Extra virgin olive oil healthiest for frying fish: study

The healthiest way to fry fish is to use extra virgin olive oil, especially if this is done in the microwave, a new study has found.

The frying techniques, the nature of the oil used and the fish species have been shown to exert a great influence on the changes that take place during the process.

Researchers from University of the Basque Country in Spain have shown that the choice of cooking oil is hugely important owing to its impact on the lipid profile in the fish and on the possible generation of toxic compounds in the oil during frying, which can influence food safety and human health.

To conduct this research, fillets of European seabass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata) were shallow-fried in a frying plan and in a microwave oven using extra virgin olive oil and refined sunflower oil.

The changes that took place in the lipid composition of the fish and of the frying oil were studied by means of Proton Nuclear Magnetic Resonance Imaging (H-1 NMR).

During the shallow-frying of the fish under domestic conditions, not only do the fish lipids migrate to the frying oil, the components of the oil are also transferred to the fillet of fish.

As a result, the composition of the oil used for frying is modified. It is enriched by the acyl groups (fatty acids) that are present in a higher concentration in the fish fat than in the original oil, and simultaneously it is depleted in the acyl groups present in a higher concentration in the original oil than in the fish fat.

So after having been used for frying, the extra virgin olive oil was richer in omega-3, omega-1 acyl groups, linoleic and saturated fats (from the fish) and poorer in oleic, which is the main acyl group in olive oil.

Likewise, after having been used for frying, the sunflower oil was richer in all the acyl group types (coming from the fish) except linoleic, which is the majority acyl group in sunflower oil.

Both types of oil were enriched by small amounts of cholesterol from the fish.
Besides the migration of lipids during frying, because these oils are subjected to high temperatures (170 degrees Celsius) in the presence of oxygen, certain small-scale thermal oxidation may take place in them.

In the extra virgin olive oil used for frying fish, this thermal oxidation reaction did not occur as it is more degradation-resistant than sunflower oil.

Yet in the sunflower oil used for frying fish in the frying pan, secondary oxidation compounds (aldehydes) were formed; some of them are regarded as potentially toxic depending on the concentration in which they are found.

It should be pointed out that these compounds did not form in the sunflower oil in the microwave oven.

Therefore, in view of the results obtained and bearing in mind the generation of these compounds that are potentially harmful for health, the healthiest option for frying is to use extra virgin olive and fry in the microwave, researchers said.

The study was published in the journal Food Research International.