Abstract
The formation and properties of alginate fluid gels produced using in-situ calcium release and a defined shear field are investigated.Results show that, while the rate of particle growth during formation increases significantly upon lowering the shear rate, the rate of inter-particle interaction post-production remains unaffected by the magnitude of shear.The longer polymer chain allows a greater number of possible sites for calcium crosslinking per chain which enhances the formation of a percolating network and increases the number of rheologically-effective network crosslinks. As a result, high Mw alginate fluid gels exhibit faster gelation kinetics and greater viscosities than those of low Mw alginate fluid gels.Particle stiffness increases with increasing CaCO3 concentration until a critical value is reached. Quiescent gels display the same CaCO3 critical value, suggesting that the number of crosslinks within an individual fluid gel particle is equivalent to that of their counterpart quiescent gel. This is due to the fast coil-dimer transition which, under the applied shear field, leads to kinetically trapped fluid gel structures. However, their textural response differs from that of a quiescent gel which is likely caused by the bridging between fluid gel particles.In addition to advancing the understanding of the production of fluid gels from alginates, this work shows for the first time how the ordering kinetics and the resulting particle properties can be manipulated by the choice of alginate Mw and CaCO3 concentration, potentially allowing the design and delivery of specific structures with desirable attributes.
Highlights
Alginates are linear polyuronic polysaccharide extracted from brown seaweed (Phaeophyceae), consisting of (1-4)-linked blocks of poly-b-D-mannuronic acid (M) and poly-a-L-guluronic acid (G)residues at different proportions and with different sequential occurrence (Draget, Skjåk-Bræk, & Stokke, 2006; Stokke et al, 2000).Quiescent gelation of alginates with different M/G ratios has been extensively studied, where alginates with high content of guluronate have been shown to form strong, brittle gels, whereas alginates rich in mannuronate residues give softer and more elastic gels (Draget, Skjåk Bræk, & Smidsrød, 1994; Matsumoto et al, 1992; Smidsrød, 1974)
Little attention has been paid to the influence of polymer molecular weight (Mw) on the time dependent gelation mechanism of alginate gels and, to our knowledge, no study has reported the effect of polymer Mw and Calcium on the formation of suspensions of alginate microgelled particles
The aim of the present work is to investigate the structural events dictating the formation of such particulate structures and the different properties that emerge from varying either processing conditions or the material
Summary
Alginates are linear polyuronic polysaccharide extracted from brown seaweed (Phaeophyceae), consisting of (1-4)-linked blocks of poly-b-D-mannuronic acid (M) and poly-a-L-guluronic acid (G)residues at different proportions and with different sequential occurrence (Draget, Skjåk-Bræk, & Stokke, 2006; Stokke et al, 2000).Quiescent gelation of alginates with different M/G ratios has been extensively studied, where alginates with high content of guluronate have been shown to form strong, brittle gels, whereas alginates rich in mannuronate residues give softer and more elastic gels (Draget, Skjåk Bræk, & Smidsrød, 1994; Matsumoto et al, 1992; Smidsrød, 1974). Recent molecular model investigations have demonstrated that calcium ions preferentially bind to the G-blocks which are stiffer and possess a more extended chain conformation compared to the M-blocks due to a greater hindered rotation around the glycosidic linkages (Braccini, Grasso, & Pérez, 1999; Braccini & Pérez, 2001). The effects of both calcium and alginate M/G ratio on the rheological behaviour of alginate gels are discussed in great detail in the work of Mitchell and Blanshard (1976). The aim of the present work is to investigate the structural events dictating the formation of such particulate structures and the different properties that emerge from varying either processing conditions or the material (alginate Mw and calcium concentration)
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