The general processing flow for edible vegetable oils is: Pressing/Extraction — Degumming (acid washing to remove phospholipids) — Alkali Refining (removing fatty acids, etc.) — Bleaching (using adsorbents like activated carbon to remove soaps, pigments such as chlorophyll, etc.) — Deodorization (steam vacuum deodorization) — Finished Oil.
Activated Carbon (AC) is frequently used alone or in combination with bleaching earth (bentonite). For example, to remove chlorophyll, bleaching earth is mixed with oil at a ratio of 250:1 to 1000:1 at 60–90°C, followed by the addition of activated carbon at a ratio of 150:1 to 100:1 at 90–120°C. Stirring for 5–15 minutes significantly enhances the removal efficiency.
Activated carbon can effectively adsorb carotenoids in soybean, cottonseed, pumpkin seed, and sesame oils. Spent activated carbon can be mixed with oil residue to be used as feed for pigs and cattle.
In the refining of cottonseed oil, bleaching earth is first added to remove gossypol, followed by activated carbon to remove pigments like carotenoids and chlorophyll. While activated carbon is more expensive than bleaching earth, bleaching earth cannot be regenerated and recycled like activated carbon.
From the decolorization distillate of soybean and rapeseed oil, Vitamin K can be obtained through methanol extraction, activated carbon adsorption, and toluene elution.
For peanut oil decolorized with coconut shell activated carbon, the adsorption capacity increases with temperature up to 90°C.
Impregnated activated carbon containing 5% magnesium oxide (MgO) can be used to decolorize crude or degummed vegetable oils. Adding 3g of this impregnated carbon per 100g of oil and heating it to 93°C for 4 hours allows the activated carbon to adsorb almost all phospholipids and fatty acids, preventing the common flocculation phenomenon during heating. This treatment eliminates the traditional steps of alkali neutralization, water washing, and bleaching. When used in a bed-type device, granular impregnated activated carbon requires lower dosages and has a longer service life. The adsorption capacity of granular impregnated carbon for pigments like chlorophyll is approximately five times higher than that of powdered activated carbon.
Utilizing a pre-coating filtration process with activated carbon and silica removes pigments, gums, soaps, and phospholipids from edible oil. This method is time-efficient and avoids the production of soapstock wastewater typical of alkali refining, thereby saving on the costs of complex effluent treatment.
Activated carbon is also utilized in the supercritical carbon dioxide (SC-CO2) extraction of oils and fats.
It should be noted that soybean oil treated with activated carbon may show reduced oxidative stability because tocopherols (antioxidant Vitamin E) are also adsorbed during the process.
In the refining of coconut oil, a mixture of bleaching earth and activated carbon is commonly used. If the goal is to retain Vitamin A in the oil, the processing temperature should not exceed room temperature.