Effect of frying cycles on polar components in soybean oil and absorbed lipids of fried potatoes

Abstract: Frying is a traditional and popular processing method for the food preparation throughout the world. Frying can not only confer good ?avor and color, but also generate many kinds of reaction products, affecting the quality of the oil and the food. A lot of published literatures focused on the quality of the frying oil. However, for direct consumption, the quality of the fried food affects human's health and safety, and thus addressing the study on the quality of the fried food is needed. As we all know, the content of total polar compound (TPC) is one of the most valid and objective criteria for the evaluation of deterioration of oils during deep frying. Thus, in this study, our research was mainly focused on the content and composition of polar compounds in the lipids absorbed on the surface of fried food during deep frying. Preparative flash chromatography, as a convenient and fast way, was adopted to separate polar compounds from the oil and the absorbed lipids of fried food. The obtained polar compound was further analyzed by high performance size-exclusion chromatography to determine its main compositions. The results indicated that the contents of TPC from both the fried oil and the absorbed lipid of fried food were gradually increasing with the number of frying times. No significant difference of TPC content between the rest oil of treatments and the sample of control was observed for the number of frying times of 1-30 (the heated oil without adding French fries was the control). However, TPC content of the control was significantly higher than that of the rest oil of treatments for the number of frying times of 40-60, indicating that the addition of potatoes to a certain extent could help inhibit the formation of polar compounds. In addition, frying times significantly changed the distribution of polar components, which were oxidized triglyceride oligomers (TGO), oxidized triglyceride dimers (TGD), oxidized triglyceride monomers (ox-TG), diacylglycerols (DG), free fatty acids and sterols (FFA & sterols), and some unknown compounds with small molecule, not only in the frying oil but also in the absorbed lipid of fried food. The contents of TGO, TGD and ox-TG in samples of the control, the rest oil of treatments, and the absorbed lipid of fried food were significantly (P<0.05) increasing with the number of frying times. But under the same condition, the contents of TGO, TGD and ox-TG in the absorbed lipid of fried food were much lower than those in the rest oil of treatment, while the difference of the contents of TGO, TGD and ox-TG between the control and the treatment was affected by the number of frying times. On the other hands, a little change was observed in the content of DG and FFA & sterols of all the samples for the frying times of 60. More kinds of unknown compounds with small molecule were generated with the increase of frying times. In conclusion, the content and the composition of TPC in the frying oil and the absorbed oil of fried potatoes are seriously affected by frying times. And in some extent, the health value of the fried potatoes is also influenced. Taking into account of the potential threat of polar compounds to human health, the frying times must be strictly controlled.