Edible oil refining: from crude to refined oil, step by step

Edible oil refining turns crude oil into an oil fit for food through a fixed sequence: degumming, neutralisation (chemical refining only), bleaching, deodorisation, and for some oils winterisation or fractionation. The lab follows the oil through these steps, because each step removes one group of impurities and each can leave its own trace in the result.

Why the crude oil is refined at all

Crude oil from crushing or from a mill holds more than triglycerides: phosphatides (gums), free fatty acids, pigments, traces of metals, oxidation products and volatile compounds that give flavour and odour. FEDIOL, the association of the European vegetable oil industry, describes refining as the removal of these components, step by step. Codex Alimentarius sets what a refined oil may still contain in its Standard for Named Vegetable Oils. The Codex figures in this article are from the 2015 amendment of CODEX STAN 210-1999, which has been amended since; check the current edition before you quote a limit in a contract.

The steps and what the lab tests

Step	What happens	What the lab tests, and why
Degumming	Water or food-grade processing aids, at an elevated temperature that depends on the method, hydrate the phosphatides, proteins, carbohydrates and trace metals so they can be separated	Phosphorus (ISO 10540-1 or AOCS Ca 12-55): shows how much gum is left
Neutralisation	Caustic soda converts free fatty acids into soap, which is separated as soapstock	Free fatty acid (AOCS Ca 5a-40 or ISO 660) before and after; soap (AOCS Cc 17-95) after washing
Bleaching	Bleaching earth takes out colour bodies, residual soap and gums, trace metals and oxidation products	Colour (AOCS Cc 13e-92), soap, peroxide value, iron and copper
Winterisation or fractionation	Controlled cooling and filtration: waxes out of sunflower oil; palm oil split into olein and stearin	The contract's own tests; iodine value to characterise the fractions
Deodorisation	Vacuum steam distillation removes volatiles, at 0.5 to 8 mbar and 180 to 270 °C according to FEDIOL	Free fatty acid, peroxide value, moisture and volatile matter, colour, and the process contaminants below

Degumming

The lab measures phosphorus as a proxy for the gums left in the oil. Crude oils are normally bought on contract terms that state their own limits, so the test is as much about agreeing the method as about the number.

Neutralisation and physical refining

In chemical refining the free fatty acid figure falls here, which is why the lab reads it on both sides of the step. Physical refining has no neutralisation step. The free fatty acids are distilled off in the deodoriser instead, so the oil goes to the deodoriser with a higher FFA. The Malaysian Palm Oil Board gives deodorisation temperatures of 200 to 240 °C for chemical refining and 240 to 270 °C for physical refining. The free fatty acid figure and how labs express it is covered in free fatty acid and iodine value.

Bleaching

After bleaching the oil is checked for colour and for what bleaching earth can leave behind. Codex's supplementary quality factors for refined oils (2015 text) give maximum levels for soap (0.005% m/m), iron (1.5 mg/kg) and copper (0.1 mg/kg), because copper in particular speeds up oxidation, as FOSFA's Code of Practice for Member Superintendents notes.

Winterisation and fractionation

Sunflower oil holds waxes that crystallise in the cold and cloud the oil. Winterisation cools the oil and filters them out, often with a filtration aid such as perlite or diatomaceous earth. Palm oil is separated by controlled cooling into a liquid olein and a solid stearin with different melting points and fatty acid composition (see palm oil density for the physical side). Not every oil goes through this step.

Deodorisation

Deodorisation leaves the oil with its final flavour and odour. The lab looks at the same indicators as before, plus the two classes of contaminant that form when oil is held hot. Codex's code of practice CXC 79-2019 notes that glycidyl esters start to form at about 200 °C and more so above 230 °C, and notes that oil can be deodorised below 230 °C to avoid significant glycidyl ester formation. In the 2015 Codex text, peroxide value for refined oil is capped at up to 10 milliequivalents of active oxygen per kg, acid value at 0.6 mg KOH/g, and matter volatile at 105 °C at 0.2% m/m.

Contaminants and trans fats

3-MCPD esters and glycidyl esters are measured by GC-MS (ISO 18363-1 or AOCS Cd 29c-13). Regulation (EU) 2023/915 sets maximum levels for the sum of 3-MCPD and its esters and for glycidyl fatty acid esters in vegetable oils and fats; Annex I, section 5 lists them per category, and lower limits apply to oil intended for infant food. Regulation (EU) 2026/1825 of 29 July 2026 amends Regulation (EU) 2023/915 in this area. We could not open the text of that amendment, so we do not describe it in detail; from secondary sources it mainly concerns baby food, cereal-based infant food and the oils and fats made for them, with part of it applying from 1 January 2027. We do not quote limit values because they differ per category; look them up in the current consolidated text.

Trans fatty acids are a separate regulation, (EU) 2019/649: a limit of 2 g per 100 g of fat in food for the final consumer and for retail. ISO 15304 was written to evaluate trans isomers formed during high-temperature refining and hydrogenation, and has since been replaced by the ISO 12966 series.

Where the surveyor meets this

A refinery lab works along the process. A cargo surveyor meets the result: a parcel of refined oil in a tank or a ship, sampled and analysed against the contract, often a FOSFA form (see what FOSFA is and what a cargo surveyor does). The spec there is the contract's, and Codex or the destination law may add to it.

Quality specs and what else is analysed

Quality specifications serve the trade contract: they show whether the oil meets the agreed quality. Beyond those, refineries and laboratories analyse for impurities and hazardous substances. When a new ship with a coating arrives, the previous coating can be tested too, for coating solvents. On top of that sit the food-safety specifications set by the EU, which the oil has to meet.

Practical rule

Write the method and its expression beside every result: "FFA by AOCS Ca 5a-40, as palmitic, %" rather than "FFA 0.08". The same oil gives different numbers under different methods and reference acids, and a figure without its method is hard to defend when a parcel is disputed.

Based on the working practice of our sector specialist: 25 years in the port, 20 of them as a cargo surveyor. Refineries analyse for contaminants and hazardous substances, and for a new ship with a coating also for coating solvents; the EU food safety specifications apply on top.

From nomination to certificate

For the parcel that leaves the refinery, the test results only help if they sit with the right nomination and end on the certificate. FOSS, short for Field Operational Services System, is the software from Operium for that document flow: entry on board, also offline, the path from nomination to certificate, built-in calculation methods as used in the sector, document generation and a client portal that shows the client nomination, progress and documents. An audit log records who entered which value. See FOSS or get in touch.