Around 200 billion pounds of fish and shellfish are caught each year from the world’s oceans. Most of these catches are for animal and human consumption, but other uses include the creation of fish oils and as ornamental pet fish. Fish must be eaten fresh or frozen to preserve it as it is highly perishable.
Freezing is a great technique for preserving fish, but the quality of fresh fish is better and can be sold at a higher price than its frozen counterpart. To increase their profits, this price difference has resulted in numerous retailers and producers selling frozen, then thawed (frozen-thawed) fish as fresh.
In 2002, the European Commission issued regulation number 178 stating that consumers should get an accurate description of the food they purchase, in order to try and stop this.
To establish whether businesses had violated this regulation, analytical techniques were developed along with this governing measure, to distinguish between fresh and frozen-thawed fish.
Freezing causes biochemical changes in fish muscle
Water in the muscles turns into ice crystals when fish is cooled to less than -1.5 °C. When the fish is subsequently thawed, this can cause cell membranes to rupture. This damage can result in enzymes and other cellular components leaking into the exudate and changing the extracellular metabolic and biochemical processes.
Research has demonstrated that differences between fresh and frozen-thawed fish can be identified by measuring the activity of different enzymes or modifications to the hematocrit. Methods to perform these analyzes usually damage the product, so it cannot be sold as they need a sample of the food to be removed.
Researchers at the Norwegian University of Science and Technology in Norway have recently shown that nuclear magnetic resonance (NMR) spectroscopy is a tool that is emerging in this field.
This method is able to avoid any damage to the product by measuring the metabolic products formed from the alterations in enzymatic processes non-invasively and efficiently distinguish fresh fish from frozen-thawed fish.
NMR spectroscopy can detect metabolic changes after thawing
The team carried out NMR analysis on fresh and frozen-thawed Atlantic Salmon over two weeks using a Bruker Avance 600-Mhz spectrometer to monitor the metabolic changes between the two samples.
The frozen-thawed sample sets were frozen at temperatures ≤ –20 °C overnight, then thawed and kept at 4 °C for up to 18 days after slaughter. On the other hand, the fresh fish samples were kept at 4 °C throughout the experiments. 1D 1H-NMR analysis was carried out at regular intervals starting from the day after the frozen samples were thawed.
The team compared samples from different parts of the same fish first. Over time, both sample sets had an increase in phenylalanine, with significantly higher concentrations found in the frozen-thawed fish samples.
The fumarate concentration in the fresh samples steadily decreased over time, but the concentration of fumarate in the frozen-thawed samples also increased but reached a maximum level at day three before decreasing to zero.
Aspartate concentration is different in fresh and frozen-thawed fish
The researchers thought that monitoring the alterations in aspartate concentration is the most helpful technique as there is the largest difference between samples, though both these metabolites could be employed to distinguish between fresh and frozen-thawed fish.
Aspartate grew until it reached a maximum concentration of 3.6–3.8 mg per 100 g on the third day after thawing before decreasing in frozen-thawed fish, in a similar way to fumarate. In contrast, no alteration was seen in aspartate concentration measured in fresh fish.
This technique was repeated by comparing Atlantic salmon from different packages. The results gathered were comparable to those from the initial experiment: the aspartate concentration increased in the frozen-thawed packet, reaching a maximum amount of 3.6 mg per 100 g muscle on day three, then decreased to zero. In the fresh packet, no aspartate formation was seen.
Aspartate formation in frozen-thawed fish is due to increased enzyme activity
The reason for this change between samples could be because of bacterial growth and not metabolic changes in the fish muscle. This possibility is low, as a similar amount of aspartate was observed in all samples independently from when the fish was frozen.
The researchers suggest that it is more likely that the reason is because of an increase in mitochondrial aspartate aminotransferase activity. This enzyme catalyzes the transfer of the amine group from 2-oxoglutarate to L-aspartate and has been previously discovered to heighten in activity in fish tissue fluid after freeze-thawing.
NMR spectroscopy is sensitive enough to distinguish between fresh and frozen-thawed fish, as the data shows. The non-invasive nature of this method makes it highly desirable for employment in food fraud analytics to ensure that consumers receive the quality of fish they pay for.
- 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.
- www.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
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