An acid value determination test report provides objective evidence of the free fatty acid level and hydrolytic deterioration status of edible oils, animal fats, and fat-containing foods. This guide explains the testing principle, applicable standards such as GB 5009.229 and ISO 660, laboratory procedures, calculation formulas, and result interpretation typically documented in a professional acid value experiment report.
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 an oil or fat sample, expressed in mg KOH/g. It is one of the most fundamental quality indicators in lipid chemistry, reflecting the extent of hydrolytic degradation, the efficiency of the refining process, and the overall freshness of the material. As triglycerides break down through moisture, heat, enzymatic action, or oxidation, free fatty acids accumulate and the acid value rises accordingly.
In a formal experiment report, the acid value serves both as a pass-fail compliance parameter against food safety and product standards and as a process-control metric for refining, storage, and frying operations. The acidity can also be expressed as free fatty acid (FFA) content, most commonly calculated as percent oleic acid, which is directly convertible from the acid value.
The determination is governed by several internationally and nationally recognized standards, including:
- GB 5009.229-2016 — National food safety standard: determination of acid value in food (cold solvent indicator titration, cold solvent potentiometric titration, and hot solvent procedures)
- GB/T 5530 — Animal and vegetable fats and oils: determination of acid value and acidity
- ISO 660 — Animal and vegetable fats and oils: determination of acid value and acidity
- EN ISO 660 — European adoption of the ISO method for fats and oils
- AOCS Cd 3d-63 — Acid value of fats and oils (titrimetric method)
- ASTM D664 and ASTM D974 — Acid number determination by potentiometric and color-indicator titration, widely applied to industrial oils and biodiesel feedstocks
- EN 14104 — Determination of acid value of fatty acid methyl esters (FAME)
Testing Samples
Edible Vegetable Oils
Routine testing covers soybean oil, rapeseed oil, peanut oil, sunflower oil, corn oil, olive oil, sesame oil, camellia oil, and palm oil. Refined grades are typically expected to show very low acid values, while crude and pressed oils naturally carry higher free fatty acid levels before neutralization.
Animal Fats and Marine Oils
Tallow, lard, chicken fat, butterfat, fish oil, and other animal-derived lipids are tested to monitor hydrolytic rancidity, refining efficiency, and suitability for food, feed, or oleochemical use.
Deep-Frying Oils and Fat-Containing Foods
Used frying oils from commercial kitchens and food plants are tested to verify whether the acid value remains within discard thresholds. For fat-containing foods such as snack products, instant noodles, biscuits, and fried nuts, the lipid fraction is first extracted with a suitable solvent (for example, petroleum ether), and the extracted fat is then analyzed.
Industrial and Non-Food Lipid Materials
Biodiesel feedstocks and fatty acid methyl esters, soap-stock intermediates, distilled fatty acids, feed-grade fats, and lubricant base stocks are tested under ASTM or EN methods appropriate to their matrix and expected acidity range.
Testing Items
The core item of the experiment report is the acid value in mg KOH/g, determined to two decimal places for low-acidity samples. Closely associated reporting items include:
- Acid value (AV), mg KOH/g — the primary quantitative result
- Free fatty acidity, expressed as % oleic acid or another specified fatty acid
- Supporting indicators frequently reported together: peroxide value, moisture and volatile matter, saponification value, and insoluble impurities
- Method identification, titrant concentration, sample mass, blank correction, and duplicate results with their arithmetic mean
A complete report documents sample identification, sampling date, analytical date, the standard method applied, the calculated result with units, the detection basis, and statements regarding measurement uncertainty or repeatability where required.
Testing Methods
Principle
The sample is dissolved in a pre-neutralized mixed solvent, typically ethanol and diethyl ether (1:1) or ethanol and light petroleum ether. Free fatty acids in the sample are titrated with a standardized potassium hydroxide solution in ethanol. The endpoint is detected visually using phenolphthalein, indicated by a faint pink color persisting for at least 30 seconds, or instrumentally by potentiometric detection of the equivalence point using a pH electrode. The acid value is calculated from the volume of titrant consumed.
Cold Solvent Indicator Titration
This is the primary method of GB 5009.229 and corresponds to the indicator method of ISO 660. It is suitable for light-colored, liquid oils. The sample mass is selected according to the expected acid value: larger masses (approximately 15-20 g) are used when the expected value is below 1 mg KOH/g, with progressively smaller masses (down to about 0.1 g) for highly acidic materials so that titrant consumption remains within an optimal range.
Hot Solvent Indicator Titration
For samples that are solid at room temperature or poorly soluble in cold solvent, such as tallow, palm stearin, and hard butters, ethanol is heated to approximately 70-80 degrees Celsius before titration to keep the fat fully dissolved and ensure complete reaction of the free fatty acids.
Cold Solvent Potentiometric Titration
Dark-colored oils, such as used frying oils, sesame oil, and crude palm oil, mask the phenolphthalein endpoint. In these cases the titration is performed with a pH electrode and automatic titrator, and the endpoint is determined from the potentiometric inflection curve. This variant improves objectivity, precision, and traceability of the raw data.
Calculation Formula
The acid value is calculated as follows: AV = (V − V0) × c × 56.1 / m, where V is the titrant volume consumed by the sample in milliliters, V0 is the blank titration volume in milliliters, c is the exact molar concentration of the KOH standard solution in mol/L, 56.1 is the molar mass of potassium hydroxide in g/mol, and m is the sample mass in grams. For FFA expression as oleic acid, multiply the acid value by 0.503.
Quality Control and Precision
- Standardize the ethanolic KOH titrant regularly against potassium hydrogen phthalate, and record its concentration
- Verify that the mixed solvent is neutralized to the phenolphthalein endpoint before use
- Perform duplicate determinations; the absolute difference between replicates must not exceed the repeatability limit specified by the applied standard
- Run reagent blanks with each batch and control the laboratory temperature, since solvent evaporation and CO2 absorption affect the endpoint
- Participate in proficiency testing or use certified reference materials periodically to confirm method accuracy in accredited third-party laboratories
Applications
Acid value determination supports decision-making across the entire edible oil supply chain and in related industries. Typical applications include:
- Refining process control — evaluating the performance of degumming, alkali neutralization, and deacidification stages
- Incoming raw material inspection at crushing plants, refineries, and food manufacturers
- Finished product release testing against national food safety standards and graded product specifications
- Frying oil management — deciding when to discard or refresh oil in industrial and catering frying operations
- Shelf-life and accelerated storage studies — tracking hydrolytic and oxidative stability under defined temperature and humidity conditions
- Regulatory compliance and dispute resolution — providing legally defensible data from accredited third-party laboratories
- Biodiesel and oleochemical production — confirming that feedstocks and FAME products meet acid number specifications
Under GB 2716-2018, the acid value of edible plant oils shall not exceed 3 mg KOH/g, while graded product standards for individual oils commonly specify stricter limits of 0.20 to 0.50 mg KOH/g for premium refined grades. Used frying oils are typically discarded once the acid value approaches 3 mg KOH/g or exceeds the internal control limit established by the operator.
FAQ
What is the difference between acid value and peroxide value?
The acid value measures free fatty acids produced mainly by hydrolysis of triglycerides, whereas the peroxide value measures primary oxidation products. The two indexes are complementary: the acid value reflects hydrolytic rancidity, while the peroxide value reflects oxidative deterioration, and a complete freshness assessment usually reports both.
Why does the acid value increase during storage and frying?
Moisture in the oil, elevated temperatures, lipase enzyme activity in unrefined materials, and repeated heating all accelerate the splitting of ester bonds in triglycerides, releasing free fatty acids. Longer frying times and higher water content from fried foods markedly speed up this hydrolytic process.
How is the acid value converted to free fatty acid content?
Free fatty acid content as a mass fraction equals the acid value multiplied by the molar mass of the reference fatty acid and divided by 561. When expressed as oleic acid (molar mass 282 g/mol), FFA (%) equals AV multiplied by 0.503.
Which method should be selected for dark-colored oils?
Visual phenolphthalein endpoints are unreliable in dark matrices, so cold solvent potentiometric titration with an automatic titrator is preferred for used frying oils and strongly pigmented oils, as prescribed by GB 5009.229 and ISO 660.
What result precision is expected in a valid experiment report?
Duplicate determinations must agree within the repeatability limit of the applied standard, typically an absolute difference not exceeding approximately 5-10 percent of the mean value depending on the acidity level. The reported result is the arithmetic mean of acceptable duplicates, and outlier data must be investigated and retested rather than averaged blindly.
How can a high acid value be reduced?
Industrial deacidification through alkali refining, physical (steam) refining under high vacuum, or adsorption and neutralization processes lowers the free fatty acid content. Preventive measures include dry storage conditions, protection from heat and light, minimized water contact, and prompt use of oils after opening.