Steps in the Extraction and Refining of Rice Bran Oil

Rice bran oil (RBO) is one of the most nutritionally rich vegetable oils, yet it starts its journey as a simple byproduct of rice milling. The outer bran layer, which is stripped away during the polishing of white rice, holds an oil packed with bioactive compounds that are rare in other edible oils. Turning that crude, impurity-laden extract into a clean, food-grade oil involves a carefully sequenced set of extraction and refining steps – each serving a precise purpose. Here is a clear breakdown of how it all works.

Why rice bran oil needs special handling 🔗

Rice bran is a low oil-content material, with an oil content of only about 20%. Because of this, a simple mechanical press leaves behind too much residual oil in the spent meal to be economically viable. There is also a more pressing problem: rice bran contains active lipase enzymes. If the bran is not stabilized promptly after milling, these enzymes begin breaking down the oil into free fatty acids (FFAs), making it rancid very quickly. This is why every step – from pretreatment to final refining – is designed not just to maximize yield but also to protect the oil’s quality and nutritional value.

RBO is notable for its content of tocotrienols, gamma-oryzanol, and phytosterols, bioactive compounds linked to antioxidant activity, cholesterol reduction, and anti-inflammatory effects. Preserving these compounds through extraction and refining is one of the central challenges in RBO processing.

What makes rice bran oil valuable: key bioactive compounds 🔗

Gamma-oryzanol 🔗

Gamma-oryzanol is a mixture of ferulic acid esters of sterols and triterpene alcohols, and it is found in rice bran oil at a concentration of 1 to 2%, where it functions as a natural antioxidant. It is largely unique to rice bran oil and is not found in meaningful quantities in other common vegetable oils. Research has shown that gamma-oryzanol is present in rice bran at levels up to 10 times higher than vitamin E, making it the dominant antioxidant in this oil. Studies indicate that it can lower total cholesterol, LDL cholesterol, and triglyceride levels, thereby reducing the risk of coronary heart disease.

Tocotrienols 🔗

Tocotrienols belong to the vitamin E family and are among the key health-promoting components in rice bran. The bran fraction of rice contains tocotrienols alongside tocopherols, phytosterols, and a range of B vitamins and minerals. Delta-tocotrienol, in particular, has been associated with anti-inflammatory effects. These compounds can be partially lost during high-temperature refining, which is one reason why physical refining at controlled temperatures is increasingly preferred for premium-grade RBO.

Extraction of rice bran oil 🔗

Pre-treatment: preparing the bran for extraction 🔗

Before any oil can be drawn out, the raw bran must go through several preparatory steps. First, it is cleaned to remove impurities such as stones, broken rice, dust, and metal fragments. It is then subjected to heat treatment – usually steam cooking at temperatures above 100°C – to stop lipase activity and prevent hydrolytic rancidity.

After cooking, the bran is conditioned for moisture and then fed into an extruder or puffing machine. This puffing step transforms the powdery bran into porous, granular pellets through high-pressure mechanical shear and steam injection, increasing surface area by 300-400% and improving solvent penetration. The expanded material is then dried and cooled to 50-55°C before entering the extractor.

Solvent extraction 🔗

For commercial-scale production, solvent extraction using hexane is the most widely used method, because it can reduce residual oil in the spent meal to below 1%, compared to roughly 7% for mechanical pressing. The puffed bran pellets are loaded into an extractor where hexane is sprayed or percolated through them in a countercurrent pattern – fresh solvent contacts the most depleted bran, and the most concentrated solvent-oil mixture (called miscella) is collected from the bran with the highest oil content. This maximizes extraction efficiency.

The resulting miscella (a mixture of oil and hexane) is then processed through a series of evaporators and a stripping tower under vacuum to separate the solvent from the oil. The stripping tower removes remaining solvent from the miscella, yielding crude rice bran oil (CRBO) with a purity exceeding 99.5%. The recovered solvent is cooled and recycled back into the process. The spent wet meal is processed in a DTDC (Desolventizer-Toaster-Dryer-Cooler) system to remove solvent traces and is then sold as protein-rich animal feed.

Refining of crude rice bran oil 🔗

Crude rice bran oil is not suitable for human consumption and must be refined – a process that involves dewaxing, degumming, deacidification, bleaching, and deodorization. Each step targets a specific class of impurities.

Step 1: Dewaxing 🔗

Unlike most vegetable oils, rice bran oil has a notably high wax content – typically around 1-5%. These waxes, if left in the oil, make it appear turbid or cloudy, especially at low temperatures, and give it an unpleasant mouthfeel. In dewaxing, the oil is slowly cooled to temperatures between 5°C and 10°C in crystallization tanks and held there for 24-48 hours to allow wax crystals to form and grow. The solidified wax is then filtered out, leaving the oil clear and stable. This step ensures the oil remains transparent even in cold conditions, an important quality parameter for a consumer-grade product.

Step 2: Degumming 🔗

Crude RBO contains phospholipids (gums), trace metals, and proteins that affect its stability and shelf life. Degumming is an essential preliminary step because traces of metals and phosphorus in the oil affect its stability. Water degumming hydrates the phospholipids, causing them to swell and separate from the oil. For RBO, enzymatic degumming is also used, where enzymes react with the phospholipids and break them into oil-soluble and water-soluble fractions that can be easily separated. Water degumming has been shown to achieve approximately 92% removal of phospholipids from rice bran oil. The gums removed during degumming contain significant concentrations of gamma-oryzanol and tocopherols, making their recovery a commercially valuable side-stream.

Step 3: Deacidification (neutralization) 🔗

Rice bran oil typically has a high free fatty acid (FFA) content – often exceeding 10% in crude form – due to the rapid lipase activity that occurs in unstabilized bran. For deacidification, alkali refining or steam refining are typically used. Alkali refining neutralizes FFAs using sodium hydroxide solution in a batch or continuous process, converting them into soap (soapstock) that can be centrifuged away. Physical deacidification – steam distillation under high temperature and vacuum – is increasingly favored for high-acid RBO because it improves gamma-oryzanol retention by over 30% and produces no soapstock, reducing waste and preserving more of the oil’s nutritional value.

Step 4: Bleaching (decolorization) 🔗

After degumming and deacidification, the oil still retains color pigments such as chlorophyll and carotenoids, along with residual metals and oxidation products that affect both appearance and flavor. In a vacuum bleaching tower, activated clay (1-3%) is added to the oil at 105-110°C to adsorb and remove these pigments and trace impurities. The spent bleaching earth is then filtered out. Some operations also use activated carbon alongside bleaching earth for more thorough decolorization. This step is carried out under vacuum to prevent oxidation of the oil at elevated temperatures.

Step 5: Deodorization 🔗

Even after bleaching, the oil carries off-flavors and odors – primarily from residual FFAs, peroxides, aldehydes, and ketones formed during oxidation. Deodorization involves steam stripping to remove these objectionable odors and the characteristic rice oil flavors. The oil is heated to high temperatures (230-260°C) under high vacuum while live steam is injected through it. The volatile compounds – including remaining FFAs and off-flavor compounds – are carried away with the steam. This is the final stage of refining and produces an oil with a mild, neutral flavor suitable for both cooking and food manufacturing. However, because this step involves high temperatures, careful process control is needed to avoid degrading the gamma-oryzanol and tocotrienol content of the oil.

Physical vs. chemical refining: a key decision 🔗

There are two broad approaches to refining RBO: chemical refining and physical refining. Chemical refining of crude RBO yields better results in terms of color, cloud point, and other physical characteristics. Physical (steam) refining, on the other hand, avoids the use of alkali and produces less waste, and – critically – it better preserves bioactive compounds like gamma-oryzanol and tocotrienols. For a high-value nutraceutical product, physical refining is generally preferred. The choice between the two often comes down to the acid value of the incoming crude oil and the intended end-use of the refined product.

Uses of refined rice bran oil 🔗

Once refined, RBO is a highly versatile oil. Its high smoke point makes it widely used in Asia for cooking, frying, and as a salad oil. Beyond the kitchen, it finds use in the production of nutraceuticals (gamma-oryzanol and tocotrienol supplements), cosmetics (skin creams and sunscreen formulations), and pharmaceuticals. The byproducts of refining – including soapstock, spent bleaching earth, and deodorizer distillate – are also processed to recover gamma-oryzanol concentrates, tocopherols, and fatty acids, making RBO processing a relatively low-waste operation when managed well.

What do you think? Given that physical refining preserves more of rice bran oil’s nutritional compounds than chemical refining, should food processors be more widely mandated to adopt it for oils marketed as health products? And with rice bran being generated in enormous quantities as a milling byproduct, what other strategies could help ensure this resource is more fully utilized rather than discarded?

References

1. https://www.sciencedirect.com/topics/medicine-and-dentistry/gamma-oryzanol
2. https://cookingoilmillmachinery.com/FAQ/an_overview_of_rice_bran_oil_solvent_extraction_process_65.html
3. https://pubs.acs.org/doi/10.1021/acsfoodscitech.4c00781
4. https://pubmed.ncbi.nlm.nih.gov/9754398/
5. https://pubmed.ncbi.nlm.nih.gov/11308370/
6. https://www.rxlist.com/supplements/gamma_oryzanol.htm
7. https://www.drugs.com/npp/gamma-oryzanol.html
8. https://www.agrifarming.in/rice-bran-oil-extraction-solvent-process-flow-chart
9. https://www.cnhuataigroup.com/news/industry-news/rice-bran-oil-extraction-process.html
10. https://www.cnhuataigroup.com/news/industry-news/rice-bran-oil-solvent-extraction-plant.html
11. https://www.sciencedirect.com/science/article/abs/pii/S1383586614002834
12. https://cookingoilmillmachinery.com/FAQ/main_process_of_making_rice_bran_oil_50.html
13. https://www.spectecindia.com/rice-bran-oil-refining-process/
14. https://www.sciencedirect.com/article/abs/pii/S1383586614002834
15. https://www.researchgate.net/publication/310540370_Degumming_and_Neutralization_of_Rice_Bran_Oil
16. https://www.researchgate.net/publication/225667949_A_novel_process_for_physically_refining_rice_bran_oil_through_simultaneous_degumming_and_dewaxing