The present study involved the measurement of fish muscle texture by both objective and subjective means. Reliable methods for the evaluation of fish texture as well as interpretation of results are discussed. The study demonstrated that significant correlations between the methods could be achieved if carefully controlled conditions *were maintained. The presence of dimethylamine in frozen hake (Urophycis chuss) appeared to be a good chemical indicator of toughness whereas the extractable protein nitrogen was not as reliable. Data suggest that although the enzymatic formation of formaldehyde was a major factor in the toughening of red.hake, other factors probably contribute to the textural deterioration observed during cold storage. It is evident that haddock (Melunogrammus aeglefinus) toughens when stored at relatively high temperatures (-5°C) and like hake, experiences a loss of water-holding capacity although no formaldehyde accumulates in the tissues. The molecular basis of toughening in fish was examined by SDS gel electrophoresis. Apparently, the formaldehyde produced by the TMAO-ase enzyme system in red hake resulted in the covalent cross linking of troponin and myosin light chains, forming higher molecular weight aggregates. Changes at the molecular level were not detected by this method in haddock. Textural changes in this species are not as pronounced as those of hake and are most likely due to secondary bonds such as hydrogen or electrostatic.
The initial stages of thermally-induced aggregation of cod myofibrils resulted from noncovalent intermolecular cross-linking as demonstrated by SDSelectrophoresis. The nature of the noncovalent bonds was studied by introducing a zero length cross-linker, l-ethyl-3-(3-dimethylamino propyl) carbodiimide (EDC). This allowed the examination of the noncovalent interactions by SDS electrophoresis and quantitative densitometry. Initially, during heating, about 50 % of the myosin heavy chain was cross-linked to form a polymerized complex before the involvement of actin, regulatory proteins, or the myosin light chains. Inhibition of thermally-induced noncovalent crosslinking with triglycerides or Triton X-100 as well as the enhancement of aggregation at higher ionic strength, suggested the importance of hydrophobic interaction in this reaction. Cod myosin heavy chains were prepared and the head regions labelled with the fluorescent probe, yV-[7(dimethylamino)-4-methyl-3-coumarinyl] maleimide (DACM)which reacts specifically with two thiol groups near the active sites for Ca2+ and EDTA-ATPase. Chymotryptic cleavage of the thermally aggregated myosin heavy chains suggested the involvement of the tail region of the molecule rather than the head in the noncovalent cross-linking reactions.
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