Conformational Changes And Quality Control Of Peanut Protein During The High-moisture Extrusion Process

High-moisture extrusion technology for moisture contents higher than 40%has the advantages of lower energy input,no waste discharge,high efficiency,and higher quality of the texturized products,so is now considered as one of the new developmental trends in the field of food extrusion.However,for more than 30 years,high-moisture extrusion technology is still in the research stage.The main reason is that the high-moisture extrusion process is a multi-input and multi-output process,which has been considered as a“black box”.Controlling the extrusion process precisely is still very challenging since the mechanism for forming a fibrous structure is not clear.Moreover,there is a lack of meat substitutes product design that should be popular with the consumers.Thus,the objectives of this study were to provide a new insight into the high-moisture extrusion process from the aspect of energy input ways;to make the whole extrusion process visible using a multi-scale method to show the process of forming meat-like fibrous structure using 2D and 3D perspectives;to investigate the fibrous structure improvement mechanism of peanut protein by TG enzyme induction and polysaccharide modification;to explore the the texturization behavior of peanut-soybean/wheat protein mixtures during high-moisture extrusion process;to design and develop a new series of peanut protein-based meat substitutes in line with Chinese consumption habits by designing the high-moisture extrusion process;to establish the mathematical models for the fibous structure forming process of peanut protein during the high-moisture extrusion process using a hierarchical augmented principal component analytical method.The main conclusions are as follows:1.Extrusion energy input had a significant impact on system response parameters,especially the die pressure and specific mechanical energy(SME)with a variation range of 1.20~4.80 Mpa and557.24~1135.67 kJ/kg.Shear ways played a decisive role in the die pressure and die temperature,followed by the extrusion temperature,and finally the moisture contents.SME was mainly determined by the moisture contents and the energy input intensity.Shear ways can significantly improve the fibrous structure(fibrous degree higher than 1.20).Extrusion temperature was the decisive factor in the tensile properties and springiness.While the moisture contents played a more important role in the color values and the hardness.Under different energy input ways,the relationship of the system response parameters and properties of the high-moisture texturized peanut protein would be changed.2.During the high-moisture extrusion process of peanut protein powder(PPP),the protein molecules underwent dramatic structural changes and unfolded in the extruder barrel,which created favorable conditions for molecular rearrangement in the subsequent zones.From the mixing zone to the melting zone,the protein molecular structure was opened,and closed to each other(aggregation),and the original intermolecular forces,especially the stronger forces,were destroyed.At the same time,the?-turn structure and the random coil structure transform to the?-sheet structure.It was confirmed that the meat-like fibrous structure started to form at the junction of the die and the cooling zone,and that this structure was caused by the phase separation and rearrangement of protein molecules in the cooling zone.From the die to the cooling zone,the phase separation occurred and the protein molecules were reordered to form a new protein conformation mainly maintained by the interaction of hydrogen bonds and disulfide bonds.The ratios of?-helix and?-sheet were 45.05%and 36.26%,respectively.It created the favorable conditions for stabilizing and strengthening the newly formed protein conformation after the cooling zone,which was also the key to form a fibrous structure.Finally,the fibrous structure of high-moisture texturized peanut protein was mainly maintained by non-covalent bond interaction(mainly hydrogen bond and disulfide bond interaction)with?-helix>?-sheet>?-turn>random coil.Of the two main peanut proteins,arachin played a greater role in forming the fibrous structure than the conarachin,especially those subunits of arachin with a molecular weight of 42,39,and 22 kDa.3.The TG enzyme can impact the formation of the fibrous structure by affecting the speed of peanut protein-gel forming.In the extruder barrel,TG enzyme can promote the expansion,aggregation,and cross-linking of protein molecular chains(mainly arachin),help to destroy the hydrogen bonds and disulfide bonds,but enhance the hydrophobic interaction.The conarachin molecular chain was flexible and the protein molecules were more disorderly arranged,and the protein secondary structure was transformed from?-helix to?-turn and random coil.In the die,when the protein molecules were rearranged,the addition of 0.1%or 0.2%TG enzyme can avoid the rebound phenomenon of the protein molecular chains,promote the formation of new hydrogen bonds and disulfide bonds.The structure is dominated by?-sheets.From the cooling zone to the extrudate,the TG enzyme was beneficial to protein molecular chain stretching and forming large molecular weight protein subunit(66 kDa),while the main force for maintaining the fibrous structure is converted from the interactions of hydrogen bonds and disulfide bonds to hydrogen bonds and hydrophobic interaction,and the four protein secondary structures were present with?-sheet>?-helix>?-turn>random coil.4.For the peanut protein/polysaccharide mixtures,0.1%carrageenan could significantly improve the tensile resistant force(about 1.92 kg),but it was not conducive to the fiber orientation.0.1%sodium alginate could significantly improve the fibrous degree(up to about 1.24)and springiness,which was conducive to the fiber orientation,and it would also increase its tensile resistant force,but the hardness and chewiness increased significantly.The larger the amount of wheat starch(0~8%),the smaller the fibrous degree was,and the hardness and chewiness were both significantly reduced.Carrageenan,sodium alginate and wheat starch can protect the molecular structure of conarachin and arachin by increasing the thermal transition temperature T_p or increasing the enthalpy change?H,slowing down the thermal transition speed of peanut protein.The polysaccharide facilitates protein-lipid interactions,promoted the cleavage of intramolecular disulfide bonds,and stabilized the new conformation.Three polysaccharides,especially the wheat starch,can significantly increase the consistency of the blend system,promote protein molecule aggregation,interfere with its flow behavior,change the main force maintaining the protein structure by enhancing the hydrophobic interaction.Three polysaccharides all had a larger effect on the protein subunit with a molecular weight of 22 kDa,promoting the degradation of protein subunits with the molecular weight of 42 kDa and 39 kDa,while wheat starch can also promote the synthesis of large molecular weight protein subunits(66 kDa).The three polysaccharides can promote the opening of the?-helix and gradually transforming into a?-turn and random coil structure.When blended with 0.1%carrageenan,the four protein secondary structures were present with?-sheet>?-helix>?-turn>random coil in the extrudate.When blended with 0.1%sodium alginate or2%wheat starch,the order was?-sheet>?-turn>?-helix>random coil.5.Extrusion response parameters(SME,die temperature and die pressure,etc)can be affected by the type and ratio of proteins in the material,which was related to the total protein concentration in the mixtures and mass flow behaviour upon the thermal and mechanical energy input during the extrusion.With soybean protein isolate(SPI)added,it increased the total protein concentration and enhanced the gel property of the melt,which led to a smoother and denser structure in the extrudates.While with wheat gluten(WG)added,it promoted to form a melt with a higher viscosity during the extrusion,showing a rougher and looser structure in the extrudates.Compared with WG,SPI played a major role for the texturization behavior of PPP-SPI-WG mixtures during high moisture extrusion.Both SPI and WG are required to improve the textural quality of texturized peanut protein.Furthermore,the extrudates with PPP 80%,SPI 10%and WG 10%showed rich fibrous structure,lower hardness,and higher springiness.6.The prediction model of the qualities of high-moisture texturized peanut protein can achieve accurate prediction of the color(the fitting degree was above 0.838),and the fitting degree of tensile resistant force,hardness and chewiness were all above 0.905.The system response parameter prediction model can accurately predict the system response parameters(SME,torque and MFR)and the fitting degree was around 0.900.For the model indirectly predicting the qualities of high-moisture texturized peanut protein through system response parameter,the fitting degree of the qualities can be improved,which was beneficial to improve the prediction accuracy.The prediction model of the peanut protein conformational changes in each extrusion zone can be correlated with each other,which can predict the variation of peanut protein conformation during the high-moisture extrusion process.7.By improving the extrusion equipment,two new meat substitutes,a peanut protein-based dry tofu and a peanut protein-based vegetarian sausage,have been successfully developed with a higher springiness(about 0.90)and comparable textural properties,respectively compared with ham sausage.These two new meat substitutes can be shaped,flavored,and colored during the extrusion process by modifying the equipment,which would also greatly shorten the processing procedure without producing pollution or increasing nutrient losses.