Acid value and peroxide value testing is the cornerstone of quality and safety evaluation for edible oils, animal fats, and fat-containing food products. These two parameters quantify hydrolytic and oxidative deterioration, respectively, and are mandatory indicators under national food safety standards such as GB 2716-2018 and international specifications including Codex Stan 19. This article explains the technical principles, sample types, test methods, applicable standards, and typical acceptance criteria used by accredited third-party laboratories and in-house quality control teams.
Technical Overview
Lipids deteriorate through two dominant pathways: hydrolysis, in which triglycerides break down into free fatty acids and glycerol, and autoxidation, in which unsaturated fatty acids react with oxygen to form hydroperoxides. Acid value and peroxide value are the two most widely used indicators for monitoring these degradation processes in oils and fats.
The acid value (AV) is defined as the mass of potassium hydroxide, in milligrams, required to neutralize the free fatty acids present in one gram of sample. A rising acid value signals hydrolytic rancidity, inadequate refining, excessive processing temperature, moisture contamination, or raw material spoilage.
The peroxide value (PV) measures the concentration of hydroperoxides, the primary oxidation products formed during the early stages of lipid oxidation. Results may be expressed in millimoles of active oxygen per kilogram (mmol O2/kg), milliequivalents per kilogram (meq/kg), or grams of iodine per 100 grams of sample (g/100g). A high peroxide value indicates poor storage conditions, exposure to light, heat, air, or pro-oxidant metals.
Because hydroperoxides decompose into secondary oxidation products such as aldehydes and ketones in advanced oxidation, the peroxide value may peak and then decline. For this reason, professional laboratories interpret PV results together with acid value, p-anisidine value, and other oxidation indicators to obtain a complete picture of lipid quality.
Testing Samples
Edible Vegetable Oils
Soybean oil, rapeseed (canola) oil, palm oil, peanut oil, sunflower oil, corn oil, olive oil, sesame oil, camellia oil, rice bran oil, and blended cooking oils. Both refined and crude grades can be evaluated to verify refining efficiency and storage stability.
Animal Fats and Marine Oils
Lard, tallow, butterfat, chicken fat, fish oil, krill oil, and algal oil. These matrices are typically rich in saturated or long-chain polyunsaturated fatty acids and require careful sampling to prevent oxidation before analysis.
Used Frying Oils
Deep-frying oil collected from food processing lines, quick-service restaurants, and central kitchens. Repeated heating accelerates both hydrolysis and oxidation, making AV and PV essential for discard-point decisions.
Fat-Containing Processed Foods
Instant noodles, biscuits, potato chips, pastries, moon cakes, margarine, shortening, chocolate and cocoa products, dairy products, infant formula, meat products, frozen fried foods, pet food, and animal feed. The fat fraction is first extracted from the food matrix and then analyzed.
Oilseeds, Nuts, and Raw Materials
Testing Items
Core testing items and typical regulatory criteria for oils and fats include:
- Acid value (mg KOH/g): GB 2716-2018 sets a maximum of 3 mg KOH/g for edible vegetable oils; many refined oil specifications are stricter, at 0.2 to 0.6 mg KOH/g.
- Free fatty acids (FFA, expressed as percent oleic acid): the acidity equivalent of acid value, commonly specified for olive oil and animal fats.
- Peroxide value: GB 2716-2018 sets a maximum of 0.25 g/100g (approximately 10 mmol O2/kg or 20 meq/kg) for edible vegetable oils; Codex Stan 19 sets 10 meq/kg for refined oils and 15 meq/kg for cold-pressed and virgin oils.
- p-Anisidine value (ISO 6885) and TOTOX value (2 × PV + p-AV): indicators of secondary oxidation products.
- Oxidative stability index (ISO 6886, AOCS Cd 12b-92): accelerated Rancimat test for shelf-life prediction.
- Supporting parameters: polar compounds (GB 5009.202) for frying oil, carbonyl value, moisture and volatile matter, and insoluble impurities.
Testing Methods
Acid Value Determination (GB 5009.229-2016, ISO 660)
The national method GB 5009.229-2016 provides three procedures. In the cold solvent indicator titration, the sample is dissolved in an ethanol-diethyl ether or isopropanol-toluene solvent system and titrated with standardized potassium hydroxide solution to a phenolphthalein endpoint. The cold solvent potentiometric titration replaces visual endpoint judgment with a pH-electrode titrator and is preferred for dark-colored oils where the indicator endpoint is difficult to observe. The hot solvent indicator titration is applied to solid or deeply colored fats such as palm oil, dissolving the sample in heated ethanol. Internationally, ISO 660:2020 (also adopted as EN ISO 660) and AOCS Cd 3d-63 follow the same neutralization principle with defined solvent and endpoint requirements.
Peroxide Value Determination (GB 5009.227-2016, ISO 3960)
The reference technique is iodometric titration. The sample is dissolved in a chloroform-glacial acetic acid mixture (GB 5009.227-2016) or an isooctane-acetic acid system (ISO 3960:2017, EN ISO 3960, AOCS Cd 8b-90). Saturated potassium iodide is added, and the hydroperoxides oxidize iodide to free iodine, which is titrated with standardized sodium thiosulfate solution using starch as indicator. Endpoints can also be detected potentiometrically for improved precision. A reagent blank is deducted, and the analysis is performed under subdued light to prevent photolytic peroxide formation or decomposition. Results are calculated as g/100g, mmol O2/kg, or meq/kg depending on the reporting standard.
Sample Preparation and Handling
Solid or high-moisture foods are homogenized and the lipid fraction is extracted with suitable solvents before titration. To preserve sample integrity, oils should be filled into clean, dry amber glass bottles with minimal headspace (ideally nitrogen-flushed), kept cool during transport, and analyzed promptly. Proper sampling is critical because mishandling can artificially elevate the peroxide value before the test even begins.
Quality Assurance
Accredited third-party laboratories perform these determinations under ISO/IEC 17025 management systems, using calibrated titration equipment, certified reference solutions, controlled reagent blank correction, replicate determinations, and participation in proficiency testing to guarantee accurate and legally defensible results.
Applications
Acid value and peroxide value testing supports decision-making throughout the entire edible oil and food supply chain:
- Refining process control: verifying the performance of degumming, neutralization, bleaching, and deodorization stages.
- Incoming raw material inspection: accepting crude oils, oilseeds, and nut ingredients at goods receipt.
- Finished product release: confirming compliance with GB 2716-2018, Codex Stan 19, customer specifications, and export requirements.
- Shelf-life and stability studies: comparing packaging formats, antioxidant formulations, and storage conditions through accelerated and real-time protocols.
- Frying oil management: establishing discard points and validating oil turnover practices in industrial and food-service frying.
- Regulatory supervision: market surveillance, batch arbitration, and dispute resolution based on accredited third-party laboratory reports.
- Research and development: evaluating novel antioxidants, oil blends, and processing technologies.
FAQ
What is the difference between acid value and peroxide value?
Acid value measures free fatty acids produced by hydrolysis, while peroxide value measures hydroperoxides produced by oxidation. A high acid value usually indicates moisture, enzymatic activity, or refining deficiency, whereas a high peroxide value indicates exposure to oxygen, light, heat, or metal contamination.
How are peroxide value units converted?
The three common units are g I2/100g, mmol O2/kg, and meq/kg. One meq/kg equals 0.5 mmol O2/kg, and 0.25 g I2/100g corresponds to approximately 10 mmol O2/kg or 20 meq/kg. Always confirm which unit a specification uses before comparing results against limits.
Can solid foods such as biscuits or nuts be tested?
Yes. The sample is homogenized and the fat is extracted with an appropriate solvent, after which the extracted oil is titrated in the same way as bulk oil. The report should state the basis of the result, for example per gram of extracted fat.
How much sample is required for testing?
For bulk oils, approximately 50 to 100 grams is typically sufficient for replicate acid value and peroxide value determinations. For fat-containing foods, 100 to 200 grams of the homogenized product is usually required to obtain an adequate amount of extracted fat.
Why does the peroxide value sometimes decrease while oil quality worsens?
In advanced oxidation, unstable hydroperoxides decompose into secondary products such as aldehydes and ketones, causing the peroxide value to fall even as rancidity intensifies. Laboratories therefore recommend supplementing PV with p-anisidine value, TOTOX value, and sensory evaluation for aged or heavily processed oils.