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Production test study on full fat soybean extrusion processing
Full-fat soybean is a product obtained through the 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 with 15.6 MJ/kg and microwave-treated soybeans with 15.4 MJ/kg. The comprehensive amino acid digestibility of extruded full-fat soybean is as high as 92.5%, while lysine digestibility reaches 90.6%, which is significantly higher than that of soybean meal. Although the nutrient composition of full-fat soybean is similar to raw soybeans, the moisture content is lower due to heat treatment, leading to relatively higher concentrations of other components. However, methionine remains insufficient. The nutritional profile of expanded soybeans is comparable to other raw materials (see Table 1).
Cheva-Lsakaru and Tangtaweeipat compared the effects of different processing techniques on meat chickens. Their study indicated that extrusion of full-fat soybeans offers two main advantages: first, it effectively destroys trypsin inhibitors, with a damage rate exceeding that of steam and drying by 11.2% and 74.5%, respectively; second, it improves feed conversion rates significantly compared to soybean meal, steam-processed, and dry-fried full-fat soybeans.
Based on laboratory tests, this paper explores how the structure of the extruder, raw material preparation, and key process parameters during extrusion affect the final extrusion outcome. The goal is to identify a more stable and efficient processing method while improving both yield and quality.
Materials and Methods:
Test Materials: Soybeans and raw soybean meal were provided by Haiyan County Tongyuan Feed Factory in Zhejiang Province. Their main components are listed in Table 2.
Main Reagents: All reagents used were of analytical grade.
Equipment: A 135 dry extruder was used, with some parts modified, including the die, which was externally heated.
Analytical Methods:
- Crude fat: Determined using Soxhlet extraction.
- Crude protein: Measured via the Micro-Kjeldahl method.
- Pepsin enzyme assay: Followed GB8622-88 standards.
- Yield: Calculated in kg per hour.
Results and Discussion:
The Influence of the Screw Pressure Ring on Extrusion Performance
During the extrusion process, the pressure ring plays a critical role in controlling flow, increasing local pressure, and influencing the stability of the process and yield. This experiment tested raw soybeans (ground through a 2.5 mm sieve, moisture adjusted to 15.5%) and raw bean cake (ground through a 2.5 mm sieve, moisture at 18%) at temperatures of 160±5°C and 140±5°C, respectively. Urease activity (UA) and yield were used as evaluation indicators. The results showed that the number and diameter of pressure rings significantly affected UA and yield. Using three pressure rings (φ135 mm, φ130 mm, φ135 mm) resulted in low UA but poor yield and unstable extrusion. A φ130 mm pressure ring caused excessive UA due to insufficient material processing. A φ138 mm pressure ring was found to be more balanced, though UA still exceeded the standard slightly. In subsequent tests, a φ138 mm pressure ring was placed in the middle of the screw.
For raw bean cake, since its initial UA was much lower, only one φ130 mm pressure ring was sufficient to meet the standard. This setup was used in further tests.
The Influence of Screw Composition on Extrusion Effect
The combination of the screw structure affects the residence time and force applied to the material during extrusion. A 5-section single-threaded screw was used for the previous pressure ring test. To investigate the effect of screw design, the first section near the die was replaced with a double-threaded screw. The results showed that using a double-threaded screw reduced yield but also significantly lowered UA. Considering both quality and output, a combination of one double-threaded and four single-threaded screws was found to be optimal and used in subsequent tests.
The Influence of Raw Material Particle Size on Extrusion Effect
Particle size affects surface area, physical properties, and heat/mass transfer during extrusion. Tests were conducted on whole soybeans, coarsely ground soybeans (6.0 mm sieve), finely ground soybeans (2.5 mm sieve), coarsely crushed bean cakes, and finely divided bean cakes. Results showed that finer grinding led to better reduction of anti-nutritional factors. For example, whole soybeans produced extrudates with high UA, but fine grinding brought UA within standard limits. Although finer grinding increases power consumption, it also significantly improves yield, reducing overall energy use. Additionally, smaller particle size reduces machine wear. Therefore, fine grinding is recommended in practice, provided the extruder is properly adjusted.
Effect of Raw Material Moisture on Extrusion
As shown in Table 6, increasing moisture content improved the inactivation of anti-nutritional factors and increased yield. However, excessively high moisture can lead to higher moisture content in the final product, which is not ideal for storage. It is recommended to maintain soybean moisture at around 15% and bean cake moisture at about 18%.
Effect of Extrusion Temperature on Extrusion
Table 7 shows that extruding raw soybeans at 170±5°C and raw bean cake at 140±5°C resulted in products with UA values within acceptable limits.
Conclusion
For the extrusion of full-fat soybeans, we used a self-made die and an external heater (2kW), selected a φ138 mm pressure ring, and employed a double-threaded screw in the first section and single-threaded screws in the rest. The raw material was finely ground (2.5 mm sieve), moisture adjusted to 15%, with 0.5% additive added during water adjustment. The extrusion temperature was controlled at 170±5°C, producing full-fat puffed soybeans with UA < 0.4, achieving an output of 800–1000 kg/h using a PHG-135 extruder.
For raw bean cake extrusion, the same equipment was used except for the φ130 mm pressure ring. The raw material was also finely ground, moisture adjusted to 18%, with 0.3%–0.5% additive. The extrusion temperature was set to 150°C, resulting in a high-quality product with an output of 1000–1200 kg/h.