The magazine Food Research International has published an article based on a study conducted by researchers at the University of the Basque Country (UPV/EHC) where it was demonstrated that:
”The selection of cooking fat is of great importance due to its impact on the fat composition of the fish and the possible generation of toxic compounds in the oil during the frying process. This can effect food safety and human health.”
In the study, fillets of sea bass and bream were subjected to shallow frying in either a saucepan or a microwave, using either extra virgin olive oil or refined sunflower oil. The changes that occurred in the fat composition of the fish and the cooking oil were studied using nuclear magnetic resonance (NMR). The researchers explained that during the frying process under normal conditions not only do the fish fats migrate to the cooking oil, but components in the oil are transferred to the fish itself. Consequently, the composition of the oil is modified. On the one hand, it is enriched in terms of those acyl groups (fatty acids) that are more concentrated in the fish oil than in the cooking oil. On the other hand, the quality of acyl groups found in greater concentration in the cooking oil is simultaneously reduced.
In addition to this fat migration during frying, because the oils are subjected to high temperatures (170°C) in the presence of oxygen, a little thermal oxidation may occur. In the case of extra virgin olive oil, this reaction did not happen, owing to its greater resistance to degradation than that of sunflower oil.
However, in the case of sunflower oil heated in a frying pan, the formation of aldehyde compounds occurred. This did not happen, though, when the fish was cooked in a microwave.
Therefore, taking these results into account, and especially the generation of potentially harmful compounds, the better option of the two for frying fish is extra virgin oil. It was also observed that the type of fish is also a significant factor in the process of fat migration during frying. The fat content of the bream was actually found to have decreased after frying, whereas that of sea bass remained around the same or increased.