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Determine oxidative stability of oils and fats based on AOCS Cd 12b-92

1st April 2013


In order to assess the quality of edible oils and fats and to check their storage properties one often needs a test that determines the stability of the products at short notice and requires a minimum of work. Werner Schneider reports.

Oils and fats have a certain resistance to oxidation. This means that depending on the degree of saturation and the natural or added antioxidants present, the storage periods may be increased to a greater or lesser extent.

In the Rancimat the oils and fats are prematurely aged by thermal decomposition (Fig. 1). The resulting decomposition products are blown out by an air stream and transferred to a measuring cell filled with distilled water.

This measuring cell contains a conductivity measuring cell. As soon as the volatile decomposition products reach the cell the conductivity increases sharply. The time which elapses before such decomposition products appear is known as the induction time.

Evaluation is carried out by the Rancimat fully automatically; it can, however, also be carried out manually with a software-supported tangent method.

Method of choice

For the determination of the oxidation stability of oils and fats the Rancimat method is the method of choice: it is the subject of the aAOCS Official Method Cd 12b-92' of the American Oil Chemists' Society, the aJOCS Official Method 2.4.28.2-93' of the Japan Oil Chemists' Society as well as ISO standard 6886.

The determination and evaluation are carried out accurately and fully automatically. The PC-controlled wet chemistry unit, which has the facilities for eight samples, usually stands in a fume cupboard and is separated from the control unit PC.

Double determination

Metrohm has investigated seven different vegetable oils as well as a cooking butter with the Metrohm 743 Rancimat (see Table 1).

Each sample was subjected to a double determination at four different temperatures (100oC, 110oC, 120oC and 130oC).

With the aid of the 743 PC software the following quantities were then calculated automatically:

€ Induction times at the different temperatures as a measure of the oxidative stability of the sample.

€ Standard factor, that is the exact temperature dependence of the induction time for the particular sample.

€ The standard factor is ca. 2, ie the induction time is approximately halved if the temperature is increased by 10oC.

€ Storage stability at 0oC and 4oC by extrapolation from the measured data.

€ Experimental conditions: sample weight: 3 g; air flow: 20 L/h; absorption solution: 60 mL dist. water. u

ENQUIRY No 50

Werner Schneider is with Metrohm Ltd, Herisau, Switzerland. <a href=“http://www.metrohm.ch“ target=_blank>www.metrohm.ch</a>





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