Accelerated oxidation tests: the Schaal method

The speed of oxidation, the duration of storage until the development of rancidity, the important factor in the processing and commercialization of fats, oils and fats that contain fat.

The degradation and rancidity of oils and fats can be sensed from the beginning, once volatile compounds appear. Exposing oils, fats or products to high temperatures accelerates the oxidation of the compounds. Therefore, methods are used to determine the oxidation efficiency of the structure, particularly when evaluating the efficacy of antioxidants to delay rancidity in these products.
There are several types of oxidation tests, such as the Rancimat Method, the Schaal Method, etc. In other articles, we have talked about the Rancimat Method which is a method that has been developed as a variation of the system AOM (active oxygen method), to determine the time of induction of fats and oils; with the typical application of Rancimat being the determination of the oxidation stability of fats and oils.

The Schaal test, also known as the oven test, was developed for the cookie industry. Through the test it was possible to choose butters that demonstrated the greatest resistance to rancidity. This test is used for animals, creams and products. Furnace tests at lower temperatures are used for products such as essential oils and cheese pastes, whereas elevated temperatures are aimed at volatile ingredients, such as Mamluk emulsions or modifications to the other original characteristics of the product.

Throughout this article we will explain what is the Schaal method and why it is widely used for its simplicity.

What is the Schaal method?

This accelerated oxidation test, called in English “Schaal Oven”, consists of storing the samples at a constant temperature for several days. The samples were kept at a constant and controlled temperature, above room temperature, normally at 60/63 degrees Celsius. The increase in temperature acts as a gesture and allows oxidation reactions, as well as measuring their evolution, both organoleptically (color, smell, taste, etc.) and by chemical analysis.

The samples are controlled for a period of days that may vary, depending on the degree of oxidation of the samples. This method is very common in companies in the food sector and is widely used due to its simplicity.

Throughout the duration of the test, fat is extracted cold from a fraction of the sample with petroleum ether. With this step already carried out, it is possible to determine the peroxide index to control the oxidation state.
The samples that are stable for more days will have more chances of success in the extensive tests, with their antioxidant systems being the best.

Exponential oxidation graphs are made from the data obtained in these measurements.
To improve the understanding of this essay, we will analyze its general execution procedure.

Process

To perform a trial with the Schaal method, we must follow the following steps:

  • Set the oven to the desired storage temperature.
  • Label a sufficient number of 4-ounce bottles with proper identification to provide 3-5 bottles for each sample to be tested. To obtain valid comparative results, control samples (without antioxidants) should be included in this test. Code the samples to eliminate any bias from the organoleptic members of the panel. (Caution: Make sure that the labels used adhere to the bottles and remain legible during prolonged storage at elevated temperatures.)
  • Record in a laboratory notebook the identification of the sample and the date of the start of the test.
  • Fill marked jars from one third to half full with the desired test samples and cover the jars.
  • Place the jars in the oven to allow free circulation of hot air in the closed oven.
  • Evaluate the odors and / or colors (flavors) of the samples at appropriate intervals, using a trained organoleptic panel composed of at least three members. The duration of the interval between evaluations will depend on the nature of the individual sample. However, samples that have a relatively short Schaal furnace life (1 week or less) should be evaluated at 24-hour intervals, while samples that have a longer life in the furnace can be evaluated twice a week. A sample is removed from the oven when a stale odor or color has been detected by most of the panel members.
  • Record in the laboratory notebook the date on which each sample is removed from the oven.
  • Calculate for each sample the average days of rancidity (by smell or color) when all the replicas have been removed from the oven.
  • Calculate the results of the report as “Storage stability, such as days to develop rancid odor at a certain temperature.

Advantages and disadvantages of the Schaal Method

The main advantage is that it is a simple test, and does not require very specific equipment. It is also an assay that can be used for both oils and fats as well as for food samples.
One of the disadvantages is that although the use is simple, this test is very variable and is not practical as a routine analysis system. The reaction conditions are difficult to control in the case of repeated handling and the oxidations are very sensitive to the trace of contaminants in the crystals, additionally, it is not an automated test.

It is recommended if you need to obtain general estimates of oxidative stability.