Raman analysis of surimi gelation by addition of wheat dietary fibre Ignacio Sánchez-González a , Arantxa Rodríguez-Casado b , Mercedes Careche a, * , Pedro Carmona c a Instituto del Frío – CSIC. c./José Antonio Novais 10, 28040 Madrid, Spain b IMDEA Alimentación, 28049 Madrid, Spain c Instituto de Estructura de la Materia – CSIC. c./Serrano 121, 28006 Madrid, Spain article info Article history: Received 16 November 2007 Received in revised form 27 March 2008 Accepted 16 May 2008 Keywords: Surimi Surimi gels Dietary fibre Protein structure Raman spectroscopy Interactions abstract Raman analysis has been carried out to study the effects of Vitacel Ò wheat dietary fibre (WDF) during gelation of surimi. The main results reveal the following: (a) Vitacel Ò comprises natural cellulose I as major component; (b) hydration of WDF leads to mCH frequency upshifting and decreasing intensity. On the basis of these spectral features it is suggested that water transfer from protein to WDF can occur in surimi gels. WDF hydration can be interpreted in the sense that this fibre either takes water that is delivered from the gel protein upon heat-mediated formation of b-sheets and hydrophobic contacts and/or or acts as an active dehydrating agent. An increase of solvent-exposed hydrophobic side chains is observed in the sol phase, upon the addition of WDF, which may cause breaking of intermolecular pro- tein hydrophobic contacts; a subsequent change upon WDF-containing gel formation is the reduction in the mCH intensity, which may be indicative of increasing hydrophobic WDF-protein contacts. Interest- ingly, these results constitute molecular data, to be considered when designing restructured fish products with these fibre ingredients. 1. Introduction Dietary fibres (DF) have a positive association for consumers as a functional ingredient, and the scientific evidence for their bene- ficial health effects are well-known (Dreher, 2001; Gallaher, 2000). DF is at least quantitatively the principal ingredient used in functional foods (Saura-Calixto & Goñi, 2005). Wheat dietary fi- bre (WDF) is insoluble, neutral in taste and odour and has been used, for example, in formulations of restructured fish products (Sánchez-Alonso, Haki-Maleki, & Borderías, 2007). This fibre, like other biopolymers, can also be used technologically as a water- controlling agent, in order to obtain desirable textures, physical stability and thus contribute to the final characteristics of the for- mulated foods (Lee, 2002; Sánchez-Alonso, Haki-Maleki, & Borderí- as, 2006). Surimi technology allows the formulation of tailor-made sea- food products, which can be considered as excellent carriers of some functional ingredients, provided they do not completely interfere with the gelation of the myofibrillar proteins. The pres- ence of WDF in surimi gels leads to changes in the rheological prop- erties of these products (Sánchez-González, 2008). When WDF is incorporated into the formulation, an increase of initial and resid- ual stress, elastic and viscous moduli can be observed at equal surimi (and therefore protein) concentrations. However, if the WDF is added at equal water concentrations, that is, replacing part of the surimi (or protein) by WDF, the rheological parameters men- tioned above may not reach the values obtained with the control. The above results suggest that the effect of WDF addition on the formulations could be a balance of (a) the strengthening effect of the fibre on the gel structure either by acting as a filler or by its high water absorbing capacity and (b) the increase of the heteroge- neity of the protein network, maybe due to the aggregation/coag- ulation of the surrounding protein matrix. One way to study the molecular interactions between biopoly- mers and proteins in the above models is the use of Raman spec- troscopy. This technique gives information based on both the relative intensities and the frequencies of vibrational motions on the amino acid side chains, polypeptide and polysaccharide back- bone. It has been proposed as a useful tool to investigate molecular interactions, structural changes in proteins and water in seafood products during storage (Careche, Herrero, Rodríguez-Casado, Del Mazo, & Carmona,1999; Herrero, Carmona, & Careche, 2004), as well as in the study of the alterations in protein side chains and secondary structure in surimi, gels, or during gelation of actomyo- sin (Bouraoui, Nakai, & Li-Chan, 1997; Sánchez-González et al., 2008; Thawornchinsombut, Park, Meng, & Li-Chan, 2006). It has also been used in the study of the structure of celluloses (Pron- iewicz et al., 2001). The changes from surimi to gels in relation to protein structures include increase of hydrophobic interactions, decreasing of a-helical structure and concomitant increasing of Abbreviations: DF, dietary fibre; WDF, wheat dietary fibre. * Corresponding author. Tel.: +34 91 549 2300; fax: +34 91 549 3627. E-mail address: mcareche@if.csic.es (M. Careche).