Handbook of Food Science and Technology 3

Liver 
Dogfish 
20.4 
71.2 
55 
16.2 
Cod 
15–19 
76–80 
47–60 
21–29 
Oil 
Anchovy 
30.5 
69.7 
32.6 
37.1 
Herring 
26.1 
73.5 
55 
16.2 
Mackerel 
27.5 
72.5 
48.9 
23.6 
Table 2.4.
 Saturated and unsaturated fatty acid composition of
some fish products (% of total fat) (according to [PIC 87]) 

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From Muscle to Meat and Meat Products 73 
High level of non-protein nitrogen in fish 
The main components of the non-protein nitrogen fraction of fish are 
ammonia, trimethylamine oxide (TMAO), creatine, free amino acids, 
nucleotides, purine bases and urea in the case of cartilaginous fish. They 
result in the formation of degradation compounds, which are often 
detrimental to quality.
TMAO is present in very large amounts in cartilaginous fish but absent in 
crustaceans, freshwater fish and mammals. It decomposes to trimethylamine 
by bacteria, thereby serving as a quality index for fish. In quantitative terms, 
creatine is also very important as, in its phosphorylated form, it provides 
energy for muscle contraction.
Free amino acids are present in significant amounts in fish (1.3 – 3.8% 
compared to 0.1 – 0.6% in meat). They are of relative importance that varies 
depending on the species: taurine, alanine, glycine and imidazole amino 
acids are prevalent in most fish. Histidine, a particularly abundant amino 
acid in clupeids and scombroids fish, has been the subject of numerous 
studies because its microbial decarboxylation results in histamine, a known 
source of food allergy. In addition, its heat resistance protects it from 
destruction during processing, which is why histamine is subject to very 
strict regulation.