Frontal elution liquid chromatography of a "total artefacts" peak in frying oils At present, colour reactions and other chemical and physical tests are used to decide when an oil should be discarded lWG. None of these tests is very satisfactory, however, as several different mechanisms of oil deterioration may operate at. the same time, and normally only one @oup of products is sensitive to one colour reaction.
Thermal degradation of oils leads to thermal dimeric triglycerides and higher polymers.
Oxidative degradation may produce oxidized triglycerides containing polar functional groups and dimeric triglycerides linked by one or more oxygen atoms. Hydrolytic processes will lead to diglycerides and free fatty acids. Depending on fat composition, type of food, frying temperature, agitation of the oil, presence or absence of antioxidants, oil surface area and access of air, cooling periods between usage, and the presence or absence of a layer of water vapor at the oil-air interface, the relative concentration of a particular class of artefacts may vary considerablys. A new approach was the introduction of gel permeation chromatography (GE)
for the analysis of used frying oils 0--Lo. The importance of this technique lies in the fact that a peak at molecular weight ~Soo, which is not present in fresh fats, increases considerably during the frying procedure and constitutes a measure of oil deterioration.
This peak represents the sum of both non-polar (thermal) and polar (oxidized) dimeric triglycerides.
In a similar approach, a method was developed for the simultaneous elution of polar compounds by liquid chromaiography (IX) using an ethanol front?. The resolution of osidized oils into several peaks by LC on silica gel columns has also been described in two earlier paperslO*L"-.
Both of the two types of bulk frying oil investigation described, GPCO-0 and frontal elution LCll, have the shortcoming that they include several, but not all. of the important classes of degradation products.. GPC does nof determine oxidized triglycerides, and the type of LC describedll does not include non-polar dimetic triglycerides and products of low polarity in the polar peak I?. We wanted to develop a method that would include 'ihis class of products in a similar frontal elution peak. This would then constitute a "total artefacts" peak that combines all of the major classes of degradation products.
A composite peak can be quantitated only by a detector that has a similar response for all components of the peak. The Pye detector comes close to this requirement, while UV and refractive index detectors are too specific and may indicate only one or a few components.
Non-polar dimeric triglycerides can be separated from the normal monomeric triglycerides by adsorption chromatography13 on dry, active silica gel, e.g., in mixtures of light petroleum and diethyl ether Id. It is difficult to separate the two classes of