No changes in actomyosin Ca 2+ -, Mg 2+ -, or Mg 2+ -Ca 2+ -ATPase activities were observed during iced storage of Pacific whiting fillets, but Mg 2+ -EGTA-ATPase increased with a loss of Ca 2+ -sensitivity. Surface hydrophobicity of actomyosin increased substantially within 2 days, but not total sulfhydryl (SH) content. During longer storage, the SH content decreased gradually, but surface hydrophobicity remained constant. Autolytic degradation products increased in fish muscle with storage time. Myosin heavy chain (MHC) was degraded by 45% within 8 days, but no noticeable difference was observed in actin. Results indicated that autolysis may be the main cause of physicochemical changes in Pacific whiting muscle proteins during iced storage.

MATERL4LS & METHODSCathepsin B was the most active cysteine protease in Pacific whiting fish fillets; cathepsin L was predominant in surimi. Cathepsin L showed highest activity at SY'C in both fish fillets and surimi, indicating its function in myosin degradation during conventional heating of fillers and surimi gels. Washing during surimi processing removed cathepsin B and H but not cathepsin L. Myosin heavy chain was the primary substrate during autolysis of surimi paste and actin and myosin light chain showed limited hydrolysis during 2 hr incubation. Purified Pacific whiting cathepsin L hydrolyzed myofibrils, myosin and native and heat-denatured collagen. The degradation pattern of myofibrils by the protease was the same as the autolytic pattern of surimi. Fish and surimi samplesPacific whiting (A4erlucciusproductu.s) were harvested off the Oregon coast, stored on-board in aerated slush ice, and off-loaded within 24 hr of capture. The round fish were mechanically "butterfly-cut" in a local surimi processing plant and transferred in ice to the Oregon State University Seafood Laboratory. The fish were hand-filleted, vacuum-packed, and kept frozen at -20°C until use before 6 mo. Pacific whiting surimi without additives was obtained from a local surimi processing plant. Washed mince was collected from the processing line, kept in ice, and transferred to the OS&Seafood Laboratory. Samples were used on the day received unless otherwise specified.

Commonly used protease assays and substrates were compared for sensitivity and simplicity in analyzing proteolytic activity in Pacific whiting causing gel weakening of surimi during heat-setting. Assay based on detection of trichloroacetic acid (TCA)-soluble products, using azocasein as substrate, showed highest sensitivity. By that assay, optimal pH of the protease was 5.5, and optimal temperature, 55°C. The validity of the assay for measuring activity was confirmed by pH profiles of residual proteolytic and autolytic activities of uncooked surimi. These analyses showed pH profiles similar to those of fish juice with a pH optimum of 5.5.

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