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1.2.1.3. N – containing extractives
The N-containing extractives can be defined as the water-soluble, low molecular weight, nitrogen-containing compounds of non-protein nature. This NPN-fraction (non-protein nitrogen) constitutes from 9 to 18% of the total nitrogen in teleosts. The major components in this fraction are: volatile bases such as ammonia and trimethylamine oxide (TMAO), creatine, free amino-acids, nucleotides and purine bases, and urea in the case of cartilaginous fishes.
Compound in mg/100g wet weight Fish Crustaceans Poultry Mammalian muscle
Cod Herring Shark species Lobster Leg muscle 1) Total extractives 1200 1200 3000 5500 1200 3500 2) Total free amino – acids 75 300 100 3000 440 350 <10 <10 <10 750 <20 <10 20 20 20 100-1000 <20 <10 <10 <10 <10 270 55 36 <1.0 86 <1.0 -- <10 <10 <1.0 <1.0 <1.0 750 <10 <10 3) Creatine 400 400 300 0 -- 550 4) Betaine 0 0 150 100 -- -- 5)Trimethylamine oxide 350 250 500-1000 100 0 0
The amount of TMAO in the muscle tissue depends on the species, season, fishing ground, etc. In general, the highest amount is found in elasmobranchs and squid (75-250 mg N/100g) followed by cod (60-120mg N/100g),while flatfish and pelagic fish have the least. Pelagic fish (sardines, tuna, and mackerel) have their highest concentration of TMAO in the dark muscle while demersal, white fleshed fish have a much higher content in the white muscle. Quantitatively, the main component of the NPN-fraction is creatine. In resting fish, most of the creatine is phosphorylated and supplies energy for muscular contraction. |