Abstract:This study was performed in order to assess the effect of early post mortem structural changes in the muscle upon the liquid-holding capacity of wild cod, net-pen-fed cod (fed cod) and farmed salmon. The liquid-holding capacity was measured by a low speed centrifugation test. Transmission electron microscopy was used to discover ultrastructural changes both in the connective tissue and in the myofibrils. Differential scanning calorimetric thermograms of the muscle proteins were recorded to elucidate whether fundamental differences did exist between the proteins of the raw material tested. Multivariate statistics were used to explicate the main tendencies of variations in the thermograms. The salmon muscle possessed much better liquid-holding properties than the cod muscle, and wild cod better than fed cod regardless of the storage time. Both fed cod and farmed salmon, underwent the most severe structural alterations, probably caused by the low muscle pH values. The higher liquid-holding capacity of the salmon muscle was related to species specific structural features and better stability of the muscle proteins. The myofibrils of the salmon muscle were denser and intr.a-and extracellular spaces were filled by fat and a granulated material. The differences in thermograms of muscle from wild and fed cod were largely explained by the variations in pH. The severe liquid loss of fed cod is due to a low pH induced denaturation and shrinkage of the myofibrils. Post mortem degradation of the endomysial layer and the sarcolemma may have further facilitated the: release of liquid.

Low-field NMR T(2) and Fourier transform infrared (FT-IR) measurements were performed on meat samples of two qualities (normal and high ultimate pH) during cooking from 28 degrees C to 81 degrees C. Pronounced changes in both T(2) relaxation data and FT-IR spectroscopic data were observed during cooking, revealing severe changes in the water properties and structural organization of proteins. The FT-IR data revealed major changes in bands in the amide I region (1700-1600 cm(-)(1)), and a tentative assignment of these is discussed. Distributed NMR T(2) relaxation data and FT-IR spectra were compared by partial least-squares regression. This revealed a correlation between the FT-IR peaks reflecting beta-sheet and alpha-helix structures and the NMR relaxation populations reflecting hydration water (T(2B) approximately 0-10 ms), myofibrillar water (T(21) approximately 35-50 ms), and also expelled "bulk" water (T(2) relaxation times >1000 ms). Accordingly, the present study demonstrates that definite structural changes in proteins during cooking of meat are associated with simultaneous alterations in the chemical-physical properties of the water within the meat.

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The aim of this study was to investigate the correlation patterns between Fourier transform infrared (FT-IR) and Raman microspectroscopic data obtained from pork muscle tissue, which helped to improve the interpretation and band assignment of the observed spectral features. The pork muscle tissue was subjected to different processing factors, including aging, salting, and heat treatment, in order to induce the necessary degree of variation of the spectra. For comparing the information gained from the two spectroscopic techniques with respect to the experimental design, multiblock principal component analysis (MPCA) was utilized for data analysis. The results showed that both FT-IR and Raman spectra were mostly affected by heat treatment, followed by the variation in salt content. Furthermore, it could be observed that IR amide I, II, and III band components appear to be effected to a different degree by brine-salting and heating. FT-IR bands assigned to specific protein secondary structures could be related to different Raman C-C stretching bands. The Raman C-C skeletal stretching bands at 1,031, 1,061, and 1,081 cm(-1) are related to the IR bands indicative of aggregated beta-structures, while the Raman bands at 901 cm(-1) and 934 cm(-1) showed a strong correlation with IR bands assigned to a alpha-helical structures. At the same time, the IR band at 1,610 cm(-1), which formerly was assigned to tyrosine in spectra originating from pork muscle, did not show a correlation to the strong tyrosine doublet at 827 and 852 cm(-1) found in Raman spectra, leading to the conclusion that the IR band at 1,610 cm(-1) found in pork muscle tissue is not originating from tyrosine.

Fourier transform infrared (FT-IR) microspectroscopy and low-field (LF) proton NMR transverse relaxation measurements were used to study the changes in protein secondary structure and water distribution as a consequence of aging (1 day and 14 days) followed by salting (3%, 6%, and 9% NaCl) and cooking (65 degrees C). An enhanced water uptake and increased proton NMR relaxation times after salting were observed in aged meat (14 days) compared with nonaged meat (1 day). FT-IR bands revealed that salting induced an increase in native beta-sheet structure while aging triggered an increase in native alpha-helical structure before cooking, which could explain the effects of aging and salting on water distribution and water uptake. Moreover, the decrease in T2 relaxation times and loss of water upon cooking were attributed to an increase in aggregated beta-sheet structures and a simultaneous decrease in native protein structures. Finally, aging increased the cooking loss and subsequently decreased the final yield, which corresponded to a further decrease in T2 relaxation times in aged meat upon cooking. However, salting weakened the effect of aging on the final yield, which is consistent with the increased T2 relaxation times upon salting for aged meat after cooking and the weaker effect of aging on protein secondary structural changes for samples treated with high salt concentration. The present study reveals that changes in water distribution during aging, salting, and cooking are not only due to the accepted causal connection, i.e., proteolytic degradation of myofibrillar structures, change in electrostatic repulsion, and dissolution and denaturation of proteins, but also dynamic changes in specific protein secondary structures.

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