Langmuir 2006, 22, 8873-8880
8873
Surface Structure Smoothing Effect of Polysaccharide on a Heat-Set Protein Particle Gel Kooshan Nayebzadeh,† Jianshe Chen,* Eric Dickinson, and Thomas Moschakis Procter Department of Food Science, UniVersity of Leeds, Leeds LS2 9JT, U.K. ReceiVed February 13, 2006. In Final Form: August 3, 2006 This work investigates surface properties of a protein particle gel and effects of polysaccharide on the surface microstructure of such a protein gel. Whey protein isolate (WPI) was used as the primary gelling agent, and a polysaccharide (xanthan) was investigated for its surface smoothing effects. The surface properties of heat-set WPI gels with and without the presence of xanthan (0, 0.05, and 0.25%) were characterized using a surface friction technique. The surface friction force of a gel against a stainless steel substrate was found to be highly dependent on the sliding speed for all three gel samples, and the addition of xanthan caused a general reduction of surface friction. The gel containing no xanthan has the largest surface friction and behaved in the most load-dependent manner, whereas the gel containing 0.25% xanthan has the lowest surface friction and showed the least load dependency. It was inferred that the WPI gel containing no xanthan has the roughest surface among the three samples and the presence of xanthan leads to a smoother surface with probably a thinner layer of surface water. Surface features derived from surface friction tests were confirmed by surface microstructure observation from confocal laser scanning microscopy (CLSM) and environmental electron scanning microscopy (ESEM). Surface profiles from CLSM images were used to quantify the surface roughness of these gels. The mean square root surface roughness Rq was calculated to be 3.8 ( 0.2, 3.0 ( 0.2, and 1.5 ( 0.2 µm for gels containing 0, 0.05, and 0.25% xanthan, respectively. The dual excitation images of protein and xanthan from CLSM observation and images from ESEM observation indicate a xanthan-rich layer at the surfaces of the xanthan-containing gel samples. We speculate that the creation of the outer surface of a particle gel is based on a different particle aggregation mechanism from that leading to network formation in the bulk.
Introduction Many semisolid composite foods can be classified as particle gel systems. Ice cream, cheese, and yogurt are few typical examples of food particle gels.1 The main structural elements of these gels are proteins, starch granules, fat droplets, ice crystals, sugar crystal, and others. The mechanisms of particle interaction, particle aggregation, and network formation have received extensive attention in the literature, and the mechanical and rheological properties as well as the microstructural character of such gel systems are reasonably well understood.2,3 For example, it is well known that particle aggregation can follow either a diffusion-limited mechanism or a reaction-limited mechanism.4 Particle gels could be visually transparent or translucent depending on the nature of the microstructure or the size of the particle clusters. A transparent gel is often associated with a network of fine strands or a strings of beads, whereas a translucent gel often has a network of large particle clusters.5 Particle gels are mechanically viscoelastic with relatively short linear regimes.6 Unlike polymer and biopolymer gels, particle gels often become mechanically weaker under deformation or sometimes fracture at relative small strains. It is also well known that 3D networks of many particle gels are fractal in nature and have a fractal dimension between 2 and 3.7 * Corresponding author. E-mail:
[email protected]. † Current address: Department of Food Science and Engineering, Faculty of Biosystem Engineering, University of Tehran, Iran. (1) Stainsby, G. In Food Polymers, Gels, and Colloids; Dickinson, E., Ed.; Royal Society of Chemistry, Cambridge, U.K., 1991; p 450. (2) Bremer, L. G. B.; Bijsterbosch, B. H.; Walstra, P.; van Vliet, T. AdV. Colloid Interface Sci. 1993, 46, 117. (3) McGuffey, M. K.; Foegeding, E. A. J. Texture Stud. 2001, 32, 285. (4) Lin, M. Y.; Lindsay, H. M.; Weitz, D. A.; Ball, R. C.; Klein, R.; Meakin, P. Nature 1989, 339, 360. (5) Doi, E. Trends Food Sci. Technol. 1993, 4, 1. (6) Chen, J.; Dickinson, E.; Langton, M.; Hermansson, A.-M. Lebensm.Wiss.-Technol. 2000, 33, 299.
Despite the understanding of the physicochemical properties of bulk particle gels, little is known about the surface properties of such gel systems. For example, it is not precisely clear yet how the surface of a particle gel is created, what are the differences in particle aggregation at the surface and within the bulk phase, what are the main influencing factors, and so on. A large quantity of research (experimental studies and computer simulation) has been carried out on particle aggregation on 2D surfaces,8-10 but because of the lack of the third dimension, it is difficult to make direct links between the findings from these studies and what happens at the surface of a bulk gel system. The lack of progress in surface studies of particle gels is largely due to the potential complexity of the surface character and the limited availability of techniques for surface characterization.11 Surface delicacy and surface deformability are the main challenges in the surface characterization of particle gels. In particular, most food particle gels are mechanically weak, and close surface contact under a relatively high surface load could cause severe surface distortion or even surface damage. Another major problem for the surface characterization of food particle gels is the existence of surface moisture. A thin layer of water at the surface will tend to interfere with the results obtained from surface-contacting techniques (such as tribology, atomic force microscopy, surface force apparatus, etc.) and makes some surface imaging techniques (such as electron scanning microscopy) less reliable.11 Nevertheless, various efforts have been made during past few years to study the surfaces of wet, soft, deformable biogels. Gong and co-workers have successfully used surface friction force measurements to characterize surface properties of polymer (7) Ikeda, S.; Foegeding, E. A.; Hagiwara, T. Langmuir 1999, 15, 8584. (8) Wijmans, C.; Dickinson, E. Langmuir 1999, 15, 8344. (9) Semenova, M. G.; Chen, J.; Dickinson, E.; Murray, B. S.; Whittle, M. Colloids Surf., B 2001, 22, 237. (10) Murray, B. S. Prog. Colloid Polym. Sci. 1997, 103, 41. (11) Chen, J. CRC Crit. ReV. Food Sci. Nutr. 2006, in press.
10.1021/la060419o CCC: $33.50 © 2006 American Chemical Society Published on Web 09/07/2006
8874 Langmuir, Vol. 22, No. 21, 2006
Nayebzadeh et al.
gels.12-16 They found that the measured surface resistance of a polymer gel against a solid substrate depends not only on the surface load12 but also on the interfacial interaction13 and surface charge.14,15 Although surface friction increases with increasing sliding speed,16 separate studies demonstrated17 that this speed dependence vanishes once polymer chains at the gel surface are in coiled conformations, a state that can be achieved by adding salt or oppositely charged surfactant to reduce the mesh size or to condense polymer chains at the surface. Baumberger et al.18,19 applied a surface friction technique to characterize the surfaces of gelatin gels. They found that the surface deformability of a soft polymer gel was more evident when the gel was slid at an extremely low speed against a solid substrate. At these low speeds, the gels exhibited stick-slip behavior against the solid substrate. One key observation from the surface friction tests is the deviation from the classical description of the surface friction. Classical theory predicts that the surface friction increases in proportion to the surface load,
that the former has no polymer chains stretching out from the surface so the contribution from polymer chain adsorption/ repulsion to the surface friction force should be minimal. Here we extend our investigation to particle gels of mixed biopolymers. Whey protein isolate, a milk protein, was used as the primary gelling agent of particle gels, and xanthan gum, a polysaccharide widely used in food products as a thickening agent, stabilizer, and texture modifier, was incorporated for its potential surface smoothing effects. The intention is to enhance the understanding of the surface creation of more complex particle gels containing mixed biopolymers and to explore the possibility of designing particle gels having specific surface functionalities, such as textural functionality (smoothness/roughness, brightness, etc), adhering functionality (stickiness to packaging and contacting materials), mass transfer functionality (for controlled release of flavor/aroma/moisture), and so forth.
F ) µW
Materials. Whey protein isolate (WPI) (Lacprodan DI-9224) was obtained from Arla Foods (Videbaek, Denmark). The sample contained at least 93.5% protein and other minor components such as fat (
