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The two most commonly used norms for determining the oxidation stability of animal fats and vegetable oils are AOCS Cd 12b-92 and EN ISO 6886. The standard method recommended by Metrohm for this is based on EN ISO 6886.

This Application Note describes the determination and comparison of the oxidation stability of sunflower oil according to AOCS Cd 12b-92, EN ISO 6886, and the recommended method from Metrohm with an 892 Professional Rancimat.

Despite different parameters of the norms and the Metrohm method, it is shown that there is no significant difference found between the results of these experiments.

The sunflower oil sample is measured directly with the Rancimat without any preparation steps.

892 Professional Rancimat (right view). Contains picture 3576
Figure 1. 892 Professional Rancimat equipped with measuring and reaction vessels for the determination of oxidation stability.

For analysis, an appropriate amount of the raw sunflower oil is weighed into the reaction vessel and the analysis is started.

With the Rancimat method, the sample is exposed to an airflow at a constant temperature of 100–180 °C (Figure 1). Highly volatile secondary oxidation products are transferred into the measuring vessel with the airflow where they are absorbed in the measuring solution. Here, the conductivity is continuously registered.

stability measurement, oxidation stability graph
Figure 2. Determination of the oxidation stability of sample 4. Induction time is determined to be 2.54 h.

The formation of secondary oxidation products leads to an increase in the conductivity. The time until occurrence of this marked conductivity increase is referred to as the «induction time», which is a good indicator for the oxidation stability (Figure 2).

Table 1. Overview of the different measuring parameters for the samples. Sample 1 is prepared with 60 mL measuring solution, and samples 2–6 are made with 50 mL measuring solution.
Sample According to Sample size (g) Gas flow (L/h)
1 Metrohm 3.00 ± 0.01 20.0
2 EN ISO 6886 3.00 ± 0.01 10.0
3 AOCS Cd 12b-92 2.50 ± 0.01 9.0
4 AOCS Cd 12b-92 5.00 ± 0.01 9.0
5 AOCS Cd 12b-92 2.50 ± 0.01 20.0
6 AOCS Cd 12b-92 5.00 ± 0.01 20.0
Table 2. Results of the oxidation stability of sunflower oil with the 892 Professional Rancimat. Determinations were carried out in four-fold for each parameter set mentioned in the norms.
Sample (n = 4)
 Mean value (h) SD(abs) in h SD(rel) in %
Sample 1 2.57 0.05 1.8
Sample 2 2.51 0.06 2.4
Sample 3 2.53 0.08 3.4
Sample 4 2.51 0.04 1.5
Sample 5 2.75 0.06 2.1
Sample 6 2.56 0.04 1.5

A mean value of 2.57 h induction time is found over all samples (n = 24), with SD(abs) = 0.06 h and SD(rel) = 2.1%. These values meet both the repeatability and the reproducibility requirements listed in AOCS Cd 12b-92 and EN ISO 6886.

Furthermore, all demonstrated methods delivered acceptable values for all samples with SD(rel) ≤10% (Table 2).

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Internal reference: AW ST CH-0177-082022