A used cooking oil (UCO) specification looks short, but each number on it maps directly onto yield, pretreatment cost or compliance risk at the receiving plant. This article explains what the common parameters mean, the ranges buyers typically see, and how to write them into a contract so that both sides measure the same thing. For the wider feedstock picture, see our buyer's guide to waste and residue feedstocks.
What a UCO specification is for
UCO is a collected waste. Its composition depends on the original frying oils, how long they were used, what was cooked, and how the oil was stored, settled and filtered. A specification does not make that variability disappear; it sets the limits within which the buyer's process can absorb it, and the basis on which price adjustments or rejection apply.
Free fatty acids (FFA)
Frying breaks some triglycerides down into free fatty acids through hydrolysis. FFA is reported as a percentage by mass, normally expressed as oleic acid. It is closely related to acid value (AV, in mg KOH/g): FFA as % oleic acid is approximately AV divided by 1.99.
Typical traded UCO has FFA of about 2–10%, and many contracts set a maximum of 5% or a basis with price adjustments above it. FFA matters differently depending on the route:
- Biodiesel (FAME): above roughly 0.5–1% FFA, the alkaline catalyst forms soap instead of ester, so the plant must esterify the FFA first. More FFA means more pretreatment cost.
- HVO and SAF: hydrotreating converts FFA to hydrocarbons, so FFA is tolerated, but high-FFA oil is corrosive and needs suitable tanks and lines.
MIU: moisture, impurities and unsaponifiables
MIU groups three contaminants that add no fuel value:
- Moisture: free and dissolved water, usually measured by Karl Fischer titration or oven drying. Water promotes hydrolysis, so wet oil gains FFA in storage.
- Insoluble impurities: food particles, carbon and other solids that do not dissolve in a solvent such as petroleum ether or hexane, measured by filtration.
- Unsaponifiable matter: material that does not react with alkali to form soap, such as sterols, hydrocarbons and some polymers, measured by solvent extraction after saponification.
Contracts commonly cap MIU at 2–3%, sometimes with a separate cap of about 1% on moisture and impurities combined. Some contracts pay on a net basis and deduct MIU above an agreed threshold from the invoiced weight.
Iodine value (IV)
Iodine value measures unsaturation: the grams of iodine that react with 100 g of oil (g I2/100 g). It reflects the source oils. UCO rich in palm oil or animal fats has a lower iodine value, while UCO from sunflower, soybean or rapeseed oil is higher. Indicatively, traded UCO ranges from about 60 to 130.
For biodiesel, iodine value matters because EN 14214 limits the finished ester to 120 and because higher unsaturation lowers oxidation stability. Lower iodine value, on the other hand, usually means poorer cold-flow properties in the ester and a UCO that solidifies more readily in transit. For HVO, iodine value mainly affects hydrogen consumption.
Sulphur
Fresh vegetable oils contain very little sulphur, and most UCO is low as well. Sulphur can rise with food residues, some cooking processes and contamination during collection. Indicatively, contracts set a maximum of about 20–50 mg/kg.
For biodiesel, sulphur is a compliance parameter: EN 14214 and EN 590 diesel both limit sulphur to 10 mg/kg, and FAME plants have limited ability to remove it. HVO and SAF units remove sulphur during hydrotreating, so it is less critical for them, although it still consumes hydrogen and creates hydrogen sulphide that must be handled.
Other parameters buyers add
- Phosphorus and metals (sodium, potassium, calcium, magnesium, iron): important for HVO catalysts and for meeting EN 14214 limits on phosphorus and metals in biodiesel.
- Chlorine: a corrosion concern in hydrotreaters, often specified by HVO and SAF buyers.
- Polyethylene: plastic from packaging that bleaching does not remove; some HVO buyers set a limit.
- Density and appearance: simple checks that can reveal adulteration or mislabelling.
Typical UCO specification at a glance
These values are indicative; they illustrate common commercial ranges, not a standard.
- FFA (as oleic acid): typically 5% maximum, with lower-grade lots traded up to about 10%.
- Moisture and impurities: typically 1–2% maximum combined.
- MIU: typically 2–3% maximum.
- Iodine value: typically about 60–130 g I2/100 g, reported rather than capped unless the buyer's product requires it.
- Sulphur: typically 20–50 mg/kg maximum.
- Phosphorus, metals, chlorine and polyethylene: specified by HVO and SAF buyers, usually in mg/kg.
How test methods affect the numbers
Two laboratories can report different values for the same sample if they use different methods. Name the method for each parameter in the contract, for example ISO 660 for acid value and FFA, ISO 3961 for iodine value, ISO 663 for insoluble impurities and Karl Fischer titration for water. For sulphur at low levels, ultraviolet fluorescence methods designed for fuels give more reliable results than general elemental screening.
Sampling and acceptance
Draw samples from every compartment or flexitank, at loading and at discharge, under a procedure both parties have agreed. Retain sealed samples until the lot is accepted and name an independent referee laboratory in advance. UCO settles in storage, so a sample from the top of a tank can differ markedly from the bottom.
Documentation beyond the analysis
For regulated fuel markets, the analysis is only half of the file. UCO is listed in Annex IX Part B of the Renewable Energy Directive, and its value depends on sustainability evidence: registered and audited points of origin, a proof of sustainability, and a traceable chain of custody. Ask for the certificate scope with your quotation, and read our explainer on Annex IX Part A vs Part B for how the category is used. To discuss a specification for a used cooking oil lot, or the requirements of your HVO plant, contact our team.
Frequently asked questions
What does MIU stand for in a UCO specification?
MIU stands for moisture, insoluble impurities and unsaponifiable matter. It is the share of the oil that contributes no fuel value, and it is often used as a price-adjustment parameter.
What FFA level is acceptable for biodiesel?
Alkaline transesterification works best below about 0.5–1% FFA. Plants that buy UCO with higher FFA use acid esterification or another pretreatment step first; the capacity of that step sets the maximum FFA they can accept.
Why does the iodine value of UCO vary so much?
Iodine value reflects the oils originally used for frying. Palm oil and animal fats give lower values; sunflower, soybean and rapeseed oils give higher values. Mixed collections fall in between.
Is sulphur a problem for HVO plants?
Less than for biodiesel, because hydrotreating removes sulphur. It still consumes hydrogen and forms hydrogen sulphide, so many HVO buyers set a limit anyway.
Should the contract use the seller's or the buyer's analysis?
Either can work, provided the contract says which governs, names the test methods and appoints an independent referee laboratory for disputes.