Fillet quality and health of Atlantic salmon (Salmo salar L.) fed a diet supplemented with glutamate

Atlantic salmon were fed a standard extruded dry feed (Control) or the same feed supplemented with 1.5% l-glutamate (Glu) (triplicate net pens per diet) from May 2009 (body weight = 105 g) to May 2010 (body weight = 3.1 kg). No significant differences were observed in growth (TGC = 3.1) or feed conversion ratio (1.0) between the dietary treatments. Instrumental texture analyses showed that Glu supplementation resulted in significantly (P ≤ 0.05) firmer fillets after ice storage (10.1 vs. 9.1 N) and after frozen storage (8.7 vs. 6.3 N). Additionally the Glu group had less organ adhesions (score = 0.5 vs. 1.1), lower hepato-somatic index (0.91 vs. 0.99%) and less fat accumulated in the livers (1.8 vs. 2.1 g). The condition factor, carcass and fillet yield, and cardio- and spleen-somatic indices were unaffected by dietary treatment. Hepatocellular vacuolization, intestinal inflammation and muscle degeneration were observed in the Control (50, 13, and 5%, respectively) and the Glu group (40, 7, and 25%, respectively). No abnormal observations were found in the spleen or kidney. Plasma analyses revealed significantly lower activity of creatine kinase (3.3 vs. 5.8 U/mL) and alanine aminotransferase (5.8 vs. 7.5 U/L) in the Glu fed group. Muscle pH was significantly higher in the Glu group (6.22 vs. 6.19), but the fat-, protein-, amino acid-, and collagen contents were similar. The Glu group had significantly higher concentrations of saturated fatty acids and docosapentaenoic acid (1.9 vs. 1.7 and 17.4 vs. 17.1% of fatty acids, respectively), and n− 3 fatty acids and n− 3/n− 6 ratio in the muscle tended to be higher (P < 0.09). Collagen properties determined as degree of glycation, solubility, thermal behaviour, pyridinoline bonds and structure were similar for both dietary groups. Compared with the Control, the skeletal muscle of salmon fed the Glu supplemented diet showed up-regulation of genes involved in stress response, mitochondrial functions, and amino acid and lipid metabolism, whereas several genes involved in cytoskeletal structure were down-regulated. Glu supplementation resulted in firmer fillets, coinciding with altered energy metabolism and improved health related parameters. It is suggested that optimal dietary amino acid levels for growth may differ from optimal levels for good fish health and flesh quality.