stability,and good mechanical properties.Compared with RBP and chitosan films,the infrared spectra of LBL films were significantly different,indicating that LBL technology improved the interaction among chitosan,RBP,and RBP aggregate molecules in the film.Except for static electricity,there is still participation of covalent and non-covalent bonds including disulfide and hydrogen bonds during the film formation process.These results indicated that the protein aggregates with different morphological structures played the major role in properties of LBL films,while the effect of chitosan was not significant.?4?The results of antioxidant,antibacterial and degradation propertiesof films showed that the LBL films showed the highest antioxidant activity,followed by the RBP aggregation films,and films prepared by the amorphous aggregates exhibited the highest antioxidant activity values?DPPH: 32.55 %,FRAP: 0.18?.Also,LBL films with chitosan showed significant antibacterial activity,while the morphological structure of protein aggregates has no significant effect on the antimicrobial properties of LBL films.Additionally,the LBL films showed the best degradability,and no significant difference was found between the RBP and RBP aggregation films.In the test time range,the final weight loss rate of films was 72.86% to 88.37%.These results indicated that both RBP aggregates and RBP aggregates-chitosan LBL films have good antioxidant activity and degradability,and LBL films also have good antimicrobial properties.This study provides theoretical basis and information for application practice of rice bran protein film,which may help in promoting the application of RBP in preparing of edible films and other food products. |
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