Oil and Fat Acid Value Testing Methods: Principles, Standards, and Procedures

The acid value is one of the most important quality indicators for oils and fats, quantifying the free fatty acids released through hydrolysis and oxidative degradation. Reliable acid value determination, performed in accordance with international standards such as ISO 660 and GB 5009.229, underpins edible oil grading, refining process control, frying oil management, and food safety compliance. This technical article explains the principles, sample types, testing procedures, and practical applications of acid value testing for oils and fats.

Technical Overview

The acid value (acid number) of an oil or fat is defined as the mass of potassium hydroxide, in milligrams, required to neutralize the free fatty acids present in one gram of sample. It is expressed in mg KOH/g and serves as a direct measure of the extent of triglyceride hydrolysis, during which free fatty acids are liberated from the glycerol backbone. Elevated acid values indicate raw material deterioration, incomplete or improper refining, moisture exposure, enzymatic lipase activity, or thermal abuse during processing and frying.

Because free fatty acid accumulation is closely linked to rancidity development and off-flavor formation, the acid value is a primary acceptance criterion in edible oil specifications and national food safety standards. Most food safety regulations for refined vegetable oils set a maximum acid value of 3 mg KOH/g, while premium categories such as extra virgin olive oil are subject to stricter commercial limits. The acid value can also be converted to free fatty acid (FFA) content; when expressed as oleic acid, the approximate relationship is FFA (%) = acid value / 1.99.

The principal testing standards include ISO 660 (Animal and vegetable fats and oils — Determination of acid value and acidity), which is harmonized as EN ISO 660, GB/T 5530 for animal and vegetable fats and oils, and GB 5009.229 for the determination of acid value in foods. Potentiometric acid number determination, widely applied to dark-colored oils and biodiesel feedstocks, follows the approach established in ASTM D664.

Testing Samples

Edible Vegetable Oils

Routine acid value testing covers all major vegetable oils, including soybean, rapeseed (canola), peanut, sunflower, corn, cottonseed, sesame, rice bran, camellia, olive, and palm oil products. Both refined oils and cold-pressed or virgin oils are evaluated, with virgin oils typically assessed against tighter commercial thresholds.

Animal Fats and Marine Oils

Lard, tallow, chicken fat, butterfat, and fish oils are frequently tested because animal-derived materials are prone to rapid lipase-mediated hydrolysis. Acid value results guide refining decisions and shelf-life evaluation for these fats.

Crude and Semi-Refined Oils

Crude pressed and solvent-extracted oils, degummed oils, and neutralized intermediates are monitored at multiple stages of the refining line. Acid value data determine alkali dosing during neutralization and verify the effectiveness of degumming, washing, and deodorization steps.

Used Frying Oils and Special Matrices

Frying oils from food service operations, oilseeds, oilseed meals, margarine, shortening, fatty acid distillates, acid oils, waste cooking oils, and biodiesel feedstocks are also common test samples. Dark or highly polymerized samples require special method selection, as visual indicator endpoints may be obscured.

Testing Items

The core testing item is the acid value expressed in mg KOH/g. Closely associated determinations include:

• Free fatty acid (FFA) content, expressed as oleic acid for most vegetable oils, as lauric acid for coconut and palm kernel oils, or as palmitic acid for palm oil trade specifications
• Peroxide value, which characterizes primary oxidative degradation and is commonly tested alongside the acid value
• Moisture and volatile matter, since water accelerates hydrolysis and elevates the acid value during storage
• Saponification value and iodine value for complete characterization of oil quality in grading and identification studies

Testing Methods

Cold Solvent Indicator Titration

This is the primary reference method described in ISO 660 and the first method of GB 5009.229. A accurately weighed test portion is dissolved in a neutralized 1:1 (v/v) mixture of ethanol and diethyl ether, or another approved solvent mixture, and titrated with standardized 0.1 mol/L potassium hydroxide (or sodium hydroxide) solution using phenolphthalein as indicator. The endpoint is the appearance of a pale pink color that persists for at least 15 to 30 seconds with continued shaking. The acid value is calculated as AV = (V − V0) × c × 56.1 / m, where V is the titrant volume, V0 the blank volume, c the titrant molarity, 56.1 the molar mass of KOH, and m the sample mass in grams. Sample mass is selected according to the expected acid value, typically from about 20 g for very low values down to 1 g or less for highly acidic samples, so that titrant consumption falls within an optimal range.

Hot Ethanol Titration

For fats and oils that do not dissolve completely at room temperature, the hot solvent method is applied. Neutralized ethanol is heated to approximately 70 to 80 degrees Celsius or to gentle boiling, the test portion is dissolved in the hot solvent, and titration is performed while warm. This procedure, given in ISO 660 and the corresponding hot solvent method of GB 5009.229, ensures complete dissolution of solid fats such as tallow, palm stearin, and hard margarine stocks and prevents fatty acid crystallization errors.

Potentiometric Titration

For dark-colored, highly pigmented, or emulsified samples where the phenolphthalein endpoint cannot be observed reliably, potentiometric titration with a glass electrode and a suitable reference electrode is used, typically on an automatic titrator. The titration is performed in ethanol or isopropanol medium, and the endpoint is detected from the inflection of the titration curve or a fixed pH criterion. This approach, reflected in the potentiometric method of GB 5009.229 and consistent with the principles of ASTM D664 for acid number determination, improves precision and eliminates subjective color judgment.

Sample Preparation and Key Operational Controls

Representative sampling and careful handling are decisive for valid results. Solid fats should be melted at a temperature no more than about 10 degrees Celsius above their melting point and homogenized before weighing. Samples must be protected from light, heat, and air, analyzed promptly, and filtered if sediment or impurities are present. All solvents must be neutralized immediately before use, blank determinations should be conducted, and duplicate results must agree within the repeatability limits stated in the applicable standard. Titrant concentration should be verified regularly, with 0.01 mol/L or 0.05 mol/L solutions used for samples of very low acidity to reduce relative error.

Applications

Acid value testing is applied throughout the oils and fats value chain:

• Factory quality control and oil grading, verifying conformity with national food safety standards for edible vegetable oils and fried food products
• Refining process optimization, including alkali neutralization dosing and verification of degumming, bleaching, and deodorization performance
• Frying oil management in commercial kitchens and food plants, supporting objective discard decisions and total polar material monitoring programs
• Shelf-life studies and accelerated storage testing for packaged oils, snacks, infant formula fats, and bakery shortenings
• Biofuel production, where acid number limits in biodiesel specifications such as ASTM D6751 (maximum 0.50 mg KOH/g) are verified using potentiometric determination
• Regulatory compliance, customs and trade arbitration, and product certification, typically carried out by accredited third-party laboratories under ISO/IEC 17025 management systems
• Raw material screening for cosmetics, pharmaceutical excipients, and animal feed fats, where hydrolytic degradation directly affects product stability and safety

FAQ

What is a normal acid value for edible oil?

Refined edible vegetable oils should generally have an acid value below 3 mg KOH/g, which is the limit set by most national food safety standards for edible plant oils. Freshly refined oils typically measure well below 0.5 mg KOH/g, while virgin and cold-pressed oils may carry higher but still specification-compliant values.

Why does the acid value increase during storage?

The acid value rises because triglycerides hydrolyze in the presence of moisture, heat, and lipase enzymes, releasing free fatty acids. Prolonged exposure to air, light, and repeated heating cycles, such as in deep frying, accelerates both hydrolysis and oxidation, driving the value steadily upward.

What is the difference between acid value and free fatty acid content?

The acid value is the titrimetrically measured quantity in mg KOH/g, whereas FFA content is a percentage calculated from the acid value using the molecular weight of a reference fatty acid. For oleic acid basis, FFA (%) is approximately the acid value divided by 1.99.

Which method should be used for dark or strongly colored oils?

Potentiometric titration is preferred for dark oils such as sesame oil, toasted oils, used frying oils, and crude fish oils, because the phenolphthalein color endpoint is difficult or impossible to judge visually in such matrices.

How can the acid value of an oil be reduced?

Industrial reduction is achieved through alkali refining (chemical neutralization) or physical refining with steam distillation at high temperature under vacuum. Preventively, storing oils in cool, dry, light-protected conditions in well-filled containers and avoiding moisture contamination minimizes acid value growth.

How precise is acid value testing?

When performed to standardized procedures with calibrated equipment, duplicate determinations should agree within the repeatability limits specified in ISO 660 or GB 5009.229. Accredited third-party laboratories reporting under ISO/IEC 17025 provide results with defined measurement uncertainty suitable for compliance and arbitration purposes.