Victor J. Morris

The roles of amylose and amylopectin in the gelation and retrogradation of starch

Abstract The retrogradation of starch gels has been studied by using X-ray diffraction, differential scanning calorimetry, and measurements of the shear modulus. Starch gels were considered as composites containing gelatinised granules embedded in an amylose matrix. The short-term development of gel structure and crystallinity in starch gels was found to be dominated by irreversible (T

The gelation and crystallisation of amylopectin

Abstract A range of physical and chemical techniques, including viscometry, rheological measurements, dilatometry, turbidity measurements, X-ray diffraction, and differential scanning calorimetry, has been used to study the gelation of amylopectin. Gels form on cooling concentrated aqueous solutions to 1°. The development of gel stiffness is closely related to the association of amylopectin chains, as monitored by dilatometry and differential scanning calorimetry. X-Ray diffraction studies suggest that intermolecular association involves a crystallisation process. The association of amylopectin chains in the gel is substantial and is thermo-reversible at temperatures below 100°. Heterogenous acid hydrolysis of the gel followed by examination of the residue by gel-permeation chromatography showed that the associated regions contained branched fragments, the individual chains of which had a d.p. of 15. The combined data suggest that the amylopectin molecules associate by crystallisation of the branches with d.p. 15 to form a network.

Gelation of amylose

Abstract A range of physical techniques, including light-scattering, turbidity measurements, viscometry, dilatometry, rheological measurements, and X-ray diffraction, has been used to study the gelation of amylose. Gels form on cooling entangled amylose solutions and occur as a result of a phase separation which produces a three-dimensional polymer network. Crystallisation, as detected by X-ray diffraction, was observed to be a slower process originating in the polymer-rich phase.

Starch gelation and retrogradation

Abstract Starch-containing foods are normally cooked by heating in the presence of water. Most food scientists are aware of the dramatic changes that occur during heating and cooling (cooking), and of the textural changes (staling) that occur during storage. Recent scientific studies have provided a picture of the molecular changes that occur during such processes. This picture emphasizes the important roles played by the crystallization of the two starch polysaccharides (amylose and amylopectin). Moreover, the recognition that such crystallization seldom reaches completion is providing new insights into the description and manipulation of starch gelation and retrogradation.

Structure of the acidic extracellular gelling polysaccharide produced by Pseudomonas elodea

Abstract The gelling polysaccharide secreted by the bacterium Pseudomonas elodea contains l -rhamnose, d -glucose, and d -glucuronic acid in the molar ratios 1:2:1. Methylation analysis of native and carboxyl-reduced polysaccharide indicated (1→4)-Rha p , (1→3)-Glc p , (1→4)-Glc p , and (1→4)-Glc p A to be present in the ratios 1:1:1:1. Graded acid hydrolysis gave a series of acidic oligosaccharides that were isolated by ion-exchange chromatography and fractionated by gel-filtration. The purified oligosaccharides were analysed and characterised as their methylated alditol derivatives by e.i.-m.s. and c.i.-m.s., and also examined by fast-atom-bombardment (f.a.b.)-m.s. The tetrasaccharide repeating-unit 1 , excluding acetyl groups, is proposed.

The granular structure of C-type pea starch and its role in gelatinization

We have used a combination of techniques to study the structure and properties of C-type starch from pea seeds. It was found that all C-type starch granules contain both types of polymorph; the B polymorphs are in the center of the granule and are surrounded by the A polymorphs. During heating in excess salt solution the A and B polymorphs within C-type granules melt independently, giving a double transition in heat capacity and a two-step swelling, compared with single transitions for A- and B-type starches. It was shown that B polymorphs gave a transition with a lower peak temperature than A. The disruption of crystallinity during gelatinization began from the hilum area and was propagated along the granule, accompanied by swelling of disrupted areas. It is proposed that the swelling of disrupted parts of the granule decreases the melting temperature of the neighboring crystallites resulting in the progressive disruption of crystalline areas. The gelatinization process is dependent on the arrangement of A and B polymorphs within the granule.

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.

Molecular viscoelasticity of xanthan polysaccharide

Previous studies of xanthan polysaccharide using molecular probes of local chain geometry such as optical rotation and n.m.r. have demonstrated a cooperative disorder–order transition in aqueous solution on cooling or on addition of salt. In the present work we have investigated chain geometry of the ordered species at a ‘macromolecular level’ using quasi-elastic light scattering, transient electric birefringence and elongational flow birefringence, and have used viscoelastic measurements to probe the ‘supramolecular’ organisation responsible for the ‘weak-gel’ properties of xanthan solutions.Solution viscoelasticity at fixed xanthan concentration was drastically modified by changing the counterion to the polyelectrolyte and by treatment with urea, although in all cases the local ordered structure was unaffected. ‘Macromolecular’ studies under conditions which minimise intermolecular interactions (Na+ salt form in the presence of urea) and at substantially lower concentrations indicate a persistence length comparable to that of other highly persistent biopolymers, such as double helical DNA and the triple helical polysaccharide schizophyllan.We conclude that in aqueous solution xanthan may be regarded as a highly extended worm-like chain interacting by non-covalent association to develop a weak-gel network, which is readily reversible under shear.

Interfacial Characterization of β-Lactoglobulin Networks : Displacement by Bile Salts

The competitive displacement of a model protein (beta-lactoglobulin) by bile salts from air-water and oil-water interfaces is investigated in vitro under model duodenal digestion conditions. The aim is to understand this process so that interfaces can be designed to control lipid digestion thus improving the nutritional impact of foods. Duodenal digestion has been simulated using a simplified biological system and the protein displacement process monitored by interfacial measurements and atomic force microscopy (AFM). First, the properties of beta-lactoglobulin adsorbed layers at the air-water and the olive oil-water interfaces were analyzed by interfacial tension techniques under physiological conditions (pH 7, 0.15 M NaCl, 10 mM CaCl2, 37 degrees C). The protein film had a lower dilatational modulus (hence formed a weaker network) at the olive oil-water interface compared to the air-water interface. Addition of bile salt (BS) severely decreased the dilatational modulus of the adsorbed beta-lactoglobulin film at both the air-water and olive oil-water interfaces. The data suggest that the bile salts penetrate into, weaken, and break up the interfacial beta-lactoglobulin networks. AFM images of the displacement of spread beta-lactoglobulin at the air-water and the olive oil-water interfaces suggest that displacement occurs via an orogenic mechanism and that the bile salts can almost completely displace the intact protein network under duodenal conditions. Although the bile salts are ionic, the ionic strength is sufficiently high to screen the charge allowing surfactant domain nucleation and growth to occur resulting in displacement. The morphology of the protein networks during displacement is different from those found when conventional surfactants were used, suggesting that the molecular structure of the surfactant is important for the displacement process. The studies also suggest that the nature of the oil phase is important in controlling protein unfolding and interaction at the interface. This in turn affects the strength of the protein network and the ability to resist displacement by surfactants.

A new view of pectin structure revealed by acid hydrolysis and atomic force microscopy

Individual pectin polymers and complexes, isolated from the pericarp of unripe tomato (Lycopersicon esculentum var. Rutgers), were subjected to a mild acid hydrolysis and visualised and characterised by atomic force microscopy (AFM). The AFM images confirm earlier studies showing that individual pectic polysaccharides often possess long branches. The AFM data have been used to construct size and molecular weight distributions for the single molecules and complexes, from which the calculated number-average and weight-average molecular weights can then be compared directly with the published literature data on the rheology of bulk samples. Loss of the neutral sugars arabinose, galactose and rhamnose from the pectin samples does not significantly alter either the size or the branching density of the individual polymers, but is reflected in a breakdown of the complexes. Significant loss of galacturonic acid at long hydrolysis times was found to be accompanied by changes in the size and branching of the single polymers and further breakdown of the complexes. The results suggest that rhamnose, arabinose and galactose are not the major components of the individual polymers but are, instead, confined to the complexes. The polysaccharides represent a previously unrecognised branched homogalacturonan with a minimum mean size some three times larger than that previously reported. The complexes consist of homogalacturonans (HGs) held together by rhamnogalacturonan I (RG-I) regions. Comparison of the rate of depolymerisation of the homogalacturonans and complexes with the published data on changes in the intrinsic viscosity of bulk pectin samples, subjected to similar acid hydrolysis, suggests that the different rates of depolymerisation of RG-I and HG contribute separately to the observed changes in intrinsic viscosity during acid hydrolysis. Thus data obtained using a single molecule microscopy technique provides new insights into the behaviour in the bulk.