Palm oil is purified through a multi-step refining process that removes impurities, free fatty acids, color pigments, and odors from crude palm oil. The industry-standard method produces what’s known as RBD palm oil (refined, bleached, and deodorized), and the four core steps are degumming, neutralization, bleaching, and deodorization. Each step targets a different category of unwanted compounds, and skipping any one of them leaves noticeable quality problems in the finished oil.
Physical vs. Chemical Refining
Before diving into each step, it helps to understand that there are two main approaches to purifying palm oil. Chemical refining uses caustic soda (sodium hydroxide) to neutralize free fatty acids, converting them into soap that gets separated out by centrifuge. Physical refining skips the caustic soda entirely and instead removes free fatty acids later in the process through steam distillation under high vacuum.
Physical refining is the preferred method for palm oil, and most large-scale producers use it. The reasons are practical: it retains about 98% of the neutral oil, compared to roughly 88% for chemical refining. That 10-percentage-point difference in yield adds up fast at industrial volumes. Physical refining also uses fewer chemicals, generates less wastewater, and costs less to operate. Chemical refining is simpler to run but becomes especially wasteful when the crude oil has high acidity, since the caustic soda can attack the neutral oil itself, not just the fatty acids you’re trying to remove.
Step 1: Degumming
Degumming removes phospholipids, gums, and other colloidal impurities from crude palm oil. The standard method is acid degumming, which uses a small amount of phosphoric acid, typically 0.05 to 0.1% by weight of the oil. The acid converts non-hydratable phospholipids into a form that clumps together and can be separated out.
Optimized conditions for acid degumming call for a phosphoric acid concentration of about 0.06%, a temperature of 90°C, and a reaction time of 30 minutes with continuous stirring. Getting these parameters right matters beyond just removing gums. Research published in the journal Foods found that the degumming conditions directly influence the formation of process contaminants (harmful compounds that can develop during later high-heat stages). Proper degumming with 0.06% phosphoric acid for at least 20 minutes significantly reduced the formation of these contaminants in the finished oil.
This step also has an important secondary function: it prepares the oil for effective bleaching. Residual phospholipids left behind by poor degumming will interfere with the bleaching clay’s ability to absorb color pigments.
Step 2: Neutralization (Chemical Refining Only)
If you’re using the chemical refining route, the next step is neutralization. Free fatty acids in the crude oil react with caustic soda to form insoluble soaps, which are then spun out in a centrifuge. The amount and concentration of caustic soda depends on the free fatty acid content of the oil, with a 5 to 20% excess typically added to ensure complete neutralization.
The tricky part is using just enough caustic soda. Too little leaves free fatty acids in the oil. Too much triggers “parasitic” saponification, where the sodium hydroxide attacks the neutral oil you’re trying to keep, dragging down your yield. This is why chemical refining isn’t recommended for crude palm oil with free fatty acid levels above 15%, since the neutral oil losses become too severe to justify.
In physical refining, this step is replaced by steam distillation during deodorization, which strips out free fatty acids without any chemical reaction.
Step 3: Bleaching
Bleaching removes color pigments (mainly carotenoids and chlorophyll), residual gums, trace metals, and oxidation products. Despite the name, this isn’t a chemical bleaching process. It works through adsorption: fine clay particles bind to unwanted compounds and are then filtered out.
The standard adsorbent is acid-activated bleaching earth, a type of clay that has been chemically treated to increase its porosity, surface area, and binding capacity. For palm oil, the typical dosage ranges from 0.5% to 3.0% by weight of oil. The mixture is heated to 90°C to 120°C under vacuum and stirred continuously for about 25 to 30 minutes. The vacuum prevents the hot oil from oxidizing during the process. Afterward, the oil-clay mixture is pumped through a filter to remove the spent clay.
Activated carbon is sometimes blended with the clay for extra adsorption power, particularly when removing stubborn color compounds or polycyclic aromatic hydrocarbons. The ratio depends on the quality of the crude oil and the target specifications for the finished product.
Step 4: Deodorization
Deodorization is the final and most energy-intensive step. It removes volatile compounds responsible for off-flavors and odors, raises the smoke point, and in physical refining, also strips out the free fatty acids that weren’t removed by caustic soda.
Conventional deodorization heats the bleached oil to between 190°C and 240°C under deep vacuum (10 to 80 kPa absolute pressure) for 2 to 3 hours. Steam is injected through the oil to carry away volatile compounds. The combination of high temperature, low pressure, and steam contact is what makes it effective, but these harsh conditions are also what creates the risk of forming process contaminants.
A newer approach called multi-flash autovaporization works at significantly lower temperatures (120°C to 148°C) by cycling the oil between high pressure (up to 450 kPa) and sharp vacuum (4 to 5 kPa). The rapid pressure drops cause volatile compounds to flash off without needing the extreme heat of conventional deodorization. This gentler process better preserves the oil’s natural antioxidants and produces fewer unwanted byproducts.
Reducing Process Contaminants
One of the biggest challenges in palm oil refining is minimizing heat-induced contaminants, particularly 3-MCPD esters and glycidyl esters. These compounds form when chloride-containing impurities in the oil react at high temperatures, especially during deodorization. They’ve drawn regulatory attention in Europe and elsewhere, pushing refiners to adapt their processes.
Water washing the crude or bleached oil before deodorization removes up to 95% of the inorganic chlorides that serve as precursors. Using potassium-based salts during processing has brought 3-MCPD ester levels below 100 micrograms per kilogram of refined oil. Post-refining treatments with activated carbon, clay, and antioxidants have reduced glycidyl esters by up to 99%. Low-temperature short-path distillation as a finishing step can also bring glycidyl esters below 100 micrograms per kilogram. Combining multiple mitigation strategies, rather than relying on any single one, delivers the best results.
Quality Standards for Finished Oil
Properly refined palm oil meets several measurable benchmarks. Free fatty acid content in crude palm oil should already fall between 2% and 5% before refining, and the finished RBD oil brings this down well below 0.1%. Moisture and impurity content for edible oil should stay within 0.15% to 0.30%, with an acceptable maximum moisture limit of 0.20%. Color shifts from the orange-red of crude palm oil to a pale yellow after bleaching. The oil should have a neutral smell and a high smoke point suitable for cooking and food manufacturing.
Small-Scale Purification Methods
Not all palm oil purification happens in industrial refineries. For small-scale or artisanal producers, simpler techniques can improve oil quality without specialized equipment. The most accessible approach uses activated carbon as an adsorbent. The oil and carbon are mixed at a weight ratio of about 15:1 (oil to carbon), heated to 70°C to 80°C with stirring for 30 minutes, and then filtered while still hot (around 60°C) through a fine strainer or cloth filter.
Pre-treating the activated carbon with hydrogen peroxide solution improves its effectiveness. In one documented method, 25 grams of activated carbon was soaked in a 30% hydrogen peroxide solution, mixed for two hours, rinsed with clean water until the pH reached about 4, and then dried at 120°C for five hours. This oxidation step increases the carbon’s ability to bind impurities, color compounds, and free fatty acids.
These small-scale methods won’t match the precision of industrial refining, but they meaningfully reduce color, odor, and free fatty acid levels in crude or previously used palm oil. For better results, the process can be repeated with fresh adsorbent material.