Abstract
Post mortem storage is a necessary process for removal of pin bones without destruction of fillets, thereby avoiding volume and economic loss. However, the enzymes involved in loosening pin bones during storage have not been studied to a great extent. In this study, the activities and localization of MMPs in the connective tissue (CT) of pin bones dissected from fillet of salmon and cod were investigated. Interestingly, the enzyme activity profile in these two species was different during post mortem storage of fish fillets. Adding MMP inhibitor (GM6001) and serine protease inhibitor (Pefabloc) revealed different effects in the two species, suggesting different regulations in salmon and cod. In situ zymography with the same inhibitors verified MMP and serine protease activity in CT close to pin bone at early post mortem (6 h) in salmon. However, MMP inhibition was not evident in cod in this area at that time point. Immunohistochemistry further revealed MMP9 and MMP13 were located more to the outer rim of CT, facing the pin bone and adipose tissue, while MMP7 was more randomly distributed within CT in salmon. In contrast, all these three MMPs were randomly distributed in CT in cod. In summary, our study reveals different MMP enzyme profiles in salmon and cod in the pin bone area, influenced by serine proteases, and suggests that MMPs and serine proteases must be taken in consideration when studying the conditions for early pin bone removal.
Highlights
The consumers prefer fresh and boneless fish fillets
Our results show that the total matrix metalloproteases (MMPs) activity in salmon increased significantly short time after slaughter (6 h), followed by a dramatic reduction already after 12 h storage (Fig. 1a, left panel)
This is in contrast to the MMP enzyme profile in cod, where the activities of MMPs sustained during storage, with just a minor increase at 12 and 24 h (Fig. 1a, right panel)
Summary
The consumers prefer fresh and boneless fish fillets. a major problem associated with pin bones is the firm attachment to the muscles which can lead to fillet destruction during removal or breakage halfway into the fillet, so that only a portion of the pin bones is removed. The fish industry needs knowledge about optimal conditions for pin bone removal to save time and costs. Little is known about the enzymes responsible for weakening the connective tissue (CT) surrounding the pin bones and the attachment to the surrounding tissue. There are major differences between salmon and cod in terms of bone strength and pulling force required to remove the pin bones (Akse and Tobiassen 2002; Esaiassen and Sørensen 1996; Westavik 2009). Whether this is due to a specific difference in enzymatic profiles post mortem in the two species is currently unknown. Degradation of the CT is enzymatic, involving numerous enzymes that can be regulated by various factors including pH, temperature and ion strength and processes that affect these factors could as such impact loosening of the pin bones (Larsen et al 2008; Vargova et al 2012)
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