Oxidative stability is important for evaluating the quality of oils, fats and fatty acid methylesters (biodiesel); today the Rancimat method has largely replaced the slower AOM or Swift methods
Oxidative stability is an important criterion for evaluating the quality of oils, fats and fatty acid methylesters (biodiesel).
Today the Rancimat method developed by Hador and Zurcher has to a large extent replaced the time consuming AOM or Swift methods.
The Rancimat's fully automatic determination of induction time also allows large numbers of determinations to be carried out in less time.
This means that one can now make statements about products stabalised with antioxidants e.g prediction of induction times and the sensible choice of both the type and amount of stabilisers used plus be part of the technical specification for suppliers.
Antioxidants.
Classical antioxidants are chiefly used in food industry, particularly for the protection of animal fats against oxidative spoilage.
In addition they are also used for the food material containing sensitive essential ingredients eg, oxidation-sensitive vitamins, amino acids and unsaturated fatty acids.
Antioxidant plant phenols in food stuff also play an important role with respect to human health (oxidative stress in the body).
Important and well known antioxidants are tert-butyl-4-methoxy-phenol and butylhydrotouleneBHT, which are legally permitted food additives, vitamin C or the more lipophillic ascorbyl-palmitate and vitamin E.
Multi functional antioxidants can be obtained from extracts of rosemary or sesamin from sesame.
These highly effective absorbers of radicals are also oxygen collectors and complexing agents so that, depending on the matrix, their use is associated with considerably improved stabilisation.
Depending on the type of food, synergists can noticeably increase the antioxidant effects of anti oxidants.
Polyhydroxyl carboxylic acid eg, citric acid and their esters, poly-(phosphates) and lecithins are examples of such antioxidant synergists.
The Rancimat method.
A stream of air is blown through the sample at a temperature between 50-220C.
This oxidises the fatty acids in several stages.
In principle oxidation takes place according to a radical chain mechanism, in which easily volatile oxidation products (chiefly formic acid) are finally formed.
These are transferred by the stream of air into a measuring vessel containing deionised water, whose conductivity is continually being measured.
Plotting conductivity against time produces oxidation curves, whose point of inflection is known as the induction time.
These induction times are correlated with values determined by the more complicated active oxygen method.
Oils and fats.
The Rancimat principle is suitable for the determination of the oxidative stability of a wide range of natural oils and fats.
These are normally mixed triglycerides of both saturated and unsaturated fatty acids.
If inedible oils and fish oils are disregarded then these are usually edible vegetable oils i.e.
edible fats that are liquid at room temperature.
Edible vegetable oils primarily contain tocophenol as the natural antioxidant.
According to Souci et al, the total tocopherol concentration of various oils and fat are as follows:.
Wheat germ oil 2443.
Corn oil 1006.
Soybean oil 937.
Rapeseed oil 654.
Sunflower oil 625.
Olive oil 133.
Cocoa butter 56.
Butterfat 36.
Total tacopherol concentration in mg/kg.
In contrast to oils edible fats contain little tocopherols, ie.
rancidity is much higher, hence they must be stored appropriately and or stabilised by antioxidants.
Fatty acid methyl esters.
Fatty acids can be obtained from oils and esterification with methanol to produce mixtures of substances whose properties allow them to be used as a fuel for diesel motors.
Such fuels are generally known as Fame (fatty acid methyl ester) or Biodiesel.
The most important oil used for this purpose is rapeseed oil methyl ester.
As Fame is also subject to spoilage by autoxidation, the adulterated product having deposits in it which will block the nozzles of a modern diesel engine.
This can be avoided by the use of pure product and/or the addition of antioxidants.
The 743 Rancimat is an instrument with two separately thermostatted heating blocks, each holding up to four reaction vessels containing 2-10g of sample.
This means that samples can be run at one or two different temperatures.
Up to four different instruments can be linked to a single PC via serial interfaces, ie, up to 32 samples at eight different temperatures.
The short-chain carboxylic acids formed by the oxidation at elevated temperatures are collected in the deionised water and the conductivity measured.
The conductivity-time curve is shown live for each measuring point.
The induction time is determined automatically from the second derivative of the measuring curve.
In addition, all the induction times for a series of samples can be exported to Excel with a mouse click for statistical evaluation and presentation as a graph.
Evaluating the effects of antioxidants on edible oils and food products containing oils and fats can easily be made using the Rancimat; predictions on shelf life and the quality of raw products can be determined, which enables more efficient warehousing and stock control.
