Like a virgin: Fishy NMR fry up

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  • Published: Aug 1, 2016
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Like a virgin: Fishy NMR fry up

Fried fillet of fish

Photo by David Bradley The influence of frying technique, cooking oil and fish species on the changes occurring in fish lipids and oil during shallow-frying, studied by H-1 NMR

Proton nuclear magnetic resonance (NMR) spectroscopy has been used in experiments by scientists at the University of the Basque Country (UPV/EHU) that suggest that extra virgin olive oil is the best option for frying fish.

Bárbara Nieva-Echevarría, Encarnación Goicoechea, María J. Manzanos, and María D. Guillén of the department of Food Technology in the Faculty of Pharmacy, at the University's Lascaray Research Center have tracked the changes that take place in fish lipids and in the oil during the frying process. They hoped to determine which oils retain their nutritional value during cooking various different kinds of fish. It was previously assumed and verified to some extent that the exact lipid profile of a given cooking oil substantially affects the final nutritional level of the fish as well as whether or not potentially toxic degradation products are generated. The influence on food safety and human health is important to those offering culinary advice.

The team shallow fried fillets of European sea bass (Dicentrarchus labrax) and farmed gilthead sea bream (Sparus aurata) in a frying pan and in a microwave oven using extra virgin olive oil and refined sunflower oil. Proton NMR allowed them to follow the changes in lipid compositions of the fish itself and of the two different oils in which the samples were cooked. The team describes their procedure thus: "Proton NMR spectra of oils unheated, heated and after their use in shallow-frying, as well as of the fish lipids extracted from raw and fried fillets, were recorded on a Bruker Avance 400 spectrometer operating at 400 MHz... 200 microlitres of lipid samples were mixed in a 5 mm diameter tube with 400 microlitres of deuterated chloroform (CDCl3), which contains 0.2% of non-deuterated chloroform, and a small proportion of tetramethylsilane (TMS) used as reference compound for calibrating chemical shift at 0.0 ppm."

Fatty acids on the menu

Critically, there is migration of lipid components from the frying oil to the fish and oils from the fish itself back into the surrounding cooking oil during conventional domestic shallow-frying conditions. One result of this two-way oily traffic is that the frying oil and the fish are both modified. First, the frying oil becomes enriched by the acyl groups (the fatty acids) that are present in a higher concentration in the fish fat than in the cooking oil. Secondly, and at the same time, the level of the main acyl group that was originally present in the unused cooking oil falls. Thus, after being 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, and in other healthy extra-virgin olive oil components.

Similarly, the used sunflower oil was richer in all the acyl group types (leaching from the fish fillet) with the exception of linoleic acid, which is the major acyl group in sunflower oil. In addition, the amount of cholesterol, coming from the fillet, rises in both types of cooking oil during the frying process.

The team also explain how the fat profile of the fish fillets changes in composition during the frying process. The fish becomes enriched with the oleic acyl groups and other healthy components from the olive oil or with the linoleic from the sunflower oil. Levels of plant sterols, the vegetable counterparts of animal cholesterol also rose in the fish. At the same time, during the frying process, lipids such as omega-3 docosahexaenoic (DHA) and eicosapentaenoic (EPA) polyunsaturated groups are depleted in the fillet as cooking proceeds.

Health food

An additional factor to take into account when studying cooking is perhaps the most obvious one, the fact that the oil and food are raised in temperature. During frying, the lipids experience a high temperature of around 170 degrees Celsius in the presence of oxygen. Thus various small-scale thermal oxidation reactions can take place in the oils and in the fish. However, components of extra virgin olive oil are much more resistant to degradation by thermal oxidation reactions than those of sunflower oil, making olive oil a healthier choice on these grounds.

Indeed, after pan-frying fish in sunflower oil secondary oxidation compounds, such as trans,trans-2,4-alkadienals and trans-2-alkenals, were detected. That said, if the fish was cooked in sunflower oil in the microwave oven, no aldehydes were detected with the technique used. This suggests that if you opt for sunflower oil instead of extra-virgin olive oil, microwave cooking might be a healthier choice than the frying pan. Therefore, in view of the results obtained, the healthiest option for frying is to use extra-virgin olive and fry in the microwave.

The team also points out that their study shows that the different fish species affect considerably the oil migration processes during cooking. The fat content of the gilthead sea bream is found to have fallen after cooking while that of the European sea bass stays almost the same relative to the uncooked state.

This study shows that the frying technique, the type of oil used and the fish species exert a great influence on the changes that take place during the frying process. "Correctly selecting the oil is of paramount importance owing to its impact on the final composition of the fat in the cooked fillet and the possible generation of potentially toxic compounds in the oil during the frying process, which will greatly influence food safety and human health," the team concludes.

"[This work] is part of a broader study in which the effect of several culinary techniques on fish lipid composition are addressed. In addition, in vitro gastrointestinal digestion of these fish fillets are also underway," Guillen told SpectroscopyNOW. "The techniques used for these purposes are mainly proton NMR and solid-phase microextraction-gas chromatography mass spectrometry (SPME-GC/MS)."

Related Links

Food Res Intl 2016, 84, 150-159: "The influence of frying technique, cooking oil and fish species on the changes occurring in fish lipids and oil during shallow-frying, studied by 1H NMR"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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