At the same time, the amount of residual oil in soybean meal also reflects the level of processing technology and production management, and indirectly demonstrates the differentiated competitiveness among enterprises. Therefore, it is highly valued by soybean oil processing enterprises.

1. Pretreatment Section

Pretreatment includes several steps such as cleaning, soybean conditioning, cracking and dehulling, flaking, extrusion, etc., which are closely related, and each process adjustment will affect the next process.

1.1 Cleaning

Soybean maybe mix with a certain amount of impurities during harvesting, transportation and storage. These impurities are mainly straw, stems and leaves, soil, metal objects, etc. The purpose of this section is to remove impurities from the oilseed.

If these impurities are not removed, on the one hand, the impurities will adsorb a certain amount of oil and remain in the cake, thereby reducing the oil extraction efficiency. On the other hand, impurities will be carried to the next section, affecting the processing effect of other sections. For example, the inclusion of iron in the raw material will not only affect the cracking and flaking effect, but also shorten the service life of the equipment and even damage the equipment.

1.2 Conditioning
The soybean hull is thin and has strong binding force with the seed kernel. Even broken under the action of external force, the hull may still adhere to the broken kernel, and it is difficult to be completely removed. The conditioner can regulate the moisture and temperature of soybean, which reduces the moisture content of soybean by 1% to 2%, and raises the temperature to 55-65 °C, so that the soybean hull becomes crisp and easy to crack. After the subsequent cracking, dehulling is easier and the efficiency is higher.

1.3 Cracking

After the cracking section, the soybeans are broken into 6-8 petals with relatively uniform size, and soybean hulls are easily detached from the surface of the beans. If the bean hull is not removed, it is easy to absorb oil during the extraction section, which leads to an increase of residual oil in soybean meal. In addition, in the large-scale edible oil production, the removal of more than 80% of bean hull can greatly improve the processing capacity of the equipment, so it is necessary to use the aspirator for dehulling after cracking.

The soybean hull content is 6%-8%, and the oil content in the soybean hull is generally less than 1%. The main component of the soybean hull is cellulose. If the dehulling is insufficient, part of the bean hull will adhere to the surface of the flakes after flaking, thus forming an isolating membrane which can hinder the direct contact between solvent and oil in the bean kernel when extracting. This is not conducive to oil extraction and will result in an increase of residual oil in soybean meal.

On the contrary, if soybeans are fully dehulled, the above drawbacks can be overcome.

1.4 Flaking

The flaking process is to squeeze the oilseed cells against each other by the pressure between the flaking rolls. So that part of the cell walls are destroyed, and the oilseed turn into thin flakes with large surface area. Thereby the flaking process can shorten the path of oil extracting from oilseed.

In theory, the larger the surface coefficient of the contact between the flakes and solvent, the better the extracting effect. If the flake is too thin (<0.3 mm), the powder degree will increase, which will affect the cyclical penetration of the miscella, eventually causing the residual oil content in soybean meal to increase. If the flake is too thick (>0.5 mm), the cell tissue will not be completely destroyed. The solvent cannot be quickly penetrated into the interior of the flakes, and the oil inside the flakes cannot be rapidly diffused outward. Therefore, the thickness of flakes should be checked frequently during the production process, and the thickness of flakes should be 0.3-0.4 mm.

1.5 Extrusion/ Expansion

The expanded oilseed flakes have good porosity and permeability, and the permeation rate of the miscella is significantly improved. The amount of penetrative miscella per square meter per hour can reach 40-50 cubic meters. Therefore, good permeability can cause the residual oil rate to decrease accordingly. After the expansion, the solvent is not easily retained when the flakes run to the leaching section of the extractor. So the load of the solvent recovered by the desolventizer and drying system is reduced, and the heat and steam consumption are also reduced.

In the actual production process, parameters such as moisture, expansion temperature, and steam volume should be adjusted to ensure the best expanding effect.

2.3 Purity of Solvent
During the extracting process, it is important to ensure that the solvent is pure. In general, there are two main reasons for causing the solvent to be impure:
① The evaporator is flooded, causing the solvent to carry oil into the water separation tank. After the water is separated, the solvent turns yellow, so that the solvent is not pure.
② The positive pressure of the water separation tank is too large, causing the solvent to mix with water. Mixing oil or water in the solvent will reduce the effect of the last fresh solvent spray.

To ensure the purity of fresh solvent, the following technical measures should be taken:
① Ensure that the steam pressure used in production process is stable, and keep the evaporator working within the specified temperature and vacuum range.
② Increase the effective volume of the water separation tank to enhance the water separation effect.

2.4 Amount of Solvent
The amount of solvent is usually measured by the solvent ratio, which is the ratio of the mass of the solvent used per unit time to the mass of the extracted material.
The large solvent ratio has a certain effect on reducing the residual oil in soybean meal, but the concentration of the miscella is lower, which increases the load on the evaporation system and the solvent recovery system. So the steam consumption is larger.

The small solvent ratio, that is, the amount of fresh solvent is reduced, and the concentration of miscella increased, so the driving force of the mass transfer process is insufficient, resulting in incomplete extracting and high residual oil in soybean meal. Therefore, it is important to choose a reasonable solvent ratio under the premise of ensuring residual oil index.

3. Conclusion

By analyzing each step of the pretreatment section and extracting section, there are many factors affecting the residual oil content in soybean meal. These factors are complicated and mutually influential. The operator should make a targeted analysis according to the actual production situation, and make timely adjustments to reduce the residual oil content in soybean meal and improve the economic benefits of the enterprise.

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