| Mung beans are traditional Chinese food ingredients,which are popular among the people.Mung bean protein,as a by-product of mung bean starch extraction,has high nutritional value and various biological functions such as improving lipid metabolism and preventing nonalcoholic fatty liver.Mung bean protein is an excellent plant protein that can be used to replace animal proteins.However,the densely packed super-large aggregates in mung bean protein significantly affect their functional properties in food systems,while sonication can effectively affect the protein aggregation behavior.Some studies have shown that thermosonication(i.e.ultrasonic and heat at the same time)has a higher effect on modifying plant proteins,but the mechanism is still unclear.For the purpose of clarifying the mechanism of thermosonication on the aggregation behavior of mung bean protein isolate(MPI),the effects of different processing conditions on the structural properties,dispersibility in solution,surface activity,and emulsification properties of MPI were explored,and finally it was applied to the ice cream model system to further clarify the mechanism.The main research contents and results are as follows:Firstly,the effects of processing time(5 min,10 min,20 min,and 30 min)of ultrasonic treatment(20 kHz,30% amplitude)at different temperatures(30°C,50°C,and 70°C)on the structural properties of MPI(10%,w/v)were studied.The secondary structure,tertiary structure,aggregation,and crosslinking behavior of MPI were determined.The results showed that the ultrasonic treatment at 30°C had little effect on the secondary and tertiary structures of MPI.After ultrasonicated at 50°C,the structure of MPI partly unfolded and the surface hydrophobicity increased,with more non-polar groups exposing.Ultrasonic treatment at 70°C significantly changed the secondary structure of MPI(P<0.05)and the protein unfolded further.When ultrasonicated for 10 minutes,the surface hydrophobicity of MPI increased by 202%.It is noteworthy that after ultrasonicated at 30°C and 50°C for 10 min,the particle size of MPI decreased by 73.0% and 75.2%,respectively.The rapid increase of surface hydrophobicity after ultrasonic treatment at 70°C induced hydrophobic aggregation among the dispersed proteins and then formed soluble aggregates.The results of polyacrylamide gel electrophoresis showed that disulfide bonds also participated in the formation of such soluble aggregates.On this basis,the effects of thermosonication on the properties of MPI solution were studied,by determining the solubility,turbidity,and thermal stability of the MPI solution.The results showed that the solubility of MPI was increased by the formation of soluble aggregates,with the highest solubility of 94.3%.The turbidity of the MPI solution decreased and it was more clear and transparent.After ultrasonicated treatment at 30°C,50°C,and 70°C for 10 minutes,the turbidity decreased by 81.2%,85.5%,and 79.1% respectively,and the thermal stability was also significantly improved.After heated at 90°C for 30 minutes,the turbidity of MPI solution decreased by 81.7%,88.5%,and 88.5% respectively,when compared with the untreated group.Considering the treatment effect and industrial cost comprehensively,the treatment time was selected to be 10 min.In soybean oil emulsion system,the effects of ultrasonic treatment on MPI interface activity and emulsification performance at different temperatures(30°C,50°C,and 70°C)were studied,by determining the interfacial tension,emulsification activity,emulsification stability of MPI and the stability,interfacial protein content,particle size,microstructure,rheological properties of MPI emulsion.The results showed that thermosonication significantly improved the emulsification and emulsification stability of MPI(P<0.05).After sonicated at 50°C and 70°C,the emulsification increased by 40.6% and 133.2% respectively and the emulsification stability was improved by 63.0% and 95.4%.The formation of protein aggregates induced more adsorption of protein to the oil-water interface,with the protein content of the emulsion interface increased by 30.8% and 32.2%,thus the stability of the emulsion was also improved.The microstructure of the emulsion showed that the particle size of the emulsion decreased significantly after thermosonication,while the degree of aggregation among oil droplets increased.Due to the unfold of MPI,the intermolecular interaction force between oil droplets was enhanced,which is conducive to the formation of the network structure.Accordingly,the viscosity of emulsion increased.After 50°C and 70°C ultrasonic treatment,the consistency coefficient of the emulsion increased by 2.5 and 22.5 times,respectively.Since ice cream is a protein interfacial active(emulsification,foaming)food system,the effect of thermosonication on the formation and properties of ice cream was finally studied.The MPI ultrasonicated at 50°C and 70°C for 10 min was applied to ice cream.The particle size,interfacial protein content,stability,rheological properties of the emulsion,and the swelling rate,hardness,and melting characteristics of ice cream were determined.The results showed that after ultrasonicated at 50°C and 70°C,the particle size of the emulsion decreased by 55.7% and 44.2% after aging for 6 h,and the interfacial protein content and viscosity were increased.During the whipping and aeration phase,the expansion rate of ice cream after ultrasonic treatment at 50°C was the highest,which was 57.6% higher than that of the untreated MPI group.The melting rate of ice cream significantly decreased after thermosonication(P<0.05).The melting amount of 50°C and 70°C ultrasonic treatment groups at room temperature for 60 min decreased by 14.1% and 11.2% respectively,when compared with the untreated group.Ultrasonic treatment at 50°C and 70°C increased the hardness of ice cream by 24.9% and 76.0%,respectively.To conclude,the results of this study proved that under proper heating conditions,ultrasonic treatment can effectively crack the densely structured super-large aggregates in the original MPI,promote the formation of new soluble aggregates,improve solution dispersibility,interface activity,emulsification ability of MPI,and expand applications of MPI in ice cream food systems. |
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