Mari Øvrum Gaarder defended her thesis on March 25th 2011

Ane Gro Siri Skjelfjord

The importance of the calpain system in post mortem Atlantic salmon (Salmo salar L.) muscle in relation to texture

Abstract from the thesis - PhD thesis 2011:15

Fillet texture is an important quality characteristic influencing the consumer acceptance and an excessive softening of the fish flesh can make it too fragile to process. During recent years, Norwegian fish farming has experienced problems in achieving a constant firm salmon fillet. The calpain system is shown to have a role in the meat tenderization process and it is also detected in fish species. The information on the calpain system in fish is scarce, even though this system is thought to be of importance for the stability of muscle texture. The overall aim of this PhD project was to detect if the calpain system was present in the Atlantic salmon (Salmo salar) muscle, and to further study this system and its possible relation to fillet texture.

From research on mammals, it is known that the calpain system consists of µ- and m-calpain and their specific inhibitor calpastatin. Research on fish species, however, is not consistent when it comes to the findings of the components in the calpain system. In the present study (paper I), separation and identification of µ-calpain and m-calpain and two forms of calpastatin from Atlantic salmon muscle were for the first time assessed. A two-step chromatographic approach starting with a hydrophobic column followed by an anion exchange column was used. The calpain isoforms differed in activity, temperature optimum and calcium requirement. The µ-calpain had the lower values of the two isoforms. In fact, m-calpain had 15 times higher activity than µ-calpain. On casein zymograms, only one calpain isoform was detected and it was most likely m-calpain. In paper IV, the stability of calpain and calpastatin in Atlantic salmon muscle were tested. The results showed that both calpain and calpastatin were stable during post mortem storage on ice for 6 days.

The Atlantic salmon farmed in Norway today is a product of an intensive program, and in paper II the aim was to investigate the calpain system in different salmon families. Different families of Atlantic salmon showed significant difference in fillet texture. The calpain activity was significantly different between textural groups, but no correlation to fillet texture was detected. Neither was there observed any significant correlation between calpastatin and the fillet texture, although there was a tendency to higher calpastatin activity in the firm texture group. The texture, however, was quite soft for all individuals in this experiment, and this might explain the lack of correlation between the calpain system and fillet texture.

Due to intensive farming of Atlantic salmon, the fish is exposed to pre-slaughter stress in form of transportation, crowding and handling. It is known that fish exposed to pre-slaughter stress generally have a softer fillet texture and shows a higher myofibre breakage. In paper III, the aim was to investigate the calpain system in Atlantic salmon exposed to three different treatments; minimal pre-slaughter crowding stress (NS), short-term pre-slaughter crowding stress (SS) and long-term pre-slaughter crowding stress (LS). Significant differences between all three groups were detected where the highest calpain activity was observed in the LS group and the lowest in the NS group. There was also a significant difference in the calpain:calpastatin ratio, where the LS group had the highest ratio. The salmon from the LS group had a softer fillet texture and a higher myofibre breakage. Together with the increased calpain:calpastatin ratio, this indicates that the salmon calpain system might play a role in the fillet softening of Atlantic salmon muscle.

A large portion of farmed Atlantic salmon is exported on ice throughout the world. To reduce the amount of ice, a method called super-chilling has been applied. In paper IV, the aim was to investigate the calpain system in ice stored and super-chilled Atlantic salmon in the post mortem period. Even though minor changes in fillets texture were observed, super-chilling had a significant effect on the calpain system, ATP and its degradation products. Super-chilled fillets had an increased level of calpain in the early phases of storage, while the calpastatin activity was lower during the whole storage period compared to the ice stored fillets. The super-chilled fillets had an increased level of myofibre breakage compared to the ice stored fillets after 6 days of storage. With these results, together with the observation in paper III, it seems that there exists a relationship between the calpain system and level of myofibre breakage.

Based on the result obtained during the present PhD work it is likely to believe that the calpain system plays a potential role in the post mortem fillet softening which occur in Atlantic salmon muscle.

Published: 10.06.11
Updated: 20.06.11
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