DIN EN ISO 3656:2011-05

This International Standard specifies a method for the determination of the absorbance of light in the ultraviolet spectrum by fats and oils. Changes in absorption in the UV region are used as quality, purity, and authenticity criteria for fats and oils. Refining causes the formation of conjugated dienes and trienes and increased values of specific extinction at 232 nm and 268 nm, which then indicate the presence of refined oils. The oxidation of linoleic and linolenic acids results in the formation of hydroperoxides in which the double bonds become conjugated. Furthermore, the formation of either carbon-carbon bonds or carbon-oxygen bonds (α,β-unsaturated carbonyl compounds) as secondary autoxidation products are observed. These compounds all lead to an increase of the absorption in the region between 225 nm and 325 nm. The third edition of this International Standard allowed the determination of the UV absorbance at 232 nm and 268 nm using three different solvents (isooctane, cyclohexane or n-hexane). However, it is known that the solvents themselves have an effect on the UV absorbance between 260 nm and 276 nm in vegetable oils. Recent investigations have shown that the measurement of specific extinction at 268 nm and 270 nm for the same oil in isooctane or in cyclohexane give significantly different results. In isooctane, the maxima appear at 267 nm to 268 nm whereas in cyclohexane, the maxima appear at 268 nm to 269 nm. In the IOC standards for the determination of the ultraviolet absorbance of (firgin) olive oils, the specified wavelengths are 232 nm and 270 nm. Taking into account the above, the revised forth edition of this International Standard specifies measurement at 268 nm when isooctane is used and measurement at 270 nm when cyclohexane is used. Moreover, the variation of the specific extinction (δ K) for olive oils has been introduced. Precision data from a new collaborative trial have also been taken into consideration for this revision. The International Standard has been prepared by ISO/TC 34/SC 11 "Animal and vegetable fats and oils" (secretariat: BSI, United Kingdom). CEN/TC 307 "Oilseeds, vegetable and animal fats and oils and their by-products - Sampling and test methods" (secretariat: AFNOR, France) is responsible for the European Standard. On the national level, NA 057-05-05 AA "Gemeinschaftsausschuss für die Analytik von Fetten, Ölen, Fettprodukten, verwandten Stoffen und Rohstoffen" (GA Fett) ("Joint Committee for the analysis of fats, oil, fat products, related substances and raw materials") of the German Society for Fat Science (DGF) and NAL are responsible for the standard.