Proteins play a very important role in stability, functionality and texture of food systems through their functional properties Jiménez-Castaño, Villamiel, & Lopez-Fandino, 2007). This functionality is governed by physicochemical and structural properties of proteins like intrinsic amphiphilic character, surface activity, molecular weight, net charge, solubility, conformational flexibility and extrinsic conditions of the aqueous medium such as pH, ionic strength and temperature (Bryant & McClements, 2000). Accordingly, functional properties of proteins, such as water holding, the ability to form gels, foams and emulsions can be improved by chemical, physical or enzymatic treatments, e.g. acidification, heating, hydrolysis, acetylation, esterification, amidation and enzymatic cross-linking, among others (Nagasawa, Takahashi, & Hattori, 1996; Nakamura, Kato, & Kobashayi, 1992). A simple and safe strategy for modifying the functional properties of proteins is through the interaction with polysaccharides (Villamiel, & Lopez-Fandino, 2007).
All functional properties of proteins are affected by their interactions with polysaccharides; interactions can occur through non-covalent (such as hydrogen and hydrophobic interactions and ionic bonds) or covalent bonds. Non-covalent bonds are rather weak, though collectively they become stronger. Covalent bonds can exist between free sulfhydryl groups of proteins to give disulfide bonds or between an amino group of a protein and a reducing carbonyl group of a polysaccharide by Maillard reaction (belongs to a group of reactions called non-enzymatic browning reactions, since they generate brown pigments). Non-enzymatic browning reactions produce positive and negative changes in the quality and nutritional value of food systems by colour changes, production of aromas and flavours, altered protein solubility, texture changes, among others (Oliver, Melton, & Stanley, 2006). It has been shown that proteinepolysaccharide conjugates had improved functional properties as compared with proteins alone. Some of the proteins that were glycosylated are: soy, fish, wheat and whey proteins, among others. The conjugated proteins obtained via Maillard reaction had better emulsifying properties than their precursor proteins and some commercial emulsifiers (Einhorn-Stoll, Ulbrich, Sever, & Kunzek, 2005; Oliver et al., 2006), as well as better foaming properties (Dickinson & Izgi, 1996), protein solubility (Shepherd, Robertson, & Ofman, 2000) and heat stability (Kato, 2002). Whey proteins are globular proteins mainly composed of blactoglobulin and a-lactalbumin, which are widely used in food for their nutritional and functional properties (de Wit, 1998).
One of the most important and studied functional properties of whey proteins is their ability to form gels (Bryant & McClements, 2000; Li, Ould Eleya, & Gunasekaran, 2006). Dextrans (DX) are neutral polysaccharides widely used in the glycosylation of proteins for two reasons: they have a reducing nature, which is one of the required conditions for the reaction to take place, and secondly, their neutral charge inhibits the formation of electrostatic complexing between proteins and polysaccharides (Dickinson & Semenova, 1992). Dextrans are composed of alpha- linear chain of glucose residues linked through a bonds (1/6). This conformation makes dextran flexible in aqueous solution, and hence unable to form gels. Considering all these characteristics, three dextrans of different molecular weight were chosen to conjugate to whey protein isolate (WPI) by Maillard reaction. The objective of this work was to study the influence of Maillard reaction and the effect of dextran molecular weight on whey proteins and dextran (DX) gel properties through uniaxial compression test.
Maillard reaction greatly affects the gelling properties of WPI/dextran gels, since mixed gels exhibited gel fracture whereas WPI/DX conjugate gels did not, under the same test conditions. The Maillard reaction established entirely new properties in WPI/DX gelling systems, weakening the structures and preventing self supporting gelation in some of them. There are several hypotheses about the causes of this behaviour, but much remains to be done in future studies to try to elucidate the structures and mechanisms that weaken the conjugate gels or even prevent their formation.