Small-Angle X-ray Scattering and Rheological Characterization of Alginate Gels. 3. Alginic Acid Gels

Abstract

Alginic acid gels were studied by small-angle X-ray scattering and rheology to elucidate the influence of alginate chemical composition and molecular weight on the gel elasticity and molecular structure. The alginic acid gels were prepared by homogeneous pH reduction throughout the sample. Three alginates with different chemical composition and sequence, and two to three different molecular weights of each sample were examined. Three alginate samples with fractions of guluronic acid residues of 0.39 (LoG), 0.50 (InG), and 0.68 (HiG), covering the range of commercially available alginates, were employed. The excess scattering intensity I of the alginic acid gels was about 1 order of magnitude larger and exhibited a stronger curvature toward low q compared to ionically cross-linked alginate. The I(q) were decomposed into two components by assuming that the alginic acid gel is composed of aggregated multiple junctions and single chains. Time-resolved experiments showed a large increase in the average size of aggregates and their weight fraction within the first 2 h after onset of gelling, which also coincides with the most pronounced rheological changes. At equilibrium, little or no effect of molecular weight was observed, whereas at comparable molecular weights, an increased scattering intensity with increasing content of guluronic acid residues was recorded, probably because of a larger apparent molecular mass of domains. The results suggest a quasi-ordered junction zone is formed in the initial stage, followed by subsequent assembling of such zones, forming domains in the order of 50 A. The average length of the initial junction zones, being governed by the relative fraction of stabilizing G-blocks and destabilizing alternating (MG) blocks, determines the density of the final random aggregates. Hence, high-G alginates give alginic acid gels of a higher aggregate density compared to domains composed of loosely packed shorter junction zones in InG or LoG system.

Alginate/sodium caseinate aqueous-core capsules: A pH-responsive matrix

“…Heteropolymeric blocks alternating MG residues did not contribute to junctions under acidic conditions [27].…”

Participation of ions in promoting intermolecular associations of cell wall polysaccharides

“…H ? -induced association/aggregation of pectin and alginate has been verified by microscopic observation and small angle X-ray scattering [11,46,47]. It is influenced by pH, ionic strength, cosolutes as well as the fine chemical structures of pectin and alginate such as DE, DA, M/G ratio, and blockiness etc.…”

Alginates in Foods

“…This is illustrated in Fig. 6.6, where measurements of dynamic moduli show acid gels prepared from a high G alginate have values that are about 2 orders of magnitude greater than those of intermediate G alginate (Draget et al, 2003). Acid gels, however, do show some variations from ionic alginate gels in the nature of gel network formation and subsequently in rheology.…”

“…A sudden decrease in pH of an alginate solution to below the pKa of the monomers causes the precipitation of alginic acid (Haug and Larsen, 1963). It is also well known that if the reduction in pH is performed in a controlled manner, it is possible to form alginic acid gels (Draget et al, 1994(Draget et al, , 1996(Draget et al, , 2003. An excellent example of a product that utilises these properties is the anti-heartburn medication Gaviscon R .…”