| High-moisture extrusion is an important method to produce texturized protein for plant-based meat analog.Current studies on high-moisture-extruded texturized proteins mainly focus on the effects of extrusion parameter and protein raw material changes on fiber and texture,as well as the molecular mechanisms of structural changes and interaction forces of protein raw materials during extrusion process.Studies on the thermal and cooking stability of the texture and fiber structure of texturized proteins in the subsequent processing are relatively limited.In fact,the current application of texturized proteins made from vegetable proteins such as soy protein,pea protein,and gluten protein suffer from poor textural thermal stability and the weakening,destruction,or even disappearance of the fiber structure during thermal processing.This severely limits the application of high-moisture extrudates.Based on this situation,this thesis aims to improve the stability against pasteurization and sterilization while maintaining the fibrous structure of the extrudates.A soy protein isolate-wheat gluten blend(SPI-WG)with a weight ratio of 7:3 was used as the basic material to produce texturized protein.The improvement of thermal stability was carried out by enhancing viscous flow temperature(T_f),increasing texturization degree,promoting phase separation and protein crosslinking and so on.The details of the study are as follows:Firstly,the effects of starch/maltodextrin with different molecular weight distributions on the structure and thermal stability of the texturized protein were investigated.Large molecular weight wheat starch(WS),medium molecular weight maltodextrin(MD10),and small molecular weight maltodextrin(MD20)was added to the texturized protein system at three amounts.The effects of saccharides were characterized investigated by means of textural,rheological,confocal laser scanning microscopy,water activity,low-field NMR,p H and color difference analysis,etc.The addition of 5%MD10 to SPI-WG system had the best effect of improving of the structural regularity of the sample fibers and the thermal stability of the texture.The addition of 10%and 15%small molecule maltodextrin MD20 lowered the viscous flow temperature of the blends,improved the fluidity of the texturized protein during cooling,promoted the phase separation of the water-rich and water-poor phases,and improved the texturization degree of the fresh samples.15%WS improved the thermal stability of the texturized protein,indicating that wheat starch in the high-moisture system,when thoroughly gelatinized during subsequent sterilization,improved fibrous structure and thermal stability of the texturized protein.In this study,the Maillard reaction between starch/maltodextrin and protein was not significant.These results suggested that the promotion of phase separation by large molecular weight sugars changed the viscous flow temperature of the blends and significantly improved the texture and thermal stability of the texturized proteins.Based on the starch/maltodextrin study,eleven common commercial polysaccharides of three major categories with different gelling properties were selected in this thesis to investigate their effects on the texture and thermal stability of extrudates.Each of the eleven polysaccharides was added to the SPI-WG system at 3%of the total system weight to obtain extrudates.The addition of 3%non-gelling high-ester pectin,konjac gum and locust bean gum to the system helped improve the thermal stability of extrudate;locust bean gum had the most significant effect among them--it not only improved stability,but also slightly increased texturization degree after pasteurization.Although locust bean gum did not form gel during extrusion,its excellent thickening and phase separation promotion effects were able to increase the viscosity of the system,mediate the formation of phase separation structures,improve the water distribution and participate in the Maillard reaction.It increased the T_f of the material blend from 107.16°C(control sample)to 114.51°C,thus improved the fibrous structure of the extrudate and the thermal stability of the texture.The polysaccharide,which can form gel during extrusion,did not significantly improve the thermal stability of the texturized protein because the gel formed a mechanical barrier to the SPI-WG protein phase separation system,resulting in a loose structure of the product and poor stability.The synergistic effect between the expansion-contraction of the material and the thermal movement of the polymer during sterilization at high temperature and high pressure were also the cause.In order to improve the stability of extrudates with oil addition,two kinds of powdered oil with different emulsification stability were prepared,and liquid soybean oil was used as a control to study the effects of powdered oil.The liquid oil,due to its strong plasticizing effect,severely softened hardness and weakened the thermal stability of extrudate.The oil encapsulated by the wall material of sugar and protein mixture not only helped to improve the structure of the texturized protein,but also can improve its hardness and thermal processing stability.Microstructural and rheological results illustrated that liquid oil decreased protein material T_f to about 100°C due to strong plasticizing effect,promoted the material flow in the cooling phase and therefore improved the structural regularity of the texturized protein.However,the network structure became thin and fragmented due to the physical barrier effect of the oil,coupled with its low T_f value,thus the thermal stability was weakened.Among them,the restricted plasticizing effect of powder oil PO2 and oil-protein interaction made the T_f of the warming section increase from 107.16℃to 116.42～118.42℃,which was 10℃higher than the sample with the addition of equal amount of liquid oil,but still in the working temperature range of the extruder.And the system could still enter the viscous flow state,so the regularity of the texturized protein was less damaged,and the protein network structure is dense and thick,causing improved product hardness and thermal stability.The high encapsulation degree and stability of the powder oil helped to reduce the Maillard reaction caused by dextrin in the wall material,and the product has the best microscopic regularity and thermal stability.To further enhance the thermal stability of texturized proteins,common acid coagulants gluconic-δ-lactone(GDL)and salt coagulants magnesium chloride,calcium sulfate and calcium chloride,as well as sodium chloride,which may change the gelling of soy protein,were selected and added to the SPI-WG texturized protein extrusion system at 0.3%or 1.5%of the total system weight,respectively,and their effects on the thermal stability of wet texturized proteins were investigated.The results showed that the addition of coagulants could only improve the texture of fresh samples,but it was difficult to maintain the stability of the texturized protein thermal processing,especially sterilization.The addition of 0.3%Na Cl and 1.5%Na Cl to the SPI-WG system increased the T_f of the warming section of the system from 107.16°C to116.42°C and 122.78°C.The pasteurization stability of both sample hardness and texturization degree were also improved compared to Control60,and the texturization degree after sterilization with 0.3%Na Cl showed a significant increase to 1.68.Combining the hardness and texturization performance of the two thermal treatments,the 0.3%Na Cl extrudate had the best thermal stability.The study in this section also showed that the role of the enhanced T_f value of the product on the thermal stability of the texturized protein also depended on the microstructure of the material;too much phase separation and too much material stiffness,or T_f values above the processing temperature range,were not conducive to fibrous structure generation and stability enhancement of the texturized protein product.In the last part of the thesis,four schemes of starch-maltodextrin,polysaccharide,oil and salt ions were combined to optimize the thermal stability of the texturized protein.The best optimized sample 4 was produced with the formulation of 2.5%MD10,2.5%WS,0.3%Na Cl,0.3%konjac gum,34.4%SPI-WG,5.0%PO2,and 55.0%moisture content.The appearance,textural properties,and thermal stability of optimized sample 4 were close to that of beef,and the texturization degree after thermal treatment was about 1.4.With less beany taste and rich taste and texture of meat and fat,the overall acceptability of the sensory evaluation was good,with a score of 7.38 for the fresh sample and 7.76 after sterilization,making it particularly suitable for meat analogs.The above results indicated that the three pathways to improve thermal stability of the texturized protein,including enhancing the T_f of the product,reducing the influence of potential factors causing stability decrease,and enhancing extrudate’s texturization degree,were all feasible,whereas they all had some limitations.However,the combined use of multiple pathways for optimization not only enhanced the thermal stability of the product,but also led to a significant improvement in both the structure and flavor of the product. |
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