Soybean Dehulling Process and Technology Development
- Purpose and Significance of Soybean Dehulling
Soybeans contain 6%–8% hulls (seed coats), which consist of approximately 12.5% protein, 0.6% oil, and up to 40% crude fiber. The core purpose of soybean dehulling is to produce high-protein, low-fiber feed meal, while also delivering significant processing benefits: increased extraction yields and reduced consumption of steam, electricity, and solvent.
Dehulling processes are classified into pre-dehulling and post-dehulling based on where they occur in the processing flow, and into cold, hot, and warm dehulling based on processing temperature.
- Post-Dehulling Process
Post-dehulling is carried out on the defatted soybean meal stage, separating meal from hulls based on differences in particle size and bulk density through screening, grinding, and air classification. This process yields two products: approximately 25% meal with 39% protein content and 75% meal with 44% protein content (high-protein meal).
Advantages: No drying or conditioning required, resulting in low steam consumption.
Limitations: Only suitable for feed meal production. With the widespread application of extrusion-expansion technology in soybean pretreatment, post-dehulling has become considerably more difficult and is now rarely used.
- Pre-Dehulling Processes
3.1 Cold Dehulling Process
The standard cold dehulling process flow is: Drying → Tempering → Cracking → Hull-Kernel Separation → Conditioning. Soybeans received at extraction plants typically have a moisture content of 11%–13%. At this moisture level, hulls adhere strongly to the kernels and are highly tough, so drying and tempering are required to reduce hull-kernel adhesion.
(1) Drying Typically carried out in a drying tower, where air heated by burning gas or oil is used for counter-current drying. Hot air temperature is generally controlled at 60–80°C, with bean temperature reaching 50–60°C. Excessive temperatures can darken the color of oil and meal and reduce the NSI (Nitrogen Solubility Index) of the meal.
(2) Tempering Dried soybeans are transferred to tempering bins for equilibration and maturation, allowing moisture to distribute evenly within the beans and further shrinking of hulls and kernels. Residence time in the tempering bins depends on the amount of moisture removed and the drying rate, typically ranging from 3 to 7 days.
(3) Cracking A double-roll crusher with triangular, angled serrated grooves on the roll surface is used to crack soybeans into 6–8 pieces. After drying and sufficient tempering, hulls and kernels are easily separated.
(4) Hull-Kernel Separation The cracked material is fed to a double-deck Vibrating Screen or rotary sifter. Overs from the top deck (coarse hulls and larger bean pieces) have hulls removed by a suction device, while the bean pieces fall through and are returned for re-cracking. Overs from the bottom deck (medium-sized hulls and broken kernel pieces) are separated using a Kice aspirator. Throughs from the bottom deck (fine hulls and fines), accounting for approximately 5%–10% of the bean flow, are difficult to separate and are typically combined back with the kernels for downstream processing.
(5) Conditioning In cold dehulling, conditioning is essentially a softening step aimed at reducing roll vibration during flaking, minimizing fines in the flakes, and adjusting moisture content for extraction. Large-scale lines use horizontal rotary tube bundle heaters, while small to medium-sized operations use vertical stack cookers. Conditioning time is approximately 20 minutes, with discharge temperature at 50–65°C and moisture at 9%–11%.
3.2 Hot Dehulling Process
The hot dehulling process streamlines the dehulling procedure. In the Escher-Wyss process, soybeans are dried in a fluidized bed to approximately 80°C and then sent directly for cracking and hull-kernel separation. The Buhler and Crown processes preheat the soybeans prior to drying.
(1) Preheating Carried out in a conditioning tower equipped with 6–9 independent heating units containing horizontal steam-heated flat tubes. Soybeans descend by gravity, coming into contact with the heated tubes, raising their temperature and causing internal moisture to migrate to the surface while softening the hulls. Steam pressure generally does not exceed 0.1 MPa, with a residence time of 20–30 minutes.
(2) Drying
Fluidized Bed Drying (Escher-Wyss): Hot air at approximately 120°C passes through a perforated deck, fluidizing the soybeans and removing surface moisture, causing hulls to crack and loosen due to high temperature. Discharge temperature is 75–92°C, with moisture around 10%.
Jet Drying (Crown): A specially designed belt-type jet dryer is used, with hot air at 121–149°C passing upward through the bean bed from beneath the moving belt (speed ~0.05 m/s), creating a boiling/fluidized effect. Residence time is only 1–3 minutes, with discharge temperature around 88°C.
(3) Splitting A single-roll crusher is typically used to split soybeans into halves along natural cracks, followed by a Hammer Mill where high-shear impact separates hulls from kernels.
(4) Hull-Kernel Separation The split material is separated in a Kice-type aspirator, where kernels drop for further processing and hulls are pneumatically conveyed to a cyclone for collection.
(5) Cracking Similar to cold dehulling, kernels are cracked into 6–8 pieces. Since the kernels are hot, very few fines are generated during cracking.
(6) Conditioning Escher-Wyss performs conditioning in the fluidized bed, while Crown uses the aspirator—both effectively cooling the hot kernels. Discharge temperature is approximately 65°C.
3.3 Warm Dehulling Process (Evolution and Mainstream Choice)
The cold dehulling process was first promoted by Desmet at a crushing plant in Guangzhou. Subsequently, several engineering firms introduced hot dehulling processes. However, the hot air drying step in hot dehulling carries a risk of spontaneous combustion. After 2002, newly built crushing plants generally adopted the warm dehulling process—increasing the number of layers in the conditioning tower to enhance its dewatering capacity while eliminating the fluidized bed or jet dryer. Many plants that originally used cold dehulling have also been retrofitted to warm dehulling.
The warm dehulling process avoids the spontaneous combustion risk associated with the drying section while retaining the advantages of preheating and efficient hull-kernel separation, making it the current mainstream choice for soybean dehulling.
- Scikoon's Solutions in Soybean Dehulling
As a national high-tech enterprise established in 2003, Guangzhou Scikoon Industry Co., Ltd. has long been dedicated to the R&D and manufacturing of oilseed preprocessing equipment. The company is headquartered in the Huadu Airport Economic Development Zone in Guangzhou, with a self-built modern production facility covering over 13,000 square meters.
In the soybean dehulling sector, Scikoon provides comprehensive solutions covering key stages including cleaning, conditioning, cracking, and hull-kernel separation. Among these, the aspirator is one of Scikoon's core products. It features a built-in staggered steel tube arrangement that enhances material friction and impact, significantly improving hull removal and separation efficiency. Equipped with an energy-efficient circulating air system, it reduces energy consumption while removing impurities. Its sealed feeding and counterweighted discharge system ensures uniform and controlled feeding with zero dust emission. During separation, it also gently adjusts the temperature and moisture of the kernels, creating optimal material conditions for subsequent flaking and extraction.
Scikoon's dehulling equipment has been deployed in numerous large-scale oilseed processing projects both domestically and internationally, helping customers achieve efficient, safe, and clean soybean dehulling operations.
