Research advances on food extrusion equipment, technology and its mechanism

Abstract: Extrusion technology has a potential for becoming the most important food processing technology in future which can be exploited. Food extrusion technology with the characteristics of high efficiency, energy saving and pollution free, can be used for processing the extruded texturized vegetable protein and puffing food. The aim of this review was to give the detailed outlines about the research advances of food extrusion equipment and technologies, and the relationship and mechanism between inputting energy and protein conformational transition during food extrusion process. The tendency on the development of food extraction technology was proposed, with the following 3 areas being advanced: (1) Supercritical fluid extrusion (SCFX), two-stage or multi-stage extrusion and the combination of extruder and 3D (three-dimensional) printer, with rich nutrient, high efficiency and low energy consumption as characteristics, have attracted great attention. In the future, through improving the construction of extrusion equipment, and combining infrared device, rheometer, Raman spectrometer and other equipment, it will help extruder to improve the level of intelligent controlling with the advantage of versatility and visibility. Through the software simulation system, the whole extrusion process can be fully controlled. (2) The extrusion way and inputting energy were the key points of extrusion process. For high moisture extrusion, temperature as well as moisture and die aperture significantly influenced the quality of products, while low moisture extrusion usually required high shear force. Extrusion assisted with supercritical carbon dioxide (SC-CO2) did not need high extrusion temperature and high shear force, and the nutrient loss was small. For two-stage or multi-stage extrusion, the extruders were connected in series or in parallel and the inputting energy could be controlled according to the material properties. When combined with 3D printer, the shaping speed of the extruder would be faster, but the extruder required high extrusion temperature to run. Currently, controlling the way or the amount of inputting energy in the extrusion process was the main problem to be solved, which was also the key point for the process amplification. (3) It was necessary to build the theoretical model related to the inputting energy, the change of food structure and the quality of products, in order to realize the precise and scientific regulation for extrusion process. Extrusion processing could improve or change protein quality. In the extrusion process, the temperature was the promoting power for the stable transition of protein conformation, the pressure was the key factor for the formation of porous or fibrous structure, and the shear force promoted the molecular chains to unfold, break down and interact with each other. The SC-CO2 played an important role in the process of protein conformational transition, which helped to reduce the inputting energy. In the two-stage or multi-stage extrusion process, the protein conformational transition happened respectively, which improved the work efficiency. But high inputting energy was required for changing the rheological properties of vegetable protein in the process of extruding with 3D printer.