Acid oils are the concentrated fatty acid streams that come out of vegetable oil refining, and esterified acid oils are those streams upgraded by esterification into material a biodiesel plant can actually run. Two things govern the trade. Commercially, total fatty acid is the pricing basis, not tonnage: you are buying the usable fraction, not the drum. Regulatory, the origin of the parent oil decides the Annex IX position and therefore much of the value.
The family, and where esterification fits
The technical reason esterification exists: conventional transesterification uses a base catalyst, and base catalysts do not tolerate high free fatty acid, which saponifies instead of converting. Acid-catalysed esterification converts the FFA to methyl ester first, so a stream that would ruin a batch becomes a feedstock. That single processing step is the difference between a disposal problem and a traded commodity.
- Soapstock, the alkaline by-product of chemical refining. Not yet an acid oil.
- Acid oil, soapstock acidulated so the fatty acids are released and concentrated. This is the traded product.
- Palm acid oil (PAO) and palm fatty acid distillate (PFAD), from palm refining. PFAD comes from physical refining and is a distillate rather than an acidulation product, which is a genuine distinction rather than a labelling one.
- Esterified acid oil (EAO), acid oil that has been esterified, converting the free fatty acids into methyl esters. This is what makes very high FFA material usable in a conventional biodiesel plant.
What decides the price
| Total fatty acid (TFA) | The pricing basis. Everything else is secondary. |
|---|---|
| FFA content | Percentage, and which acid it is stated as. |
| Ester content | For esterified material: how far the conversion went. |
| Moisture and impurities (M&I) | Settlement weight and processing yield. |
| Unsaponifiable matter | The ceiling on usable yield. |
| Sulphur | Critical where the material is heading to hydrotreatment. |
| Acid value | Corrosivity and residual conversion. |
| Iodine value | Unsaturation; drives cold-flow downstream. |
| Phosphorus and metals | Pretreatment load and catalyst impact. |
| Parent oil origin | Palm, soy, rape, sunflower, corn or mixed. Decides certification and Annex IX position. |
| Residual methanol | For esterified material: safety, transport and classification. |
| Certification | ISCC EU with the scope covering the actual material. |
The classification question worth settling first
Acid oils sit in an awkward place. Whether a given stream counts as a residue, a co-product or a processing by-product affects how it is treated under EU renewable fuel rules, and that treatment is decided nationally rather than at EU level.
This is not a technicality. The same tank of acid oil can be economic in one member state and not in the next, and the answer can change between delivery years. Before signing anything multi-year, confirm the position for the specific stream in the specific destination market, in writing. We will tell you what we understand it to be and where we are not certain, which on this product is more often than either of us would like.
Related: vegetable and technical oils for the wider refining side stream picture, and Annex IX for the lists.
Frequently asked questions
What is an esterified acid oil?
An acid oil that has been through esterification, converting its free fatty acids into methyl esters. This matters because conventional base-catalysed transesterification cannot handle high free fatty acid content, which saponifies instead of converting. Esterification turns a stream that would ruin a batch into a usable biodiesel feedstock.
How is acid oil priced?
On total fatty acid content, not on tonnage. You are buying the usable fraction, so a lower-TFA parcel at a lower headline price is frequently the more expensive option. Always compare on a TFA basis rather than on price per tonne.
What is the difference between acid oil, soapstock and PFAD?
Soapstock is the alkaline by-product of chemical refining. Acid oil is soapstock that has been acidulated to release and concentrate the fatty acids. PFAD, palm fatty acid distillate, comes from physical refining and is a distillate rather than an acidulation product. All three price on fatty acid content.
Do acid oils qualify under Annex IX?
It depends on the specific stream and on national implementation. Whether a stream is treated as a residue, a co-product or a processing by-product affects its position, and member states have taken different views. Confirm the treatment for your stream in your destination market, in writing, before contracting on a multi-year basis.
Why does the parent oil origin matter?
Because it decides both the certification route and, in many cases, the Annex IX position and the market access. Palm-origin acid oil and corn-origin acid oil can be technically similar and commercially very different. Always state the parent oil on the offer.
Can you place high FFA material that nobody else wants?
Often, yes. High FFA is a problem for a conventional biodiesel plant and not necessarily for an esterification plant, an oleochemical buyer or an energy outlet. The useful question is not how bad the FFA is but which processing route is available nearby, because that decides whether it is a discount or a disposal cost.
Who to ask about Esterified acid oils
Ask us. Acid oil to place? Send the total fatty acid, the moisture and the origin of the parent oil. We answer the same working day, in English or Dutch, and there is no charge and no obligation for a market read or a second opinion on a specification.
You get Bart van den Brug directly, not a call centre and not a form that goes nowhere. We are an independent broker: we never take title, we do not trade our own book, and we are paid a commission only on business that actually concludes. That is why we will also tell you when the answer is no.
+31 6 115 83 448
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Sustainable Commodities 3 B.V., Lemmer, the Netherlands
Last reviewed 09 August 2026 by Bart van den Brug, Sustainable Commodities 3 B.V. (KvK 99665042), Lemmer, the Netherlands. Regulatory references are given for orientation and are not legal advice; verify against the current Official Journal text before contracting.