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Diffusion coefficients are utilised to provide a realistic approach in the quantification of mass transport phenomena, which are important for the delivery of bioactivity from high-solid biopolymer systems. Following this mechanistic consideration, we employ spray drying to produce microcapsules of waxy maize starch that suspend homogeneously tocopheryl acetate. An array of physicochemical techniques including dynamic oscillation in-shear or compression mode, microDSC, WAXD, FTIR, scanning electron or optical microscopy, and UV-vis assays were utilised to relate structural properties of the macromolecular network to diffusivity of the bioactive compound. A modified version of the free-volume theory designed to address molecular diffusion was evaluated over a wide temperature range. Predictive capabilities were facilitated by proposing a mathematical relationship between diffusion coefficient of tocopheryl acetate and fractional free volume of waxy maize starch. Moreover, the theoretical approach is able to accurately describe the extent of cooperativity in the vitamin-biopolymer interaction that determines transport kinetics.

This study optimises a protocol for the estimation of α-amylase activity in a condensed starch matrix in the vicinity of the glass transition region. Enzymatic activity on the vitrified starch system was compared with that of a reference substrate, maltodextrin. The activity was assayed as the rate of release of reducing sugar using a dinitrosalicylic acid procedure. The condensed carbohydrate matrices served the dual purpose of acting as a substrate as well as producing a pronounced effect on the ability to enzymatic hydrolysis. Activation energies were estimated throughout the glass transition region of condensed carbohydrate preparations based on the concept of the spectroscopic shift factor. Results were used to demonstrate a considerable moderation by the mechanical glass transition temperature, beyond the expected linear effect of the temperature dependence, on the reaction rate of starch hydrolysis by α-amylase in comparison with the low-molecular weight chain of maltodextrin.

Chia seed gum (CSG) was extracted from the seed coat of Salvia hispanica, purified in the laboratory and its chemical composition and functional properties were investigated. CSG was found to comprise 93.8% carbohydrate consisting of xylose, glucose, arabinose, galactose, glucuronic acid and galacturonic acid as monosaccharide units. The presence of uronic acids was reflected in the anionic behavior of the CSG solution over a wide range of pH (≥ 1.8). The solubility of CSG increased slightly with temperature and pH of the aqueous medium. CSG was able to resist pyrolytic decomposition at temperatures well in excess of 250 °C, and exhibited a high water holding capacity (23 times of its own weight). The surface activity and emulsifying properties of CSG were found to be either superior or comparable to other common gums and industrial polysaccharides indicating the potential of CSG as an effective thickener and stabilizer of processed foods.

The work dealt with the diffusional mobility of thiamin embedded in a high-solid matrix of κ-carrageenan with glucose syrup. It utilized thermomechanical analysis in the form of modulated differential scanning calorimetry and small-deformation dynamic oscillation in shear, Fourier transform infrared spectroscopy, wide angle X-ray diffraction, scanning electron microscopy and UV-vis spectrophotometry. The structural properties of the matrix were assessed in a temperature induced rubber-to-glass transformation. A thiamin-dye binding assay was employed to monitor the diffusion process of the vitamin from the high-solid preparation to ethylene glycol. The relationship between mechanical properties of the carbohydrate matrix and vitamin mobility was assessed via the application of the combined framework of the free volume theory and the predictions of the reaction rate theory. Results argue that the transport of the micronutrient is governed by the structural relaxation of the high-solid matrix. These were further treated with the concept of Fickian diffusion coefficient to provide the rate of the bioactive compound mobility within the present experimental settings.

The physicochemical properties of lentil starch were measured and linked up with its functional properties and compared with those of corn and potato starches. The amylose content of lentil starch was the highest among these starches. The crystallinity and gelatinization enthalpy of lentil starch were the lowest among these starches. The high amylose: amylopectin ratio in lentil starch resulted into low crystallinity and gelatinization enthalpy. Gelatinization and pasting temperatures of lentil starch were in between those of corn and potato starches. Lentil starch gels showed the highest storage modulus, gel strength and pasting viscosity than corn and potato starch gels. Peleg's model was able to predict the stress relaxation data of these starches well (R(2)>0.98). The elastic modulus of lentil starch gel was less frequency dependent and higher in magnitude at high temperature (60 °C) than at lower temperature (10 °C). Lentil starch is suitable where higher gel strengthened pasting viscosity are desired.

The structural properties of five ethyl cellulose polymers (EC) in mixture with the non aqueous solvent of propylene glycol dicaprylate (PGD) have been investigated with a view to facilitating topical drug delivery. Total concentration of the polymer varied from 12 to 20% (w/w) and its weight average molecular weight was in the range of 64–223 kDa. Experimental techniques utilised were small-deformation dynamic oscillation in shear, attenuated total reflectance Fourier transform infrared spectroscopy (ATR FTIR) and wide-angle X-ray diffraction. The nature of the time and temperature dependence on viscoelastic functions of all samples could be treated with the Chambon and Winter criterion allowing determination of the critical gelation temperature in relation to polymer concentration and molecular weight. A further refinement of this school of thought allowed correlation between the viscoelastic relaxation exponent and fractal dimensions in thermally treated EC/PGD matrices. Tangible evidence on the packing arrangements of the ethyl cellulose network and the molecular nature of its interaction with the non aqueous solvent as a function of polymer concentration and molecular weight was provided by X-ray diffraction and infrared studies.

The structural properties and morphology of mixed gels made of aqueous preparations of agarose and whey protein were modified by changing thermal treatment and pH. The conformationally dissimilar polymers phase separated and this process was followed by small-deformation dynamic oscillation in shear, differential scanning calorimetry and environmental scanning electron microscopy. Experimental protocol encourages formation of a range of two-phase systems from continuous agarose matrices perforated by liquid-like whey protein inclusions to phase inverted preparations where a soft protein matrix suspends hard agarose-filler particles. These distinct morphologies have widely different mechanical moduli, which were followed by adapting a theoretical analysis (isostress–isostrain and Lewis–Nielsen blending laws) from the literature in synthetic block polymers and polyblends. Based on this framework of thought, reasonable predictions of the elastic moduli in the composite gels were made that led to patterns of solvent partition between the two polymeric networks. It was shown that proteins, in mixture with polysaccharide, exhibit favorable relative affinity (P-factor) for water molecules at a pH above their isoelectric point. This is an unexpected outcome that adds to the central finding of a single P value for the distribution of solvent between the continuous matrix and discontinuous inclusions of binary gels. It was thus proposed that phase continuity and solvent distribution in agarose/whey protein systems are under kinetic control that can be heavily governed by pH changes in the aqueous environment.

The structural properties of ethyl cellulose and propylene glycol dicaprylate mixtures were investigated with a view to facilitating use of the system as excepient for topical drug delivery. The working protocol included small-deformation dynamic oscillation in combination with the principle of time–temperature superposition, micro and modulated differential scanning calorimetry, wide-angle X-ray diffraction patterns, infrared spectroscopy, and optical profile analysis in the form of gel particle roughness. In contrast to thermoreversible gelation upon heating of aqueous ethyl cellulose solutions reported widely in the literature, replacing water with propylene glycol dicaprylate and mixing with the polymer yields gels that revert to the solution state with increasing temperature. Time effects were also probed; the continuous increase in viscoelasticity of preparations as a function of time of observation at ambient temperature was accompanied by structural disintegration of the polymeric particles. This was rationalized by proposing that specific polymer–solvent interactions result with aging in particle erosion and the release of polymeric strands that are able to form a three-dimensional structure. It was thus documented that the time–temperature equivalence was active in the system producing a rubbery state in the master curve of viscoelasticity, which extends from ambient to subzero temperatures and should facilitate pharmaceutical applications.

The structural properties of the glass dispersion in agarose, κ-carrageenan and deacylated gellan with co-solute (glucose syrup) at 80.0% (w/w) solids were studied. Investigative techniques were small-deformation stress relaxation and dynamic oscillation on shear. Vitrification was monitored between −2 and −50 °C with continuous thermal runs and isothermal frequency or time sweeps obtained at constant temperature intervals. The time–temperature superposition principle was utilized to compose master curves. The Williams, Landel and Ferry equation was able to pinpoint the network T g for these systems as the turning point from the predictions of the free volume theory in the transition region to those of the reaction rate theory at the glassy state. Further insights into the physics of intermolecular interactions at the vicinity of T g were obtained using the coupling model of molecular dynamics in the form of the Kohlrausch, Williams and Watts function. The model described well the spectral shape of the local segmental motions in polysaccharide/co-solute samples at the short-time region of the stress–relaxation master curve. Analysis provided the intermolecular interaction constant and apparent relaxation time, which are valuable parameters for the elucidation of structural morphology at T g .

This study indicates that controlled depolymerization of plant (PC) and bacterial (BC) celluloses can be achieved by employing suitable ultrasonication settings. Size exclusion chromatography results indicate that reduction in the molecular weight of the two polymers was accompanied by a parallel drop in the polydispersity index of PC and an unexpected increase in the said index of BC. X-ray diffraction patterns of the fractionated materials were found to be Cellulose II crystals whereas experimentation on microcrystalline cellulose unveiled the Cellulose I conformation. The crystallinity index revealed no obvious changes in PC as a function of the time of sonication whereas a major increase in the crystalline component was encountered for BC. Furthermore, thermal degradation using TGA and FTIR spectra suggest that the processes of dissolution and regeneration in cuprammonium hydroxide of PC and BC followed by ultrasonication do not affect the chemical fingerprints via oxidative reactions of the cellulosic materials.

The dependence of relaxation processes, as manifest in changes of the glass transition temperature, were examined under pressure (0.1–700 MPa) for the gelatin/co-solute system (part I of this series) and, currently, for preparations of agarose, κ-carrageenan and deacylated gellan in the presence of co-solute (part II). Structural properties were monitored using modulated differential scanning calorimetry and small-deformation dynamic spectroscopy on shear. Response curves as a function of hydrostatic pressure were treated with the combined framework of reduced variables and WLF equation/free volume theory. Shift factors derived from viscoelastic spectra and empirically treated thermograms clearly demonstrate that the effect of increasing pressure is detrimental to the stability of intermolecular polysaccharide associations. Diminishing values of the glass transition temperature with increasing pressure argue that the concept of time–temperature–pressure equivalence applicable to amorphous synthetic polymers is not operational in the structural functions of high-solid polysaccharide gels.

Aqueous and high sugar gels of agarose and κ-carrageenan were prepared to investigate the changing nature of polymeric matrices. Transmission electron microscopy documented a transformation from an aggregated network in the aqueous environment to a lightly cross-linked structure in high solids. The latter possesses an elastic consistency based on the increasing values of yield strain from high- to low-water systems recorded using compression testing. The work provides further confirmation of the increased amorphicity of polysaccharide networks in high solid preparations.

Small deformation dynamic oscillation, large deformation compression and creep testing were carried out to examine the structural properties of agarose in the presence of 50:50 preparations of sucrose and glucose syrup. The content of co-solute varied from 10 to 93% in the mixture. Increasing additions of sugar transform gradually the enthalpic, brittle networks of the polysaccharide to ‘rubbery’ structures of enhanced thermal stability. At saturated levels of solids there is a phase inversion from the gel-like characteristics of the agarose network to the liquid-like response of the continuous phase of co-solute. The latter undergoes vitrification upon cooling thus exhibiting the mechanical glass transition region and the glassy state. Given time, vestigial development and eventual three-dimensional linkage of agarose helices reverts the high viscosity liquids to rubbery gels exhibiting considerable yield strain (140% deformation). The work is compared to sugar/polysaccharide mixtures in the literature with the view to rationalizing underlying molecular processes and defining universal effects.

The influence of pH on the mechanical relaxation of LM-pectin in the presence of co-solute has been investigated by means of differential scanning calorimetry, ζ-potential measurements and small deformation dynamic oscillation in shear. pH was found to affect the conformational properties of the polyelectrolyte altering its structural behaviour. Cooling scans in the vicinity of the glass transition region revealed a remarkable change in the viscoelastic functions as the polyelectrolyte rearranges from extended (neutral pH) to compact conformations (acidic pH). This conformational rearrangement was experimentally observed to result in early vitrification at neutral pH values where dissociation of galacturonic acid residues takes place. Time-temperature superposition of the mechanical shift factors and theoretical modeling utilizing WLF kinetics confirmed the accelerated kinetics of glass transition in the extended pectin conformation at neutral pH. Determination of the relaxation spectra of the samples using spectral analysis of the master curves revealed that the relaxation of macromolecules occurs within ~0.1 s regardless of the solvent pH.

A high-solid matrix of κ-carrageenan with polydextrose was developed to entrap α-linolenic acid, which is an omega-3 bioactive compound. Physicochemical analysis of this system utilised modulated DSC, dynamic oscillation in shear, ESEM, FTIR and WAX diffraction. The carbohydrate matrix was conditioned through an extensive temperature range to induce changes in molecular morphology and identify the network glass transition temperature. Thermally induced variation in phase morphology was employed to rationalise transportation patterns of the bioactive compound within the high-solid preparation. Thus, experimental observations using UV–vis spectroscopy modelled diffusion kinetics to document the mobility arresting effect of the vitrifying matrix on the micro-constituent. Within the glass transition region, results argue that free volume theory is the molecular process governing structural relaxation. Further, Less Fickian diffusion follows well the rate of molecular transport of α-linolenic acid as a function of time and temperature of observation in the condensed matrix.

Transmission electron microscopy was employed to examine the hypothesis, reached earlier using rheological and thermal analysis, of the changing nature of a polysaccharide network with increasing levels of sugar. Mixtures of deacylated gellan (

The aim of this work is to demonstrate that the phase behaviour of the gelatin/maltodextrin gel can be controlled by means of the molecular weight distribution of the polymeric constituents and the cooling rate. In doing so, the technique of small deformation dynamic oscillation was used to monitor the mechanical strength of the binary gels and their melting profiles upon subsequent heating. Introduction of high molecular weight fractions induced a transformation from weakened deswelled gels to reinforced composite gels. The phase inversion point from gelatin to maltodextrin continuous gels was manipulated by decreasing the length of molecular chains of the protein and/or increasing that of the polysaccharide. The formation of maltodextrin continuous structures was further assisted by quenching the binary solutions from high temperature as opposed to slow cooling. For a given concentration of gelatin, it was possible to reduce to a third the amount of maltodextrin required for phase inversion in the mixture.

The physical properties of deacylated and high acyl gellan systems were examined using small deformation rheology, optical rotation and differential scanning calorimetry. It is demonstrated that the drop in strength of deacylated gellan structures at high levels of counterions is caused by the formation of ordered nuclei which require low temperatures (

Small deformation dynamic oscillation and bright field microscopy were used to examine the structural properties of single and mixed high methoxy pectin and gelatin systems in the presence of sucrose/glucose syrup blends. Co-solute concentrated (≥78%) systems of the polysaccharide form rubbery structures which are readily transformed into glassy consistencies according to the time-temperature superposition principle. Increasing amounts of co-solute in the gelatin samples induce changes in viscoelasticity from that of conventional hydrogels to mechanical traces that cover much of the plateau region and the beginning of the glass transition area. Furthermore, manipulation of the protein/ sugar ratio can result in strong crystalline matrices, or viscoelastic solutions where the co-solute forms the continuous phase and the gelatin inclusions can undertake a conformational transition. The properties of the single components were used to rationalise the phase behaviour of their mixtures. Upon triggering the gelation of pectin, mixtures can be made where either gelatin or both components form a continuous phase. Results are discussed in the light of evidence obtained from the ethylene glycol work in Part I.

A sensitive in-house rheometer and two measuring configurations were employed to examine the structural properties of gellan/sugar preparations at stoichiometric levels of added calcium ions. Perforated cylinders allowed recording of the gelation process in slippery gellan gels (from 0 to 40% sucrose) whereas systems with greater amounts of co-solute could also be analysed with a flat cone-and-plate geometry. The sharp sigmoidal transition of the highly enthalpic aggregated helices in aqueous or low sucrose (up to 30%) gellan gels was observed at temperatures below 60 °C upon cooling (1 °C/min). This gave way at 60% sucrose and beyond (high solids regime of sucrose plus corn syrup) to a thermally stable structure with a low modulus monotonic temperature profile (from 90 to 5 °C). At the intermediate range of co-solute (30 to 60%) a bimodal cooling profile was recorded which was rationalised on the basis of a composite network comprising lightly cross-linked and therefore flexible gellan chains, and rigid aggregates. In the absence of external calcium, at the intermediate level of solids, only the aggregated type of network was formed at the low temperature end, apparently supported by the batch salts. Therefore, the added calcium facilitated stabilisation at the top temperature end of limited intermolecular associations between the thermally vibrant gellan chains in a sugar (>40%) environment.

The structural properties of maltodextrin in mixtures with native whey protein have been investigated using dynamic oscillation and a sequence of experimental time-temperature, frequency and strain sweeps. Cooling of the binary system results in a gelled maltodextrin matrix surrounding the liquid whey protein inclusions. Subsequent heating denatures the protein and the formation of a gelled filler changes the viscoelastic functions of the mixture dramatically. Modelling of the two distinct microstructures shows a clear transition from an isostrain condition with a solid-like maltodextrin matrix to an isostress arrangement where the protein filler forms the strongest phase. Estimation of the pattern of solvent partition between the two phases suggests that there is a linear, positive dependence between the relative amount of solvent kept in the liquid inclusions of whey protein and its polymer concentration. However, gelation of the protein immobilizes the mixed system and prevents, within the experimental timescale, solvent diffusion into the maltodextrin network, thus creating a constant value for the relative solvent distribution between the two phases.

The small and large deformation properties of commercial low fat spreads and traditional full fat products have been investigated in order to develop a background understanding of the changes in viscoelastic properties and the structural organisation occurring as a result of addition of biopolymers to the aqueous phase of low fat dispersions. Parameters have been derived from compression analysis, dynamic oscillation (frequency, strain and temperature sweeps) and creep compliance testing. It seems that the direct replacement of fat with a biopolymer-structured aqueous phase does not imitate the plastic rheology of butter and margarine. Thus, the ratio of plastic to maximum stress ( σ p σ m ) of butter and margarine is substantially higher (0.96–1.0) than the ratio of inflectional to maximum stress ( σ i σ m ) of commercial low fat spreads with a strong, gel-like character (up to 0.83). Additionally, some commercial embodiments with reduced amounts of structural components have stress-strain profiles resembling those of viscous solutions instead of a plastic product. Dynamic oscillatory measurements have characterised the mechanical properties of water-continuous low fat spreads. Dispersions reproduced the mechanical profile of three-dimensional biopolymer gels with a high elastic component (tan δ ≈ 0.04) and a substantial linear response to increasing amplitude of oscillation (up to 10% deformation). Products comprising hydrolysed starch as one of the functional ingredients show long melting profiles upon heating, which contrast strongly with the ‘melt in the mouth’ properties of butter. In accordance with the above, butter requires lower initial strain to exhibit negligible recovery of shape after the removal of stress, than do commercial low fat spreads with a pronounced elastic element, during a creep compliance experiment.

An investigation of the ability of the enzyme α- d -glucosidase to act on the substrate 4-nitrophenyl α- d -glucopyranoside (pNPG) while embedded in glassy carbohydrate matrices (deacylated gellan with polydextrose and polydextrose alone) is presented. Physicochemical characterisation of the matrices was achieved using the techniques of modulated differential scanning calorimetry, small deformation dynamic oscillation on shear, Fourier transform infra-red spectroscopy, wide angle X-ray diffraction and scanning electron microscopy. A UV–vis spectrophotometric procedure was adapted for the analysis of the activity of α- d -glucosidase in hydrolysing pNPG in the condensed carbohydrate systems. In order to derive a relationship between the structural properties of the matrix and the enzymatic activity, mechanical spectra were recorded using the combined framework of the Williams, Landel and Ferry equation with the time–temperature superposition principle. Theoretical modelling and experimental observations strongly argue for a pronounced effect of the gelling polysaccharide/co-solute mixture on enzymatic activity near the mechanical Tg of the matrix.

The accepted mechanism of alginate gelation at ambient temperature is by formation of egg-box junctions between pairs of polyguluronate chain sequences and an included array of site-bound calcium ions. We now present evidence (circular dichroism and low-amplitude mechanical spectroscopy data) of similar regular interactions at high temperatures where all the active ingredients (alginate, salt and sequestrant) are hydrated in a hot medium. During controlled cooling, a second transition is obtained at low temperatures, which may be attributed to the lateral interaction of dimeric chains and thus the development of an extended, three-dimensional network. A subsequent temperature increase melts the second wave of structure formation, thus producing substantial thermal hysteresis, but preserves the ‘weak gel’ properties of the alginate-calcium reaction observed at elevated temperatures. At levels of salt higher than 40% calcium conversion aggregation can occur even at 90°C, leading to precipitation rather than a homogenous network. Prolonged refrigeration (72 h at 4–5°C) sustains a gradual molecular rearrangement to a self-supporting (visual evidence), firmer (frequency sweeps) network. Surprisingly, the strongest high guluronate alginate is not only more elastic than its low molecular weight counterpart (strain sweeps in dynamic oscillation) but it also exhibits higher yield strain on compression testing than the high molecular weight, high mannuronate sample. The distinct differences in the long-range properties of high guluronate and high mannuronate alginate samples with comparable molecular weights might serve as a guide for the development of reformed products with the desired characteristics.

The gel properties of high-methoxy pectin from pumpkins have been investigated to assess the potential of this material as a hard-currency export from the former Soviet Union. Comparison was made with commercial slow-set, medium-set and rapid-set pectins from citrus peel. Gels were formed by cooling pectin solutions (pH 3·0; 60% (w/w) sucrose; 5% (w/w) corn syrup) from 95°C to 25°C, and the time-temperature course of network formation was monitored by small-deformation oscillatory measurements of storage modulus (G′). At concentrations above ∼1% (w/w) the pumpkin pectin gave weaker gels than the other three samples, but its minimum critical gelling concentration (c0) was found to be much lower (by at least a factor of five). Compression testing gave similar results, with pumpkin pectin giving useful breaking-stress (‘hardness’) at concentrations down to ∼0·5% (w/w), about a factor of two lower than for the citrus samples. Its gelation was also less rapid, giving G′ values below those of the other three samples at temperatures down to ∼60°C, but then setting sharply; this behaviour could be useful in avoiding ‘pregelation’ in commercial processing. The commercial slow-set pectin showed typical ‘weak gel’ properties in the solution state at 95°C, with systematic reduction in gel-like character with increasing ester content in the other samples. The rigidity of the final gels also decreased systematically through the series: rapid-set

The method of dynamic oscillation was employed to investigate changes in the mechanical properties of deacylated gellan gels in the presence of sucrose-corn syrup blends. The effect of sugar, at concentrations between 0 and 50% in the mixture, is to raise gradually the gelling temperature and the mechanical strength of the gellan network. However, there is a sharp increase in thermal stability (formation of a structure at the highest experimentally accessible temperature of 90°C) and substantial reduction in shear modulus development at 60% co-solute in the system which is discussed in terms of a disproportionate conformational ordering and diminishing water-supported, intermolecular junction zones of the gellan strands in a low solvent environment. The dramatic change in physical properties of the gellan-solute blends at 90°C produces a spectacular temperature dependence of the in-phase and out-of-phase viscoelastic components during subsequent cooling. Thus, the high-solids gellan network undergoes transformation from a rubber-like consistency to a glassy state in the manner observed for entangled networks of high molecular weight synthetic polymers. The onset of glass transition is enhanced by increasing amounts of sugars, occurring at about 37°C for the 0·5% gellan plus 85% solute system at an experimental frequency of 10 rad/s. Mechanical spectra of sucrose-corn syrup mixtures at this concentration remain Newtonian at 5°C, the lowest experimentally accessible temperature. According to the time-temperature superposition principle, vitrification of the above gellan sample commences at ≈17 Hz at 90°C. Following the empirical procedure developed by Williams, Landel and Ferry, the temperature at which the gellan-solute glass forms (Tg) was found to be close to the vitrifiication temperature of a sucrose preparation at 15% moisture content (≈ −25°C).

Bacterial levan, a highly branched, high molecular weight polymer of fructose, was purified from culture supernatants of Pseudomonas syringaepv.phaseolicola grown in a liquid high-sucrose medium, and the predominance of β-(2 → 6) linkages was confirmed by 13C NMR. The solution properties of this material resembled those of disordered linear polysaccharides in the response to low-amplitude oscillatory shear (frequency dependence of G′ and G″); the absence of any detectable conformational change with temperature (as monitored by optical rotation); close superposition of steady-shear viscosity (η) and complex dynamic viscosity (/gh∗) at equivalent values of shear-rate (γs−1) and frequency (ωrad s−1); a similar form of shear-thinning (giving linear plots of η versusηγ0·76); and the onset of semi-dilute behaviour at a closely comparable degree of space-occupancy (c[η] ≈ 3·6). The intrinsic viscosity, however, was unusually low ([η] ≈ 0·17 dl g−1) and the concentration dependence of ‘zero-shear’ viscosity in the semi-dilute regime unusually high (η0 ∼ c9·3), as anticipated from the densely packed, branched molecular structure. Solutions of levan and pectin, matched to approximately the same initial viscosity, showed a substantial reduction in viscosity when mixed. Similar behaviour was observed for mixed solutions of levan with locust bean gum (LBG), chosen for its structural similarity to cellulose and hemicelluloses of the plant cell wall. Viscosity reduction was eliminated at low concentrations (indicating that it does not arise from heterologous association), but became very pronounced at high concentrations, and was then accompanied by resolution into levan-rich and LBG-rich solution phases. This evidence of strong thermodynamic incompatibility and exclusion behaviour with (1 → 4)-linked plant polysaccharides suggests that the primary role of levan during pathogenesis may be as a barrier to intimate morphological contact (recognition) between plant cell walls and those of the parasite, thus inhibiting initiation of a hypersensitive response by the host.

Co-gels of potato maltodextrin Paselli SA-6 with gelatin were prepared by rapid quenching of mixed solutions from 90°C. At fixed setting temperature and fixed concentration of gelatin, the time required to form a self-supporting network showed an initial steady decrease with increasing concentration of SA-6 (as expected from polymer exclusion), but then increased dramatically before again decreasing. The interpretation of this behaviour as phase inversion from a gelatin-continuous network with SA-6 inclusions to a (more slowly-forming) SA-6 network with gelatin inclusions was confirmed by differential scanning calorimetry (showing both components melting separately, with no evidence of specific interaction), mechanical spectroscopy (showing that the mixed gel network was destroyed completely by melting of the gelatin component at low concentrations of SA-6, but only weakened at SA-6 concentrations above the inversion point) and by light microscopy (showing the expected changes in distribution of the two polymers). In similar studies using the faster-gelling potato maltodextrin Paselli SA-2, microscopy and gel-melting profiles again showed phase-inversion from a gelatin-continuous network at low concentrations of SA-2 to a maltodextrin-continuous network at higher concentrations. Inversion, however, occurred at a lower concentration of maltodextrin than in the gelatin/SA-6 systems, and the accompanying change in gelation rate was confined to a sharp decrease in the dependence of gel-time on SA-2 concentration.

The effect of thermodynamic incompatibility in mixed solutions of gelatin and Paselli maltodextrins SA-6 and SA-2 has been studied at a temperature (45°C) where the individual polymers are stable as disordered coils. Concentrated mixtures of SA-6 and gelatin showed classic phase separation into two co-existing liquid layers, with compositions lying along a well-defined binodal. On decreasing SA-6 concentration below the binodal, however, a substantial proportion (up to 60%) of the maltodextrin was precipitated, with normal single-phase solutions occurring only at much lower concentrations of both polymers. SA-2 showed a more extreme version of the same behaviour, with precipitation of up to 100% of the maltodextrin and no evidence of co-existing liquid phases at any accessible concentrations. In both cases, the amount of maltodextrin precipitated was proportional to the square of its initial concentration and to the first power of gelatin concentration, indicating that gelatin drives self-association and aggregation of maltodextrin when both polymers are present in a single liquid phase. 1 H NMR showed the precipitated maltodextrin to be higher in molecular weight and in degree of branching than the material remaining in solution, and particlesize analysis indicated that the volume of the individual maltodextrin particles increased linearly with the total mass precipitated.

As a prelude to studies of co-gelation with galatin, the gelation behaviour of Paselli maltodextrins SA-6 and SA-2 (DE ≈ 6 and 2, respectively) was mapped out over the experimentally-accessible range of temperature (T) and concentration (c), using a simple visual method to determine the time required for formation of a self-supporting network (tg). For both samples, log tg decreased linearly with log c and increased linearly with T. At equivalent temperatures and concentrations, SA-2 gelled between 20 and 60 times faster than SA-6. Selected samples were monitored more rigorously by mechanical spectroscopy, taking tg as the time at which elastic response (G′) became greater than viscous response (G″). In all cases the values of tg obtained by this procedure were lower than those from visual inspection, by a constant factor of about 3·4. The concentration-dependence of gel moduli (G′) for SA-2 and for gelatin (second-extract limed ossein; LO-2) fitted accurately to the form anticipated from cascade theory for normal polymer networks. For SA-6, by contrast, log G′ varied linearly with log c over the entire range at which measurements could be made, indicating a different mechanism of structure-formation (such as the agglomeration of short, aggregated helices).

The storage moduli ( G ′) of phase-separated co-gels formed by quench-cooling mixed solutions of gelatin and potato maltodextrin (Paselli SA-2 and SA-6) have been related quantitatively to the experimentally-determined concentration-dependence of G ′ for the constituent polymers. Distribution of water between the phases was examined explicitly by using polymer blending laws to derive calculated moduli for gelatin-continuous and maltodextrin-continuous networks over the entire range of solvent partition. Allowance was made for the direct contribution of polymer chains, and for density differences between the phases, in calculating relative phase-volumes. The effect of gel formation prior to phase-separation was calculated using classical theory for network de-swelling. Good agreement with observed moduli for more than 30 gelatin/maltodextrin combinations was achieved using a single adjustable parameter, p (the ratio of solvent to polymer in the gelatin phase divided by the same ratio for the maltodextrin phase), with an optimum value of p ≈ 1·8 for both SA-6 and SA-2.