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How to determine if your edible oils are rancid

How to determine if your edible oils are rancid

Rancidity is the process through which oils and fats become partially or completely oxidized after exposure to moisture, air, or even light. Though not always that obvious, foods can go rancid long before they become old. For oils, whose antioxidant properties are highly valued, such as for olive oil, this is especially problematic. A simple (and free) test for rancidity of oils can be performed at home using your own analytical instruments: your senses of smell and taste.

  1.  Pour a few milliliters of the oil into a shallow bowl or cup, and breathe in the scent.
  2.  If the smell is slightly sweet (like adhesive paste), or gives off a fermented odor, then the oil is probably rancid.
  3.  A taste test should be performed to be sure, since some oils may have a naturally sweet scent.
  4.  Ensure the oil sample is at room temperature, then sip a small amount into your mouth without swallowing. Similar to tasting wine, slurp air across the oil in your mouth, then exhale to determine if the oil has flavor.
  5.  If the oil has no flavor, it is most likely rancid. Do not consume it!

Once food has turned rancid, there is no way to go back and fix it. So, if you find out by means of the sensory test that the oil is rancid, it is already too late. For those of us who would rather skip this step to avoid having rancid food in our mouths, the possibility to accurately predict the future oxidation behavior of edible oils would be great. In fact, this is exactly what the Rancimat from Metrohm can do if you follow our tips and tricks in this article.

Rancimat to the rescue!

With the 892 Rancimat and the 893 Biodiesel Rancimat, Metrohm offers two instruments for the simple and reliable determination of the oxidation stability of natural fats and oils and of biodiesel, respectively. The method, also known as the Rancimat method or Rancimat test, is the same in both cases. It is based on a simple principle of reaction kinetics, according to which the rate of a chemical reaction (here the oxidation of fatty acids) can be accelerated by increasing the temperature.

The 892 Rancimat (L) and the 893 Biodiesel Rancimat (R) from Metrohm (click to enlarge image).

How does it work?

During the determination, a stream of air is passed through the sample at a constant temperature (e.g. 110 °C according to standard EN 14214 for biodiesel). Any oxidation products that develop are transferred by the air stream to a measuring vessel, where they are detected by the change in conductivity of an absorption solution. In addition to the temperature (both the accuracy and stability of which are guaranteed by the Rancimat system), the preparation of the measurement and the condition of the accessories also influence the quality and reproducibility of the results. In this blog post, we have compiled some practical experience in using the Rancimat to help you.

Oxidation stability: practical tips and tricks from the experts

Remove foreign particles from the reaction vessel

Foreign particles in the reaction vessel can catalyze reactions in the sample, leading to measurement results which are not reproducible. Remove foreign objects such as packaging remains from the reaction vessels using a strong gas stream (preferably nitrogen).

Weigh sample with a plastic spatula

Weigh the sample directly into the reaction vessel. Make sure that the maximum filling height does not exceed 3.5 cm. An error of ±10% in the sample weight has no influence on the final result.

Metal spatulas should not be used for weighing, as the metal ions could catalytically accelerate oxidation.

Reaction vessel lid

The green reaction vessel lid (see following image, article number: 6.2753.100) must seal the reaction vessel tightly. If this is no longer possible, the lid must be replaced. Leaky reaction vessel lids lead to incorrect and non-reproducible measurement results!

Tip: to make it easier to seal or to remove the lid, a fine film of silicone oil can be applied with a finger to the upper outer edge of the reaction vessel, to a height of about 1 cm.

Position and stability of the air tube

The stable, vertical positioning of the air tube (article number: 6.2418.100 or 6.2418.130) in the reaction vessel increases the reproducibility of the measurement results.

The air tube should protrude straight down into the vessel as illustrated in the following graphic representation (click image to enlarge).

Absorbent solution in the measuring vessel

Deionized water is used as the absorption solution with the Rancimat. Prior to beginning the analysis, the electrical conductivity of the water in the measuring vessel should not exceed 5 µS/cm.

If this value is higher, check the filters of the water system, and also ensure that there are no other sources of contamination.

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Positioning of the cannula for air supply

The PTFE (polytetrafluoroethylene) cannula for the air supply into the absorption solution (article number: 6.1819.080) must be aligned properly so that no air passes over the electrodes of the conductivity measuring cell, as shown in the graphic (click image to enlarge).

Air bubbles at the electrodes lead to noisy measurement curves that are difficult to evaluate.

Is it time to start the determination yet?

First, the temperature of the heating block (which is defined in the method) must be reached and stabilize before the reaction vessel is inserted into the instrument.

The sample identification data is then entered in the  StabNet software by the operator.

After connecting all of the tubing for the air supply, the reaction vessel can be inserted into the heating block. The sample measurement begins immediately after pressing the button on the Rancimat.

Cleaning: important for reproducible results

To obtain reliable analysis results, cleaning all accessories is of the utmost importance.

Both the reaction vessel and the inlet tube are disposable items. You can dispose of these materials immediately after cooling down. The rest of the accessories can be cleaned with a laboratory dishwasher (or equivalent) at maximum temperature and maximum drying time.

If you use glass or polycarbonate materials for the measuring vessels, you can of course also clean them in the same manner. The same applies to the measuring vessel lid with integrated conductivity electrode, the transparent silicone tubing, or the black Iso-Versinic tube, as well as the reaction vessel lid.

Tip: the silicone or Iso-Versinic tubing should be washed in a vertical position inside of the dishwasher to ensure it is thoroughly cleaned inside.

After washing, the transfer tubes and the reaction vessel lids should be heated at 80 °C for at least two hours in a drying cabinet, since the materials of these accessories absorb reaction products. This step further reduces the possibility of carryover to the next measurement which leads to unstable measurement results.

Maintenance

Depending on the use of the Rancimat, a regular visual control of the air filter on the back of the instrument is recommended. A clogged filter will lead to fluctuating air flows. The molecular sieve may also need a regular change depending on the instrument usage.

I hope that these tips have given you some helpful suggestions which will save you a little time and troubleshooting when using the Rancimat for determination of the oxidation stability of edible oils and other products. Good luck with your determinations!

Want to learn more?

Check out all of the stability measurement options offered by Metrohm.

Post written by Simon Lüthi, PM Titration (Meters & Measuring Instruments) at Metrohm International Headquarters, Herisau, Switzerland.