Advantages of avoiding the use of sulphiting agents
Sulphites are increasingly used in a number of beverages and foods including wine, beer, salads, and other prepared foods. As a result of their prevalence in foodstuffs, they could accumulate in the body above recommended limits and potentially pose a health risk for consumers. Furthermore, although other “sulphite-free” antioxidants have been developed in recent years to prevent discoloration in shrimp (for example, 4-hexylresorcinol), great caution should be exercised in the use of these new substances due their possible links with hormonal disorders.
Another important international issue is the decline of demersal fishery resources throughout the Mediterranean region. Among the solutions to this challenge are the use of more sustainable fishing practises that include, for instance, a reduction in fishing effort. This in turn could be balanced by an improvement in the quality and safety of seafood to give a higher value product.
Can oxygen-free packaging prevent melanosis?
The growing interest in new and more effective packaging methods, which could reduce or eliminate the use of chemical additives, combined with the need to encourage more sustainable fishing practises has resulted in novel food packaging techniques being tested in many parts of the world. Among this research is a recent study carried out at IAMC CNR (Istituto per l’ Ambiente Marino Costiero, Consiglio Nazionale delle Ricerche) in Mazara del Vallo, Sicily. The study, part of the Ritmare Project, was carried out on board a bottom trawler, and was designed to combine freezing and oxygen-free packaging methods to inhibit melanosis in the two most important shrimp species caught in the Mediterranean basin. The results from the trials were compared with conventional shrimp products using sulphiting agents.
The commercial bottom trawler was equipped with an automatic skin/modified atmosphere packaging system. Samples of both species, deep water rose shrimp (DWRS) and giant red shrimp (GRS), were washed in flowing seawater and quickly pre-chilled by dipping them in a 1:1 mixture of seawater and ice. The shrimp were drip dried, packaged in special oxygen barrier bags (approximately 400 g), and frozen in a blast freezer at −35°C. Roughly eight hours after being caught, the shrimp bags were thermo-sealed in an oxygen-free environment by a heat-seal plastic film. To compare results, a second batch of shrimp was processed using the preservation techniques and materials usually employed by Mediterranean fleet crews. This batch was dipped in a seawater solution (4% w/v) of commercial anhydrous sodium sulphite (the shrimp to dipping solution ratio was 1:4). Finally, all the samples were stored at −18°C and delivered to the CNR laboratory for sensorial and biochemical analysis.
After four months of storage at -18°C inspection of the GRS bags showed that the giant red shrimp specimens were perfectly preserved in an oxygen free environment with no signs of dark discoloration detectable through the skin plastic film. After five months of storage the exoskeleton (cephalothorax) and the tail of a single specimen were subjected to a detailed investigation. This clearly showed that there were no signs of blackspot and that the muscle tissue remained bright, without signs of dehydration and/or yellowing due to lipid oxidation.
Encouraging results from several indicators
The overall quality characteristics of shrimp without chemical preservative treatments and simply frozen and oxygen-free packaged were also tested using international scientific protocols for seafood control. These include measuring the changes in the melanosis score, pH, total volatile basic-nitrogen (TVB-N, a freshness indicator) as well as thiobarbituric acid (TBA, a measure of lipid oxidation) between time zero (after the catch) and six months of storage time at -18°C. These tests revealed that pH in skin packed shrimp remained close to that of fresh shrimp after six months of storage time. On the contrary, a significant increase in pH (up to 7,6) was detected in the samples treated with sulphiting agents. This is due to the high alkalinity (pH 8,5) of the sodium sulphite solution used for traditional sulphiting treatment, which directly affects the shrimp meat.
In the case of (TVB-N) no significant differences were detected between the two treatments (oxygen-free packaging vs sulphiting). More specifically, sulphited samples showed a rapid increase of TVB-N immediately after the sulphiting treatment, which could be the outcome of high pH in turn induced by sodium sulphite. During the frozen storage of both shrimp species, lipid oxidation (here measured by TBA) remained constant and well below the threshold limit for this kind of product. Furthermore, no significant differences were detected between the oxygen-free packed samples and the sulphited ones. With regard to the melanosis score, after six months of storage, the sample packed under skin did not show any sign of the classic black spotting. This result confirms that melanosis is induced by oxygen, and therefore any oxygen-free packaging method combined with freezing could be used to delay the post mortem blackening of shrimp.
Oxygen-free packaging is a potential alternative to sulphites
The results of this study reveal that early application of oxygen-free packaging techniques, especially skin and/or modified atmosphere packaging using nitrogen (100%) as compensating gas, in combination with freezing and frozen storage, should be considered for preventing melanosis and other chemical deterioration induced by oxygen. This packaging technique may promote the safety and quality of marine crustaceans as well as maximize their economic value.
Istituto per l’ Ambiente Marino Costiero, Consiglio Nazionale delle Ricerche,
Via L. Vaccara, 61, 91026 Mazara del Vallo, Italy