Abstract
Alginate is a polysaccharide obtained from brown seaweed that is widely used in food, pharmaceutical, and biotechnological applications due to its versatility as a viscosifier and gelling agent. Here, we investigated the influence of the addition of glucose on the structure and mechanical properties of alginate solutions and calcium-alginate hydrogels produced by internal gelation through crosslinking with Ca2+. Using 1H low-field nuclear magnetic resonance (NMR) and small angle neutron scattering (SANS), we showed that alginate solutions at 1 wt % present structural heterogeneities at local scale whose size increases with glucose concentration (15–45 wt %). Remarkably, the molecular conformation of alginate in the gels obtained from internal gelation by Ca2+ crosslinking is similar to that found in solution. The mechanical properties of the gels evidence an increase in gel strength and elasticity upon the addition of glucose. The fitting of mechanical properties to a poroelastic model shows that structural changes within solutions prior to gelation and the increase in solvent viscosity contribute to the gel strength. The nanostructure of the gels (at local scale, i.e., up to few hundreds of Å) remains unaltered by the presence of glucose up to 30 wt %. At 45 wt %, the permeability obtained by the poroelastic model decreases, and the Young’s modulus increases. We suggest that macro (rather than micro) structural changes lead to this behavior due to the creation of a network of denser zones of chains at 45 wt % glucose. Our study paves the way for the design of calcium-alginate hydrogels with controlled structure for food and pharmaceutical applications in which interactions with glucose are of relevance.
Highlights
IntroductionAlginate is a charged and linear copolymer consisting of (1–4) linked β-D-mannuronic acid (M) and α-L-guluronic acid (G), whose ratio varies depending on the alginate source
In order to better understand the impact that low molecular weight sugars have on the structure and gelation properties of calcium-alginate gels here, we further investigate the influence of glucose as a cosolute on the nano and microstructure of calcium-alginate gels produced by internal gelation
0.29 ± 0.01 mL/mg (Equations (1)–(4) and Figure S1). This value is lower than previously reported for other types of alginate with higher G content performed in our lab of
Summary
Alginate is a charged and linear copolymer consisting of (1–4) linked β-D-mannuronic acid (M) and α-L-guluronic acid (G), whose ratio varies depending on the alginate source. Due to its charged nature, alginate chains can form gels in the presence of low concentrations of di- and trivalent cations (Mg2+ being an exception) at a range of pH values and temperatures. The gelation properties of the alginate depend on the M/G ratio as well as the distribution of the M and G units. Calcium-mediated gelation has been broadly studied and is attributed to the chelation of Ca2+ between G units from different alginate chains via the so-called egg-box model [9]. The egg-box model involves a two-step network formation mechanism, a dimerization process followed by dimer–dimer aggregation of G units and
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