Frying oil discarding: polar content vs. oligomer content determinations

The debate on deep-fat frying is primarily focused on the point at which any oil used for frying should be discarded. Polar content (PC) and triacylglycerol oligomer content (TOC) determinations constitute the basis of legislation for oil discarding in some European countries. Most of them have set a maximum level of 25% PC while others have established a PC cut-point between 20% and 27%. The maximum TOC have been set at 10% while other countries permit 16%. The aim of the present study was to study the relationship between these two measurements and to open some debate on whether PC or TOC is preferred for oil discarding assessment. Monounsaturated oils (olive oil, extra virgin olive oil, high oleic acid sunflower oil), polyunsaturated oils (sunflower oils and a blend of olive oil and sunflower oil), and saturated oils (palm olein) used in frying of different foods under domestic controlled conditions were employed. Further application of HPSEC to the isolated PC to measure TOC was preferred to the direct evaluation of TOC. Although PC and TOC were always very high correlated (p < 0.001), the 25% PC corresponds to a 10% TOC in palm olein but to 15% in sunflower oil. These findings suggest the need to unify criteria for oil discarding. Taking into account the potential toxicity of oligomers and also that some compounds defined as polar components are not necessarily altered (e.g. diacylglycerols, free fatty acid), it can be suggested that TOC gives more precise information about the alteration of the oil and its potential toxicity than the PC.