Impartial assessment of oil degradation through partitioning of polar compounds in vegetable oils under simulated frying practice of fast food restaurants

Measurement of total polar compounds (TPC) is recognised as one of the objective indicators to gauge oil resistance against high temperature. Dependency to TPC measurement alone in arbitrating the quality and safety of oils can be misleading especially for those oils that are rich with significant amount of natural diacylglycerols (DAG). In fact, the results generated could be at the higher side when the analysis protocol involves excessive drying. Method modification has been established to isolate and quantify the individual polar compound fractions without the involvement of drying stage to protect the integrity of samples and therefore produce credible results. Performance of the established method is validated with the DAG quantified from the acylglycerols analysis. A series of vegetable oils, i.e. palm olein (POo), soybean oil (SBO), canola oil (CAN) and sunflower oil (SFO), were subjected to 9 days of intermittent frying, 8 h day−1 and 144 frying cycles. The transient of TPC across frying time was coincided with polymerised triacylglycerols (PTAG) following the linear model with coefficient of determination (R2) greater than 91% while DAG and oxidised triacylglycerols (OxTAG) showed good regression with the quadratic model (R2 > 0.97). Oils with equal amount of polyunsaturated fatty acids (PUFA) were found to have similar rate of combined PTAG and OxTAG. Whilst liquid oils have similar OxTAG contents with POo, they were much distressed with polymeric reaction given that their PTAG levels were 1.6-2.4 folds higher than POo. Blending liquid oils with POo has been proven to moderate the level of PTAG when similar frying conditions were applied.