Rheology of thermo-reversible fish protein isolate gels Tom Brenner a, * , Taco Nicolai b , Ragnar Johannsson a a Matis, Biotechnology Division, Gylfaflot 5, 112 Reykjavik, Iceland b Polymeres, Colloids, Interfaces, UMR CNRS, Université du Maine, 72085 Le Mans Cedex 9, France article info Article history: Received 13 February 2009 Accepted 26 April 2009 Keywords: Fish protein isolate Thermo-reversible gels Rheology Strain hardening Power-law creep abstract Fish protein isolates (FPI) from cod muscle were studied at pH 9 and 11. Thermo-reversible gels, stable at and below room temperature could be produced at both pH values. Application of low shear stress to the gels led to an initial elastic response followed by a power-law deformation (creep) at both pH. Above a critical shear stress gels fractured and flowed regularly with a viscosity of about 0.05 Pa s. Strain–stress curves were recorded using both flow and oscillatory measurements. For all systems strain hardening was observed followed by fracture. Fracture occurred at approximately the same defor- mation, but the amplitude of strain hardening and the yield stress decreased with decreasing rate of the stress ramp. Results obtained from oscillatory shear at 1 or 0.1 Hz were close to those obtained with con- tinuous shear during the fastest stress ramps. The structure of the gels was investigated using confocal laser scanning microscopy and turbidity measurements. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Fish muscle proteins can be isolated by solubilising at extreme pH. The protein can be precipitated at the iso-electric point, which yields a heterogeneous suspension with a protein content close to 200 g/L. Such suspensions of fish protein isolate (FPI) have been shown to undergo heat-induced irreversible gelation after addition of salt (Choi & Park, 2002; Kim, Park, & Choi, 2003; Kristinsson & Liang, 2006; Kristinsson, Theodore, Demir, & Ingadottir, 2005; Thawornchinsombut & Park, 2007; Undeland, Kelleher, & Hultin, 2002; Yongsawatdigul & Park, 2004). Recently, we have shown (Brenner, Johannsson, & Nicolai, 2008) that non-precipitated FPI solutions extracted from cod muscle at pH 11 form homogeneous gels at room temperature if their pH is lowered to between about 9.5 and 8.5 and if the protein concentra- tion is above about 6 g/L. These gels melt when heated above about 25 °C, but are reformed when cooled at the same temperature. Thermo-reversible gels can also be formed at pH 11 for C > 15 g/ L, but only if they are preheated above 30 °C. An advantage of these thermo-reversible gels is that they may be processed at higher temperatures as liquids after which gelation may be induced when desired by cooling. The dynamic mechanical properties in the linear regime have been reported in a previous work (Brenner et al., 2008) for FPI gels formed at pH 9 and 8.5 without preheating. At high frequencies (f), the storage (G 0 ) and loss (G 00 ) shear moduli are almost frequency independent with G 0 >> G 00 , but they decrease weakly at lower fre- quencies. For a given frequency, both G 0 and G 00 increase with decreasing temperature and increasing concentration. Here we report on the effect of preheating on the linear dy- namic mechanical properties of FPI gels at pH 9 and pH 11. In addi- tion, we have studied the non-linear rheology using both continuous shear and large amplitude oscillatory shear. Non-linear rheology of food systems is important for sensory attributes (Montejano, Hamann, & Lanier, 1985; vanVliet & Walstra, 1995), as well as the design of manufacturing processes. We determined complete stress–strain curves and creep measurements, which have rarely been reported for food systems. 2. Materials and methods 2.1. Sample preparation Cod white muscle tissue was homogenized at pH 11, followed by centrifugation at 15000g for 10 min. The supernatant was sep- arated over gauze. During this procedure the temperature was kept below 10 °C. Fresh cod was either used directly or frozen at 18 °C and thawed prior to sample preparation. No effect of freezing on the rheology was observed. Cod caught near Iceland’s shores, as well as near France, was used in the study. No difference in results presented here was found, but preparing protein solutions with concentrations above 20 g/L was easier using cod caught near Ice- land, independent of time of year. FPI at pH 11 was found to be sta- ble during storage at 4 °C for about 1 day, after which precipitation was usually observed. 0963-9969/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2009.04.020 * Corresponding author. Tel.: + 354 4225131; fax: +354 4225002. E-mail address: tom@matis.is (T. Brenner). Food Research International 42 (2009) 915–924 Contents lists available at ScienceDirect Food Research International journal homepage: www.elsevier.com/locate/foodres