Evaluation of frying oil subjected to prolonged thermal treatment: volatile organic compounds (VOC) analysis by DHS-HRGC-MS and 1H-NMR spectroscopy

It is known that fats and oils, when subjected to prolonged heating for frying, are subjected to a series of chemical-physical modifications, the effects of which can be observed in the variation of the sensory and nutritional characteristics. The main factors influencing the entity of these transformations are represented by the temperature and the time of the treatment, the nature of foods being fried, the presence of metals that catalyze the oxidation phenomena, and the composition of the frying oil. In order to prevent possible consumer risks due to the excessive and inappropriate use of frying oils and fats, the Italian Ministry of Health, in Circular n.1 of January 11, 1991, established that the concentration of total polar compounds (TPC), indicators of the state of triglyceride alteration, does not exceed 25 g /100 g in oils and fats used for frying food, a limit that reflects similar provisions established by legislation in other countries. In current analytical practice, the TPC evaluation method by preparative column chromatography is quite laborious and it is not repeatable and accurate. The aims of this study were to develop an accurate and repeatable method to evaluate the volatile organic compounds (VOC) which are formed during frying and find new possible markers correlated with TPC data, which should be suitable for routine use in order to determine the degree of triglycerides alteration. In particular, the markers have been studied by dynamic head space analysis (DHS) combined with high resolution gas-chromatography (HRGC) equipped with mass spectrometry (MS) and (1)H-NMR spectroscopy.