Research Article Received: 2 January 2014 Revised: 24 April 2014 Accepted article published: 30 April 2014 Published online in Wiley Online Library: (wileyonlinelibrary.com) DOI 10.1002/jsfa.6717 Protein gels and emulsions from mixtures of Cape hake and pea proteins Ana Sofia Tomé, a Carla Pires, b* Irineu Batista, b Isabel Sousa a and Anabela Raymundo a Abstract BACKGROUND: Portioning of frozen fish generates by-products such as fish ‘sawdust’ and cut-offs which can be further processed into protein concentrates and isolates. The objective of the present work was to produce gels and emulsions using recovered Cape hake protein powder (HPP). In previous works, the structures of the gels produced by HPP were found to be strong, with a high rubbery character. In this work, the addition of commercial pea proteins (PPC) to HPP gels and emulsions was studied. RESULTS: Physical properties of gels and emulsions prepared with different proportions of mixtures of PPC and HPP were evaluated. In general, gels and emulsions showed high values for whiteness and, as expected, the higher content of HPP in the protein mixtures led to higher firmness values of the gels. The gel network was rapidly formed upon heating due to the fish protein macromolecules and further reinforced by the pea protein macromolecules when cooled to 5 ∘ C. Both visco-elastic parameters, storage and loss moduli, of the produced gels increased with the HPP proportion in the protein mixtures, corresponding to more structured systems. For the emulsions, two different pH environments were studied: 3.8 and 7.0. At neutral pH a synergy was found between the vegetable and fish protein, which is not so strong when pH is lowered to 3.8, near the isoelectric point of pea proteins (pI = 4.5). This evidence was supported by the results from the texture measurements, viscosity and visco-elastic parameters. CONCLUSIONS: Gels made from Cape hake proteins showed a softer texture and were less rubbery with the addition of pea proteins. Emulsions stabilised by these mixtures showed slightly different behaviour when produced at pH 7.0 or pH 3.8. © 2014 Society of Chemical Industry Keywords: Cape hake by-products; recovered proteins; pea proteins; gels and emulsions; texture and rheological properties INTRODUCTION Fish proteins from by-products and under-utilised species are tra- ditionally used in the production of fish meal and silage but other alternatives with added value, such as the production of protein isolates by acid or alkaline solubilisation, have also been investigated. 1 Fish proteins can be used as an excellent dietary supplement to provide a highly healthy source of easily digestible proteins. 2 The high gelation capacity of fish proteins is well recognised 3 and this capacity is very important in forming desired textures in many seafood products. Protein gelation can be viewed as a process during which proteins interact to establish a net- work of molecules structured (e.g. kamaboko). These interactions include protein–water, protein–fat and protein–protein. 4 They are influenced by several factors as protein concentration, temper- ature, pH, ionic strength, additives, endogenous and exogenous enzymes and processing factors. Therefore, these factors can affect protein gelation, as well as the type and the properties of gels. The emulsifying capacity of fish proteins have also been studied by sev- eral authors. 5,6 Similarly, fish protein powder prepared by different methods also showed good gelation and emulsifying capacity. 7 – 10 Concentrate or isolate plant proteins are also widely employed in many foodstuffs due to their nutritional quality and contribution to food texture. Pea protein isolates (PPC) have gained increasing importance in recent years. They present a balanced amino acid composition, good nutritional value and have been used as a sub- stitute for meat protein or as a functional additive. 11 Several studies have reported the gelation ability of PPC. 4,12,13 The use of plant proteins to stabilise oil/water emulsions was extensively studied by several authors. 14 – 16 Those studies showed that pea proteins had high surface active properties at the oil – water interface. The capacity of PPC to stabilise food emulsions was optimised and the main operating conditions were studied by Raymundo et al. 17 Proteins from different sources have been used to modify the rheological properties of surimi or other fish protein gels. Thus, the addition of soy protein isolate affected the gel properties and modori phenomena of Alaska pollock surimi as reported by Furukawa and Ohta. 18 Chang-Lee et al. 19 also evaluated the effect of egg white, whey protein and soy protein isolate as protein ∗ Correspondence to: Carla Pires, IPMA, IP, Instituto Português do Mar e da Atmosfera, DMRM/DivAV, Av. Brasília, 1449–006 Lisbon, Portugal. E-mail: cpires@ipma.pt a CEER – Biosystems Engineering, Instituto Superior de Agronomia/Technical University of Lisbon, Tapada da Ajuda, 1349-017 Lisbon, Portugal b IPMA, IP, Instituto Português do Mar e da Atmosfera, DMRM/DivAV, Av. Brasília, 1449-006 Lisbon, Portugal J Sci Food Agric (2014) www.soci.org © 2014 Society of Chemical Industry