Role Of Water In Extrusion Texturization For Vegetable Protein

Food extrusion is a multifactors process involving temperature, pressure, mechanical shear, etc. Water is the most important factor affecting successful extrusion operation and the structure and property formation of extrudate. Feed moisture content is the main basis for demarcating high-moisture extrusion and low-intermediate-moisture extrusion. Compared with the textured protein extruded at low-intermediate-moisture content, the meat analogue extruded at high-moisture content has been paid more attention due to its visual texture and fibrous structure resembling chicken breast. Therefor, the hypothesis of―membranous gas cavities‖was proposed to explain the formation mechanism for fibrous structure in high-moisture textured protein, in which the contribution of water change & role to forming the fibrous structure in final product has been emphasized. Inspired by previous related researches and the hypothesis of―membranous gas cavities‖, feed moisture content was separated from numerous extrusion process parameters to further analyze its effects during extrusion texturization for vegetable protein. The objectives of our research are to investigate the essential differences between high-moisture extrusion and low-moisture extrusion in producing protein meat analogue, and further to propose a mechanism model for explaining the forming process of fibrous structure for high-moisture textured vegetable protein.In this research, soybean protein isolate (SPI) was processed using a pilot-scale, co-rotating, intermeshing, twin-screw extruder typed DSE-25 (Brabender GmbH & Co., Germany) under 28-60% moisture content and 140-160℃cooking temperature. By means of on-line sensor detecting and theory model deduction, the effects of moisture content on system parameters such as motor torque, pressure drop, in-line viscosity, residence time distribution (RTD) and specific mechanical energy (SME) were analyzed. By using differential scanning calorimetry (DSC) and low field nuclear magnetic resonance (LF-NMR) techniques together, the change of water states and distribution in textured soybean protein (TSP) extruded under different conditions were investigated. By adopting multiplicate analysis methods and techniques such as sensory, texture, protein solubility, electrophoresis, infrared spectra etc., the effects of moisture content on the product properties (including appearance, texture, chemical cross-linking, protein subunits (7S/11S), protein secondary structure) were analyzed. After dead-stop operation, the samples from different zones of the extruder were obtained to explore the effects of moisture content on protein properties within the extruder barrel. Finally, the canonical correlation analysis between water with different states or mobility, system parameters, product texture and protein properties were investigated, and the specific roles of water with different sates or mobility during extrusion cooking were analyzed systematically. The main conclusions drawn from this research were listed as follows:(1) After extrusion process, soybean isolate protein (SPI) cross-linked and aggregated into macromolecule polymers, resulted in solubility decreasing. The structure of TSP was hold collectively by hydrophobic interactions, hydrogen bonds, disulfide bonds and their interactions, and the contribution of non-covalent bonds outweighed covalent bonds. The four kinds of protein secondary structure all existed in TSP, and the order of their relative percentage was:β-sheet,β-turn>unordered>α-helix.(2) The critical quantity of freezable or mobile water in SPI raw materials was 35%-38% (wet base), which may be the scientific basis of moisture boundary for demarcating low-intermediate moisture extrusion and high moisture extrusion. Above the critical quantity,the water added into the raw materials was mainly converted into the freezable or mobile water in final product. In the extrusion cooking pro- cess, it is the freezable or mobile water that affects the extrusion process and the properties of texturized product. Water performed several important functions such as plastification, reducing- viscosity, con- ducting-heat, cooking, shaping-assist and reaction reagent, etc.(3) There was plenty of freezable water in the texturization process for high moisture extrusion. Water could reduce the viscosity and residence time of SPI dough within extruder, decrease the effect of mechanical energy acting on melt dough, finally the color of extrudate was light. Increasing water content could reduce the degree of aggregation and cross-linking, increase the interactions between disulfide bonds and hydrogen bonds and between disulfide bonds and hydrophobic interactions, increase the ratio of 7S to 11S, consequently increase the degree of fiberization for TSP. These may be the essential reasons why high-moisture extrusion has unique advantage over low-intermediate-moisture extrusion to produce protein meat analogue with fibrous structure.