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The sodium and potassium salts of nitrate and nitrite (E249-E252) are authorised additives for cured meat products for a variety of purposes. In the realm of meat protection, nitrite kills the bacterium Clostridium botulinum, the causative agent of botulism, as well as other Clostridium species. The active agent is nitric oxide (NO), produced by reduction of nitrite in the meat. When slow release of nitrite is desirable, nitrate is added instead. It diffuses throughout the product before being slowly being reduced to nitrite, then nitric oxide. Nitrite is also added for aesthetic reasons, to please the consumer. The NO reacts with the haem in myoglobin to produce red-coloured nitrosomyoglobin, which maintains the pink-red colour of meat and prevents it from turning brown from air oxidation of the myoglobin present. In addition, nitrite modifies meat flavour and texture. As with many food additives, there are health concerns over the addition of nitrite and nitrate. Nitrite can react with amines in heated meat (such as cooking bacon) to form carcinogenic nitrosamines, one reason why the amount added to food is regulated, generally at 200 ppm for nitrite and 500 ppm for nitrate. Checking for the amount of nitrite and nitrate that has been added to meat is tricky, because they react with so many components in the meat matrix, such as myoglobin and other proteins, lipids and sulphydryl groups. Free nitrite and nitrate are usually present at 10-15% and less than 10%, respectively. However, analytical methods target these residual levels since it would be difficult, if not impossible, to measure the total amounts present. The classical method for nitrite involves its conversion into an azo dye followed by spectrophotometric detection. Recently, capillary electrophoresis, differential pulse voltammetry, anion-exchange chromatography and ion chromatography have also been employed. The principal advantage of chromatographic methods is the in-built separation of components and this has prompted researchers in Portugal to try and devise a simple, rapid HPLC method for residual nitrite and nitrate in meat, specifically ham. Isabel Maria Pinto Leite Viegas Oliveira Ferreira and her colleague S. Silva from the University of Porto recognised that the method should include minimal sample manipulation in order to reduce the possibility of nitrite oxidation. So, they tested five protocols on cooked turkey ham, cooked pork ham and dried ham bought from a local supermarket. In the chosen procedure, homogenised ham was dried in an oven for 1 h then mixed with activated charcoal and hot water (50-60 °C) for 15 min. This solution was deproteinised with acetonitrile and filtered. Recoveries of 93.6-97% and 99.5-104.3% were achieved for nitrite and nitrate, respectively. The extracts were analysed by HPLC with diode array detection at 205-215 nm. A reversed-phase C18 column was combined with an isocratic mobile phase of n-octylamine in tetrabutylammonium hydrogensulphate. Good nitrate/nitrite separation was achieved within 8 minutes and no interfering peaks were observed. The detection limits were 0.019 and 0.048 mg/kg for sodium nitrite and sodium nitrate, respectively. Detection was linear over 0.0126-10 mg/L. Real world levels of nitrite and nitrate in the ham were found to be in the ranges 0.113-1.253 and 0.663-0.666 mg/kg, respectively. These were statistically similar to the amounts determined by the reference colorimetric methods ISO 2019 and 3001, although the latter were consistently higher. The researchers suggested that unremoved matrix interferences would raise the colorimetric values, highlighting the advantage of chromatographic separation. The speed of the new method, including extraction and analysis, combined with its excellent selectivity and lack of spectral interferences, suggest that it will be useful for routine assays of nitrite and nitrate in quality control labs. 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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