Water and Salt Distribution in Atlantic Salmon (Salmo salar) Studied by Low-Field 1H NMR, 1H and 23Na MRI and Light Microscopy: Effects of Raw Material Quality and Brine Salting

The effect of different Atlantic salmon raw materials (prerigor, postrigor and frozen/thawed) on water mobility and salt uptake after brine salting was investigated by using LF H-1 NMR T-2 relaxation,H-1 and Na-23 MRI and light microscopy. Distributed exponential analysis of the T-2 relaxation data revealed two main water pools in all raw materials, T-21 and T-22, with relaxation times in the range of 20-100 ms and 100-300 ms, respectively. Raw material differences were reflected in the T-2 relaxation data. Light microscopy demonstrated structural differences between unsalted and salted raw materials. For prerigor fillets, salting induced a decrease in T-21 population coupled with a more open microstructure compared to unsalted fillets, whereas for frozen/thawed fillets, an increase in T-21 population coupled with salt-induced swelling of myofibers was observed. The result implies that the T-21 population was directly affected by the density of the muscle myofiber lattice. MR imaging revealed significant differences in salt uptake between raw materials, prerigor salted fillets gained least salt (1.3-1.6% NaCl), whereas the frozen/thawed fillets gained most salt (2.7-2.9% NaCl), and obtained the most even salt distribution due to the more open microstructure. This study demonstrates the advantage of LF NMR T-2 relaxation and H-1 and Na-23 MRI as effective tools for understanding of the relationship between the microstructure of fish muscle, its water mobility and its salt uptake.