Low-field NMR and MRI studies of fish muscle: Effects of raw material quality and processing

Abstract

The present thesis aims at using non-invasive and non-destructive NMR techniques to contribute to a further understanding of fish tissue composition and its characteristics. Moreover, it aims at investigating the water dynamics and the distribution of fat and salt in fish as affected by species, raw material quality and processing from both the chemical and the physical angle at the same time.

The applicability of low-field NMR as a tool for the fish processing industry was investigated. The bench top low-field NMR instrument was found suitable for fat and water determination in small Atlantic salmon (Salmo salar) samples, whereas the portable low-field NMR surface scanner (ProFiler) was appropriate for rapid fat determination in minced muscle. Thus, low-field NMR was proven to be good measuring technique, and with the introduction of the NMR surface scanner concept, online quality control may become feasible in the future.

Transversal (T2) NMR relaxometry has been demonstrated to contain valuable information about water dynamics in Atlantic salmon and Atlantic cod (Gadus morhua) tissue. The thesis contributes to a further understanding of the relationship between water distribution and microstructure of fish flesh. It has been established that the method is sensitive to fish species, ante-mortem handling, rigor status, freezing/thawing, heating, and brine salting. The tissue T2 relaxation characteristics have been linked to microstructure, salt distribution and salt uptake. It is shown that T2 relaxation components correlate well with water holding capacity during salting. It has been suggested that entrapped and free water, and fat when present, give rise to the main relaxation components in fish muscle tissue. The understanding of the tissue water distribution and dynamics has been improved. However, the clarification of the relaxation characteristics in fish flesh is still an active area of research.

In fatty fish, both fat and water contributes to the T2 NMR relaxation signal.  A two dimensional map of the diffusion versus T2 relaxation proved to be a good technique to increase the understanding of water and fat distribution in salmon muscle tissue, by clear separation of the NMR signals from water and fat components into different populations.

MR imaging was probed for investigation of fat and salt distribution. 1H MRI was successfully applied to produce separate quantitative water and fat images. Combined 1H and 23Na imaging of brine salted Atlantic salmon revealed that the uptake and distribution of salt in the tissue was highly dependent on the spatial fat distribution.

An evident relation was observed between T2 relaxation characteristics of salmon flesh and the sodium distribution in salted fillets. T2 relaxaometry and MR imaging gave further insight into the microstructure and water distribution of fish tissue of different quality and its effect on salt distribution. The combination of these NMR techniques is considered to be a useful tool to increase the understanding of the tissue water distribution and dynamics and for optimization of salting processes.