Acid Value & Free Fatty Acids

What acid value and FFA are

When an oil’s triglycerides break down — through moisture, heat, enzymes or age — they release free fatty acids. The acid value is the amount of a base (potassium hydroxide) needed to neutralise those free acids in a gram of oil, so it is a direct measure of how much the oil has degraded. It is usually reported either as the acid value in mg KOH per gram, or as % FFA expressed as a reference acid (commonly oleic). A low value means fresh, well-kept oil; a rising value signals hydrolysis and lower quality.

How it is measured — ISO 660

ISO 660 is the international standard method for determining acid value and acidity in animal and vegetable fats and oils; the widely used AOCS equivalent is Ca 5a. In outline the oil is dissolved in a solvent and titrated with a standardised alkali to a colour or instrumental end-point, and the result is calculated as acid value or % FFA. We describe the method’s role here rather than reproducing it — for the exact, current procedure and calculation, consult the official ISO 660 (or AOCS) document.

Why FFA matters in processing

FFA is one of the most consequential numbers in an oil mill. High-FFA crude oil means greater refining loss, because free acids must be removed — by caustic in chemical refining or by steam stripping in physical refining. In fact, an oil’s FFA level often decides the refining route: high-FFA oils such as palm are commonly physically refined. FFA also reflects how fresh and well-handled the raw material was, since poor storage and delayed processing raise it. Watching acid value from crude through to finished oil is a core quality-control practice.

Indicative limits — and why they vary

Finished, fully refined oils are held to a very low acid value, while crude and some unrefined or virgin oils carry higher, defined maxima. The exact figures are set by the relevant standard — Codex for named vegetable oils, plus national and buyer specifications — and they differ by oil type, grade (crude, virgin, refined) and the standard’s revision. Because of that variation we do not state universal numbers here: treat any limit as indicative and confirm against the specific standard and grade you are working to.

Acid value vs peroxide value

Acid value is easy to confuse with peroxide value, but they measure different things and it helps to keep them apart. Acid value / FFA tracks hydrolytic breakdown — triglycerides splitting to release free fatty acids, driven by moisture, heat, enzymes and time. Peroxide value tracks oxidative breakdown — the primary oxidation products that form as the oil reacts with oxygen, an early marker of rancidity. An oil can be low in one and rising in the other, so quality control usually watches both: FFA tells you about hydrolysis and refining load, peroxide value about freshness and oncoming rancidity. See the test-methods overview for how each is determined.

Units: acid value vs % FFA

Two units appear in practice and it is worth knowing how they relate. Acid value is expressed as milligrams of KOH per gram of oil — the amount of base needed to neutralise the free acids. % FFA expresses the same free acidity as a percentage by weight of a reference fatty acid, most often oleic acid (sometimes palmitic or lauric, depending on the oil). The two are proportional, related by the molar mass of the chosen reference acid, so a specification should state which unit and which reference acid it uses. When comparing figures between sources, first check they are on the same basis — otherwise the numbers are not directly comparable.

Tracking acid value through the process

Acid value is not a single checkpoint but a number followed along the line. It is measured on incoming crude to grade the raw oil and plan refining; monitored in process to confirm neutralization or steam stripping is working; and checked on finished oil to prove it meets specification. A rising acid value in storage warns that oil is hydrolysing and should be used or reprocessed. Because it both grades the input and verifies the output, acid value is one of the first tests an oil laboratory sets up — and one of the clearest single indicators of how well the whole handling and refining chain is performing.

What raises FFA — and how to limit it

Because high FFA is costly, it helps to know what drives it up. The main causes are moisture (which fuels hydrolysis), heat, the seed’s or fruit’s own enzymes acting after harvest, microbial activity, and simple time — delay and poor storage. Some raw materials are inherently prone: oil-palm fruit, for example, begins developing free fatty acids quickly after harvest, which is why prompt sterilization and milling matter. To limit FFA, processors keep seed and oil dry and cool, process fresh material promptly, degum and refine without long delays, and store finished oil away from heat, light and moisture. Controlling FFA is therefore as much about good handling and timing as about the refining step that removes it.

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