Recovery of proteins from fish waste products by alkaline extraction

Waste from fish-filleting units is used in the production of fish meal or directly used in animal feeds but more frequently is simply discarded. In order to upgrade the by-products of the filleting of hake and monkfish, a systematic study was made of the recovery of proteins by chemical extraction and precipitation with HCl and sodium hexametaphosphate [Na-6(PO3)(6)]. The extraction process was studied to determine the effect of pH, type of alkaline solution used [NaOH or Ca(OH)(2)], concentration of NaCl, ratio of extracting media to raw material, and temperature on the percentage of protein solubilised. Proteins present in the extract were recovered by precipitation with HCl and the effect of pH on the percentage recovered was examined. The effect of Na-6(PO3)(6) concentration on the recovery of soluble proteins present in the supernatant after isoelectric precipitation was also studied. The raw material and products were characterised by their approximate chemical composition and amino acid profile.