Freshness and Contamination with Formalin of Mackerel marketed in Dar es Salaam

Abstract Background Fish constitutes a nutritious food that deteriorates quickly when poorly preserved. Several biochemicals, including formaldehyde, naturally accumulate in the fish post-mortem. Apart from this natural formaldehyde, reports indicate the unlawful addition of formalin (37% formaldehyde solution) to the stored fish to prolong freshness. This is risky since formaldehyde is carcinogenic, genotoxic, and a potentiator of other carcinogens. Aim This study aimed to investigate both the freshness and the extent of contamination with formaldehyde of mackerel sold in Dar es Salaam. Methods A total of 60 mackerel samples were conveniently and equally obtained from the local markets, street vendors, and supermarkets in five districts of the Dar es Salaam region. Freshness was evaluated based on organoleptic characteristics. Formaldehyde analysis was done by High-Performance Liquid Chromatography (HPLC). Analysis of variance of formaldehyde concentration in fish flesh by source outlet and district was subsequently run. Results The analyzed mackerel samples had acceptable levels of freshness (2.46 ± 0.50) and a mean formaldehyde concentration of 10.89 ± 2.44 mg/kg. On average, the samples from supermarkets were the freshest (2.20 ± 0.21) however the most contaminated with formaldehyde (16.07 ± 4.68 mg/kg), while those from local markets were the least contaminated (3.91 ± 1.86 mg/kg) (p=0.000). Moreover, 0% (n=0), 20% (n=4), and 35% (n=7) of samples from local markets, street vendors, and supermarkets respectively, had formaldehyde concentrations above 20 mg/kg, the previously estimated highest concentration for natural formaldehyde in fish. Conclusion Mackerels found in Dar es Salaam have acceptable freshness but are substantially contaminated with formaldehyde. Whether this formaldehyde is natural or artificially added, our findings are inconclusive, given the conflicting global cut-off values for natural formaldehyde in fish. We, therefore, recommend a contextualized study to establish the time dynamics of formaldehyde formation in the stored fish. In the meantime, we advise the public to dwell on the local markets for fish rather than the supermarkets and street vendors.