The acid value is one of the most decisive quality indicators for edible oils, animal fats, and industrial oil products, quantifying the free fatty acids released through hydrolytic degradation. An oil and fat acid value analysis report documents sample information, testing standards, analytical procedures, and measured results that determine whether a product meets safety, commercial, and regulatory specifications. This article explains the technical basis of acid value testing, the applicable ISO, ASTM, and GB standard methods, sampling guidance, result interpretation, and answers to frequently asked questions.
Technical Overview
The acid value (AV), also known as the acid number, is defined as the mass of potassium hydroxide, in milligrams, required to neutralize the free fatty acids contained in one gram of oil or fat. The result is expressed in mg KOH/g and is directly proportional to the concentration of free fatty acids that have been liberated when ester bonds in triglyceride molecules are cleaved by hydrolysis. In this sense, the acid value is the principal indicator of hydrolytic rancidity, whereas the peroxide value tracks oxidative rancidity, and the two parameters are usually reported together for a complete degradation profile.
Fresh, well-refined oils typically exhibit an acid value below 0.5 mg KOH/g, because efficient alkali refining and deodorization remove nearly all free fatty acids. During storage, transport, or processing, the acid value rises as hydrolysis proceeds. The main drivers of this increase include:
- Moisture ingress combined with native lipase enzyme activity in oilseeds and crude oils
- Prolonged exposure to elevated temperatures, for example in continuous deep frying
- Oxidative damage that weakens ester bonds and accelerates secondary degradation
- Microbial contamination in improperly stored raw materials and by-products
- Incomplete or failed refining steps such as degumming, neutralization, and washing
- Repeated heating and cooling cycles that promote water condensation inside storage vessels
A high acid value signals deteriorated quality, reduced smoke point, soapy or bitter off-flavors, shortened shelf life, and potential non-conformity with food safety limits. For industrial oils, elevated acidity can also indicate corrosion risk in fuel systems and process equipment. Accredited third-party laboratories therefore treat acid value determination as a core release test for oils and fats across the food, feed, cosmetic, pharmaceutical, and energy sectors.
Testing Samples
Edible Vegetable Oils
Routine testing covers soybean oil, rapeseed and canola oil, palm oil and palm olein, sunflower oil, peanut oil, corn oil, sesame oil, camellia oil, rice bran oil, coconut oil, olive oil, and blended cooking oils. Both refined products ready for retail and bulk shipments at the port or factory intake are commonly submitted for acid value verification against purchase specifications.
Animal Fats and Marine Oils
Lard, tallow, chicken fat, butterfat fractions, fish oil, and krill oil are analyzed because animal-derived fats generally carry higher initial free fatty acid levels and are more susceptible to enzymatic lipolysis. These matrices often require potentiometric detection because their color can mask a visual titration endpoint.
Used and Deep-Frying Oils
Monitoring acid value in oils collected from commercial fryers helps food service operators decide when to extend use or discard the oil. Rising acidity in frying oil reflects cumulative water ingress from fried food and thermal stress, and it is frequently evaluated alongside total polar compounds for a complete frying-life assessment.
Oilseeds, Crude Oils, and Processing Intermediates
Crude pressed oils, solvent-extracted oils, degummed oils, and semi-refined intermediates are tested at each refining stage to evaluate process efficiency. Oilseed meals, oil powders, and encapsulated oil powders can also be characterized after suitable solvent extraction of the lipid fraction.
Specialty and Industrial Oils
Cosmetic base oils, pharmaceutical excipients, dietary supplement softgel fills, biodiesel and its vegetable feedstocks, synthetic esters, and lubricants are analyzed under the appropriate product standard, for example ASTM D664 for petroleum-derived and related matrices where the acid number specification for biodiesel is 0.50 mg KOH/g maximum.
Testing Items
A standard oil and fat acid value analysis report typically centers on the following determinations, with complementary parameters added depending on the purpose of testing:
- Acid value, expressed in mg KOH/g, the primary test item
- Free fatty acid content, expressed as percent oleic acid, derived from the acid value
- Peroxide value, to characterize primary oxidative rancidity alongside hydrolytic degradation
- Saponification value and iodine value, for full characterization of the lipid profile
- Moisture and volatile matter, because water content directly accelerates hydrolysis
- Insoluble impurities and phosphorus content, relevant to crude oil trade contracts
Testing Methods
Hot Ethanol Indicator Titration (GB 5009.229)
In the hot solvent method specified in GB 5009.229 for foods, the sample is dissolved in hot neutralized ethanol and titrated with a standardized potassium hydroxide or sodium hydroxide solution using phenolphthalein as the indicator. Heating keeps the lipid fully dispersed in the solvent and ensures a sharp, reproducible endpoint. This method is well suited to lightly colored, edible-grade vegetable oils.
Cold Solvent Indicator Titration (ISO 660, EN ISO 660, GB/T 5530)
The international reference method ISO 660 and its Chinese equivalent GB/T 5530 dissolve the sample in a neutralized mixture of ethanol and diethyl ether or ethanol and light petroleum, then titrate with ethanolic KOH to a phenolphthalein endpoint that must persist for approximately thirty seconds. Cold solvent titration is the standard approach for international trade lots and provides results directly comparable between laboratories.
Potentiometric Automatic Titration (ASTM D664, GB 5009.229)
Potentiometric titration detects the equivalence point through the inflection of the pH or potential curve rather than a visual color change, which makes it the method of choice for dark-colored oils such as crude oil, sesame oil, fried oils, and animal fats. Automatic titrators improve precision, eliminate operator subjectivity in endpoint judgment, and generate traceable titration curves that are attached to the analysis report.
Rapid Screening Methods
Portable rapid test kits, colorimetric strips, and thermometric titration systems are used for on-site screening during production, frying operations, and incoming inspection. Screening results that approach specification limits should always be confirmed by standard titration in an accredited laboratory before a lot disposition decision is made.
Regardless of the method, the acid value is calculated as AV = (V1 minus V0) multiplied by c, multiplied by 56.1, divided by m, where V1 is the titrant volume consumed by the sample, V0 is the blank volume, c is the molar concentration of the KOH standard solution, 56.1 is the molar mass of potassium hydroxide in g/mol, and m is the sample mass in grams. Accredited laboratories run certified reference materials or quality control samples with each batch and report measurement uncertainty alongside the result.
Result Interpretation and Quality Limits
Interpretation always depends on the applicable product standard, but the following reference points are widely used across the industry:
- Fresh refined edible vegetable oils typically show an acid value below 0.5 mg KOH/g, with premium grades often below 0.2 mg KOH/g
- The national food safety standard for edible vegetable oils, GB 2716, sets a general maximum of 3 mg KOH/g
- Extra virgin olive oil is limited to 0.8 percent free fatty acidity, equivalent to approximately 1.6 mg KOH/g calculated as oleic acid
- Biodiesel must meet an acid number of 0.50 mg KOH/g or lower under ASTM D6751 and EN 14214
- Frying oils in service are commonly flagged for closer monitoring or discard as the acid value climbs above approximately 2 to 3 mg KOH/g, in combination with total polar material limits
To convert between the two reporting conventions, free fatty acid content as percent oleic acid equals the acid value multiplied by approximately 0.503. For example, an acid value of 2.0 mg KOH/g corresponds to about 1.0 percent free fatty acids. Because limits are periodically revised, clients should always verify the current edition of the governing standard before making compliance judgments.
Applications
Acid value analysis reports support decisions at every point of the oil and fat supply chain:
- Incoming raw material inspection and supplier qualification for food and feed manufacturers
- Release testing of finished bottled oils against national food safety standards and product specifications
- Frying oil lifecycle management in restaurants, central kitchens, and snack production lines
- Refining process optimization, including evaluation of degumming, neutralization, and deodorization efficiency
- Shelf-life studies and accelerated storage trials during product development
- Trade contract settlement and independent verification for import and export shipments
- Supporting documentation for regulatory submissions, customer audits, and product certification
- Quality investigation of customer complaints involving rancid odor or off-flavor defects
FAQ
What is considered a good acid value for fresh oil?
Properly refined vegetable oils should test below 0.5 mg KOH/g, and premium refined grades frequently register below 0.2 mg KOH/g. Cold-pressed and virgin oils are naturally higher because they are not alkali refined, so they should be judged against their own product category limits.
Why does the acid value increase during frying?
Water released from fried foods enters the hot oil and hydrolyzes triglycerides into free fatty acids, while sustained temperatures above 160 degrees Celsius accelerate the reaction. Acid value therefore grows steadily with frying time and provides a practical index of cumulative oil abuse.
How is acid value different from peroxide value?
The acid value measures free fatty acids produced by hydrolysis, whereas the peroxide value measures hydroperoxides produced by oxidation of unsaturated fatty acids. A complete rancidity assessment reports both, because a sample can be oxidized without significant hydrolysis, and vice versa.
How much sample is required and how should it be submitted?
Most methods require only a few grams per determination, but laboratories typically request 50 to 100 grams or milliliters to allow replicate testing and confirmation. Samples should be filled to the top of an amber glass bottle to minimize headspace air, kept cool and dark during transport, and analyzed promptly after receipt.
How are acid value and free fatty acid content related?
They describe the same property in different units. Percent free fatty acid as oleic acid equals the acid value multiplied by approximately 0.503, so an acid value of 4.0 mg KOH/g corresponds to roughly 2.0 percent free fatty acids.
What is the typical turnaround time for a report?
Standard acid value testing is usually completed within three to seven working days of sample receipt at accredited third-party laboratories, with expedited service available for urgent release decisions. Reports include sample identification, the method and standard applied, the result with units, measurement uncertainty, and a conformity statement against the specified limit.