LF 1H NMR T2 relaxation rate as affected by water addition, NaCl and pH in fresh, frozen and cooked minced hake

Abstract

The applications of low field nuclear magnetic resonance relaxometry of proton (LF 1H NMR) to the study of seafood are increasing. The transverse relaxation times (T2) provide information about the physical-chemical nature and spatial organization of muscle components and T2 has been suggested to be particularly useful as a probe to estimate changes in water distribution in a variety of cases. However, the interpretation of T2 in fish muscle is not straightforward. In order to gain a better understanding of the significance of these changes observed in situations of interest in fish technology, there is a need for a systematic study of the changes of T2 as affected by protein concentration, NaCl addition, pH or denaturation of muscle proteins, since many of the scenarios in which LF 1H NMR is applied involve modification of one or more of the above factors. This in turn makes it possible to establish the relation of T2 with other parameters known to give relevant information about muscle changes. LF 1H NMR T2 was monitored in hake (Merluccius merluccius) mince with different thermal histories (unfrozen, frozen, cooked) as affected by pH, water and NaCl addition, and it was related to water holding capacity (WHC). T2 changed in terms of bands shape, relaxation times and relative abundance of the bands. The relaxation rate of the major band (1/T21) increased significantly upon frozen storage or pH increase, whereas water or NaCl addition had the opposite effect. WHC decreased with freezing or water addition and increased with NaCl or basic pH. Linearity found for pH and protein concentration with 1/T21 was consistent with chemical exchange being responsible for these changes. The significance of these results for technological situations where compositional and biochemical changes are occurring is discussed.