Detoxification of tuna from mercury through l-cysteine: a speciation-based study

Mercury (Hg) contamination in tuna, especially as methylmercury (MeHg), remains a persistent food safety concern. This study assessed the effectiveness of l-cysteine in reducing total Hg (tHg) and MeHg in canned tuna and fresh fillets. Samples were treated with a 1.2% (w/w) cysteine solution and analysed by ICP-MS for tHg and frontal chromatography ICP-MS for MeHg. Untreated canned tuna contained 0.066-0.225 mg kg^-1 tHg, whereas fresh fillets reached 1.036 mg kg^-1. Cysteine treatment significantly reduced Hg levels, with mean removal of 48% in canned tuna and 64% in fillets. MeHg decreased proportionally with tHg, indicating cysteine binds both Hg and MeHg without species selectivity. Removal was more consistent in fillets than canned tuna, likely due to processing effects on protein binding. Overall, cysteine treatment represents a simple, low-cost, food-safe intervention that lowers Hg exposure and reduces concentrations below regulatory limits, providing additional protection for vulnerable consumers (pregnant women and infants).