In rice milling, rice bran is the main by-product, accounting for only 5%–8% of the paddy weight yet containing over 60% of the grain’s nutrients. Rice bran contains about 16%–22% oil, with unsaturated fatty acids making up as much as 70% of the total. It is also rich in gamma-oryzanol – a natural active compound that helps inhibit platelet aggregation and reduce cholesterol absorption.
But rice bran is also an extremely unstable raw material. From the moment it leaves the rice mill, an invisible chemical reaction begins.
Under natural moisture content and ambient storage conditions, free fatty acid (FFA) levels in rice bran rise by about 1% per hour in the first few hours after milling. If the bran moisture exceeds 12% and it is stored at 25–35°C for just 12 days, 60%–80% of the oil can be broken down. One stability study showed that untreated rice bran stored at room temperature for six months saw FFA content soar from 2% to 68.27%.
For rice bran oil processors, this means: the window between milling and pressing may be only 4 to 24 hours. Miss that window, and you are left with a batch of high-acid, hard-to-refine, low-value raw material.
This article systematically explains the biochemical mechanism of rice bran rancidity, the temperature and time window for stabilization, and how extrusion stabilization extends rice bran shelf life from 24 hours to over 30 days.
1. The Root of Rancidity: The “Fatal Contact” Between Lipase and Oil
To understand why rice bran spoils so easily, we need to look at the cellular structure of the rice grain.
In an intact rice kernel, lipase and oil are stored in separate cellular compartments – lipase in the aleurone layer and seed coat, and oil mainly in the oil bodies of the germ and aleurone layer. They are separated by cell walls and membrane structures and coexist peacefully.
But milling destroys this natural separation. When the rice is milled and the bran is stripped away, cell walls rupture, and lipase immediately contacts the oil, catalysing the hydrolysis of triglycerides into free fatty acids and glycerol. This process is called hydrolytic rancidity.
The action of lipase is extremely fast. The optimal reaction temperature for rice bran lipase is around 30°C, with an optimal pH of 7.0. Under ambient conditions, in the first few hours after milling, FFA content rises by about 1% per hour, and the acid value climbs rapidly from about 4 mg KOH/g to over 10 mg KOH/g.
This means: the greatest enemy of a rice bran oil processor is not the competition – it is time.
2. 4–24 Hours: How Narrow Is the Window?
The “safe processing window” for rice bran depends on several variables:
Temperature. At ambient temperatures of 25–35°C, the acid value of rice bran rises significantly within hours. If the ambient temperature is higher (as in tropical regions), lipase activity is stronger and rancidity accelerates. At lower temperatures (e.g., 4–5°C), rancidity slows considerably, but this is not feasible for large-scale production.
Moisture. Moisture is an “accelerator” for lipase activity. Higher-moisture bran provides a more favourable environment for enzymatic reactions. Studies show that when rice bran moisture exceeds 12%, storing it at 25–35°C for 12 days can lead to a 60%–80% oil decomposition rate.
Microorganisms. In addition to endogenous lipase, microbial contamination also accelerates rice bran rancidity. During storage, lipases secreted by microorganisms further catalyse oil hydrolysis.
Taking these factors together, the ideal time window from milling to pressing is 4 to 24 hours. It should be noted that this window is based on undried, natural-moisture rice bran. If the bran is thoroughly dried and kept at low moisture, it can be stored for at least four months without excessive FFA increase. Beyond 24 hours without drying, the acid value rises to a level that is difficult to refine economically; beyond 72 hours, the bran may no longer be suitable for edible oil production.
3. Stabilization: Pressing “Pause” Before Lipase Does Its Damage
Since the root cause of rancidity is the catalytic action of lipase, the core logic of stabilization is to inactivate the lipase before it causes irreversible loss.
Common industrial methods for rice bran stabilization include extrusion, high-temperature roasting, microwave heating, infrared heating, and radio frequency heating. Among these, extrusion is the most widely used and reliable method in industrial production.
4. Extrusion Stabilization: Parameters, Mechanism, and Effects
The core principle of extrusion stabilization is to use the shear force, friction, and pressure inside the extruder to generate instantaneous high temperature, denaturing and inactivating the lipase in a very short time.
Key Process Parameters
| Parameter | Typical Range | Role |
|---|---|---|
| Barrel temperature | 120–140°C | The key variable for lipase inactivation. Under conventional extrusion conditions, lipase activity is completely lost above 128°C, regardless of the initial moisture content of the bran |
| Screw speed | 140–250 rpm | Affects shear force and material residence time |
| Initial bran moisture | 14%–19% | Higher moisture improves heat transfer and lipase inactivation, but excessive moisture accelerates degradation of heat-sensitive nutrients |
Temperature is the most critical factor for lipase inactivation. Under conventional extrusion conditions, lipase activity is completely lost above 128°C, regardless of the initial moisture content. In practice, barrel temperatures are usually set at 130–140°C to ensure stabilization.
It should be noted that later research identified a thermostable lipase in stabilized rice bran that still shows maximum activity at 80°C and retains most of its secondary structure at 90°C. This suggests that while conventional extrusion can achieve complete lipase inactivation, process parameters still need to be optimized and verified for different rice bran varieties.
Stabilization Effects
After extrusion, rice bran shelf life is significantly extended:
- Dry-heat-treated rice bran can be stored for 30 days, after which FFA content rises significantly. This is attributed to incomplete destruction of lipase by dry heat, with residual enzyme activity reactivating under storage conditions.
- Extruded rice bran shows no significant increase in FFA content over a 60-day storage period, proving that extrusion destroys the lipase system more thoroughly than dry heat treatment.
- Untreated rice bran saw FFA content rise from 4.05% to 64.60% over 60 days of storage, while extruded-stabilized bran showed no significant change over the same period.
- Under accelerated storage conditions of 32°C and 85% relative humidity, rice bran extruded at 130°C maintained stable FFA levels over 28 days.
- After 375 days of storage at ambient temperature, all extruded rice bran samples had FFA levels below 7%, while untreated bran exceeded 70%.
The “Bonus” of Extrusion: Improved Downstream Extraction Efficiency
Extrusion not only solves the rancidity problem but also brings an important process advantage: significantly improved solvent extraction efficiency.
Extruded rice bran forms a porous pellet structure with higher bulk density and greatly improved solvent permeability. Studies show that the percolation rate of extruded bran can be up to 9 times that of untreated bran, and the extraction time required to reach 1% residual oil is reduced from 100 minutes to 10 minutes.
In addition, compared with unextruded bran, extrusion can directly increase oil yield by 2%–3%.
5. Comparison with Other Stabilization Methods
Besides extrusion, other stabilization methods have their own characteristics:
High-temperature roasting: Roasting at 130°C for 30 minutes gives a lipase inhibition rate of about 72%. Equipment investment is lower, but processing time is long, energy consumption is high, and stabilization is less thorough than extrusion.
Microwave heating: Short-time microwave treatment (720 W, 90 seconds) can achieve a lipase inhibition rate of up to 85%, the best stabilization effect. However, microwave equipment has limited throughput and is not suitable for large-scale continuous production.
Infrared and radio frequency heating: These can reduce lipase activity to 20%–30% of fresh bran, with good nutrient retention, but industrial application is not yet widespread.
6. From Stabilization to Pressing: The Complete Rice Bran Oil Processing Chain
Once stabilization is complete, the rice bran can enter the normal oil extraction process:
Pressing method: Stabilized rice bran is roasted in a roaster/conditioner, then fed into a screw oil press. Our rice bran oil presses feature precise pressing temperature control, strong pressing pressure, and precision-designed barrel clearances – improving oil yield while effectively preserving the unsaturated fatty acids in the oil.
Solvent extraction method: For plants processing 20–30 tonnes per day or more, solvent extraction is recommended. Stabilized bran is fed into a Rotocel or Loop extractor, where residual oil is extracted with solvent, reducing meal residual oil to below 1% – far better than the 5%–7% typical of pure pressing.
Refining and dewaxing: The crude rice bran oil still needs to go through degumming, deacidification, bleaching, deodorisation, and winterisation (dewaxing). Rice bran oil contains 3%–5% waxes; without a dedicated dewaxing unit, the oil will appear cloudy at room temperature, directly affecting its commercial value.
7. Operating Costs and Profitability
In a rice bran oil plant, raw material procurement accounts for about 80% of operating costs. The equipment’s “residual oil rate” is the most critical indicator of return on investment (ROI).
- Steam consumption: 280–450 kg per tonne of rice bran (depending on heat recovery system configuration)
- Solvent loss: Can be controlled below 2 kg per tonne of material processed
- Power consumption: Approximately 28–35 kWh per tonne
By keeping the residual oil rate below 1%, more oil goes into the finished oil tank rather than remaining in the meal – establishing a competitive advantage in a market where rice bran prices fluctuate sharply.
FAQ
Q1: Can rice bran stabilization completely inactivate lipase?
Under conventional extrusion conditions (above 128°C), lipase activity can be completely lost. However, later research identified a thermostable lipase in stabilized rice bran that still shows maximum activity at 80°C. This suggests that extrusion parameters need to be optimized for specific raw material characteristics and cannot simply be applied uniformly.
Q2: Does stabilization destroy nutrients in rice bran?
Extrusion inactivates lipase but also causes some loss of heat-sensitive components. Studies show that under extreme conditions of 150°C, anthocyanin C3G loss in black rice bran can reach 54.73%. However, under optimized conditions of 140°C, lipase inactivation can reach 76% while C3G retention reaches 61%. The key is to find the balance between inactivation efficiency and nutrient retention.
Q3: Do small rice bran oil mills also need stabilization?
Yes. Regardless of scale, unstabilized rice bran will rapidly increase in acid value after pressing, leading to difficult refining, poor oil quality, and low oil yield. For small mills, a smaller-capacity rice bran extruder can be selected, with manageable investment costs.
What KMEC Can Do for You
Whether you are planning a 10 TPD small pressing plant or a 100 TPD large refinery, KMEC offers complete rice bran oil processing equipment solutions.
Our product line covers the entire process from stabilization to refining:
- Stabilization and pre-treatment: Rice bran extruder (110–140°C precise temperature control), roaster/conditioner, cleaning equipment
- Pressing and extraction: Screw oil press, Rotocel/Loop extractor, evaporators, solvent recovery system
- Refining and dewaxing: Degumming tank, deacidification tower, bleaching tank, deodoriser tower, winterisation crystalliser, winterisation filter press
- Auxiliary equipment: Conveying systems, automation control systems, oil quality testing equipment
KMEC engineers can provide the optimal equipment configuration based on your capacity targets, raw material characteristics, budget, and product positioning – from single machines to complete turnkey EPC plants.
Contact KMEC today for a customised rice bran oil processing equipment solution and quotation!
Data sources: [1] Loeb, J.R. et al., “Rice bran oil. IV. Storage of the bran as it affects hydrolysis of the oil,” Journal of the American Oil Chemists’ Society, 1949. [2] Kim, C.J. et al., “Optimization of Extrusion Rice Bran Stabilization Process,” Journal of Food Science, 1987. [3] Bhardwaj, K. et al., “Identification, purification, and characterization of a thermally stable lipase from rice bran,” Plant Physiology, 2001. [4] Gujral, H.S. et al., “Physico-Chemical Characteristics of Rice Bran Processed by Dry Heating and Extrusion Cooking,” Journal of Food Processing and Preservation, 2004.