In order to understand the corn oil extraction process and how corn oil is extracted during ethanol production, it helps to first understand how corn is processed.
Broadly speaking, corn processing is broken up into wet milling and dry milling.
Processing Corn by Wet Milling
Wet milling is primarily used to make food ingredients: sweeteners, starch, corn oil, and so on. (Ethanol can be produced this way, but it’s less common; only about 10 percent of U.S. ethanol production starts with wet milling.)
This process is called “wet milling” because the corn is pre-soaked to soften the kernel and begin separating it into its constituent parts. It’s then fed through a series of mills and centrifuges. These separate the germ, starch, fiber, and gluten proteins. These are then separately processed to create various food additives and other products:
The corn oil that comes out of the wet-milling process is primarily intended for human consumption, and is held to those standards. It’s often called “crude corn oil” (CCO).
Wet milling is a very versatile process—it can be tuned to produce many different co-products in differing ratios. But it is also capital intensive, and relatively slow.
Processing Corn Dry Milling
Dry milling provides the bulk of U.S. ethanol, in addition to animal feed, and some corn oil. This corn oil—used to supplement feed, or as the basis of biodiesel—is often called “distiller’s corn oil” (DCO), or sometimes “technical corn oil” (TCO).
In contrast to wet milling, dry milling starts with grinding the entire corn grain to a fine flour. This is then mixed with water and other ingredients to break it up into starches and simple sugars. Those are fermented to create ethanol, which is then purified via distillation and dehydration. The remaining “distillers grains” are spun down through a series of centrifuges to extract corn oil, and the remaining stillage is mixed with any nutrient-rich wastewater from the earlier steps, then dried to become animal feed (called “dry distillers grain and solubles” or “DDGS”).
Dry milling is a more limited process—it only has a few possible products, primarily animal feed, and ethanol. That said, it is also extremely efficient, pumping out large volumes to feed high demand.
How Is Corn Oil Extracted Differently in Different Plants?
Wet milling—as primarily a food production process—relies on a traditional food-production friendly solvent-based method of extracting corn oil, which usually relies on hexane. Hexane is extremely effective in this role (capturing 90 percent of the corn oil), and relatively easy to remove from the resulting extract, leaving pure corn oil behind. That’s good news if you’re looking to make food-grade oil. While a solvent-based extraction is very consistent, it’s also a slow and expensive process.
Dry-grind plants take a different approach, largely relying on two-phase centrifuges. This is fast and inexpensive—but the yields can seem to vary quite unpredictably. That’s because this process is newer—something ethanol producers really only started reliable pursuing over the last dozen or so years—and subject to a large number of contributing factors that (everything from daily variations in stillage to centrifuge speed to ambient temperature).
Today, most dry-grind ethanol plants are extracting corn oil—it’s an important way to protect their profit margins as ethanol prices dip and dive. Those who are doing so effectively are relying on emulsion breakers to smooth out this variability and drastically increase yields. While most emulsion breakers are polysorbate-based, a new generation of non-polysorbate based chemistry is gaining traction. These new breakers are much more efficient for many plants, increasing yields while drastically reducing the amount of chemistry needed to extract corn oil.