The Metrohm 743 Rancimat measures the oxidative stability of cooking oil by accelerating the rate of oxidation until the sample becomes rancid.
It can therefore be used to monitor and predict the lifetime of process frying oils.
During the measurement, a stream of air is passed through the sample that is contained within a sealed and heated reaction vessel.
These conditions accelerate the rate of oxidation of the sample, with peroxides being initially formed as the primary oxidation products.
After some time, the sample degrades completely; the second oxidation products formed include low-molecular organic acids in addition to other volatile organic compounds.
These are transported in a stream of air to a second vessel containing distilled water.
The conductivity in this vessel is recorded continuously.
The organic acids can be detected by the increase in conductivity.
The time elapsed until these secondary reaction products appear is known as the induction time or induction period, the point where the oil has become rancid.
The peroxide number (the primary oxidation products mentioned above) is a good measure of the amount of damage suffered by the oil (from storage, oxidation by light or heating) and, using a Metrohm auto titrator, peroxide number can be calculated, as well as the oxidative stability, to maximise the lifespan of a cooking oil.
The 743 Rancimat can also be used to measure the effectiveness of any antioxidants in the oil.
Evaluating the effects of antioxidants using the Rancimat is done by first determining the induction time of the fat or oil at several different temperatures.
The antioxidant is then added in different amounts (or equal amounts of different antioxidants are added) and the measurements are repeated.
A measure of the effectiveness of the antioxidant at the corresponding temperature is then obtained as is a stability index.
The correct choice of stabiliser can then be determined, as can the stability at storage temperature by extrapolating the storage temperature along a linear regression of known induction times.
Therefore, the Rancimat can replace storage tests that may be prolonged.
