Abstract
Atlantic salmon (Salmo salar L.) with soft fillets are not suited for manufacturing high quality products. Therefore fillets with insufficient firmness are downgraded, leading to severe economic losses to the farming and processing industries. In the current study, morphological characteristics of salmon fillets ranging from soft to hard were analysed. Different microscopic techniques were applied, including novel methods in this field of research: morphometric image analysis, periodic acid Schiff staining, immunofluorescence microscopy, transmission electron microscopy and fourier transform infrared microscopy. The results showed that the myocytes of soft muscle had detached cells with mitochondrial dysfunctions, large glycogen aggregates and enlarged inter cellular areas, void of extracellular matrix proteins, including lower amounts of sulfated glycoproteins. Myofibre-myofibre detachment and disappearance of the endomysium in soft muscles coincided with deterioration of important connective tissue constituents such as Collagen type I (Col I), Perlecan and Aggrecan. In summary our investigations show for the first time an association between soft flesh of Atlantic salmon and massive intracellular glycogen accumulation coinciding with degenerated mitochondria, myocyte detachment and altered extracellular matrix protein distribution. The results are important for further understanding the etiology of soft salmon.
Highlights
Texture quality is important for consumer acceptability of Atlantic salmon and insufficient firmness causes downgrading in the processing industry [1]
The fish were reared throughout their entire production cycle in a farming cage that is similar to commercial production units at Nofima research station (Averøy, Norway), which is approved by the Norwegian Animal Research Authority (NARA)
The results showed that hard muscle differed significantly from soft muscle within the spectral region of 800– 1000 cm21 (PCA score plot, Fig. 2A), which represents the typical area of sulfated glycosaminoglycans [21]
Summary
Texture quality is important for consumer acceptability of Atlantic salmon and insufficient firmness causes downgrading in the processing industry [1]. The amount and composition of connective tissue and muscle fibre density are among inherent characteristics found to affect muscle texture [2,3,4,5,6,7]. Transcriptome profiling of muscle atrophy in rainbow trout has identified transcriptional responses and pathways involved, including up regulation of genes involved in proteolysis, aerobic metabolism and decreased extracellular matrix collagens [12]. In line with these results, recent gene expression profiling of farmed salmon revealed that sufficient firmness of salmon muscle was largely dependent on an efficient aerobic metabolism and rapid removal of damaged proteins [13]
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