Scientific Alert Note

Upgrading of fish by-products and underutilized species

Within the PROPEHEALTH project, the possibility of using native proteins recovered from seafood by-products and underutilized species was studied. This is an important goal in order to improve the utilisation of fish resources and to reduce the environmental impact of fish processing. A special method, called the pH shift process, was applied to obtain proteins with the highest functionality. The protein isolates obtained were characterised by chemical and physical methods and applied in different seafood products.

Author: Irineu Batista, IPIMAR, Portugal

When processing seafood, a lot of by-products are generated. As much as 50% of the whole fish being processed may end up as by-products. Underutilized species of fish could also represnt a great raw material source for production of high quality seafood products. At the moment, underutilized species and by-products are used as fishmeal for animal feed. However, the proteins present in these raw materials, could be used in products for human consumption as well. For recovering these proteins, a process developed by Hultin & Kelleher (1999) should be used. In this way one may produce functional protein isolates from low value fish sources. This process involves solubilising the fish proteins at a low pH (pH2-3.5) or high pH (pH10.5-11.5), removing solid components (bones, scales and insoluble protein material) and neutral fat by centrifugation, and recovering soluble proteins by isoelectric precipitation at pH 5.5.

: Recovered proteins

The work developed in the SEAFOODplus project PROPEPHEALTH aims at upgrading fish by-products and underutilized species by using the pH-shift methods described to produce protein isolates. For this reason, the application of the recovered proteins in different seafood products was studied.

The yields achieved from when recovering proteins from hake and blue whiting by-products using the alkali-aided process were 63 % and 51.2 %, respectively. These yields are considerably higher than those obtained by the traditional surimi process. The gels obtained with the isolated proteins may be considered of medium quality. In the case of hake by-products it was sometimes observed that the proteins obtained by the traditional process pH shift process did not gellify properly. However, by applying microbial transglutaminase (ca. 0.5 U/g protein) to the recovered proteins the gel forming capacity improved considerably.

: Fish sausages produced with recovered protein isolates

The proteins remaining in solution after isoelectric precipitation were also recovered and spray dried. The dried proteins showed functional properties similar to those from other protein sources. The protein isolates were used to prepare Frankfurter-type sausages and it was concluded that the best formulations were obtained when 20 % of the minced fish was replaced by these proteins. The sausages were also enriched with n-3 fatty acids by the addition of cod liver oil.