Original article Cold-set whey protein gels induced by calcium or sodium salt addition Ka ´tia Regina Kuhn, A ˆ ngelo Luiz Fazani Cavallieri & Rosiane Lopes da Cunha* Department of Food Engineering, Faculty of Food Engineering, University of Campinas (UNICAMP), PO Box 6121, 13083-862 Campinas, SP, Brazil (Received 28 August 2009; Accepted in revised form 17 November 2009) Summary Cold-set whey protein isolate (WPI) gels formed by sodium or calcium chloride diffusion through dialysis membranes were evaluated by mechanical properties, water-holding capacity and microscopy. The increase of WPI concentration led to a decrease of porosity of the gels and to an increase of hardness, elasticity and water-holding capacity for both systems (CaCl 2 and NaCl). WPI gels formed by calcium chloride addition were harder, more elastic and opaque, but less deformable and with decreased ability to hold water in relation to sodium gels. The non linear part of stress–strain data was evaluated by the Blatz, Sharda, and Tschoegl equation and cold-set gels induced by calcium and sodium chloride addition showed strain- weakening and strain-hardening behaviour, respectively. The fractal structure of the gels indicated a weak- link behaviour. For WPI gels results suggest intrafloc links, formed at heating step, which were more rigid than the interfloc links, promoted by salt addition. Keywords Blatz, Sharda, and Tschoegl equation, cold gelation, fractal analysis, mechanical properties, salts, whey proteins. Introduction Whey proteins (WP) are widely used in food products because of their high nutritional value and technological properties as gelling agents, emulsifiers, texture modifi- ers, thickening and foaming agents (Bryant & McCle- ments, 1998). Protein gelation is an important feature to obtain desirable sensory and textural properties in foods. Different processing variables such as protein concentration, pH and mineral content exert influence on whey protein gels. There is a great deal of informa- tion about the effect of these variables on heat-induced gelation of whey proteins (Kinsella & Whitehead, 1989; McSwiney et al., 1994; De Wit & Van Kessel, 1996), but there is not much information about the cold-set gelation of these proteins induced by different salts addition, mono and divalent, mainly regarding the comparison of the influence of these salts on the properties and microstructure of cold-set whey protein gels (Marangoni et al., 2000). Cold-set gelation consists of a process with two consecutive steps. In the first step the solution of native globular proteins is heated at neutral pH (well above the iso-electric point), low ionic strength and protein con- centration lower than the minimum required for gela- tion, leading to a loss of their native structure with partial unfolding and subsequent aggregation. Despite this aggregation, the proteins remain soluble after cooling and do not gel, due to predominant electrostatic repulsive forces amongst the formed aggregates. In the second step, changes in solvent quality induce gelation. The structure and properties of cold-set WP gels depend on the gelation kinetics (Alting et al., 2004; Cavallieri & Cunha, 2008), the kind of induction (salt or acidic) (Ju & Kilara, 1998b), the type (Veerman et al., 2003; Bolder et al., 2006) and the properties (Alting et al., 2003) of protein aggregates formed in the first heating step, as a consequence of the temperature and heating time. In the case of salt-induced cold gelation, ionic strength seems to be the main factor influencing gel properties. Both monovalent and divalent salts screen electrostatic interactions between charged protein mol- ecules, leading to a decrease in electrostatic repulsions and consequent protein aggregation (Bryant & McCle- ments, 1998). Nevertheless, divalent cations, such as Ca 2+ , may also induce aggregation because of their ability to act as bridges between the negatively charged carboxylic groups on neighboring whey protein mole- cules (Hongsprabhas & Barbut, 1996, 1997a,b; Bryant & McClements, 1998, 2000; Marangoni et al., 2000). The salt concentration used to form a cold-set gel is a major *Correspondent: Fax: +55 19 3521 4027; e-mail: rosiane@fea.unicamp.br International Journal of Food Science and Technology 2010, 45, 348–357 348 doi:10.1111/j.1365-2621.2009.02145.x Ó 2010 The Authors. Journal compilation Ó 2010 Institute of Food Science and Technology