28 Fish and Sea foods

Kulsum Jan

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Post mortem changes in fish and sea foods

  • Postmortem biochemical processes are directly linked to final quality attributes.
  • Postmortem tenderization of fish muscle.
  • Loss of freshness.
  • Muscle spoilage.
  • Autolysis.
  • Fat hydrolysis.

 

Proteolysis in muscle

  • Myofilament degradation.
  • Degradation vary from one fish species to another.
  • Degradation of titin, desmin,nebulin, dystrophin, α-actinin release, myosin proteolysis, and tropomyosin delocalisation.
  • Costameres also degrades within 24 hr postmortem.
  • Proteolysis of sarcoplasmic 16Da protein.
  • Connective tissue collagen degradation.

 

Postmortem muscle structural changes

  • Very little structural change occurs in fish myofibrils during postmortem.
  • Fish myofibrils are structurally stable.
  • Both fish and mammals show myofiber to connective tissue (endomysium) detachments within 24h postmortem.
  • Perimysium shows some weakening.
  • Myocommata is also mechanically weakened during storage

 

Role of proteases in postmortem autolysis of fish muscle

  • Physical, chemical, biochemical and microbial processes combinely deteriorate the fish flesh.
  • Proteolysis of muscle protein by enzymes.
  • Fat hydrolysis.
  • Bacterial contamination is minimum

 

Changes in fish flesh biochemistry post mortem

  • Rigor mortis
  • Autolysis
  • Bacterial attack
  • Protein denaturation
  • Decreasing flesh pH
  • TVB-Total Volatile Base

 

Rigor mortis

  • On death, the circulatory system stops and the ATP levels drop.
  • Resulting in rigid muscles.
  • Actin and myosin, combine in the presence of calcium ions to form actomyosin.
  • ATP is used to supply energy for contraction.
  • After loss of ATP, permanent actomyosin complex is formed.

 

Autolysis

  • Autolytic reactions by enzymes occurs, decomposing various compounds.
  • Tasteless and bitter one compounds -By enzymes in flesh.
  • Soupy mess – By gut enzymes.

 

Bacterial attack

  • On autolysis, bacteria start to decompose muscle.
  • Anaerobic  bacteria- Foul type spoilage.
  • Slimy texture.

 

Protein Denaturation

  • The destruction of its secondary, tertiary and quaternary structure.
  • By slow freezing and variability of storage conditions.
  • Impaired water holding ability..Dull, white and spongy.
  • Fibrous and tasteless.

 

Decreasing flesh pH

  • A living fish has a flesh pH of 7.0.
  • Decrease in pH after death- Glycolysis.
  • At 6.6 –soft texture.
  • Below this level, firm and eventually tough.

 

TVB-Total Volatile Base

  • Total amount of nitrogen-containing substances which are produced during storage.
  • For example TMA (Trimethylamine).
  • Fishy smell.
  • Odourless and tasteless.
  • During frozen storage- TMA oxide converted to dimethylamine (DMA) and formaldehyde.
  • Formaldehyde denatures the muscle structure.
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 Suggested readings

  • Obatake, A., Doi, T. and Ono, T. 1988. Post-mortem change in inosine monophosphate concentration and related enzyme activity in the dark muscle fish. Bull. Jpn. Soc. Sci. Fish. 54, 283–288.
  • Pedrosa-menabrito, A. and Regenstein, J.M. 1988. Shelf-life extension of fresh-fish spoilage of fish. J. Food Qual. 11, 117–127.
  • Saito, T., Arai, K. and Matsuyoshi, M. 1959. A new method for estimating the freshness of fish. Bull. Jpn Soc. Sci. Fish. 24, 749–750.
  • Tanikawa, eiichi, akiba, minoru, ishiko, hirotoshi, yoshitani, setsuko. Studies on post-mortem changes in the chemical constitution of the meat of sea cucumber (stichopus japonicus selenka): Changes in the Amounts of Lactic Acid and Glycogen in the. Meat of Stichopus japonicus during the Period of Rigor Mortis. Bulletin of the faculty of fisheries hokkaido university, 6(1): 52-56 Issue Date 1955-05.