Geoffrey J. Brownsey

X-Ray fibre-diffraction studies of synergistic, binary polysaccharide gels

Abstract X-Ray fibre-diffraction studies have been used to examine the proposal that intermolecular binding occurs between different polysaccharides in certain synergistic binary gels. No evidence for intermolecular binding was found in studies of tara-kappa carrageenan, carob-kappa carrageenan, tara-furcellaran, or carob-furcellaran gels. Present experimental data suggest that the most likely model for such gels consists of a galactomannan solution contained within a carrageenan or furcellaran network. However, evidence for intermolecular binding was found in fibres prepared from tara-xanthan and carob-xanthan gels. Gelation has been taken to involve an interaction of the cellulosic backbone of xanthan and the mannan backbone of the galactomannan. Models for the junction zones of the mixed polymer network are discussed.

Structural relaxation and physical ageing of starchy materials

The structural relaxation during the ageing of an amorphous maltose and a starch-sorbitol mixture was examined using a range of physical techniques. Heat capacity, measured by differential scanning calorimetry, showed an overshoot in the glass-transition region, the size of which was temperature and time dependent. Volume relaxation measurements were made at different ageing temperatures in the range T(g) -15 to -30 K. The volume decreased with increasing ageing time, in an essentially linear fashion with log time. The mechanical behaviour of the materials showed a progressive embrittlement on ageing. For both materials, the mechanical relaxation time increased with ageing, and the material became stiffer. Investigation of the effect of physical ageing on transport properties was also performed using conductivity measurements on a maltose-water-KC1 mixture. A decay in conductivity, which was almost linear with log time, was observed. The structural relaxation was modelled using the Tool-Narayanaswamy approach to describe the calorimetric data.

Comparative studies of the mechanical properties of mixed gels formed by kappa carrageenan and tara gum or carob gum

Abstract Tara gum has been considered as a possible replacement for carob gum in the formation of kappa carrageenan — galactomannan mixed gels. The mechanical properties of tara — carrageenan mixed gels have been measured and compared with the mechanical properties of carob — carrageenan mixed gels.

Gelation of sugarbeet and citrus pectins using enzymes extracted from orange peel.

Abstract Sugarbeet pectin is shown to form gels in the presence of calcium using an enzyme preparation extracted from orange peel. The gels were transparent and exhibited no syneresis. The mechanism of gelation is chain association arising from both lowered pectin solubility and from formation of a limited network of calcium-linked junction zones. The gelation reaction involves limited pectin demethoxylation, the release of acetate presumably from C-2 or C-3 of galacturonyl residues, and a decrease in pH. The enzymes responsible are pectinesterase (EC 3.1.1.11) and pectin acetylesterase. We suggest that the latter is a novel activity associated with triacetin acetylesterase (EC 3.1.1.6). The gels are compared to citrus pectin gels made in the same way.

Some observations (or problems) on the characterization of gellan gum solutions

Abstract Both acetylated and deacetylated gellan gum form molecular solutions in 90% dimethyl sulphoxide at concentrations less than 4 mg ml−1. The molecular weights are (1–2) × 106 for the deacetylated and acetylated forms. The polymers adopt the configuration of stiff polymer coils. Light scattering and viscometry suggest radii of gyration of the order of 0.1 μm. Polymer-polymer entanglement is observed at concentrations above 4 mg ml−1.

X-ray fibre diffraction studies of chitosan and chitosan gels

Abstract Fibres obtained from freshly prepared films of ungelled chitosan yielded X-ray diffraction patterns indicative of high crystallinity and characteristic

Possible biological roles for Rhizobium leguminosarum extracellular polysaccharide and cyclic glucans in bacteria-plant interactions for nitrogen-fixing bacteria

Abstract The extracellular polysaccharide (EPS) secreted by Rhizobium leguminosarum forms an aqueous solution. Addition of electrolyte can lead to the formation of transparent thermoreversible gels. A non-specific role for EPS gelation in the initial attachment of the bacteria to plant material is suggested. Techniques have been developed for the preparation of cyclic glucans from EPS − R.leguminosarum . The preparation, purification and characterization of ring sizes will be described. Evidence will be presented that cyclic glucans solubilize naringenin, a flavonoid known to induce nodulation genes. A biological role for cyclic glucans in accumulating naringenin is proposed.

Molecular origins of acetan solution properties

Acetan is a branched acidic heteropolysaccharide secreted by Acetobacter xylinum. X-ray diffraction studies of oriented fibres suggest non-crystalline helices with fivefold symmetry and a pitch of 4.8 nm. Optical rotation and circular dichroism studies are consistent with the retention of the helical structure in solution and a helix-coil transition upon heating and cooling. Aqueous solutions yield high low shear rate viscosity and shear-thin upon shearing.

Cation-dependent gelation of the acidic extracellular polysaccharides of Rhizobium leguminosarum: A non-specific mechanism for the attachment of bacteria to plant roots

Abstract The extracellular acidic heteropolysaccharides produced by various species of Rhizobium leguminosarum possess the same backbone but have different sidechains. Aqueous solutions of these polysaccharides form gels in the presence of excess of salt and divalent cations are more effective at inducing gelation than are monovalent cations, although the moduli of the gels are similar at comparable ionic strength. Gelation of the extracellular polysaccharide is proposed as a mechanism for attaching the bacteria to the tips of plant roots.

A study of the mechanical behaviour and microstructure of a mixed starch—egg-white protein gel

Abstract The mechanical behaviour of a composite gel consisting of washed gelatinized starch granules embedded in an egg-white protein gel matrix has been investigated. The shear modulus of the composite gel is dependent on the shear modulus of the matrix, the relative deformabilities of particle and matrix and the volume fraction occupied by the particles. The strength of this gel is affected by stress localization at the filler—matrix interface. Mixed starch—egg-white protein gels contain solubilized amylose, the immiscibility of the amylose and protein also affecting mechanical behaviour.