Nowadays the consumption of oil is very common, so it is necessary to know its origin and its characteristics since these can affect our health. Its elaboration requires a rigorous control of both the raw material and the preparation conditions, since there are many factors that can alter the quality of the final product, often influenced by both the quality of the fruit or seed from which the oil is extracted and the storage conditions of the finished product, since light and heat can initiate some undesired reactions in the oils that alter their flavor.
One of the most important parameters to take into account in the quality of an edible oil or fat is the peroxide value, since it measures the degree of initial oxidation of the oil, especially in the primary oxidation phase. This is fundamental since oxidation of lipids is one of the main causes of chemical aging of this type of food, which results in the development of unpleasant odors and flavors, which in this product is usually associated with rancidity. But the problem is not only limited to the perception of a product as unpleasant, the by-products generated from these reactions can be very dangerous to health, for example, the peroxidation of certain fats are associated with inflammations that cause arthritis and cancer.
According to the FAO, the maximum limits for the peroxide value range between 10 and 15 milliequivalents of active oxygen/kg of oil, depending on whether it is refined or pressed oil, respectively.
These foods are more easily oxidized, and it occurs due to chain reactions that occur mainly with unsaturated fatty acids, the process can be initiated with small amounts of oxygen, so avoiding it is extremely difficult, however there are methods that can delay the initiation of the process. Then, due to the consequences that the oxidation of these foods can cause and to the high degree of consumption that oils and fats have in our diet, it is extremely important to keep a strict follow-up of this parameter; and although there are several methods for the determination of the peroxide index, the method by titration of the iodine released in the reaction between a lipid extract and a solution of potassium iodide is the most used at laboratory level.
The free fatty acids in oils and fats react with atmospheric oxygen and produce peroxides, these are compounds whose general formula is expressed as ROOH; these compounds give rise to a series of consecutive reactions that generate volatile substances that produce unpleasant odor and flavor in oils and fats, and that we associate with the degree of rancidity of the fats. These reactions are accelerated by temperature and exposure to light.
The amount of peroxide in the oil or fat sample indicates the degree of primary oxidation, and is related to the likelihood of the oil becoming rancid. Thus, the higher the peroxide value, the greater the degree of oxidation of the sample, while a low value is indicative of good product quality and proper preservation.
The reason for this alteration in the quality of the oil or fat is essentially due to the fact that the double bonds contained in the fatty acids that constitute them react with oxygen to form substances that then decompose into other compounds that give rise to the unpleasant characteristics of oxidized fats.
In official methods, such as the AOAC method Cd 8-53(90), peroxides are reacted with iodide ions to form iodine, then the peroxide value is determined by titration of the liberated iodine with thiosulfate using starch as an indicator.
The rate is calculated from the mL of sodium thiosulfate expended to reduce the iodine formed in the first reaction by the action of the peroxides present. The peroxide value is usually expressed as the milliequivalents of peroxide per kilogram of lipid, although it can also be divided by two and expressed as millimoles of active oxygen per kilogram of lipid.
Method of analysis
The following method is described in the Venezuelan standard for vegetable oils and fats using the iodine reaction method and titration with sodium thiosulfate.
- Graduated cylinder
- Mixture of glacial acetic acid and chloroform. 60% acetic acid and 40% mL of chloroform.
- Saturated solution of potassium iodide.
- 0.1 N sodium thiosulfate solution.
- 0.01 N sodium thiosulfate solution.
- 1% starch indicator solution.
1.- 5 g of the sample are weighed in an erlenmeyer with lid. If it is a pasty sample, it can be heated gently until the fusion begins.
2.- 30 ml of the solution of acetic acid and chloroform are added with the help of a pipette, and it is shaken.
Sample Prepation. Source: @yusvelasquez
3.- 1 mL of the potassium iodide solution is added, it is covered, shaken and it is let rest 1 minute.
4.- After the time, 30 ml of distilled water are poured. It is valued with the solution of sodium thiosulfate 0.1 N or 0.01 N depending on the amount of liberated iodine, using 0.5 mL of the starch solution as indicator. At this moment it will be observed that the solution becomes darker.
Sample with indicator. Source: @yusvelasquez
5.- Shake the erlenmeyer vigorously to liberate the iodine of the chloroform layer and to continue adding the solution of sodium thiosulfate.
6.- Without stopping shaking and adding the thiosulfate solution drop by drop keep the sight in the solution contained in the erlenmeyer, the change of coloration to colorless will indicate the final point of the titration. To register the spent volume of sodium thiosulfate.
Color change at the end point. Source: @yusvelasquez
7.- In parallel a blank assay is made, for it the steps from 1 to 4 are repeated. The valuation of this blank must not consume more than 0.4 mL of the solution 0.01 N of sodium thiosulfate.
The following formula is used to calculate the peroxide value:
Ip: peroxide value in meq O2/Kg
N: normality of the sodium thiosulfate solution
V: volume of the sodium thiosulfate solution spent in the titration, in mL
Vl: volume of sodium thiosulfate solution spent in the titration of the blank, in mL
P: weight of the sample in grams.
The starch indicator solution should be prepared immediately before use to avoid decomposition.
Conclusion and contribution
The peroxide value is a very important parameter in the edible oils and fats industry to determine the quality of an oil, since the peroxide content defines the primary oxidation state of the product and thus gives us an estimate of its tendency to rancidity, a condition that can be produced by conditions such as prolonged exposure to air, high temperatures and/or direct sunlight. Therefore, it is a reaction that can occur very easily, hence the importance of constant monitoring of this parameter.
This article allows us to socialize how this analytical method is performed, which is based on redox volumetry and is widely used in food chemical analysis laboratories; therefore, it serves as a guide for students of analytical chemistry and chemistry in general who require information on the determination of peroxides in samples of oils and fats, and also serves to inform the general public about the importance of this parameter to determine the quality of an oil or fat.
This is the end of this post, I hope you find the information presented here very useful, thank you for reading!
- Rojano, B. (1997). Lipid oxidation and antioxidants. National University of Colombia.
- FAO (1999). Standards for specified vegetable oils.
- Venezuelan Standard COVENIN 508:2001. Vegetable oil and fat. Determination of peroxide value.
- Asosiation of Official Analytical Chemist (A.O.A.C), method Cd 8-53(90).