Accelerated oxidation tests: the Rancimat method
The decomposition of vegetable oils and animal fats, which can be perceived in the initial stage through a deterioration of smell and taste (rancidity), is a result of a series of chemical alterations caused mainly by the effect of oxygen. These oxidation processes that progress slowly at ambient temperatures are called auto-oxidation.
They begin with radical reactions on unsaturated fatty acids and are in a process that involves multiple stages, resulting in various decomposition products, in particular peroxides as primary oxidation products and alcohols, and aldehydes and carboxylic acids as secondary oxidation products.
The Rancimat method consists of a measure of the conductivity of the volatile compounds that are formed from the oxidation. This method has been developed as a variant of the complex AOM system (Active Oxygen Method) in order to determine the induction time of fats and oils.
The determination of the stability to the oxidation of oils and fats is the typical use of Rancimat. In addition to oils and vegetable fats, with Rancimat it is also possible to analyze animal fats and determine their stability to oxidation.
Precisely the pure substances of oils and fats contained in foods are subject to oxidation, which contribute to the deterioration of food. Rancimat can also be used to determine the oxidation stability of fats and oils in food.
Stability to oxidation indicates the resistance to oxidation of oils and fats and foods containing fat. It is a standard parameter of quality control in the production of oils and fats in the food industry or for food inspection in modern processing plants.
Oxidative Stability Procedure: the Rancimat Method
With the Rancimat method, a sample exposes an airflow at a constant temperature between 50-220 ° C.
To determine the oxidation stability an air stream is passed through the sample of fat or oil at a high temperature. This causes the oxidation of the sample’s fat molecules in volatile organic compounds and other products.
Highly volatile secondary oxidation products (mostly formic acid) are transferred to the measuring vessel with the airflow, where they are absorbed into the measuring solution (distilled water). Here the conductivity is continuously recorded. Organic acids can thus be detected by increasing conductivity. The time that passes until appearance of these secondary reaction products is called induction time or induction period, which is a good indicator for oxidation stability, as it characterises the resistance of fats and oils to oxidation.
In this technique, the conductivity, recorded continuously, draws a curve whose inflection point marks the period of induction from which a drastic increase of the same is produced, linked to the increase of volatile oxidation products, as shown in the following figure :
Figure obtained from the manual 743 Rancimat.
In addition to the term induction time, the expression is used Oxidative Stability Index (OSI – Oil Stability Index). The Rancimat method is also known as the “Rapid Oxidative Stability Test”.
Precautions
This is a standardised and commonly accepted method, but can lead to errors, such as any trace being left in the tubes or even the vaseline used to close them, which can increase the conductivity.
Certain free fatty acids of low molecular weight that make up some oils can even volatilise at those temperatures (100 ° -120 ° C) also giving an increase in conductivity.
On the other hand, certain antioxidants are volatilised at those temperatures, which causes not only an increase of the conductivity, but also they are totally ineffective because they do not remain in the oil that they are meant to protect.
It may interest you: What is a natural antioxidant and why is it useful in food?
Application examples
- Stability to oxidation of fats and oils: if fats and oils are exposed to air and light for a long time, they oxidize and are subject to hydrolysis reactions. Fats and oils then have an unpleasant taste and smell and become stale. Stability to oxidation makes it possible to estimate the speed with which a grease or oil becomes rancid. By using Rancimat it is also possible to characterise the effectiveness of antioxidant additives.
- Oxidation stability of instant noodles: an example of the use of Rancimat is the determination of the oxidation stability of fast cooking pastas. The pastas are fried in a fat or oil bath during the processing process to allow rapid cooking by the consumer. Because of this previous frying, instant noodles are high in fat (up to 22%) and become stale after a while.
- Stability to Walnut Oxidation: the microstructure of fresh, intact walnuts prevents rapid oxidative deterioration. But this microstructure is destroyed during the processing of nuts. This accelerates the oxidation of fats and reduces the shelf life. Before the oxidation stability of walnuts can be determined, it is necessary to separate the fat-containing phase from the rest of the walnut using petroleum ether. The isolated fat is analyzed in the Rancimat.
- Stability to oxidation of biscuits or other bakery products: the Rancimat method is a simple technique for determining the oxidation stability of fats contained in cereals, biscuits, breads and other baked goods.
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