Changes in the f j i n g medium (rapeseed oil) during controlled deep-frying of slicedpotatoes and codfillets and oil heated without them were compared. Fifteen f j i n g cycles were conducted each day for five days. Results showed that oil after f j i n g of cod fillets contained 10% less triacylglycerol (TAG) polymers and 40% less TAG dimers than other oils. Other parameters (iodine and anisidine values and absolption value at 233nm) showed that less thennooxidative products formed duringfjling of food rich in proteins. Oil after f j i n g of cod fillets was darker and its acid value was significantly higher than that for other oils. Content of TAG polymers and dimers for oil afterfjling of slicedpotatoes and oil heated without food was equal. The relative loss of C18:2 acid and decrease in iodine value showed that oil after heating without food underwent more intensive thermooxidative transformation than other oils.

The increase in consumption of food fried in the so‐called ‘deep‐frying‐oil’entails the necessity of knowledge of both thermooxidative transformation occuring in the frying medium and chemical composition of oil absorbed by the fried product. The aim of this study was to correlate the quality of frying medium and oil extracted from potato fritters fried under rigorously controlled laboratory conditions in liquid and partially hydrogenated rapeseed oils. Oxidation and polymerization reactions predominated during deep frying of potato fritters in both frying media, but hydrolysis occured only to a small degree. The peroxide value was not a suitable quality control indicator for monitoring the thermooxidative transformation during deep frying. The most suitable method for examining such transformations was to study the content and composition of the polar fraction. A correlation existed between the amount of polymers and oxidized triacylglycerols (TAGs) and the amount of polar fraction. Small differences existed in the content of thermooxidative transformation products in the frying medium and the oil extracted from potato fritters. By monitoring anisidine value (AnV), E1%1cm and the content and composition of the polar fraction in the frying medium, it was possible to evaluate the quality of the fat in the fritters. Thus, the use of partially hydrogenated compared to liquid rapeseed oil in deep frying process may be preferred because at the same content of polar fraction and its components partially hydrogenated rapeseed oil contained a lesser amount of secondary oxidation products.