| Soy protein isolate (SPI) is one kind of plant proteins with broad sources and potentialfunctionalities. SPI has been used as a major functional ingredient to improve the yield, theoil-water-holding capacity of meat products to meet the low fat-high protein demand ofconsumers. At present, the commercial manufactured SPI in our country is almost the gellingtype SPI used in meat products, while the quality is instable and cannot compete with otherSPI products made in USA or Japan. In addition, many researchers have reported that theun-denatured SPI with native structure has destructive effects on the gelling property ofmyofibrillar protein (MP) or quality of meat products. The application effects of different SPIfrom various commercial manufacturers on the gelling property of MP or practical meatproducts were not consistent. As MP is mostly closed to the actual proteins extracted by addedsalt from meat during processing, studies on the impacts of SPI on the gelling property of MPmodel system can reflect the effects of different SPI in the actual processing of meat products.Studies on the structure characteristics and SPI-MP interaction mode could supply somesuggestion for the production of special SPI for gelled meat products. Therefore, the effects ofgelling abilities, aggregation states of SPI and pre-emulsified SPI on the gelling properties ofcomposite MP+SPI, as well as the effects of various SPI on the quality of emulsified sausagewere investigated in this study.Firstly, in order to estimate the effect of gelling abilities of SPI on the gelling property ofMP, it was need to prepare a series of basic SPI with specific structure, modified method andsignificantly different gelling abilities. Commercial SPI is generally prepared with alkalineextraction–isoelectric precipitation, followed by neutralization then spray drying. In this study,preheating treatments at50–80°C for1h were firstly applied during alkaline extraction. Itwas found that preheating at60°C for1h could obtain the highest elastic modulus and gelstrength of SPI, as well as maintain a fine protein solubility. Ultrahigh temperature (UHT,130or140°C for4s) treatment during neutralization induced the formation of a large amount ofsoluble aggregates, and significantly improved the solubility. Outstandingly, the sequentiallow and ultrahigh temperature two-stage preheating treatment (alkaline heating at60°C for1h+neutralization heating at130°C for4s) could not only make up for the solubility losscaused by alkaline preheating, but also further promote the protein solubility. The two-stagepreheated sample SPI60+130contained abundant soluble aggregates maintaining bynon-covalent bond, disulfide bond and a few other covalent bonds, which notably increasedthe gel rigidity but decreased the gelling temperature. It indicated that the low temperaturepreheating during alkaline isolation is favorable for the entirely denaturation by subsequentUHT treatment during neutralization stage, therefore, enhanced the textural property of SPI.Various SPI with significantly different gelling abilities were used to examine their impacts on the gelling property of MP, and estimate the molecular forces involved in theformation and maintenance of composite gel of MP and SPI. The results showed that SPI60+130with the highest gelling ability exhibited the best application. The aggregate of SPI60+130,aggregate of MP, heavy chain of myosin,-actin were the main subunits which involved inthe formation of composite MP+SPI gel. The result of gel solubility indicated thatnon-covalent bond (hydrogen bond and hydrophobicity interaction) were the major molecularforces to maintain and stabilize the MP+SPI gels, while disulfide bond was less than10%.The microstructure result observed by immunohistochemistry showed that the gel networkpores were bigger when NEM blocked sulfhydryls in whole SPI and partial MP. It indicatedthat disulfide bond played certain roles during the formation of composite gel, but the impacton maintaining and stabilizing set gel was not notable.As SPI60+130exhibited fine application in the MP gelation, and also contained a largeamount of soluble aggregates (about70%), we continued to study the impact of SPI aggregatestate on the gelation of MP. The soluble and insoluble aggregates which were ineluctablyformed during the production process were used as the material. The results showed thatsoluble aggregate could act as―soft‖filler in the continuous MP matrix with less randommicrostructure to significantly improve the water-holding capacity of composite MP+SPI gel.Insoluble aggregate of SPI could act as―rigid‖filler in MP matrix to significantly increase theelastic modulus and gel strength. Therefore, it would be possible that the application effects ofSPI from different manufactures on the quality of meat products are instable because of thedifferent SPI composition.Considering the filling effect of SPI, the effect of SPI particle size on the gelling propertyof MP was investigated. Hydrolysis, mild preheat treatment and pre-emulsification were usedto prepare various SPI solution (nm size) and emulsion (μm size). The results showed thatpre-emulsification could obviously improve the elastic modulus, gel strength andwater-holding capacity of MP. The microstructure result observed by environmental scanningelectron microscope exhibited that SPI emulsion could act as filler to form the compactstructure of MP gel matrix. The emulsion of native SPI and SPI hydrolysate with small sizeseemed to escape outside, while big size emulsion could be blocked and filled in MP gelmatrix. The emulsion with bigger size was more visible to enhance the MP gelation. It mightbe because the emulsifier (SPI) and continuous protein (MP) were different, and theirinteraction was not as strong as two same proteins, therefore, the mechanical filling effect ofemulsion granules looked more important.In the last, the effects of soluble hydrolysate, soluble/insoluble aggregates of SPI on thequality of emulsified sausage were studied. The results demonstrated that incorporate theabove SPI all significantly improved the cooking yield of emulsified sausage. The solublehydrolysate of SPI remarkably destroyed the texture, and induced the loose, less strand structure of emulsified sausage. The soluble aggregate of SPI had no obvious effect on theelastic modulus and texture of emulsified sausage. However, the insoluble aggregate of SPIsignificantly enhanced the elastic modulus and texture of emulsified sausage with the mostcompact and continuous structure.In conclusion, this study found that the effects of SPI on the gelling property of MPpartly came from the interaction of MP and SPI, while the major factor was the filling effectmode of SPI in the continuous MP gel matrix. The importance of filling effect of SPI wasproved in the actual emulsified sausage. The results of this research could make somesuggestion on the structure characteristic of special SPI for gelled meat products as follows.SPI with high gelling ability, high solubility and low gelling temperature could be used ininjection type meat products. Soluble and insoluble aggregates of SPI would be respectivelyimprove the water-holding capacity and texture of meat products using roll kneading method.Pre-emulsified SPI could significantly improve the microstructure and quality of emulsifiedsausage using roll kneading or chopping methods. In all, the results of this study would hascertain guiding for the industrial production of special SPI for various meat products, as wellas the application of other plant proteins in meat products. |
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