CATHEPTIC ACTIVITY, TEXTURAL PROPERTIES AND SURFACE ULTRASTRUCTURE OF POST-MORTEM BEEF MUSCLE

Catheptic enzyme release was studied in beef psoas muscle as a function of post-mortem aging. Textural qualities (tensile and shear) and fiber surface ultrastructure were examined. A crude (12-fold purification) catheptic fraction was prepared that exhibited the expected pH and temperature optima and was activated by dithiothreitol, EDTA and Triton X-100. When beef muscle was fractionated according to its solubility characteristics the susceptibility to proteolysis decreased: water soluble > salt soluble > insoluble. During post-mortem aging proteolytic activity decreased slightly at rigor, increased to day 6 and subsequently decreased. Shear force, breaking strength, ATP, break elongation and elasticity all dropped during the first 4 days, but the latter two parameters increased thereafter. The major alteration in fiber appearance through 2 days was shortening. Later, however, transverse striations became depressed and beginning at day 5 and continuing through the 2-wk period extensive fiber breakage occurred. The possibility that cathepsins play a role in these post-mortem events is considered.     

TEXTURE OF FISH MUSCLE

Morphological and chemical aspects of fish muscle texture are reviewed and differences from red meat are pointed out. A unique feature of fish muscle is its low connective tissue content which accounts for easy disintegration of fish flesh on heating. Thus, the muscle fibers are the main textural elements in cooked fish meat. Because of this, most of the popular texture testing instruments are not applicable to fish. Best results are obtained with the thin blade shear/compression cell.
Texture of fish muscle is affected by the species, age and size of the fish within the species, and nutritional state. Postmortem factors influencing texture include glycolysis, rigor mortis and the accompanying contraction of the muscle often leading to the separation of muscle segments (gaping), temperature profile during storage, temperature of cooking, pH and presence of NaCl.     

EFFECTS OF CATHEPTIC ENZYMES FROM SPLEEN ON THE MICROSTRUCTURE OF BOVINE SEMIMEMBRANOSUS MUSCLE

Fresh bovine semimembranosus muscle was treated with catheptic enzymes derived from bovine spleen and examined by light and scanning electron microscopy. The results indicate that the degree of microstructural changes in both the muscle fibers and the tissue surrounding the sarcolemma was related to the concentration of the enzyme. The structural changes in the myofibrils were restricted to the region of the Z bands.     

HYDROXYL RADICAL MODIFICATION OF FISH MUSCLE PROTEINS

A xanthine oxidase-based system was used to generate hydroxyl free radicals in washed, minced cod muscle. Oxidation of protein was measured by increase in protein carbonyl content; the system used produced approximately 0.1 mol of carbonyl groups per 10⁵ g of protein. This degree of oxidation had only minor effects on the SDS-PAGE patterns of the muscle proteins. The solubility of the proteins was not affected by this amount of oxidation unless they were also subjected to a freeze/thaw cycle. With a freeze/thaw cycle, a highly significant decrease in protein solubility occurred compared to that which took place in a sample not exposed to the free radical system. Lowering the pH from 6.8 or 6.5 to 6.0 or 5.5 had a strong negative impact on protein solubility. Protein oxidation appeared in two phases in washed cod mince, an initial rapid increase followed by a second phase that may have been linked to oxidation of the small amount of lipid in the sample. Comparison of protein carbonyl formation in stored mackerel fillets or mince indicated that the range of oxidation studied in the cod model system was similar to what occurs in stored mackerel muscle postmortem.     

ROLE OF INITIAL MUSCLE pH ON THE SOLUBILITY OF FISH MUSCLE PROTEINS IN WATER

The solubility of the myofibrillar and cytoskeletal proteins in water was determined for the muscle tissue often species offish. The flesh of six white-muscled fish had pH's at the time of processing above pH 6.6 and greater than 80% of their myofibrillar/cytoskeletal proteins were soluble in water. The flesh of three pelagic species and a shark had pH values when processed below 6.6 and the water solubility of their myofibrillar and cytoskeletal proteins was less than 40%. When the washed minced muscle of one of the white-fleshed species, cod, was exposed to low pH, the solubility of its myofibrillar and cytoskeletal proteins decreased substantially. The water solubility of the cod myofibrillar and cytoskeletal proteins could be reestablished by washing the acid-treated cod flesh with neutral salt solutions. It is suggested that pH values below 6.6 modify certain proteins which prevent the water-extractability of the rest of the myofibrillar and cytoskeletal proteins from being expressed.     

POSTMORTEM CHANGES IN CYTOSKELETAL ELEMENTS OF FISH MUSCLE1

Transmission electron microscopic examination of samples from fresh and 7 days-cold-stored white croaker skeletal muscle did not reveal apparent changes in overall structure of contractile elements of muscle. However, when purified myofibrils obtained from fresh and stored muscles were extracted with 0.6 M KI and the residues were examined in the scanning electron microscope, a progressive disappearance of exosarcomeric longitudinal filaments connecting successive Z structures was observed. Electrophoretic analysis of myofibrillar proteins obtained from fresh and stored muscles showed a marked degradation of nebulin, a major component of the endosarcomeric cytoskeletal network, whereas the other protein constituents of myofibrils remained practically unchanged. These alterations in elements of both cytoskeletal lattices of muscle may have important effects on physical properties of fish meat.     

BONE PARTICLE CONTENT OF SOME MINCED FISH MUSCLE PRODUCTS

Minced fish muscle recovered by flesh separators may vary in bone particle content depending on the type of machine and the size of the openings that the fish are passed through to screen out the skin and bones. Orderly marketing of these materials will require some measure of control of the bone particle content. The method presented here for quantifying the bone particle content of minced flesh by gravity-flotation involves (1) shredding the flesh with a low-speed stirring device and (2) gravity separation of the bone, cartilage and other high-density components from the lower-density floatable muscle fibers. Experimental samples obtained by passing various fish materials through flesh separators and then through 1- and 2-mm flesh strainers were analyzed for their bone particle content by the proposed method. The bone particle content of flesh containing 25 to more than 100 particles per pound could be reduced by 70–100% by passage through strainers. Imported commercial samples displayed bone particle contents of 4 to over 400 per pound. Sensory panels could not detect more than 10% of the particles found by objective analysis. The gravity-flotation method may serve as a useful quality control tool.     

STABILITY OF FISH MUSCLE PARVALBUMINS AT DIFFERENT pH VALUES

Parvalbumins of herring (Clupea harengus) and carp (Cyprinus carpio) are essential components of the pattern ofsarcoplasmic proteins used for fish species identification by isolelectric focusing. In fishery products oflowpH, like marinated herring (pH ～ 4), parvalbumins were not detectable. It was demonstrated for herring and carp that parvalbumins were not precipitated at pH 4, but were degraded by fish muscle proteases.     

COMPARATIVE MICROSTRUCTURE OF RED MEAT, POULTRY AND FISH MUSCLE

Since muscle food researchers tend to focus on disciplines such as red meats, poultry or fish; there is little shared information across commodity lines. Comparisons among these three groups of muscle foods are discussed.     

FISH MUSCLE LIPOLYSIS—A REVIEW

Lipolysis occurs extensively in fish muscle post-mortem and is associated with quality deterioration in the frozen tissue. Fish muscle lipolysis is presented in the context of basic fish muscle physiology and lipid composition. Phospholipase A and lipases from fish muscle have been described and characterized. The reaction is usually followed by measurement of free fatty acid production, and several colorimetric assays are available. Processing and storage treatments have been shown to influence the extent and rate of lipolysis in fish muscle; most of the research in this area has been directed at the effects of frozen storage. The interaction of lipolysis and lipid oxidation is a particularly intriguing area of study as triglyceride hydrolysis leads to increased oxidation while phospholipid hydrolysis produces the opposite effect.     

CATHEPTIC ENZYME ACTIVITY IN AGED COUNTRY-STYLE HAMS AS INFLUENCED BY PRE-CURING TREATMENT

Chemical composition and catheptic activity were determined in fresh chilled hams and four lots of cured hams subjected to different pre-curing treatments. Hams were sampled after aging (storage) of 2, 4 and 6 months at 29 °C. Moisture decreased, while protein, salt and free amino acid concentrations increased with aging time of cured hams, but treatment effects were similar. Frozen, thawed cured hams aged for 2 months exhibited the highest specific enzyme activity but decreased in enzyme activity after 4 and 6 months of aging. Free amino acid concentration after aging was higher in hams frozen 1–3 yr (= 2.3) at −29 °C than for those frozen 0 or 2 days.