Acid value testing is one of the most important quality and safety parameters for animal fats and oils, reflecting the level of free fatty acids generated by hydrolysis and degradation. This article provides a comprehensive technical overview of animal fat acid value testing, covering sampling, testing items, internationally recognized methods such as ISO 660, GB 5009.229 and ASTM D664, industrial applications, and frequently asked questions.
Technical Overview
The acid value (acid number, AV) is defined as the mass of potassium hydroxide, in milligrams, required to neutralize the free fatty acids present in one gram of fat or oil. For animal fats such as lard, tallow, poultry fat and fish oil, the acid value is a primary indicator of hydrolytic rancidity, refining efficiency, and overall freshness. Elevated acid values typically result from enzymatic or microbial lipolysis of triglycerides, prolonged storage at elevated temperatures, moisture exposure, or the use of degraded raw materials.
The acid value is calculated from the titration data according to the following relationship:
AV (mg KOH/g) = (V − V0) × c × 56.1 / m, where V is the titrant volume for the sample, V0 is the blank titration volume, c is the exact molar concentration of the KOH or NaOH standard solution, 56.1 is the molar mass of KOH in g/mol, and m is the sample mass in grams.
Because each unit of acid value corresponds to a defined quantity of free fatty acids, the result is often converted to a free fatty acid (FFA) content expressed as a specific fatty acid equivalent. For most animal fats, whose dominant fatty acids are C16–C18 acids, the conversion is approximately: %FFA (as oleic acid) ≈ AV × 0.50, or %FFA (as palmitic acid) ≈ AV × 0.456.
Key standards governing the determination include ISO 660 (Animal and vegetable fats and oils — Determination of acid value and acidity), GB/T 5530 (the Chinese method standard harmonized with ISO 660), GB 5009.229 (the Chinese national food safety standard for acid value determination in foods, comprising cold solvent indicator titration, potentiometric titration, and hot ethanol titration), ASTM D664 (potentiometric acid number determination), and AOCS Cd 3d-63. For edible animal fats, national food safety standards such as GB 10146 commonly specify a maximum acid value of 2.5 mg KOH/g, while industrial and feed-grade specifications define their own acceptance limits based on end use.
Testing Samples
Edible Animal Fats
This category includes refined lard (pig fat), edible beef tallow, mutton fat, chicken fat, duck fat, and goose fat intended for direct food use or as food ingredients. These products are subject to strict food safety limits on acid value, and testing is routinely performed for batch release, shelf-life verification, and regulatory compliance.
Animal-Derived Specialty Oils
Fish oils, cod liver oil, and anhydrous milk fat (butter oil) are frequently tested for acid value to assess refining quality and oxidative or hydrolytic deterioration during storage and transportation. These oils often require potentiometric methods because they may contain pigments or vitamins that interfere with visual endpoint detection.
Industrial and Feed-Grade Fats
Technical tallow, greases, rendered animal by-products, and waste animal fats used as feed ingredients, oleochemical raw materials, or biodiesel feedstock are tested to classify quality grades, calculate neutralization losses, and verify conformity with purchase specifications. Crude rendered fats naturally carry higher free fatty acid levels, so larger dilution ranges and different acceptance criteria apply compared with edible products.
Testing Items
Acid value is the core testing item, and it is usually evaluated alongside a panel of related parameters to fully characterize the quality and degradation state of animal fats and oils:
- Acid value (mg KOH/g) — the primary parameter quantifying free fatty acids
- Free fatty acid content (% FFA, expressed as oleic, palmitic, or lauric acid equivalent)
- Peroxide value — indicator of primary oxidative rancidity, complementary to acid value
- Saponification value — average molecular weight of the triglycerides
- Iodine value — degree of unsaturation
- Moisture and volatile matter — a driver of hydrolysis and elevated acid value
- Insoluble impurities and unsaponifiable matter — refining quality indicators
Testing Methods
Accredited third-party laboratories select the titration method based on the physical state, color, and expected acid value range of the sample. The principal approaches are described below.
Cold Solvent Indicator Titration
The sample is dissolved at room temperature in a neutralized solvent mixture such as ethanol/diethyl ether (1:1) or isopropanol/cyclohexane, then titrated with a standardized 0.1 mol/L potassium hydroxide or sodium hydroxide solution using phenolphthalein as the indicator. The endpoint is the persistent faint pink color lasting at least 30 seconds. This method corresponds to GB 5009.229 Method 1 and ISO 660 Method A and is suitable for light-colored, fully soluble fats.
Hot Ethanol Indicator Titration
For solid or waxy animal fats that do not dissolve completely at room temperature, the sample is dissolved in hot neutralized ethanol (or an ethanol solvent system) and titrated while warm to keep the fat in solution and prevent precipitation of fatty acid soaps before the endpoint. This approach corresponds to GB 5009.229 Method 3 and improves accuracy for tallow and high-melting fractions.
Potentiometric Titration
The sample dissolved in a suitable solvent mixture is titrated with alcoholic KOH while the pH or potential is monitored with a glass electrode, and the endpoint is determined from the inflection point or a defined pH value (commonly around pH 8.2–9.2). This method, aligned with GB 5009.229 Method 2, ASTM D664, and the potentiometric option in ISO 660, eliminates subjective color judgment and is strongly preferred for dark, highly pigmented, or strongly colored fats where visual indicators are unreliable. It also enables automation and better reproducibility.
Sample Preparation and Quality Control
Solid samples are melted at the lowest practical temperature, typically not exceeding about 10 °C above the melting point, homogenized, and filtered if necessary. The sample mass is adjusted to the expected acid value per the guidance in ISO 660 and GB/T 5530:
- Expected AV below 1 mg KOH/g: approximately 20 g of sample
- Expected AV of 1–4 mg KOH/g: approximately 10 g of sample
- Expected AV of 4–15 mg KOH/g: approximately 2.5 g of sample
- Expected AV of 15–75 mg KOH/g: approximately 0.5 g of sample
- Expected AV above 75 mg KOH/g: approximately 0.1 g of sample
Critical quality control measures include neutralizing solvents immediately before use, excluding carbon dioxide, performing blank corrections, regularly standardizing the titrant, running duplicate determinations, and observing the repeatability limits specified in ISO 660. Results are reported in mg KOH/g, normally to two significant figures.
Applications
Animal fat acid value testing supports quality and safety decisions across the entire supply chain:
- Food safety compliance — verifying that edible lard, tallow, and poultry fats meet national maximum acid value limits for market release and import/export inspection
- Rendering and refining control — monitoring free fatty acid levels in crude rendered fat and confirming the effectiveness of deacidification and neutralization steps
- Shelf-life and storage studies — tracking hydrolytic deterioration of fats during warehousing, transport, and retail distribution
- Feed industry — qualifying feed-grade fats and blended feed oils against purchase specifications for free fatty acids and oxidation status
- Oleochemicals and biodiesel — assessing tallow and waste animal fat feedstock for soap, fatty acid, ester, and biodiesel production, where FFA content determines pretreatment requirements such as esterification before transesterification
- Regulatory and trade inspection — providing accredited test data for customs clearance, certification, supplier audits, and dispute resolution
- Research and product development — evaluating antioxidant performance, processing stability, and novel fat formulations
FAQ
What is the difference between acid value and free fatty acid content?
The acid value is the mass of KOH in milligrams needed to neutralize the free fatty acids in 1 gram of fat, while free fatty acid content expresses the same quantity as a mass percentage of a specific fatty acid equivalent. For most animal fats, %FFA as oleic acid is roughly half of the acid value, so an acid value of 2.0 mg KOH/g corresponds to approximately 1.0% FFA as oleic acid.
Why does the acid value of animal fats increase during storage?
Hydrolysis of triglycerides releases free fatty acids, and this reaction is accelerated by moisture, residual enzymes from animal tissue, microbial activity, heat, and long storage times. A rising acid value therefore signals hydrolytic deterioration even when peroxide value, which tracks oxidation, remains unchanged, which is why both parameters should be tested together.
Which method should be used for dark-colored tallow or fish oil?
Potentiometric titration is recommended for dark or highly pigmented samples because the phenolphthalein endpoint is difficult or impossible to observe visually. The electrode-based endpoint also provides better precision and allows automated titration, in line with GB 5009.229 Method 2 and ASTM D664.
What are typical acceptance limits for animal fats?
Edible animal fats are generally limited to an acid value of about 2.5 mg KOH/g or lower under national food safety standards such as GB 10146. Feed-grade and technical fats commonly permit substantially higher free fatty acid levels, often expressed as several percent FFA, depending on the buyer specification and end use. The applicable limit should always be confirmed against the governing standard or contractual specification.
How should samples be taken and preserved before testing?
Samples should be representative of the batch, taken with clean dry tools, filled into airtight opaque containers with minimal headspace, and protected from heat, light, and moisture. Solid fats are melted gently and homogenized before subsampling, and testing should be performed as soon as possible after sampling to avoid continued hydrolysis or oxidation.
Can acid value testing detect adulterated or reprocessed fats?
An abnormally high acid value is a strong indicator of degraded, poorly refined, or reprocessed fat, but acid value alone cannot conclusively prove adulteration. It is normally combined with peroxide value, moisture, impurities, fatty acid profile by gas chromatography, and other traceability checks to build a complete assessment of authenticity and quality.
How long does an acid value test take?
Acid value determination by titration is a rapid procedure, and accredited third-party laboratories can typically complete the analysis and issue a report within a few working days of sample receipt, with same-day or next-day results available for urgent quality control situations.