Abstract
Tuna fish meat is an expensive and highly perishable sea food. Fresh meat has a bright red colour which soon turns into an unsightly brown during storage. To prolong the aspect of freshness, the red colour is stabilised or even enhanced e.g. with carbon monoxide or nitric oxide, the product of a nitrite / ascorbic acid treatment, which bind as a ligand to myoglobin. These procedures are illegal. Here we present a method for identifying tuna meat samples, which have undergone fraudulent wet salting with nitrite. The method uses headspace-gas chromatography/mass spectrometry (GC/MS) for the determination of nitrous oxide, which is formed as the final product of the two-step reduction nitrite (added agent) to nitric oxide (ligand) to nitrous oxide (target compound). Complex bound nitric oxide is set free with sulfuric acid, which also promotes the reduction to nitrous oxide. The method was validated using 15N labelled nitrite as well as treated and untreated reference fish samples. A survey of 13 samples taken from the Swiss market in 2019 showed that 45% of all samples were illegally treated with nitrite.
Highlights
As Tuna fish species have been overfished on the one hand, and their meat being a sought-after but highly perishable food stuff on the other hand, the seafood industry has felt itself constrained to undertake efforts in increasing the shelf life of this commodity
We report on our method for the detection of nitrite treatment, which originates from the method for carbon monoxide (CO) determination and determines nitrous oxide (N2O) as the reduction product of nitric oxide (NO), stemming from the complex Mb- Fe+II-NO (Figure 1)
Headspace – gas chromatography/mass spectrometry (GC/MS) The sealed vials were placed into the incubation unit of the CombiPAL autosampler (CTC Analytics, Zwingen, Switzerland) for 60 min at 25°C and analysed with a Trace 1310 GC / ISQ quadrupole system (Thermo Scientific, U.S.A.) under the following conditions: 500 μL of headspace was injected at 70°C onto a Porous Layer Open Tubular (PLOT) column (30 m × 0.32 mm inner diameter, 12 μm film thickness, HP-PLOT Molsieve, Agilent Technologies, cat. no. 19091PMS4), Palo Alto, CA, U.S.A.)
Summary
Identification of nitrite treated tuna fish meat via the determination of nitrous oxide by head space-gas chromatography/mass spectrometry [version 2; peer review: 2 approved].
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