🍎 Acidity of fats (acid value)

Food Chemistry PN-EN ISO 660

Overview

STATUS (as of 03.11.2026). PN-EN ISO 660:2021-03 — Polish version (published on 21.11.2023; English version on 09.03.2021), implements EN ISO 660:2020 and ISO 660:2020 [IDT] and replaced PN-EN ISO 660:2010; the PKN catalogue card has no note of withdrawal. ISO 660:2020 (fourth edition, March 2020) has the stage “International Standard confirmed” in the iTeh catalogue. The fourth edition added to the scope the exclusion of milk and milk products (ISO 1740 applies to them), information on the CMR classification of coloured indicators and data from a comparison of indicators (Annex B).

SCOPE (clause 1). Three methods — two titrimetric and one potentiometric — for the determination of acidity in animal and vegetable fats and oils; the result is expressed preferably as acid value or, alternatively, as acidity calculated conventionally. The standard applies to refined and crude fats and oils, soap stock fatty acids and technical fatty acids; it does not apply to waxes or to milk and milk products (or fat from them). The methods are completely non-specific — the acid value also includes any mineral acids present.

DEFINITIONS AND PRINCIPLE (clauses 3, 4). Acid value — the number of milligrams of potassium hydroxide required to neutralise the free fatty acids present in 1 g of fat (expressed in mg/g). Acidity — the content of free fatty acids as a percentage by mass; reported without further explanation, it is by convention the acidity based on oleic acid. The sample is dissolved in a suitable solvent mixture and the acids present are titrated with an ethanolic or methanolic solution of sodium or potassium hydroxide; the methods of 9.1 and 9.2 are reference methods.

METHODS (clause 9). Cold solvent method with indicator (9.1, reference): the test portion from Table 1 is dissolved in a 250 ml conical flask in 50–100 ml of the neutralised solvent mixture, with gentle warming if necessary (high melting point samples — in an ethanol–toluene mixture), the indicator is added and the solution is titrated with constant swirling until a single drop of alkali produces a slight but definite colour change persisting for at least 15 s. Cold solvent method with potentiometric titration (9.2, reference): test portion in a 150 ml beaker, the same mixture, a combined electrode, stirring for at least 30 s before the titration and reading of the consumption at the equivalence point. Hot ethanol method with indicator (9.3): 50 ml of ethanol with 0.5 ml of indicator are heated to boiling and neutralised with a 0.1 mol/l solution while the temperature is still above 70 °C, added to the test portion, brought to the boil and titrated with vigorous agitation; short-chain fatty acids are volatile under these conditions.

TEST PORTIONS (Table 1). Refined vegetable oils and animal fats (acid value 0–1): 20 g, alkali 0.1 mol/l, weighing accuracy 0.05 g; crude vegetable oils (1–4): 10 g, 0.1 mol/l, 0.02 g; technical grade animal fats (4–15): 2.5 g, 0.1 mol/l, 0.01 g; soap stock fatty acids (15–75): 0.5 g with 0.1 mol/l or 3.0 g with 0.5 mol/l, 0.001 g; technical fatty acids (above 75): 0.2 g with 0.1 mol/l or 1.0 g with 0.5 mol/l, 0.001 g.

SOLVENTS AND INDICATORS (clause 5). A mixture of equal volumes of ethanol (about 96 %; as a replacement propan-2-ol about 99 %) and peroxide-free diethyl ether (as a replacement tert-butyl methyl ether, light petroleum with a boiling range of 40–60 °C or toluene); for hard or animal fats, 1 volume of ethanol and 3 volumes of tert-butyl methyl ether or toluene are recommended. The mixture is neutralised just before use with potassium hydroxide solution in the presence of 0.3 ml of indicator solution per 100 ml. Indicator: thymolphthalein or alkali blue 6B (2 g/100 ml in ethanol) or, failing that, phenolphthalein (1 g/100 ml in ethanol); phenolphthalein is classified as a CMR substance and should only be used when no alternative is available, and alkali blue or thymolphthalein shall be used for dark-coloured fats.

CALCULATION (clause 10). Acid value wAV = 56.1 × c × V / m; acidity wFFA (as a percentage by mass) = V × c × M × 100 / (1000 × m), where M is the molar mass of the acid chosen for expressing the result according to the fat type (Table 2 of the standard); approximately wFFA = 0.5 × wAV. The result is reported to two decimal places for values up to and including 1, to one decimal place for values above 1 up to and including 100 and as a whole number above 100.

REGULATIONS. Commission Delegated Regulation (EU) 2022/2104 (Annex I part A) allows an acidity of ≤ 0.80 % for extra virgin olive oil and ≤ 2.0 % for virgin olive oil. The method used for determining the acidity of olive oil is, however, COI/T.20/Doc. No 34 (determination of free fatty acids, cold method), specified in Annex I to Commission Implementing Regulation (EU) 2022/2105.

WHAT WE DO NOT GIVE HERE. Table 2 (molar masses of acids for expressing the acidity of individual fat types), precision data (clause 11, Annex A) and the comparison of indicators (Annex B) — they are outside the available text of the standard. Typical acid values of individual fats.

Method principle

A test portion of fat is dissolved in a neutralised solvent mixture, usually equal volumes of ethanol and diethyl ether. The free acids are titrated with an ethanolic or methanolic solution of sodium or potassium hydroxide against an indicator or potentiometrically. The acid value is calculated from the formula 56.1 × c × V / m, and the acidity as a percentage by mass, expressed as the acid chosen for the fat type.

Applications

• Animal and vegetable fats and oils, refined and crude (clause 1)
• Soap stock fatty acids and technical fatty acids (clause 1, Table 1)
• Dark-coloured fats — titration against alkali blue or thymolphthalein (5.6, 9.3.3)

Key parameters

Standard

Standard number

PN-EN ISO 660:2021-03

Title (PL)

Oleje i tłuszcze roślinne oraz zwierzęce — Oznaczanie liczby kwasowej i kwasowości

Title (EN)

Animal and vegetable fats and oils — Determination of acid value and acidity

Step-by-step procedure

1. Sample

   Prepare the test sample according to ISO 661; if it contains volatile fatty acids, do not heat or filter it (clause 8).

2. Test portion

   Select the test portion mass and the alkali concentration from Table 1 according to the expected acid value and weigh into a 250 ml conical flask (9.1.1, 9.1.2).

3. Dissolution

   Add 50–100 ml of the solvent mixture neutralised just before use and dissolve the test portion, with gentle warming if necessary; dissolve high melting point samples in an ethanol–toluene mixture (5.3, 9.1.3).

4. Titration with indicator

   Add the indicator and titrate with constant swirling using NaOH or KOH solution until a single drop of alkali produces a slight but definite colour change persisting for at least 15 s (9.1.4).

5. Potentiometric variant

   Dissolve the test portion in a 150 ml beaker as above, immerse the combined electrode, stir for at least 30 s and titrate to the equivalence point (9.2).

6. Calculation

   Acid value = 56.1 × c × V / m; acidity = V × c × M × 100 / (1000 × m), approximately 0.5 × acid value; choose the number of decimal places according to 10.1 (clause 10).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Diethyl ether is very flammable and may form explosive peroxides — the standard requires it to be used with great caution (5.2)
• Phenolphthalein is classified as a carcinogenic, mutagenic or toxic for reproduction (CMR) substance — use it only when no alternative is available (5.6)
• The standard draws attention to regulations on the handling of hazardous substances and requires technical, organisational and personal safety measures (clause 5)

Frequently asked questions