Rapeseed oil production begins in the field, but the factors that truly determine oil yield and crude oil quality are often not the extractor itself – they are the few dozen minutes before the seed enters the extractor.
Rapeseed contains approximately 38%–48% oil on a dry matter basis. But seed coming straight from the silo cannot be sent directly to the extraction section. It carries impurities, uneven moisture, and low temperature. If pretreatment is skipped, the extractor will face poor solvent penetration, accelerated equipment wear, and fluctuating crude oil quality.
A complete rapeseed pretreatment process consists of three core stages: cleaning → drying → heating. Each stage has clear process targets and quantitative standards.
I. Cleaning: Removing Impurities and Protecting Downstream Equipment
Cleaning is the first step in pretreatment. The goal is simple: remove everything that is not rapeseed.
During harvest and transport, rapeseed picks up various impurities: metal fragments (from harvesting machinery wear), stones and soil (from field harvest), straw and pod shells (from incomplete threshing), and dust (from storage and conveying). If not removed in advance, these impurities cause three consequences:
- Equipment damage: Metal fragments and stones entering the screw oil press or extractor accelerate wear on screw shafts, pressing cages, and conveying components.
- Lower crude oil quality: Soil and dust increase suspended solids in the crude oil, adding burden to downstream degumming and filtration.
- Reduced extraction efficiency: Straw and pod shells occupy valuable space inside the extractor, lowering the oil content per unit volume of material.
Typical cleaning equipment configuration:
- Magnetic separator: Removes ferromagnetic metal fragments
- Vibrating screen: Separates stones, soil, and broken pod shells by particle size
- Destoner: Uses density differences to separate stones
After cleaning, the impurity content of rapeseed is typically controlled below 1% before it can proceed to the next stage.
II. Drying: Reducing Moisture to Below 8% – Not “Drier Is Better”
Drying is the most misunderstood stage of pretreatment. Many assume “drier is better,” but for rapeseed, the precision of moisture control directly affects downstream pressing and extraction efficiency.
Why Is the Drying Target Set at 8%?
The safe storage moisture for rapeseed is typically 8%–9%. If moisture exceeds 10%, the seed is prone to mould during storage, and free fatty acids rise rapidly. But moisture should not be reduced too far either – excessive drying makes the seed more brittle, causing it to shatter in the downstream flaking mill and generate excessive fines, which actually reduces extraction efficiency.
8% is a balance point: it ensures storage safety while retaining the plasticity needed for flaking. Industry studies show that for every 1% moisture above the 6%–8% range, oil yield can decrease by nearly 0.5%.
Quantitative Impact of Drying on Extraction Efficiency
A study presented at the 13th International Rapeseed Congress specifically examined the effect of drying rapeseed before flaking and cooking on pressing and solvent extraction performance. The study found that properly dried seed achieved a press cake residual oil content as low as 1.7%, while undried seed had noticeably higher residual oil. Drying reduced the plasticity of the press cake, allowing greater compression and higher oil recovery.
Typical drying equipment:
- Fluidised bed dryer: Uses hot air to suspend and rapidly dehydrate seed, suitable for continuous production
- Rotary dryer: Suitable for large-scale processing with a wide temperature control range
- Plate dryer: Suitable for small to medium scale with flexible operation
After drying, rapeseed moisture should be stabilised at 7%–8%, creating optimal conditions for subsequent heating and flaking.
III. Heating: 70–90°C – More Than Just “Applying Heat”
Heating is the most technically demanding step in pretreatment. On the surface, it simply raises the seed from ambient temperature to 70–90°C, but the process logic behind it is far more complex than “applying heat.”
Three Purposes of Heating
First, soften the cell structure to prepare for flaking. Cold seed shatters easily when entering the flaking mill, producing excessive fines. Heating to 70–90°C softens the cell walls and increases plasticity, producing flakes that are more uniform and consistent in thickness. Uniform flake thickness is a prerequisite for uniform solvent penetration.
Second, reduce oil viscosity and improve extraction kinetics. Oil viscosity decreases at higher temperatures, improving flowability. When heated seed enters the screw oil press for pre-pressing, oil is more easily expelled; when it enters the extractor, solvent penetrates the cell interior more readily.
Third, inactivate endogenous enzymes to protect crude oil quality. Rapeseed contains endogenous enzymes such as phospholipase and lipoxygenase. If not inactivated, these enzymes catalyse phospholipid hydrolysis during downstream processing, generating non-hydratable phospholipids (NHP). NHP enters the crude oil during extraction, leading to difficult degumming and increased refining losses.
A study published in the Wiley journal Lipid / Fett specifically examined the inactivation of endogenous enzymes in rapeseed by moist heat treatment. The research team verified at pilot scale that moist heat treatment of flaked rapeseed using a dedicated conditioning unit can substantially inactivate phospholipase and lipoxygenase, thereby inhibiting NHP formation during pre-pressing and solvent extraction. Crude oil treated with this conditioning unit had extremely low phospholipid content and could be directly physically refined after simple water degumming.
Heating equipment:
- Indirect heater: Heats seed indirectly through a steam jacket or thermal oil, with precise temperature control
- Direct hot air contactor: Hot air directly contacts the seed for rapid heating, but moisture control requires attention
Why Is the Heating Temperature 70–90°C and Not Higher?
Too low (<60°C): insufficient softening, poor flake quality, incomplete enzyme inactivation.
Too high (>100°C): excessive protein denaturation, potentially affecting the feed value of the meal; at the same time, excessive temperature may cause partial oil oxidation.
70–90°C is the process window, requiring a balance between enzyme inactivation, flake quality, and meal quality. In practice, heating temperature is typically set at 75–85°C, adjusted according to seed variety, moisture content, and downstream process.
IV. The Complete Chain After Heating: Flaking and Cooking – Two Different Heating Stages
After heating, rapeseed enters the flaking mill and is pressed into flakes approximately 0.3–0.38 mm thick. Studies show that flakes thinner than 0.2 mm are too fragile and break easily, while flakes thicker than 0.4 mm lead to reduced oil yield. Flaking further disrupts cell structure and increases surface area, creating channels for solvent penetration.
The flakes then enter a cooker/conditioner, where they are heated at 80–105°C for 15–20 minutes, with an optimal temperature of approximately 88°C. The core objectives of the cooking stage are:
- Further enzyme inactivation: ensuring complete inactivation of phospholipase, lipoxygenase, and myrosinase
- Moisture adjustment: reducing flake moisture from 8%–10% to 4%–6%, creating optimal conditions for solvent extraction
- Improving meal structure: moderate protein denaturation to enhance meal feed value
It is important to note: heating (70–90°C) and cooking (80–105°C) are two different process stages. Heating occurs before flaking to soften the seed for flaking; cooking occurs after flaking to adjust flake moisture and structure to optimise extraction. The two should not be confused, and their temperature windows are different.
V. The “Leverage Effect” of Pretreatment on Extraction Efficiency
Investment in the pretreatment section produces a clear “leverage effect” in the extraction section. The reason is simple: the extractor processes pretreated material, and the material state determines the ceiling of extraction efficiency.
- Incomplete cleaning: Impurities occupy extractor space, reducing effective throughput and accelerating equipment wear
- Off-spec moisture: For every 1% moisture above spec, oil yield drops by nearly 0.5%; moisture too low causes flakes to shatter, increasing fines and blocking solvent channels
- Inadequate heating: High oil viscosity and poor flowability prevent solvent from fully contacting the cell interior, raising residual oil
- Incomplete enzyme inactivation: Elevated NHP increases degumming difficulty and refining losses
These effects often do not appear immediately in extractor operating data. Instead, they accumulate slowly in the form of “slightly higher residual oil” or “a little more phosphoric acid for degumming.” For plants processing over 50 tonnes per day, these “slightly higher” values add up to a significant cost difference.
VI. Frequently Asked Questions (FAQ)
Q1: Why must rapeseed be dried to below 8% moisture before pretreatment?
8% is the balance point between storage safety and flaking plasticity. Above 10% moisture, seed is prone to mould during storage and free fatty acids rise rapidly. If moisture is too low, seed becomes brittle, shatters during flaking, and generates excessive fines, reducing extraction efficiency. Studies show that for every 1% moisture above the 6%–8% range, oil yield can decrease by nearly 0.5%.
Q2: What is the core purpose of heating to 70–90°C?
Heating serves three purposes: softening cell structure to improve flake quality, reducing oil viscosity to improve extraction kinetics, and inactivating phospholipase and lipoxygenase to prevent non-hydratable phospholipid formation. Enzyme inactivation is the key to controlling crude oil quality.
Q3: What is the difference between heating temperature and cooking temperature?
Heating (70–90°C) occurs before flaking to soften the seed for flaking. Cooking (80–105°C, optimal around 88°C) occurs after flaking to adjust flake moisture to 4%–6% and further inactivate enzymes. They are different process stages with different temperature windows and process objectives.
Q4: Where is the “leverage effect” of pretreatment reflected?
The state of pretreated material determines the ceiling of extractor efficiency. Incomplete cleaning increases equipment wear; off-spec moisture hinders solvent penetration; inadequate heating raises residual oil; incomplete enzyme inactivation increases refining losses. These effects accumulate slowly as “slightly higher residual oil” or “more phosphoric acid for degumming,” and are particularly significant for plants with large daily capacity.
VII. What KMEC Can Do for You
Whether you are planning a 100 TPD rapeseed oil plant or a 1,000+ TPD large-scale extraction plant, KMEC offers complete rapeseed pretreatment and extraction equipment solutions.
Our product line covers the entire pretreatment process:
- Cleaning section: Magnetic separator, vibrating screen, destoner
- Drying section: Fluidised bed dryer, rotary dryer
- Heating and cooking section: Indirect heater, cooker/conditioner, conditioning equipment
- Flaking section: Flaking mill (flake thickness 0.3–0.38 mm adjustable)
- Pre-pressing section: Screw oil press
- Extraction section: Extractor, evaporator, condenser, solvent recovery system
KMEC engineers can provide the optimal pretreatment equipment configuration based on your raw material characteristics (moisture, oil content, impurity content), capacity targets, and target product grade – from single machines to complete turnkey EPC plants.
Contact KMEC today for a customised rapeseed pretreatment equipment solution and quotation!
Data sources: 13th International Rapeseed Congress (Quinsac et al., 2011), Lipid / Fett (Beyer, 1997), Canola Council of Canada, Oil Mill Machinery, and peer-reviewed research literature.