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Native (also known as virgin) edible oils are obtained by means of gentle mechanical processes without added heat to preserve beneficial compounds. These oils are usually of high quality and are considered to be particularly healthy and valuable. For example, virgin olive oil is quite resistant to oxidation due to its fatty acid composition, characterized by a high monounsaturated-to-polyunsaturated fatty acid ratio—a major factor providing oil oxidative stability. It also contains some powerful antioxidants known as polyphenols. Most of these compounds are removed during refining and are present in much lower amounts in refined edible oils than in virgin oils [2].
In this blog post, you can learn what edible oil is, how it is made, how to test it, and what parameters are important to analyze for quality and safety.
Click below to jump directly to each topic:
What is edible oil?
Edible fats or cooking oils are considered suitable for human consumption and are mainly used for food or in cosmetic products. They contain important vitamins as well as saturated and/or unsaturated fatty acids. Both edible fats and oils consist mainly of water-insoluble esters of fatty acids and glycerol, called glycerides.
Fats and oils are generally classified according to whether they are solid or liquid at room temperature. A basic differentiation is made between vegetable fats and oils from the seeds and fruits of oil plants, and fats and oils derived from animal sources. However, synthetic edible fats and oils can be produced from raw materials using chemical processes such as the Fischer–Tropsch process.
In general, the higher the proportion of unsaturated fats (especially polyunsaturated fatty acids), the healthier the fat or oil. Sunflower oil, rapeseed oil, safflower oil, soybean oil, and olive oil are particularly high in unsaturated fatty acids and polyunsaturated fatty acids. Although they can be used for cooking and frying, they are best eaten in their natural state. On the other hand, coconut oil, palm kernel oil, butterfat, and palm oil are very high in saturated fats. They are mainly used for baking, roasting, frying, and for manufacturing industrial soaps or cosmetics.
Examples of edible oils (click to expand):
Sunflower oil is very popular because it can be used as a frying oil at very high temperatures. Because of its neutral flavor and high smoke point, it is often used during baking to improve the flavor and texture of baked goods. Sunflower oil is also used in skincare products due to its unsaturated fatty acid and vitamin E content, as it is an emollient, has moisturizing and anti-inflammatory effects, and protects against UV damage.
Rapeseed oil (also known as canola oil) is tasteless and retains its fluidity even at cooler temperatures. It is a common ingredient in mayonnaise because of its neutral flavor and light color, and gives mayonnaise a creamy texture. Because of its neutral flavor and high smoke point, rapeseed oil is also used to make fried food and crunchy snacks such as French fries and popcorn.
Coconut oil is often used in food products for its ability to add a slightly coconut taste and aroma and because it remains stable at high temperatures. It is a solid at room temperature due to its high saturated fat content, but melts at approximately 24 °C. For this reason, coconut oil is often hydrogenated for use in warmer climates, raising its melting point to the range of 36–40 °C. Coconut oil is particularly preferred in vegan baking as it can act as a butter alternative. It is also used in the cosmetics industry especially for hair and body moisturizers.
Storage and quality considerations
Shelf-life and product quality are very important considerations. Edible oils and fats can ferment, deteriorate during storage, become contaminated either by natural substances associated with the source of the oil or traces of pesticides, or even be adulterated intentionally.
These products can become rancid via autoxidation where long-chain fatty acids are degraded and short-chain compounds (e.g., butyric acid) are formed. Hydrolysis of fats and oils promotes the splitting of triacylglycerols to form free fatty acids (FFA), mono-, and diacylglycerols. These free fatty acids can undergo extra autoxidation. Additionally, the oxidation of triacylglycerols leads to the formation of carboxylic acids with a glycerol backbone which increases the acidity of the oil [1].
Figure 1.
Example showing how cold-pressed linseed oil is made.
Edible oils are obtained by a variety of methods, mostly using direct extraction techniques. The main processes include pressing (Figure 1), extraction with volatile solvents, and purification or refining with caustic chemicals (bleaching).
Pressing is categorized as either «cold-pressed» or «hot-pressed», resulting in completely different finished products. During cold pressing, oil is extracted at room temperature. Cold-pressed edible oils do not need to be refined as the acid value is relatively low, so the final product is obtained after precipitation and filtration. As the name suggests, hot pressing involves extracting edible oils at high temperatures. In this case, the acid value rises significantly and the oil loses most of its natural qualities – therefore hot-pressed oils are refined to make them fit for consumption.
Different categories of oils include: native (virgin), unrefined, refined, hydrogenated, transesterified, fractionated, finished (manufactured), and cold-resistant. These are explained below in more detail (click to expand each topic).
Unrefined edible oils are obtained by melting, pressing, or centrifuging. These processes are commonly used to produce edible oils of animal origin. Heat is often added or tolerated. These oils are not chemically treated and still contain many valuable components that have survived the elevated temperatures.
Refined edible oils undergo additional chemical and/or mechanical treatments. For example, they may be bleached, filtered, deacidified, and deodorized. As a result, they are generally not considered particularly healthy and are used less for direct consumption and more for industrial purposes in food and cosmetics.
Hardened edible oils are fats that have been refined and their fatty acids have been further modified by hydrogenation. They are considered to be unhealthy and have been criticized in particular because of the trans-fatty acids produced during the hydrogenation process. They can have a negative effect on fat metabolism and cholesterol levels.
Transesterified edible oils are refined edible oils (or blends thereof) that are produced under the additional influence of catalysts. This changes the arrangement of the fatty acids and the melting behavior.
Fractionated edible oils are produced from refined or unrefined edible oils by cooling and then separating the stearin from the oleic components. This process can be used to induce specific properties in the final product.
Finished edible oils (also known as manufactured edible oils) are produced by means of hydrogenation, transesterification, and fractional distillation, or a combination of these processes.
Cold-resistant or cold-stable edible oils are produced from either refined or unrefined oils by winterization. During winterization, the oil is cooled and the precipitating fractions are filtered. The filtered product can then be stored at low temperatures without flocculation.
In short, the more processed the edible oil, the poorer its quality. Edible oil quality can and should be checked and analyzed using various testing parameters.
Testing the quality of edible oils requires accurate, reproducible, and simple analysis methods that minimize human error.
There are several well-established methods available. The best-known absolute methods include titration or stability measurement, and the best-known relative methods include near-infrared spectroscopy.
Various edible oil quality testing methods are described in the following sections.
OMNIS titrators from Metrohm are suitable for edible oil analysis.
Titration is an absolute and universal method that provides quantitative results without the need for instrument-or application-specific calibration. As a quantitative method, titration is generally used as a primary reference method for other analytical techniques, such as near-infrared spectroscopy (NIRS).
At its core, titration is based on counting ions or molecules in a sample. A titrator can be equipped to determine a wide range of species, from inorganic ions to complex molecules. Reproducibility is typically less than 1% and the performance of the titration system can be further improved by automating liquid handling or sample preparation steps.
What are the chemical requirements for a successful titration? First, every titration is based on a quantitative chemical reaction between the sample (i.e., the analyte) and the reagent solution (i.e., the titrant). To calculate the amount of analyte in the sample, the stoichiometry of this chemical reaction must be known. Therefore, the sample must be completely dissolved in a suitable solvent. A suitable detection method must be available to follow the progress of the chemical reaction.
Learn more about titration in our related blog article.
The 892 Professional Rancimat is ideal for determining the oxidation stability of edible oils.
Rancidity is the process through which oils and fats become partially or completely oxidized after exposure to moisture, air, or even light. Though not always that obvious, foods can go rancid long before they age much.
The method for determining the oxidation stability of edible oils is also known as the Rancimat method. It is based on a simple principle of reaction kinetics, according to which the rate of a chemical reaction (in this case the oxidation of fatty acids) can be accelerated by increasing the temperature.
During the determination, an air stream passes through the sample at a constant temperature. Any oxidation products that develop are transferred by the air stream to a measuring vessel where they are detected by the change in conductivity of an absorption solution. The evaluation is based on the so-called induction time. This can be used for comparisons, e.g., in long-term or storage tests. Ultimately, it provides information about the oxidation stability and quality of an edible oil.
There are three basic Rancimat methods: direct measurement (most used for edible oils), indirect measurement (e.g., by means of cold extraction, most commonly used for edible oils that have already been processed into food), and the PEG method (for the determination of the antioxidant content or for samples with low fat or high water content).
Read our blog article for more information about determining the oxidation stability of edible oils with the Rancimat.
Gas chromatography
Gas chromatography (GC) is used to determine the fatty acid composition of edible oils after esterification of the fatty acids to their corresponding fatty acid methyl esters (FAMEs).
GC separates various compounds in a mixture by injecting a liquid or gaseous sample into a mobile phase (inert carrier gas) which carries the volatile substances in the gas flow past an adsorbent stationary phase. The analytes have different affinities for the stationary phase and are separated before detection, often by mass spectrometry (MS) or other techniques.
Oxidation indicators at specific UV wavelengths
Ultraviolet-visible (UV/VIS) spectroscopy is used to obtain the absorbance spectra of a compound either as a solid or in solution. The UV/VIS region covers the wavelength range of 200–800 nm. Each type of edible oil has unique absorption characteristics in the wavelength region of 350–700 nm. Hence, the UV-visible region can be used to indicate and differentiate between various edible oils.
Changes in adsorption in the UV region are used as quality, purity, and authenticity criteria for fats and oils.
OMNIS NIR Analyzers can measure several chemical and physical properties of edible oils simultaneously within seconds.
Near-infrared spectroscopy (NIRS) is a fast and reliable method to measure chemical and physical properties in solids and liquids. NIR spectrometers measure the absorption of light from a sample at different wavelengths in the NIR region (780–2500 nm).
Read our blog article to find out more about NIR spectroscopy.
As a secondary technique, NIRS requires a prediction model to be created first. For this, several spectra with known concentrations or known parameter values must be measured that were gathered from a primary method such as titration. A prediction model is created from these spectra using chemometric software. Then, routine analysis of samples can begin.
How do pre-calibrations assist quick implementation of NIR spectroscopy? Find out more in this related blog article.
NIR spectroscopy is a non-destructive technique and can predict different parameters in seconds without sample preparation. In addition, it is environmentally friendly since no solvents or reagents are used.
This technique is especially sensitive to the presence of certain functional groups like –CH, –NH, –OH, and –SH. Therefore, NIRS is an ideal method to quantify chemical parameters in edible oils such as water content, iodine value, acid number, and more.
Find out more about using NIRS for the quality control of palm oil in our blog post, and by watching the video below.
Screening and quality control of palm oil with NIR spectroscopy
Edible oil analysis parameters
Several parameters are used to assess the quality and characteristics of edible oils. These include water (moisture) content, oxidation stability, iodine value, peroxide value, saponification value, acid value and free fatty acids, fatty acid composition, hydroxyl value, oxidation indicators, refractive index, and more.
Water content
Water or moisture content is a measure of the amount of water contained in a sample. This parameter is used in several fields and is expressed in % that can range from 0 (completely dry) to 100 (pure water). It can be given on a volumetric or mass (gravimetric) basis. Moisture analysis is one of the most common laboratory determinations.
The moisture content in edible oils must be kept within a narrow range to avoid spoilage by bacteria and fungi. Rancidity is likely to occur in these products when the moisture content is between 0.05% to 0.3%. Most regulations set a maximum allowable moisture content of 0.2% for edible oils. Butter, on the other hand, can contain up to 16% water.
In addition to oven drying or the radiometric method, Karl Fischer titration is often used to measure water content in various products. Coulometric Karl Fischer titration is the preferred method for this analysis because of the low water content of pure oils and fats. For spreadable fats like butter and margarine with higher water content, volumetric Karl Fischer titration is recommended. Another popular method to measure moisture content is NIR spectroscopy, as it is extremely sensitive to the –OH functional group.
- Click here for related water content applications.
Oxidation stability
Lipid oxidation is the cause of important deteriorative changes in the chemical, sensory, and nutritional properties of edible oils. Oxidative rancidity is based on the principle of reaction kinetics, according to which the rate of the oxidation of fatty acids can be accelerated by increasing the temperature. This means that the decomposition of the product – based on time, temperature, and air – can be reproduced in a few minutes to hours, providing valuable information for edible oil manufacturers. The evaluation is based on the induction time.
The oxidation stability parameter indicates the freshness of an edible oil. Fresh oils and fats contain more antioxidants and offer greater stability against increased temperature and oxygen. The Rancimat method, often used for determination of oxidation stability of oils, can also be used to compare different batches of the same product. This allows early detection of quality differences. Induction time can be also measured with NIRS [2].
Direct measurement with the Rancimat is mainly used for edible oils. The sample is exposed to an airflow at a constant temperature typically between 100 °C and 180 °C. Highly volatile secondary oxidation products are transferred into the measuring vessel along with the airflow where they are absorbed in the measuring solution. The conductivity of the measuring solution is continuously registered. The formation of secondary oxidation products increases the conductivity of the solution. The time until occurrence of this marked conductivity increase is called the induction time – a good indicator for oxidation stability.
| Sample | Induction time (hours) |
|---|---|
| Corn oil | 4–6 |
| Hazelnut fat | 10–12 |
| Hazelnut oil | 7–11 |
| Lard | 1–3 |
| Linseed oil | 0.5–2 |
| Margarine | 2–6 |
| Olive oil | 6–11 |
| Palm oil | 7–12 |
| Peanut fat | 9–10 |
| Peanut oil | 3–15 |
| Pumpkin seed oil | 6–8 |
| Rapeseed (canola) oil | 3–5 |
| Safflower oil | 1–2 |
| Sesame oil | 4–6 |
| Soybean oil | 1–7 |
| Sunflower oil | 1–4 |
| Tallow | 3–8 |
Figure 2.
Correlation diagram for the prediction of the induction time in edible oil samples using an OMNIS NIR Analyzer Liquid. The lab induction time (reference) was evaluated using the Rancimat method.
Determination of oxidation stability in edible oils is also possible by using NIR spectroscopy. Data gathered from Rancimat measurements as a primary method are used as reference values. The calculated values from NIR measurements of the same samples show a good correlation (R2 = 0.973) as described in the correlation plot shown in Figure 2.
- Click here for related oxidation stability applications.
Iodine value/number
Iodine reacts with the double bonds found in unsaturated fatty acids. The iodine value is a sum parameter which provides information about the degree of unsaturation of oils and fats, expressed as grams of iodine per 100 grams of oil.
Unsaturated fatty acids are among the healthier fatty acids. They are also critical to the shelf life of edible oils, as oxidation occurs at these double bonds.
Typical values for the iodine number in various edible oils are given in Table 3.
| Sample | Iodine value (g iodine/100 g sample) |
|---|---|
| Palm kernel oil | 12–14 |
| Tallow | 35–45 |
| Olive oil | 79–92 |
| Sunflower oil | 109–120 |
| Linseed oil | 170–190 |
Figure 3.
Correlation diagram for the calculation of the iodine value in edible oil samples using an OMNIS NIR Analyzer Liquid.
The iodine value can be determined by titrating a known amount of the edible oil, after the addition of auxiliary solutions, with a standard solution of sodium thiosulfate. The volume of titrant consumption is recorded.
The iodine value can be also calculated from the fatty acid NIR spectrum. Since other substances (e.g., carotenoids, aldehydes, ketones) also react with iodine, as is the case with cold-pressed oils, the calculated iodine value must be distinguished from the chemically determined value. For this reason, the primary method by which the iodine value was determined must be indicated. The exceptional correlation (R2 = 0.999) between lab values and NIR values are shown in Figure 3.
- Click here for related iodine value applications.
Figure 4.
Correlation diagram for the calculation of the peroxide number in edible oil samples using an OMNIS NIR Analyzer Liquid. The reference values were determined with titration.
The peroxide value is a measure of the amount of peroxide compounds in edible oils, expressed as meq O2 per kilogram of oil. Peroxides in edible oils can develop from the oxidation of unsaturated fatty acids with oxygen. The peroxide value is affected by storage conditions and increases with a product’s age, exposure to light, or elevated temperatures. Therefore, this parameter can be used to indicate the age and quality of an edible oil.
Determining the peroxide number can be done by titrating a known amount of edible oil, after the addition of auxiliary solutions, with a standard solution of sodium thiosulfate. The volume of titrant consumption is recorded.
The peroxide value can also be measured in edible oils by NIR spectroscopy. Figure 4 shows a correlation plot between peroxide values determined by titration and NIRS (R2 = 0.889). A list of typical values for peroxide number in edible oils is given in Table 4.
| Sample | Peroxide value (meq O2/kg sample) |
|---|---|
| Palm oil | 0–6 |
| Sesame oil | 1–8 |
| Olive oil (native) | Max. 20 |
| Sunflower oil | 6–16 |
| Coconut oil | 0–12 |
- Click here for related peroxide value applications.
Saponification value/number
The saponification value is a measure of the bound and free fatty acids in one gram of fat. It is expressed as milligrams of potassium hydroxide per gram of oil. The saponification number contains information about the average molecular weight of all fatty acids present in the sample. The higher the saponification value, the lower the molecular weight of all fatty acids.
This parameter is a key figure in the chemical characterization of fats and oils. It is mainly used for purity testing and quality control as it identifies edible oils.
For the determination, a known quantity of edible oil or fat is boiled at reflux with ethanolic potassium hydroxide. The excess unused potassium hydroxide is back-titrated with a standardized acid. The volume of titrant consumption is recorded.
| Sample | Saponification value (mg KOH/g sample) |
|---|---|
| Castor oil | 186–203 |
| Cocoa butter | 194–196 |
| Clarified butter | 218–235 |
| Sunflower oil | 189–195 |
| Coconut oil | 248–265 |
| Lard | 192–203 |
| Palm oil | 190–209 |
| Palm kernel oil | 230–254 |
| Rapeseed (canola) oil | 168–181 |
| Olive oil | 184–196 |
- Click here for related saponification value applications.
Acid value/number and free fatty acids (FFA)
The acid value is a measure of the amount of free fatty acids in edible oil, expressed as milligrams of potassium hydroxide per gram of oil. Free fatty acids (FFA, expressed in %) are not bound to glycerol in the oil and are formed by hydrolysis of triglycerides during oil extraction, refining, or storage steps.
Figure 5.
Correlation diagram for the calculation of free fatty acids in edible oil samples using an OMNIS NIR Analyzer Liquid.
The acid value and FFA affect flavor, odor, and shelf life of edible oils and therefore indicate the freshness, quality, and stability. A high acid value and FFA content may indicate poor extraction, refining, or storage conditions, or adulteration with lower quality oils. Furthermore, the content of free fatty acids is used for purity testing and in certain cases allows conclusions to be made about the pretreatment or suspected decomposition reactions.
Determination of the acid number is done by titrating a known amount of edible oil with a standardized alkali solution. The volume of titrant consumption is recorded. Free fatty acid analysis can be done by multiplying the acid number by a factor that depends on the molecular weight of the predominant fatty acid in the oil (e.g., lauric acid, palmitic acid, erucic acid, or oleic acid).
Free fatty acid analysis can be also done by NIRS. As shown in Figure 5, the lab values (reference) correlate quite well with those calculated by NIR spectroscopy (R2 = 0.946).
Typical values for acid value and FFA content in different edible oils are listed in Table 6.
| Sample | Acid value/number (mg KOH/g sample) | Free fatty acids (%) |
|---|---|---|
| Olive oil (Virgin) | 0.8–2 | Max. 0.8 |
| Canola (rapeseed) oil | 0.071–0.073 | 0.04–0.06 |
| Soybean oil | 0.60–0.61 | 0.030–0.040 |
- Click here for related acid value/number and free fatty acids (FFA) applications.
Fatty acid composition
Fatty acid (FA) composition describes the constitution and content (in %) of fatty acids in edible oils (e.g., linoleic acid / C18:2n-6 and linolenic acid / C18:3n-3). This is an important parameter to measure because these are essential fatty acids that cannot be synthesized in our bodies and must be derived from our diet.
The fatty acid composition of edible oils can be determined by capillary GC analysis of the methyl esters obtained by transesterification of the oils with potassium hydroxide in methanol at room temperature [3].
Fatty acid composition can also be measured much easier in just seconds without any sample preparation or chemical reagents with NIR spectroscopy. The NIRS correlation between calculated and reference values for fatty acid composition in edible oils is excellent (R2 = 0.958–0.999) as shown in Figure 6.
Figure 6.
NIRS correlation diagrams of calculated vs. reference values for oleic acid (18:1), linoleic acid (18.2), linolenic acid (18.3), and palmitic acid (16:0).
Additionally, fatty acid composition can be determined using Raman spectroscopy. The spectral information gathered by the Raman instrument is used for quantitative analysis of the concentration of various fatty acids in edible oils. Similar to NIRS, calibration models can be built using a primary method (e.g., GC-MS) for reference values.
Table 7 lists typical values for various fatty acids in different edible oils [4].
| Oil | Palmitic acid (16:0) | Stearic acid (18:0) | Oleic acid (18:1) | Linoleic acid (18:2) |
|---|---|---|---|---|
| Palm | 47 | 4 | 38 | 10 |
| Rapeseed | 4 | 1 | 17 | 13 |
| Sunflower (lolin) | 6 | 4 | 32 | 56 |
| Sesame |
9 | 5 | 45 | 41 |
| Olive |
12 | 2 | 75 | 9 |
- Click here for related fatty acid composition applications.
Hydroxyl value/number
The hydroxyl value is defined as the number of milligrams of potassium hydroxide required to neutralize the acetic acid formed when one gram of a substance containing free hydroxyl groups is acetylated. It is expressed as milligrams of potassium hydroxide per gram of oil.
This value is important because it helps to determine the stoichiometry of a system. It can also be used to calculate equivalent weight and, if the functionality is known, molecular weight. For edible oils, the hydroxyl value is primarily used as a quality characteristic.
Determination of hydroxyl number is done by titrating a known amount of edible oil, after the addition of auxiliary solutions, with a standardized alkali solution. The volume of titrant consumption is recorded. Table 8 lists acceptable ranges for the hydroxyl number in various edible oils.
| Sample | Hydroxyl value (mg KOH/g sample) |
|---|---|
| Castor oil | 160–168 |
| Coconut oil | 0–5 |
| Palm oil | 60–250 |
| Palm kernel oil | 265–279 |
| Rapeseed (canola) oil | 10–20 |
| Olive oil | 4–12 |
- Click here for related hydroxyl number applications.
Oxidation indicators (K-values)
Oxidation indicators (or K-values) are absorption bands between wavelengths of 200 nm and 300 nm that are related with diene and triene systems. Changes in absorption in the UV region are used as quality, purity, and authenticity criteria for edible fats and oils. For example, a low absorption between 200–300 nm is indicative of a high-quality extra virgin olive oil, whereas adulterated or refined oils show a greater level of absorption in this region.
Edible oil samples are measured with a UV/VIS spectrophotometer after dilution in iso-octane to determine their K-values. The K-values (K232, K266, K270, and K274) for three grades of olive oil are given in Table 9. It is clear that as the oil is processed more, the oxidation indicators increase.
| Olive oil grade | K232 | K266 | K270 | K274 |
|---|---|---|---|---|
| Extra virgin (EVOO) | 1.897 | 0.151 | 0.148 | 0.135 |
| Virgin (VOO) | 1.436 | 0.240 | 0.248 | 0.223 |
| Olive Oil (OO) | 3.000 | 0.640 | 0.832 | 0.458 |
Summary
The quality of edible oils can be estimated using several different parameters. Most importantly, the water (moisture) content, oxidation stability, iodine value, peroxide value, saponification value, acid value and free fatty acids, fatty acid composition, hydroxyl value, and oxidation indicators should be measured to determine whether an edible oil is suitable for consumption or not. There are many kinds of analytical methods available to determine these parameters, including (but not limited to) titration, stability measurement, and spectroscopy (e.g., NIR and Raman).
References
[1] Sakaino, M.; Sano, T.; Kato, S.; et al. Carboxylic Acids Derived from Triacylglycerols That Contribute to the Increase in Acid Value during the Thermal Oxidation of Oils. Sci Rep 2022, 12 (1), 12460. DOI:10.1038/s41598-022-15627-3
[2] Cayuela Sánchez, J. A.; Moreda, W.; García, J. M. Rapid Determination of Olive Oil Oxidative Stability and Its Major Quality Parameters Using Vis/NIR Transmittance Spectroscopy. J. Agric. Food Chem. 2013, 61 (34), 8056–8062. DOI:10.1021/jf4021575
[3] Cert, A.; Moreda, W.; Pérez-Camino, M. C. Methods of Preparation of Fatty Acid Methyl Esters (FAME). Statistical Assessment of the Precision Characteristics from a Collaborative Trial. Grasas y Aceites 2000, 51, 447–456. DOI:10.3989/gya.2000.v51.i6.464
[4] Australian Oilseeds Federation Inc. (AOF). Section 1: Quality Standards, Technical Information & Typical Analysis, 2022.
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