Physical and organoleptic traits in commercial size bluefin tuna (Thunnus thynnus) reared in floating cage

Abstract

The farming of captured wild red tuna (Thunnus thynnus) presents remarkable interest and by now more than one half of tuna caught in the Mediterranean Sea are finished in the fattening cages. The rearing of this species generally does not imply a significant growth of the animals, as instead an improvement of quality and value of the tunas through the restoration of elevated fat levels in flesh, a very appreciated trait by the Japanese consumers. The commercial outlet of the product is the international market, where there is a persistent demand of product for sushi and sashimi. A possibility for increasing the competitiveness of the Italian farmers is to guarantee to the market a very fresh product, characterised by elevated quality and better than the supplies from the other Countries. The aim of the present research was to describe some physical and freshness/organoleptic characteristics of red tuna, particularly the rigor mortis evolution (pre rigor, full rigor and rigor release phases) measured on the whole body. This quality indicator is not easy to measure in tunas due their generally great dimensions and there is a lack of this kind of information in literature, since tunas usually are immediately beheads, eviscerated and deprived of the tail. Specimens of red tunas captured in the Tyrrhenian Sea on October 2005 were transferred to the tuna farm (Vibo Marina, VV) in a circular floating cage, where they were fed with a mixed diet (anchovies, mackerels, sardines, herrings, cephalopods) at the feeding level of 1-4% live weight, depending on the water temperature. In April 2006, a random sample of 10 tunas (44.0±6.2 kg b. w.) was stunned/killed (one by one) using a modified diver gun, shoot by a diver directly in water and then subjected on board to bleeding and spiking and ikejime operations. On each fish, placed in ice covering immediately after bleeding, rigor index, body temperature, eye liquor pH and epaxial and red muscle pH were determined at death, 4h and 24h after death. Sensory evaluation by freshness classes (Rule CE n. 2406/96) and the measurement of rigor index were carried out at the 2nd, 5th, 6th and 7th day after death. The temperature increase with the depth within the tuna body was confirmed at death: eye liquor (21.6±1.6°C), epaxial muscle (21.4±1.3°C) and red muscle close to the vertebral column (30.2±0.9°C). Temperature diminished until 1-2°C within the first 24h in surface and only within 48h in depth. Fish were in Extra class of freshness until 3th day after death and in A class (good quality) until 7th day. Rigor index onset differed, possibly due to the different harvesting time and/or the stress conditions: some tunas, reached the full rigor at 48 hours, others reached rigor at 24h maintaining it until 48 h and the last fish caught showed the shortest full rigor onset (4h) and earlier rigor release (80% at 24h). Rigor release had a moderate decrease (70% at 120h; 54% at 144h and 50% at 168h, excluding the last tuna, which had 7%). The pH of red muscle (6.69 at death) had a fast decrease within 24h (4.99) and an increase at 48h (5.87). This behaviour could be related to a higher speed in the post mortem biochemical processes in red muscle, justified from the still high temperature recorded at 24h. Results confirmed the importance of a rapid and low stressing harvest procedure and fast chilling of red tuna body for high quality product, as already verified by the same authors in a previous trial.