Evaluation of the effect of rigor mortis on quality, nutritional value, and fatty acid profile changes of frozen farmed Acipenser persicus fillet

Sturgeon is the common name for the 27 species of fish belonging to the family Acipenseridae. The earliest sturgeon fossils date to the Late Cretaceous and are descended from other, earlier Acipenser form fish which date back to the Early Jurassic epoch. The Persian sturgeon is a species of fish in the family Acipenseridae. It is found in the Caspian Sea and to a lesser extent the Black Sea and ascends certain rivers to spawn, mainly the Volga, Kura, Aras, and Ural Rivers. Rigor mortis is a phenomenon where the fish, after death, becomes stiff. The rigor will get completed when almost all muscle fibers are contracted which lasts a while before it is resolved. The muscle then becomes more and more tender in the stage called meat tenderization. This locking of the muscles, called rigor mortis, can be slowed down by chilling the fish immediately after death. Rigor mortis starts immediately or shortly after death if the fish is starved and the glycogen reserves are depleted, or if the fish is stressed. The mechanism behind this is muscle contraction due to a shortage of ATP (adenosine triphosphate). However, ATP, carrier that provides energy to relax the muscle again, is no longer present, so the muscles remain rigid. Over time this rigidity disappears, but by then the fish has significantly deteriorated. Post-mortem factors such as glycolysis, pH, and rigor mortis have been shown to have profound effects on fish fillet texture. Rigor mortis is dependent on the fish species, temperature and handling before slaughter, slaughtering stress, the biological status of the fish, and the temperature of pre-rigor storage. The method used for stunning and killing the fish also influences the onset of rigor. Even when fishes are killed under the same conditions, there can be a high variation in the time of onset of rigor mortis. Therefore, the present study aimed to investigate the effect of rigor mortis on the nutritional, fatty acid changes, chemical, microbial and sensory characteristics of farmed A. persicus fillets during storage in freezing conditions.

The fish were caught using a cone net, and killed by pulling them out of the water. They were then transported to the laboratory by ice-containing units containing twice the weight of the fish. The fish were washed with drinking water immediately after death. To spend the period of freezing the corpse in the fish transport tanks (Chilled Sea Water), twice the weight of the fish was covered with ice powder prepared from drinking water. The fish layers were separated by ice and placed on the ice fish 5 cm thick. The water outlet valve of the CSW tank was left open during the storage period to drain the water from the ice thaw. The rigidity of the A. persicus was measured by dropping the tail from the edge of the table, and this stage ended with the softening of the fish muscles. The fish were washed after the rigor mortis had frozen. The fish were then beheaded, eviscerated, peeled, and washed again. The fish were filleted into 1-3 cm pieces weighing 250-200 g. The fillets were packed in polyamide plastics under vacuum and frozen in a fast freezing tunnel (temperature -40 ° C) for 8 hours. The farmed A. persicus fillet was used as a control before passing the coffin freezing and vacuum-packed. Experimental and control treatments were stored at -18 ° C for 6 months. Their quality was evaluated using chemical, microbial, sensory, and physical tests. The total bacterial counts, Staphylococcus, Pseudomonas, Coliform, Escherichia coli, and Clostridium were used for microbial characteristics. For chemical properties of peroxide, thiobarbituric acid, free fatty acid, total volatile nitrogen bases, polyan index, and the ratio of unsaturated fatty acids to saturated fatty acids were investigated. The quality of experimental and control fillets were assessed through physical experiments including pH and sensory such as tissue odor, color, taste and overall acceptance. Nutritional value and fatty acid profiles were determined.

The fish weight and length were 1.80 kg and 60 cm. In research and control treatments: pH, microbial, chemical, and sensory factors showed significant changes during storage (p<0.05). Pseudomonas, coliform, and Escherichia coli were not observed during the storage period. Nutritional value was not significantly different (p> 0.05). Palmitic acid was the highest (33.67 – 34.16%). The polyene index decreased in the experimental treatment (0.18) compared to the control (0.20). The ratio of unsaturated to saturated fatty acids in control and research treatments was 1.58 and 0.84, respectively. The shelf life of treatments was six months in freezing conditions. Color, texture, and overall acceptance in the experimental treatment were evaluated better than the control (p<0.05).

Increasing the shelf life of seafood is important not only for the processing industry but also for the companies that supply them. Freezing is currently one of the best fish preservation techniques that leads to maintaining the safety and quality of fish. However, pre-frozen treatment of the corpse and its storage in freezing conditions leads to the production of fillets with more contraction of the fillet length and more secretion during thawing. Compared to pre-corpse filleting, post-corpse filleting results in a threefold increase in efficiency, which is economically significant regardless of the time required for the process. Considering that microbial and chemical properties are lower in post-mortem fillets, but the sensory properties in these fillets are higher compared to fillets that have not undergone corpse. Because sensory characteristics such as color, texture, and appearance influence consumer decisions, Therefore, these properties are preferred over chemical and microbial agents. However, post-mortem processing in the industry is not easy and enough time must be taken to pass this stage, which leads to problems such as the need for a large pre-cooler as well as limiting the capacity of the processing plant. However, due to the benefits of corpse freezing on the quality of fillets, passing the corpse freeze is recommended for fish filleting.