The acid value (acid number) is one of the most important quality indices for edible oils, animal fats, and lipid-based products, quantifying the free fatty acids released by hydrolytic degradation. This article explains the acid value testing specifications for oils and fats, covering definition, sampling, titration methods under GB 5009.229 and ISO 660, calculation formulas, acceptance limits, and practical quality control requirements. It serves as a technical reference for food manufacturers, refiners, and accredited third-party laboratories performing acid value verification.
Technical Overview
The acid value (AV), also called the acid number or acid price in some trade contexts, is defined as the mass of potassium hydroxide (KOH), in milligrams, required to neutralize the free fatty acids present in one gram of an oil or fat. It is expressed in mg KOH/g (sometimes written as mg/g) and directly reflects the concentration of free fatty acids liberated by hydrolysis of triglycerides or by microbial and enzymatic activity.
An elevated acid value indicates hydrolytic rancidity, incomplete refining, raw material deterioration, or abuse during storage, transport, and frying. Because of this diagnostic power, acid value is a mandatory compliance parameter in most edible oil specifications worldwide.
The principal standards governing acid value determination include:
- GB 5009.229-2016 — Chinese national food safety standard, Determination of Acid Value in Foods (three titration methods).
- ISO 660:2020 — Animal and vegetable fats and oils, Determination of acid value and acidity.
- GB/T 5530 — Chinese national standard for animal and vegetable fats and oils, technically aligned with ISO 660.
- AOCS Cd 3d-63 — American Oil Chemists' Society method for acid value of fats and oils.
- ASTM D664 — Potentiometric titration approach referenced for acidic number determination in certain lipid and non-food applications.
- GB 2716-2018 — Chinese food safety standard for edible vegetable oils, which sets maximum permitted acid value limits (typically ≤ 3 mg KOH/g for most categories).
The analytical principle is uniform across these standards: a weighed test portion is dissolved in a neutralized mixed solvent, and the free fatty acids are titrated with a standardized alcoholic KOH (or NaOH) solution to a visual indicator endpoint or a potentiometric endpoint. The consumed titrant volume is converted into mg KOH per gram of sample.
Testing Samples
Acid value testing applies to virtually all triglyceride-based materials and lipid matrices. Proper sampling and sample preparation, generally following ISO 661 or equivalent procedures, are prerequisites for valid results.
Edible Vegetable Oils
Crude and refined oils including soybean oil, rapeseed (canola) oil, peanut oil, sunflower seed oil, corn oil, olive oil, palm oil and palm olein, sesame oil, rice bran oil, camellia oil, and blended cooking oils. Refined products normally show acid values below 0.6 mg KOH/g, while crude and pressed oils may be significantly higher.
Animal Fats and Marine Oils
Lard, tallow, chicken fat, fish oil, and fish liver oils, which are prone to faster hydrolysis and therefore require closer acid value surveillance during storage.
Used and Deep-Frying Oils
Frying media collected from commercial kitchens and food plants, where acid value is widely used as a disposal and oil-change criterion alongside total polar materials.
Oil-Bearing Materials and Process Intermediates
Extracted lipids from oilseeds, crude miscella samples, soapstock, and neutralization intermediates used to monitor refining efficiency.
Industrial and Special Matrices
Feed fats and pet food lipids, bakery shortening and margarine, oleochemical feedstocks, biodiesel precursors, cosmetic lipid ingredients, and pharmaceutical excipients.
Sample Handling Requirements
- Samples must be representative of the lot and protected from light, heat, and air ingress to prevent artificial hydrolysis and oxidation before analysis.
- Solid or semi-solid fats should be melted at the lowest practical temperature (typically not more than 10 °C above the melting point) and homogenized before weighing.
- Turbid samples or samples containing sediment should be filtered; insoluble foreign matter and free water must not be included in the test portion.
- Laboratory samples should be filled into clean, dry, airtight glass containers with minimal headspace and analyzed promptly.
Testing Items
The core deliverable is the acid value of the sample, reported in mg KOH/g, together with the derived free fatty acid (FFA) content where required. Typical testing items and closely associated parameters include:
- Acid value (mg KOH/g) — the primary index of hydrolytic degradation.
- Free fatty acidity (% as oleic acid, palmitic acid, or lauric acid depending on the fat type) — calculated from the acid value.
- Peroxide value — complementary index of oxidative rancidity, frequently tested in the same workup.
- Moisture and volatile matter — relevant co-factor accelerating hydrolysis.
- Conformance assessment against product specification or regulatory limits, such as GB 2716-2018.
A single test portion of a few grams to a few tens of grams is usually sufficient, and duplicate determinations are standard practice to demonstrate precision.
Testing Methods
GB 5009.229-2016 defines three titration variants, and ISO 660/AOCS methods follow the same underlying chemistry. Method selection depends on the sample matrix, expected acidity level, and available instrumentation.
Cold Solvent Indicator Titration (GB 5009.229, First Method)
The test portion is dissolved at room temperature in a neutralized 1:1 (v/v) mixture of diethyl ether and ethanol (or isopropanol). A few drops of phenolphthalein indicator are added, and the solution is titrated with standardized 0.1 mol/L or 0.5 mol/L KOH in ethanol to a faint pink endpoint persisting for at least 30 seconds. This method is the routine choice for most liquid edible oils.
Hot Ethanol Method (GB 5009.229, Second Method; ISO 660 Variant)
The sample is dissolved in ethanol that has been neutralized while hot and titrated hot with alcoholic KOH. Heating improves solubility and endpoint sharpness for dark, viscous, or solid fats such as tallow, palm stearin, and used frying oils.
Potentiometric Titration (GB 5009.229, Third Method)
The test portion dissolved in the mixed solvent is titrated with standardized KOH using a glass electrode system, detecting the endpoint at pH 8.3 or via inflection point analysis. This approach eliminates subjective endpoint judgment and is preferred for deeply colored oils, e.g., dark sesame oil, chili oil, and roasted nut oils.
Test Portion Selection and Calculation
The mass of the test portion is chosen according to the expected acid value so that titrant consumption remains within a reliably measurable range — typically from about 0.1 g for high-acidity crude or waste fats up to 20–50 g for highly refined oils. The acid value is calculated as:
X = (V − V₀) × c × 56.1 / m
- X — acid value of the sample, mg KOH/g
- V — titrant volume consumed by the test portion, mL
- V₀ — titrant volume of the blank, mL
- c — exact molar concentration of the KOH standard solution, mol/L
- 56.1 — molar mass of KOH, g/mol
- m — mass of the test portion, g
Free fatty acidity expressed as oleic acid can be derived using the conversion: FFA (%) ≈ AV × 0.503; other fatty acid bases use the corresponding molar mass.
Precision and Quality Control
- KOH (or NaOH) titrant must be standardized, conventionally against potassium hydrogen phthalate, and its concentration re-verified periodically.
- The ether–ethanol solvent must be freshly neutralized immediately before use, as absorbed carbon dioxide causes positive bias.
- Blank titrations are mandatory and subtracted from each determination.
- Duplicate results should agree within the repeatability limits stated in the applicable standard; results are typically reported to two decimal places.
- Control charts, certified reference materials, and participation in proficiency testing are expected of accredited third-party laboratories under ISO/IEC 17025 systems.
- Relevant safety controls include explosion-proof handling of diethyl ether and ventilation of ethanol vapors.
Applications
Acid value testing supports quality, safety, and commercial decisions across the entire fats and oils value chain:
- Refinery process control: verifying the efficiency of degumming, alkali neutralization, and washing stages; confirming that finished refined oil meets the ≤ 0.6 mg KOH/g target typical of first-grade edible oils.
- Regulatory compliance: demonstrating conformity with national food safety limits, such as the maximum acid value of 3 mg KOH/g specified in GB 2716-2018 for edible vegetable oils in China.
- Incoming inspection and supplier management: food manufacturers verifying incoming bulk oil, margarine, shortening, and dairy-fat replacers against purchase specifications.
- Shelf-life and storage studies: accelerated storage and real-time monitoring programs tracking hydrolytic stability of oils, snacks, nuts, cereals, and instant noodles.
- Frying oil management: controlling oil turnover in industrial and catering fryers and triggering discarding criteria.
- Waste cooking oil screening: elevated acid value is a key marker used in the identification of illegally recycled or adulterated oil.
- Feed, oleochemical, and pharmaceutical use: release testing of feed-grade fats, soap and fatty acid production, and lipid excipients governed by pharmacopoeial monographs.
- Trade and customs clearance: certificate-of-analysis data for export, import, and contract settlement.
FAQ
What is the difference between acid value and peroxide value?
The acid value measures free fatty acids produced by hydrolysis, whereas the peroxide value measures primary oxidation products. The two indices are complementary: a comprehensive freshness assessment of an oil normally includes both, and acid value alone cannot characterize oxidative rancidity.
How is acid value converted to free fatty acid content?
For most vegetable oils, FFA expressed as oleic acid (%) is approximately the acid value multiplied by 0.503. For palm oil reported as palmitic acid the factor is about 0.456, and for coconut and palm kernel oils reported as lauric acid it is about 0.356. The reporting basis should always be stated.
What is the regulatory limit for acid value in China?
Under GB 2716-2018, edible vegetable oils are limited to an acid value of 3 mg KOH/g, with tighter internal limits common for premium product grades. Specific product standards and customer specifications may impose stricter values.
Why must the titration solvent be neutralized before use?
Diethyl ether–ethanol and hot ethanol solvents absorb carbon dioxide and form traces of carbonic acid. If not neutralized, this acidity consumes titrant and inflates the analytical result; blanks alone do not fully compensate for the effect.
Which method should be chosen for dark-colored oils?
For deeply pigmented samples such as dark sesame oil, chili oil, or roasted oils, the potentiometric titration method is preferred because visual phenolphthalein endpoints are unreliable when the sample color masks the indicator change.
How much sample is required and how long does testing take?
A routine determination requires only a few grams to a few tens of grams of well-mixed sample, and the titration itself takes 15 to 30 minutes. Including homogenization, duplicate determination, and calculation, results are typically available within one working day at an accredited third-party laboratory.
Can acid value alone detect oil adulteration?
No. Adulteration, such as blending waste oil into fresh oil, is usually assessed through a multi-parameter scheme combining acid value with peroxide value, polar compounds, fatty acid profile, and contaminant testing, interpreted by experienced analysts.