Improved gelling properties of whey protein isolate by addition of xanthan gum
Introduction
Food products are complex systems in which texture is mainly dictated by the presence of proteins and polysaccharides. Functional properties of individual components are well known. However, the functional behaviour of mixed protein–polysaccharide systems is still little known (Dickinson & McClements, 1996). Among the various functional properties of proteins and polysaccharides, gelling is one commonly exploited by the food industry. Mixed gels are obtained from solutions of biopolymers, which are either complex (intermolecular attraction) or incompatible (intermolecular repulsion). The macromolecular distribution in solution leads to either antagonist or synergistic effects on the formation of these mixed gels. Improved gelling properties have been reported following both types of interactions (Cai & Arntfield, 1997; Ganz, 1974; Ould Eleya & Turgeon, 2000; Sanchez, Schmitt, Babak, & Hardy, 1997; Shim & Mulvaney, 2001; Smith, Nash, Eldridge, & Wolf, 1962; Wang & Qvist, 2000). Protein–polysaccharide interactions are mainly electrostatic in nature and rely greatly on pH, ionic strength (Girard, Turgeon, & Gauthier, 2002; Ledward, 1994; Weinbreck, Nieuwenhuijse, Robijn, & de Kruif, 2003) and protein–polysaccharide ratio (Turgeon, Beaulieu, Schmitt, & Sanchez, 2003).
Whey proteins have been chosen because they have been studied extensively; they are well known and have excellent functional properties (de Wit, 1998; Kinsella & Whitehead, 1989a). Whey proteins represent 20% of total milk proteins and are composed of β-lactoglobulin (β-lg) (∼50%), α-lactalbumin (∼20%), serum albumin (BSA), immunoglobulin and minor proteins in addition to lactose and minerals (de Wit, 1989). Being the main constituent of whey protein, β-lg dictates its functional properties (Ziegler & Foegeding, 1990).
The compact three-dimensional structure of β-lg contains amino acid hydrophobic lateral chains on the inside while polar lateral chains are located on the outside of the structure. Gelation of β-lg is the result of a thermal treatment applied to the solution which provokes partial unfolding exposing hydrophobic amino acid groups leading to aggregation of proteins partially or completely denatured (Totosaus, Montejano, Salazar, & Guerrero, 2002).
Xanthan is an anionic bacterial polysaccharide produced by Xanthomonas campestis whose molecular weight exceeds 106 Da. Its primary structure is a cellulose backbone on which lateral-charged chains composed of three saccharides (D-glucose, D-mannose and D-gluconate) are attached in addition to acetyl and pyruvate groups (McNeely & Kang, 1973; Stokke, Christensen, & Smidsrød, 1998). Xanthan in solution behaves as a pseudoplastic polymer and can only form a gel by two means: through interactions with other polysaccharides, such as galactomannans, and by cross-linkages in the presence of metallic ions (Stokke et al., 1998). The molecular structure of xanthan by itself prohibits gel formation (Kang & Pettitt, 1992).
Certain systems such as whey protein isolate (WPI)–acacia gum, β-lg–pectin and β-lg–carrageenan have been studied more extensively over the past years as compared to WPI–xanthan gum systems. Laneuville, Paquin, and Turgeon (2000) have studied β-lg–xanthan complexes size in solution. Sanchez et al. (1997) looked at the rheological properties using large deformation of WPI–xanthan systems and found a synergistic effect at a pH⩾7, and an antagonist effect at a pH⩽6.5. Both phenomena were related to phase separation. Bryant and McClements (2000) also found a synergistic effect, but on a heat-denatured-whey protein–xanthan gum system after cold gelation. Finally, Zasypkin, Dumay, and Cheftel (1996) observed an antagonist effect of xanthan addition on β-lg gel formation for a protein concentration higher than 10% w/w. In this paper, the effect of addition of very low concentration of xanthan (⩽0.06%) on WPI gels has been studied using small deformation measurements. The effect of pH and salt concentration on the type of systems and their microstructure are presented.
Section snippets
Material
Whey protein isolate (BIPRO™) was purchased from Davisco Foods Intl. Inc., MN, US. It is composed of 85.4% w/w protein, 4.76 mg/g of sodium, 0.87 mg/g of potassium and 0.67 mg/g of calcium. Xanthan gum (Keltrol F) was purchased from Kelco Co. San Diego, Ca, US. It is composed of 96% sugar and 4.02% protein. Its molecular weight is 26.3×106 Da (Lagoueyte & Paquin, 1998). All other chemicals used are analytical grade (ACS, Fisher Scientific, US).
Preparation of solutions
Solutions of 16% (w/v) WPI powder were prepared using
Effect of pH
Rheological properties of whey protein gels were mainly influenced by pH. Gel properties rely on pH by affecting the net charge carried by the protein (Tang, McCarthy, & Munro, 1995). Results demonstrate that the elastic modulus increases as the pH decreases between 5.5 and 6.5 (Fig. 1). Results are in agreement with Stading, Langton, and Hermansson (1993) who came to the same conclusion for β-lg gels (pH 6.5–5.58). The balance between protein–protein and protein–solvent interactions dictates
Conclusion
WPI gels can be improved by addition of xanthan. Xanthan has a significant effect at concentration as low as 0.01%. The impacts on the microstructure and rheological properties of the gel formation depend on pH and NaCl concentration. The balance between complexes formation and segregative phase separation in mixed systems lead to either a synergistic or an antagonist effect. A synergy is observed for gels where the protein structure is reinforced by segregative phase separation. However, an
Acknowledgements
NSERC is acknowledged for its financial support on the protein/polysaccharide study along with Anne-Françoise Allain and Yolande Kougioumoutsakis for their technical support.
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