Note.: Effect of metabisulphite treatments and freezing on melanosis inhibition in rose shrimp Aristeus antennatus (Risso, 1816)

Melanosis or blackspot is a dark discolouration, which is unattractive to consumers and reduces the market value of crustaceans. It has been found to be unavoidable in shrimps after the trauma of capture, string and thawing process, and it varies by species, moulting cycle, and harvesting and handling methods. Sulphiting agents are still the most effective and practical method to prevent melanosis. However, levels of residual sulphur dioxide should be low since they can produce health problems in certain individual consumers. In this study, female specimens of shrimps Aristeus antennatus were treated with HQ-Bacterol F containing 40% sodium metabisulphite on board a fishing vessel. Different concentrations, times and temperatures of immersion, and subsequent freezing storage were tested. Untreated shrimps showed the first signs of melanosis after remaining for 19 h on crushed ice and their market value decreased at 27 h, whereas shrimps treated with 2% HQ-Bacterol F for 5 min maintained their market value. Increasing the HQ-Bacterol F concentration reduced spot appearance but increased the residual SO2 in the tissues. However, less than 10% of the sulphites penetrated into the edible part (the muscle) and all conditions tested were safe from a legislation point of view. Temperature and duration of immersion in the metabisulphite solution did not affect melanosis in the test conditions. Quick freezing appeared to be a good method to prevent melanosis simultaneously and a following storage for three months did not affect the appearance of melanosis.