Characteristics of the salt-soluble fraction of hake (Merluccius merluccius) fillets stored at -20 and -30 degrees C.

Natural actomyosin (NAM) extracted in 0.6 M NaCl from hake fillets stored at -20 and -30 degrees C for up to 49 weeks was studied. The extracted protein decreased as storage progressed and became poorer in myosin while the proportion of actin remained constant. Two major peaks composed of myosin plus actin and actin plus tropomyosin plus troponins were obtained by size exclusion chromatography. SDS-PAGE analysis of the protein retained in the precolumn filter showed that there was protein aggregated by covalent bonding. Surface hydrophobicity increased while Ca(2+)-ATPase activity, apparent viscosity, and SH groups decreased as storage progressed. The loss of Ca(2+)-ATPase activity was due mainly to denaturation of the extracted myosin, whereas the minimum viscosity values occurred earlier and were not directly due to the lower proportion of myosin in the extracts. Thus, the extracted NAM exhibited changes during frozen storage. The temperature-dependent difference was mainly quantitative due to a smaller amount of protein extracted at -20 degrees C.

Structural changes of hake (Merluccius merluccius L.) fillets: effects of freezing and frozen storage.

Structural changes in hake (Merluccius merluccius L.) fillets as affected by freezing method and frozen storage temperature have been studied through Raman spectroscopy and related to changes in texture and functionality. Changes in protein secondary structure were observed due to storage temperature, accompanied by changes in apparent viscosity and shear resistance. Samples at -10 degrees C showed greater structural alteration than at -30 degrees C in terms of increase of beta-sheets at the expense of alpha-helices. An increase of unordered protein structure was found only in samples stored at -10 degrees C. Exposure of buried tryptophan residues was observed at both storage temperatures. The decrease of the deltaCH(2) band upon storage suggested an increase of hydrophobic interactions of aliphatic residues. Except for liquid air frozen fillets, all samples showed a decrease of the nuO-H/nuC-H band ratio compared to the fresh ones, this decrease being higher the harsher the conditions.