Principles of Soybean Flake Expansion
- Overview of the Expansion Process
Expansion is a process in which, under forced feeding and the introduction of high-temperature, high-pressure steam, soybean flakes undergo intense mixing, heating, compression, agglomeration, and gelatinization within the confined space of an expander chamber. This results in significant structural changes, transforming the flakes into an expanded material.
The main components of the expander include: the barrel, screw shaft, shear pins, steam injection pins, hydraulic die, hydraulic system, bearing lubrication system, and drive unit.
- Working Principle of Expansion
Inside the expander chamber is a screw shaft with intermittent, non-continuous screw flights. Shear pins extend from the inner wall of the barrel into the gaps between adjacent screw flights. During operation, the rotating screw flights and stationary shear pins work together to thoroughly mix and agitate the material within the barrel.
The screw pitch gradually decreases from the inlet to the discharge end, causing the material to occupy a progressively smaller volume as it moves forward. Under the action of the variable-pitch screw, the material is continuously compressed and kneaded, promoting better adhesion. Steam injection nozzles are installed on the barrel wall, injecting direct steam at a pressure of 4–6 bar into the material to increase its moisture content, raise its temperature, and soften it.
Soybean flakes are forcibly and uniformly fed into the expander by a Screw Conveyor. Inside the chamber, the flakes are subjected to intense compression as they are propelled forward by the screw shaft, resulting in a continuous increase in material density. Additionally, frictional heat generated between the material, the screw shaft, and the chamber wall, along with the cutting and mixing action of the shear pins and screw flights, as well as the injection of direct steam, collectively expose the material to thorough mixing, heating, pressurization, agglomeration, and gelatinization—leading to structural transformation.
The material is finally extruded as continuous rope-like strands through the die openings at the discharge end of the expander. As the material is conveyed by the screw shaft toward the conical die head, under the combined effects of compression, friction, and direct steam injection, its temperature rises to 105–115°C and its moisture content increases to 11%–13%. At this point, the internal pressure within the chamber is 13 to 40 times higher than atmospheric pressure. The moment the material is extruded through the die openings, the pressure drops abruptly from high to atmospheric. This sudden pressure reduction causes moisture to rapidly evaporate from the material's structure. As the water vapor escapes, it penetrates the material, forming numerous pores inside. The material thus undergoes intense expansion, resulting in expanded particles characterized by countless microscopic channels and surface cracks.
Under the high-temperature cooking and intensive mixing/compression conditions inside the expander, proteins are softened and gelatinized, transforming into an elastic, gel-like substance. This gel binds the material particles together, and by virtue of its elasticity, facilitates the formation of a porous structure during the expansion process.
