Oxidation stability of natural oils and fats
The Rancimat method
The determination of the oxidation stability of oils and
fats is the classical application for the 743
Rancimat. The resulting induction time that is determined
characterizes the resistance of oils and fats to oxidation.
In addition to the term «induction time», the
expression Oil Stability Index (OSI) is in
common use. The Rancimat method is also called the
automated Swift test or accelerated oxidation
test and is in general use today as an automated
version of the previously used and extremely complicated Active
Oxygen Method (AOM).
During the measurement a stream of air is passed through
the oil or fat sample contained in a sealed and heated reaction
vessel. This treatment results in oxidation of the oil or
fat molecules in the sample, with peroxides initially being
formed as the primary oxidation products. After some time
the fatty acids are completely destroyed; the secondary oxidation
products formed include low-molecular organic acids in addition
to other volatile organic compounds. These are transported
in the 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 that elapses until these secondary reaction products
appear is known as the induction time, induction period or
Oil Stability Index (OSI).
The Rancimat method yields standard quality control parameters
for the production of oils and fats in the food industry or
for incoming goods checks in further processing. In addition
to oils and fats from vegetable sources, the oxidation stability
of animal fats such as lard, tallow and fish oil can also
be checked with the 743 Rancimat as a matter of course.
Foodstuffs may contain added antioxidants
in order to delay the oxidative decomposition of their oils
and fats. With the 743 Rancimat it is possible to characterize
the effectiveness of antioxidants. This opens up a further
field of application for the instrument.
Standards
The Rancimat method is included in various national and
internationals standards, such as:
- AOCS Cd 12b-92 (AOCS – American Oil Chemists’
Society)
Sampling and analysis of commercial fats and oils:
Oil Stability Index
- ISO 6886
Animal and vegetable fats and oils – Determination
of oxidation stability (accelerated oxidation test)
- 2.4.28.2-93
Fat stability test on Autoxidation. CDM,
Japan
- Swiss Food Manual (Schweizerisches Lebensmittelbuch,
SLB), section 7.5.4
Estimating the storage stability
An extension of the previous possibilities is provided by
a new function – the so-called temperature extrapolation.
This is an aid for estimating the shelf life of oils and fats.
The extrapolation makes use of the relationship between the
measured induction time and the temperature given by van’t
Hoff’s law. Several measurements are made at different
temperatures and then extrapolated to the storage temperature.
The value obtained allows a rough estimation of the storage
stability of the oil or fat.
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