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Production test study on full fat soybean extrusion processing
Full-fat soybean is a product obtained through heat treatment of whole soybeans. According to Wisemn's research, among various heat treatments, extruded full-fat soybean has the highest metabolizable energy content at 17.9 MJ/kg, followed by baked soybeans at 15.6 MJ/kg and microwave-treated soybeans at 15.4 MJ/kg. The comprehensive amino acid digestibility of extruded full-fat soybean reaches 92.5%, while lysine digestibility is 90.6%, which is higher than that of soybean meal. The nutrient composition of full-fat soybean is similar to that of raw soybeans, but due to heat treatment, its moisture content is lower, and other components are relatively more concentrated. However, methionine remains insufficient. For detailed nutritional data, refer to Table 1.
Cheva-Lsakaru and Tangtaweeipat compared the effects of different processing techniques on meat chickens. They found that extrusion of full-fat soybeans offers two key advantages: first, it effectively destroys trypsin inhibitors, with a damage degree better than steam and drying processes, increasing by 11.2% and 74.5%, respectively; second, the feed conversion rate is significantly higher compared to soybean meal, steam-processed full-fat soybeans, and dry-fried processed full-fat soybeans.
Based on laboratory tests, this study investigates how the structure of the extruder, raw material preparation, and main process parameters during extrusion affect the extrusion outcome. The goal is to identify a more suitable and stable processing technology, while also improving the quality and yield of the final product.
Materials and Methods
Test Materials and Main Reagents
Soybeans and raw soybean meal were provided by Tongyuan Feed Factory in Haiyan County, Zhejiang Province. Their main components are listed in Table 2. All reagents used were of analytical grade.
Main Equipment
The primary equipment used was a 135 dry extruder, with some parts modified, including a die and external heating system.
Analytical Methods
Crude fat was determined using the Soxhlet extraction method. Crude protein was analyzed using the Micro-Kjeldahl method. Pulse enzyme activity was measured according to GB8622-88. Yield was calculated as kg per hour per Taiwan (a unit of measurement).
Results and Discussion
Influence of the Screw Pressure Ring on Extrusion Effect
During the extrusion process, the pressure ring plays a crucial role in regulating flow and increasing local pressure, which greatly affects the stability and yield of the extrusion. This experiment tested different pressure rings under varying conditions. Results showed that the number and diameter of pressure rings significantly impact the urease activity (UA) and yield of the extruded product. Using three pressure rings of φ135 mm, φ130 mm, and φ135 mm led to low UA but poor yield and unstable extrusion. A φ130 mm pressure ring resulted in high UA, indicating insufficient processing. A φ138 mm pressure ring provided a more balanced result, and was selected for subsequent tests.
For raw soybean meal, only one φ130 mm pressure ring was needed to meet the standard UA level.
Influence of Screw Composition on Extrusion Effect
The screw design influences the residence time and force applied to the material during extrusion. Tests were conducted using a five-section single-threaded screw. To evaluate the effect of screw composition, a double-threaded section was used near the die. Results indicated that while the yield decreased, the UA reduction was significant. A combination of one double-threaded and four single-threaded screws was found to be optimal, and was used in subsequent tests.
Influence of Raw Material Particle Size on Extrusion Effect
Particle size affects the surface area and physical properties of the material, influencing heat and mass transfer during extrusion. Tests were conducted with whole soybeans, coarsely ground soybeans (6.0 mm sieve), finely ground soybeans (2.5 mm sieve), and coarsely and finely ground bean cakes. As particle size decreased, anti-nutritional factors were more effectively removed. Fine grinding improved UA levels and increased yield, despite higher power consumption. However, overall power usage remained stable, and machine wear was reduced. Therefore, fine grinding is recommended in production, provided the extruder is properly adjusted.
Effect of Raw Material Moisture on Extrusion
Table 6 shows that higher moisture content improves the inactivation of anti-nutritional factors and increases yield. However, excessive moisture may lead to higher moisture content in the final product, affecting storage. It is recommended to maintain soybean moisture at around 15% and raw bean cake moisture at about 18%.
Effect of Extrusion Temperature on Extrusion
Extrusion temperature also significantly impacts the extrusion outcome. Soybeans were extruded at 170±5°C, and raw bean cake at 140±5°C. Both achieved acceptable UA levels.
Conclusion
For full-fat soybean extrusion, we used a self-made hole mold and an external heater (2kW). A φ138 mm pressure ring was chosen, with the first section being a double-threaded screw and the rest single-threaded. Raw materials were finely ground (φ2.5 mm sieve), moisture adjusted to 15%, with 0.5% additive added. Extrusion temperature was controlled at 170±5°C, producing full-fat puffed soybean with UA < 0.4. Output ranged from 800 to 1000 kg/hr using a PHG-135 extruder.
For raw bean cake extrusion, the same setup was used except for a φ130 mm pressure ring. Raw materials were also finely ground, with moisture adjusted to 18%, and 0.3–0.5% additive added. Extrusion temperature was set to 150°C, resulting in high-quality products with output reaching 1000–1200 kg/hr.