Physicochemical, textural, rheological and microstructural properties of protein isolate gels produced from European eel (Anguilla anguilla) by heat-induced gelation process
Introduction
Recently, several texture modifiers are used in food products, mainly based on globular proteins such as myofibrillar proteins extracted from meat and fish. Proteins constitute an interesting source of nutrients for humans and could be applied for the improvement of food products. In the fish industry, large quantities of proteins are generated by raw fish processing.
Fish protein isolate (FPI) can be extracted by solubilizing proteins at extreme pH values to separate soluble proteins, bone, skin, connective tissue, cellular membranes, and neutral lipids through a centrifugation step. After solubilization via the alkaline process, proteins are collected and recovered by isoelectric precipitation, giving raise to stable protein isolates (Kristinsson & Liang, 2006). Solubilization and precipitation can induce changes in the three dimensional (3D) structure of myosin and particularly myosin heavy chain (MHC), which have an effect on the functional properties of the proteins recovered and their miscibility during protein isolate production (Gehring, Gigliotti, Moritz, Tou, & Jaczynski, 2011). In addition, following the shifting pH treatment, the unique structure of protein produced is responsible for improved functional properties regarding gelation, emulsification and solubility. The partially unfolded/folded structure of proteins is more flexible, allowing the ability to form better protein networks on heating. Thus, during the pH-shifting process, structural changes (folding phenomena) cause an increase of surface hydrophobicity and reactive–SH groups in the produced proteins which may contribute to their technologically useful functionalities (Gehring et al., 2011; Nolsøe & Undeland, 2009).
Many studies focused on protein gelation in food, due to its interesting textural properties and its capacity to entrap molecules of interest into gel networks (Yang, Wang, Vasanthan, & Chen, 2014). Particularly, fish protein gels are complex and their properties as well as the mechanism of their formation have been the subject of several research works. Properties of protein gels are determined based on the type and number of protein–protein interactions, aggregation and arrangement of unfolded proteins, which are affected by the pH, ionic strength, protein concentration, heating and cooling rates (Xiong & Blacnchard, 1997). Gel formation requires structural and functional changes of the constituent myofibrillar proteins. In addition, the rheological and biochemical characteristics of the gel can be affected either by intrinsic (species, freshness, and degree of stress before harvest) or extrinsic factors (protein concentration, pH, ionic strength, and temperature) (Yin & Park, 2014).
The aim of this study was to produce European eel protein isolate gels by applying a heat treatment. The effect of pH, protein isolate concentration, and NaCl addition on chemical, textural, rheological and microstructural properties of protein isolate gels, were evaluated.
Section snippets
Fish sample
A. anguilla was purchased from the fish market of Sfax City, Tunisia. Samples were packed in polyethylene bags, placed in ice and then transported to the research laboratory within 30 min. Muscle was separated from viscera, rinsed with distilled water to remove salts and other contaminants and stored at −20 °C.
Preparation of European eel protein isolate
The alkaline solubilization process was used to prepare EPI as described by (Chaijan, Panpipat, & Benjakul, 2010). Fish mince was homogenized with cold water (4 °C) at a ratio of 1:9
Chemical composition of EPI
Chemical compositions of fresh muscle and freeze-dried EPI are presented in Table 1. Results show that alkali-aided process followed by centrifugation lead to extract 94.33% of EPI (based on the dry weight matter) and to eliminate about 87.40% of lipid content compared to the fresh muscle. In fact, after the alkaline processing, soluble proteins were separated from lipids on the basis of density by centrifugation. Similar findings were reported by Wang et al. (2015) who found that Krill
Conclusion
This work showed that protein isolate extracted from A. anguilla muscle by alkaline solubilization process can be a source of protein gels with good functional, textural and microstructural properties. Concentration of protein isolate and pH variation were found to significantly affect the properties of the produced protein isolate gels. In fact, gels prepared under the alkaline conditions and with high protein concentrations displayed better rheological and textural properties, the lowest
Acknowledgements
This study was funded by the Ministry of Higher Education and Scientific Research, Tunisia. Authors would also like to thank the Central Support Service for Experimental Research (SCSIE) of the University of Valencia for the electronic microscopy service.
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