Yellow grease is clean used fryer oil with free fatty acid (FFA) content under 15% — it is valuable and usable as a fuel feedstock. Brown grease is the contaminated, high-FFA material pumped out of grease traps — it is harder to process, has fewer uses, and is typically a cost to dispose of rather than a source of revenue.
The short version
Yellow grease comes from your deep fryers. Brown grease comes from your grease trap. They look different, they are classified differently under waste regulations, they require different haulers in many cases, and they have radically different economics.
Yellow grease, in detail
Yellow grease is the commercial name for used cooking oil that meets commodity specifications. The critical specs are:
- FFA (Free Fatty Acid) content below 15%. FFA rises as oil is used — heat, moisture, and food particles break down triglycerides and free up fatty acids. Oil that has been cycled too long climbs above 15% and drops out of the yellow grease category.
- Moisture content below 1–2%. Water in oil is a disposal problem and a processing problem.
- MIU (Moisture, Insolubles, Unsaponifiables) below 2%. This is the industry catch-all metric for "how clean is it."
Oil that meets these specs can go into renewable diesel, SAF, animal feed, biodiesel, and oleochemicals. It has real market value, measured in cents per pound or dollars per gallon.
Brown grease, in detail
Brown grease is what accumulates in grease traps and interceptors. It is a mixture of fats, oils, grease, water, food solids, and biological material that has been fermenting slowly. Its FFA content is high — often 40% or more — and it arrives at processing facilities mixed with water, food waste, and whatever else has washed into the trap.
Brown grease has some uses: some biodiesel producers can process it, anaerobic digesters can handle it, and it can be rendered down for limited industrial applications. But the processing costs are much higher, and the markets are much smaller.
In practice, most restaurants pay to have brown grease pumped out — it is a waste management expense rather than a commodity sale.
Why the distinction matters for your kitchen
Three practical consequences:
1. Keep them separate
The single fastest way to destroy the value of your yellow grease is to contaminate it with brown grease, water, or kitchen wash-down. Keep fryer oil in a dedicated container that never shares space with trap waste.
2. Different haulers, sometimes
Some collectors handle both; many specialize. A pure yellow grease buyer will not service your grease trap. A grease trap service provider may or may not also buy your fryer oil. Confirm up front what your hauler actually does.
3. Different regulations
State and local regulations often classify yellow grease and brown grease differently. Yellow grease is typically regulated as "used oil" with relatively light manifest and transport requirements. Brown grease is often classified as a special waste or grease trap waste, with stricter disposal rules. The paperwork your hauler generates should clearly distinguish between the two.
How to tell the difference visually
Clean yellow grease is golden to amber in color. It pours freely at room temperature. It smells like slightly used cooking oil — not pleasant, but not offensive.
Brown grease is dark — often near-black. It has layers of water and solids. It smells strongly of decomposition, even when freshly pumped. It does not pour cleanly.
Key takeaway
Yellow grease (fryer oil, FFA under 15%) is a valuable commodity that restaurants can be paid for. Brown grease (grease trap waste, FFA 40%+) is a disposal expense. Keep them separate, and understand which one your hauler is actually handling.