Changes in three of the major biochemical components – nucleotides, lipids and proteins – related to quality loss in farmed turbot, were determined during 29 days of iced storage; results were complemented with sensory analysis. Nucleotide degradation, as estimated by the K value, underwent a gradual increase until day 19, in agreement with the loss of freshness observed for the sensory scores (high quality: days 0–2; good quality: days 3–14; fair quality: days 15–19). After day 19, the fish was judged unacceptable and the K value did not show differences until the end of storage. Lipid hydrolysis and oxidation occurred at slow rates, free fatty acid contents and the peroxide value being below 20.0 g kg−1 lipids and 4.00 meq active oxygen kg−1 lipids, respectively, during the whole storage. The content of fluorescent compounds did not increase significantly until day 19, when a sharp increase was detected. The electrophoretic protein profiles of turbot muscle did not point to any major protein degradation event or any significant change in protein during storage. However, a new band, corresponding to 22 kDa, could be observed at day 2 in the low-ionic strength buffer extract, whose concentration seemed to increase at days 9 and 14 and was present until the end of the chilled storage. The results obtained in this work indicate slow and gradual biochemical changes and long shelf life and good quality times (19 and 14 days, respectively) for iced turbot; these long times would be very profitable when turbot commercialisation is carried out in places distant from production farms.
Seafood products have attracted considerable attention as a source of high amounts of important nutritional components to the human diet (Ackman, 1989; Piclet, 1987). However, in recent years the fishing sector has suffered from dwindling stocks of traditional species as a result of dramatic changes in their availability. This has prompted fish technologists and the fish trade to pay more attention to aquaculture techniques as a source of fish and other seafood products (FAO, 2000; Josupeit, Lem, & Lupin, 2001).
Assurance of both the quality and safety of seafood will be a major challenge faced by humankind in this new century. In this sense, wild and farmed fish species are known to deteriorate after death due to the action of different mechanisms (Hsieh & Kinsella, 1989; Pigott & Tucker, 1987). During fish chilled storage, biochemical changes are known to take place, such as changes in the protein and lipid fractions and the formation of amines (volatile and biogenic) and hypoxanthine. As a consequence of these events, a deterioration in sensory quality, a loss of nutritional value, and negative modifications of the physical properties of fish muscle have been reported (Bennour, El Marrakchi, Bouchriti, Hamama, & El Ouadaa, 1991; Nunes, Batista, & Morâo de Campos, 1992; Olafsdóttir et al., 1997).
Turbot (Psetta maxima, also known as Scophthalmus maximus) is a flat fish species of high commercial value found in Northern waters and widely appreciated for its firm, white, and flavourful flesh. In recent years, the increasing production of this species as an aquaculture product has made it more available to consumers, when the wild product is increasingly less consumed because of its low availability and high cost. Extensive work has been carried out on the effects of diet on turbot growth (Cáceres-Martı́nez, Cadena-Roa, & Métailler, 1984; Danielssen & Hjertnes, 1993; Regost et al., 2001) and on the development of tools for the identification of turbot with respect to other fish species (Etienne et al., 2000; Prost, Serot, & Demaimay, 1998). However, research concerning the quality changes that might occur during post-mortem storage only includes high pressure (Chevalier, LeBail, & Ghoul, 2001) and thermal (Madeira & Penfield, 1985) treatment, such that – to date – the mechanisms of damage taking place in farmed turbot during chilled storage remain relatively unknown.
In the light of this situation, in the present work, we were prompted to investigate the biochemical changes involved in the loss of quality undergone by farmed turbot during chilled storage. To this end, the changes in the most relevant biochemical components – lipids, proteins and nucleotides – were evaluated during a long storage period (29 days) in ice and complemented by sensory analysis.
Turbot specimens maintained high and good quality (categories E and A, respectively) during the first 14 days of chilled storage (Table 2). Afterwards, the quality decreased and by day 20 the fish specimens were no longer acceptable. The main concern relating to the loss of sensory quality was the segregation of mucus from the skin, which led to a very unpleasant odour. In general terms, both the aspect of the skin and the external odour proved to be the limiting factors of sensory quality, whereas the other three features examined – gills, consistency, and flesh odour – were still acceptable up to day 29 of chilled storage.The results of the sensory analysis obtained for turbot are consistent with the shelf life times obtained for other medium-sized fish species, such as albacore and hake (Pérez-Villarreal & Pozo, 1990; Ruiz-Capillas & Moral, 2001) and this shelf life is far longer than those of smaller fish species, such as sardine and horse mackerel (Aubourg, 2001; Nunes et al., 1992).
