Effect of drying temperature on quality of fish protein

Hake fillets (Merluccius gayi) obtained by freeze‐drying and oven‐drying at 105° and 170°c were analysed for their chemical composition and biological quality in relation to nitrogen and sulphur utilisation. Nitrogen utilisation was not affected by heating at 105°. Net protein utilisation (NPU) (balance) values were 79 and 77 for the freeze‐dried sample and that heated at 105°, respectively. These results showed a good correlation with available lysine contents (8.6 and 8.3 g/100 g crude protein). However, sulphur utilisation suffered slight damage at 105°. Heating at 170° for 6 h considerably reduced both nitrogen and sulphur utilisation. NPU fell from 79 to 51 and net sulphur utilisation decreased from 89 to 60. It is concluded that the intensity of heating during manufacture is a factor of critical importance in obtaining fish protein concentrate for either human or animal feeding.