The effects of sodium alginate and calcium levels on pea proteins cold-set gelation Jean-Luc Mession a, * , Coralie Blanchard a , Fatma-Vall Mint-Dah a , Céline Lafarge a , Ali Assifaoui a, b , Rémi Saurel a a Agrosup Dijon, UMR PAM 02.102, Equipe PAPC (Procédés Alimentaires et PhysicoChimie),1 Esplanade Erasme, 21000 Dijon, France b School of Pharmacy, Université de Bourgogne, 7 bd Jeanne d’Arc, 21079 Dijon, France article info Article history: Received 12 July 2012 Accepted 1 November 2012 Keywords: Globular pea proteins Sodium alginate Network Mixed gel Phase separation Rheological properties abstract A multi-scale investigation of pea proteins e alginate cold-set gels was proposed in this study. The gel preparation followed a two-steps procedure. Globular pea proteins were first denatured and aggregated by a pre-heating step. Sodium alginate was then added at different concentrations. Thereafter the in situ gelation process was induced at 20  C using glucono-d-lactone (GDL) and two calcium carbonate (CC) levels; calcium cations were released as the pH decreased. Small-amplitude rheology measurements (storage modulus G 0 ) showed that stronger mixed gels were obtained than single-biopolymer solutions. Confocal laser scanning microscopy (CLSM) revealed phase-separating microstructures of mixed gels, foremost owing to biopolymers incompatibility. Phase separation was kinetically entrapped by gelation at different evolution stages. According to the co-occurrence method and microstructure classification, image texture analysis disclosed that a continuous protein network dispersing small gelled alginate microdomains corresponded to the strongest mixed gels. Transmission electron microscopy (TEM) evidenced that during gelation, the pre-aggregated proteins were mainly associated into large agglomerates with no peculiar pattern. Higher cohesiveness between both networks was hypothesized, since protein agglomerates could expose “anchoring points” for alginate chains. Depending on both protein e alginate initial composition and calcium availability, non-specific inter-biopolymer cross-links via calcium were assumed to be the key factor of synergism within mixed gels. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Globular proteins are widely encountered in semi-solid food products, contributing to their nutritional value and their textural characteristics. Amongst these latter, the gelling ability represents one of the most relevant physicochemical properties of food proteins. Usually, protein gels are prepared by heat-treatment, whereby successively induces unfolding of native globulin compact structure, exposure of initially-buried hydrophobic resi- dues and subsequent aggregation of denatured proteins. When aggregated proteins exceed a concentration threshold, they could associate into tridimensional network thence occupying the whole solution volume. Depending on protein concentration and gelation conditions such as temperature, ionic strength and pH of the solvent, several structures of heat-set protein hydrogels are produced, related to aggregation route of unfolded proteins (Clark, Kavanagh, & Ross-Murphy, 2001; Langton & Hermanson, 1992). However, owing to concomitance of thermal denaturation and aggregation phenomena, the gelation procedure above cannot genuinely be undertaken with heat-sensitive compounds for instance in novel functional food, hydrogel-like systems (Maltais, Remondetto, & Subirade, 2009). To date, the overall tendency is to substitute animal proteins with sustainable and cheaper plant proteins such as soy proteins, whose isolates dominate the global market (Boye, Zare, & Pletch, 2010). Other alternatives thus need to be sought out; in fact globular proteins from dry pea seeds would be a valuable source and were therefore chosen for this study. About 80% of the total storage pea proteins are pea globulins, composed of both vicilin/convicilin 7S (z150 and z180e 210 kDa, respectively) and legumin 11S (z360 kDa) (Tzitzikas, Vincken, De Groot, Gruppen, & Visser, 2006). Each fraction is a complex mixture of several subunits constitutive of high molecular weight oligomers. Current studies deals with pea globulins thermal gelation (O’Kane, Happe, Vereijken, Gruppen, & van Boekel, 2004a; O’Kane, Happe, Vereijken, Gruppen, & van Boekel, 2004b; O’Kane, Happe, Vereijken, Gruppen, & van Boekel, 2004c; Shand, Ya, Pietrasik, & Wanasundara, 2007; Sun & Arntfield, 2010). Neverthe- less, genetic variation, time-consuming protein extraction and extensive heat-treatment procedures would add complexity to the use of pea globulins as a texture improver for human food. * Corresponding author. Tel.: þ33 380 774 030; fax: þ33 380 396 647. E-mail address: jean-luc.mession@laposte.net (J.-L. Mession). Contents lists available at SciVerse ScienceDirect Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd 0268-005X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodhyd.2012.11.004 Food Hydrocolloids 31 (2013) 446e457