The solvent extraction of vegetable oils like soybean oil dominates the oilseed industry as the most popular extraction method used around the world. Most large-scale solvent plants use hexane extraction to chemically separate oilseeds into liquid fat and solid protein—popular ingredients in the food industry and many other market sectors.
Amid growing concern for natural, organic, and chemical-free products, some consumers are raising questions about the prevalence of hexane extraction throughout our modern food chain. However, as the primary solvent used in global oil extraction, hexane offers plenty of advantages that maintain its dominant position in the industry.
In this blog, we’ll break down hexane extraction to explain what it is, why it’s preferred for large-scale oilseed extraction, and why some processors might seek greener alternatives.
What is hexane?
Hexane (C6H14) is a hydrocarbon extracted from crude oil. In fact, hexane is similar to the gasoline we put in our cars; it’s just a slightly lighter molecule made of six carbon chains—compared to octane’s eight. In its pure form, hexane is a colorless solvent that evaporates easily and dissolves only slightly in water. It’s incredibly volatile and highly flammable, and its vapors can be explosive.
Although it’s a potentially dangerous substance, hexane has many uses in food and other industrial applications. For example, it’s commonly used as a cleaning agent in textile and furniture manufacturing, as a degreaser in the printing industry, and as a base for paint thinners and glues, from those used in industrial shoemaking and roofing to everyday household products like rubber cement and spray adhesives.
Most notably, hexane is the primary solvent used to extract edible and industrial vegetable oils from the world’s top five commodity oilseeds: soybean, canola, sunflower seed, cottonseed, and peanut.
The hexane extraction process
After soybeans or other oilseeds are cleaned, cracked, cooked, and dried, the flaked seed material is bathed in hexane at a solvent plant. Proper pretreatment helps rupture the oil-bearing cells and increase the surface area of the seed, allowing the solvent to thoroughly penetrate the material to extract more fat. During extraction, hexane flows through a bed of flaked soybean or soybean collets (if the material has been expanded in an Anderson Solvex Expander), swiftly dissolving the oil from the solid seed material.
The hexane extraction process creates two byproducts: a mixture of oil and hexane called miscella, which contains about 25-35% fat, and the leftover solids, which are saturated in hexane in concentrations of 25-30%. Both byproducts undergo further heating and processing to recover as much hexane as possible. The de-oiled cake goes into a desolventizer-toaster that heats the meal enough to evaporate the hexane without deteriorating other nutrients. The miscella, meanwhile, is distilled with steam heat to vaporize the hexane, which is then recaptured and condensed for reuse, while the oil is further refined, bleached, and deodorized.
While most hexane is recovered from the end products, the solvent extraction process results in average hexane losses between 1.5 to 2 liters per ton of seed processed. These losses typically occur as trace amounts in the crude oil and meal or escape through vents, hot water, or other leakage.
Why is hexane used in oil extraction?
There are many reasons why processors rely on hexane as a solvent in extraction, and it starts with the idea that “like dissolves like.” Hexane, like oil, is a non-polar hydrophobic compound, meaning they don’t combine with water, so they work well together during the extraction process.
Processors generally prefer hexane as a solvent in extraction because:
- Perfect physical properties. Hexane’s high evaporation rate and low boiling point make it ideal for the extraction process. It’s liquid at ambient temperatures, making it easy to work with at nearly every temperature, and it boils easily at 69 degrees Celsius (about 156 degrees Fahrenheit). Hexane evaporates quickly, allowing it to extract oil with limited energy costs.
- Maximum oil recovery. Solvent extraction is the most efficient means of recovering the most oil possible, leaving less than 1% residual oil in the meal. For high-capacity processors looking to squeeze every drop in profit from their operation, solvent extraction offers an economic advantage at scale. Since solvent extraction is best suited for large-scale plants that process 500 to 1,000 tons of soybean per day or more, the extra oil recovery adds up quickly compared to mechanical plants that may only process 100 tons per day and leave 6% residual oil behind.
- Maintain product quality. Hexane is a selective reagent, which means it selectively extracts fat from the seed material without disturbing other nutrients, fiber, or protein content. This maintains the meal’s nutritional profile created as a byproduct of hexane extraction.
Considering its prevalence in solvent extraction for the largest-volume oilseed commodities around the world, hexane is vital to an efficient, cost-effective global food chain. But that doesn’t mean it’s a perfect process
Concerns over hexane extraction
Unfortunately, even after solvent-extracted oil and meal and processed, they may still contain trace amounts of hexane. Generally, these amounts are too low to have any toxic effect, but some government organizations have imposed limits to maintain safe levels of hexane exposure. For example, the European Union set a maximum residual limit (MRL) of 1 mg/kg of hexane residue in vegetable oils.
However, the U.S. Food and Drug Administration (FDA) does not currently monitor or regulate hexane residue in foods; it only limits how much hexane can be left in fish protein isolate (5 parts per million), hop extract, and spice resins (25 ppm). The cumulative effect of hexane exposure and consumption, even in trace amounts, remains relatively unknown—prodding questions about hexane’s long-term impact on environmental and human health. After all, it has been identified as an air pollutant, a fire hazard, and a potentially harmful substance when inhaled or ingested by humans and animals.
Classified as a hazardous air pollutant under the Clean Air Act, hexane falls under the regulation of the Environmental Protection Agency (EPA). According to the EPA, no studies have evaluated the toxicity of hexane consumption in humans or lab animals. Since hexane evaporates easily, the most common form of exposure is breathing contaminated air, so most available studies (though limited) examine the effects of hexane inhalation exposure instead of consumption. According to the EPA, this information is inadequate for assessing hexane’s carcinogenic (cancer-causing) potential.
However, the EPA does acknowledge the potential hazards of hexane inhalation. Short-term hexane inhalation exposure can cause mild effects on the central nervous system, including dizziness, nausea, and headache. In contrast, long-term exposure is associated with nerve damage, numbness, muscular weakness, blurred vision, headache, and fatigue. So, the EPA recommends 0.2 milligrams of hexane per cubic meter of air (mg/m3) as an estimate of safe inhalation exposure levels. Similarly, the EPA allows emissions between 0.2 to 1.2 gallons of hexane per ton of oilseeds processed.
To mitigate these occupational hazards, safety is essential in hexane extraction plants. Precautions include proper ventilation, personal protective equipment, and standard operating procedures to prevent fires, explosions, and other accidents.
Optimizing oil extraction methods
Although large-scale solvent processors dominate the commodity vegetable oil markets, they do have limitations: Hexane extraction plants can’t produce organic products because of the hazardous chemicals involved.
Driven by the growing demand for natural and organic ingredients, some processors are opting for greener extraction methods like aqueous-assisted enzyme extraction, extraction with green solvents like terpenes derived from citrus peels and tree oils, or mechanical expeller pressing instead of conventional hexane extraction. Mechanical extraction is growing in popularity to serve the organic market, leveraging equipment like Anderson’s High-Shear Extrusion System to physically crush oil from the seeds without any chemical exposure—creating chemical-free, expeller-pressed soybean meal and high-quality oil.
Both mechanical and hexane extraction methods have their place in the oilseed market, serving distinct industry needs and demands ranging from organic food to biofuel to hundreds of different edible and industrial applications in between. Selecting the appropriate method for your plant depends on the type of oilseed you process and the end markets you serve.
To explore safer, more efficient hexane extraction methods for your processing plant, contact Anderson International about our Solvex Expander and other equipment.