The oceans have been a source of food for humans for thousands of years and still provide 16% of the world’s protein today. However, fish is highly perishable and so must be eaten fresh or properly preserved. Freezing can preserve fish for up to 10 months, but despite this excellent preservation technique, fresh fish is still considered to be of a higher quality and can demand a higher sale price.
To increase profits, many producers and retailers thaw frozen fish and sell it as fresh. Although this fraudulent behavior is against the European Commission regulations on food law, it can be difficult to distinguish between fresh and frozen, then thawed (frozen-thawed) fish. Therefore, analytical methods that can detect the subtle biochemical changes are required.
When fish is cooled below –1.5 °C, water in the muscles turn into ice crystals. When thawed, cell membranes can rupture, causing enzymes and other cellular components to leak into the extracellular fluid. These biochemical changes can be measured by taking a sample of fish and analyzing the hematocrit or the activity of different enzymes in the tissue fluid. However, these methods involve damaging the product so it cannot be sold.
Nuclear magnetic resonance (NMR) spectroscopy is a non-invasive, high throughput technique that has recently been shown to be able to distinguish between fresh and frozen-thawed fish by measuring the metabolic products formed from enzymatic reactions in the extracellular fluid.
Using a Bruker Avance 600-Mhz spectrometer, researchers performed 1D 1H-NMR analysis on fresh and frozen-thawed Atlantic Salmon samples, firstly from the same fish and secondly from fish in different packages. The frozen-thawed samples were frozen overnight (≤ –20 °C) before thawing and storing at 4 °C for up to 18 days after slaughter. The fresh samples were kept at 4 °C throughout. 1D 1H-NMR was performed at regular intervals starting from the day after the frozen samples were thawed.
Fresh and frozen-thawed samples from the same fish, and those from different packages, were found to have different levels of aspartate (2.81–2.84 ppm). In the frozen-thawed samples, the formation of aspartate was observed until it reached a maximum concentration of 3.6–3.8 mg per 100 g on the third day after thawing and then decreased to zero. In comparison, no aspartate formation was observed in the fresh samples.
This difference is thought to be due to an increase in mitochondrial aspartate aminotransferase in frozen-thawed fish. This enzyme catalyzes the conversion of 2-oxoglutarate to L-aspartate, explaining the increase of aspartate observed in frozen-thawed samples, while no formation was seen in fresh samples.
This study shows NMR spectroscopy can readily distinguish between fresh and frozen-thawed fish, and the non-invasive nature of this technique makes it highly desirable for use in food fraud analytics.
- Shumilina E., et al. (2020). Differentiation of Fresh and Thawed Atlantic Salmon Using NMR Metabolomics. Food Chemistry. https://doi.org/10.1016/j.foodchem.2020.126227.
- marinebio.org. (2020). Ocean Resources. https://marinebio.org/conservation/ocean-dumping/ocean-resources/
www.fishforthought.co.uk (2020). Facts About Freezing. https://www.fishforthought.co.uk/blog/how-to-guides/fact-about-freezing