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Fresh and frozen-thawed cod raw materials were subjected to brining, dry salting, and rehydration. The effects of salting and desalting on fillet water-related properties, salt content, and salt distribution were studied using 23 Na MRI, 23 Na NMR, and low-field (LF) 1 H NMR. The results were compared with the Volhard titration method for salt determination, fillet pH, water content, and water-holding capacity. The rehydrated end product showed significant difference only with respect to water-holding capacity (unfrozen raw material higher) when fresh and frozen-thawed raw materials were compared. Excellent correlation was obtained between quantitative salt determinations using 23 Na NMR and Volhard titration. When using a typical salting method, it was shown that the salt uptake and salt distribution in the fillets were inhomogeneous. Proton relaxation times, obtained from LF 1 H NMR, were processed using either a 2-exponential model or the CONTIN algorithm. In several instances, a clear dependence on fillet pH, water-holding capacity, or salt content during fish processing was observed. Our results indicate that NMR and MRI can be used as useful tools to evaluate and optimize fish processing unit operations and that rapid (LF) NMR methods have the potential for replacing traditional salt and water-related analytical methods.

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 1H NMR T2 relaxation,1H and 23Na MRI and light microscopy. Distributed exponential analysis of the T2 relaxation data revealed two main water pools in all raw materials, T21 and T22, with relaxation times in the range of 20-100 ms and 100-300 ms, respectively. Raw material differences were reflected in the T2 relaxation data. Light microscopy demonstrated structural differences between unsalted and salted raw materials. For prerigor fillets, salting induced a decrease in T21 population coupled with a more open microstructure compared to unsalted fillets, whereas for frozen/thawed fillets, an increase in T21 population coupled with salt-induced swelling of myofibers was observed. The result implies that the T21 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 T2 relaxation and 1H and 23Na MRI as effective tools for understanding of the relationship between the microstructure of fish muscle, its water mobility and its salt uptake.

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