Freezing Technology

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
Arginine	<10	<10	<10	750	<20	<10
Glycine	20	20	20	100-1000	<20	<10
Glutamic acid	<10	<10	<10	270	55	36
Histidine	<1.0	86	<1.0	--	<10	<10
Proline	<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.