Robert K. Richardson

Effect of xanthan on the small-deformation rheology of crosslinked and uncrosslinked waxy maize starch

Abstract Small-deformation oscillatory measurements have been used to characterise the effect of ordered xanthan (in 0.1 M KCl) on the rheological properties of gelatinised waxy maize starch (crosslinked and uncrosslinked). The uncrosslinked material shows a single DSC endotherm on heating, centred at ~70 °C. The overall enthalpy change on gelatinisation of the crosslinked starch is the same (ΔH = 13.2 J/g), but occurs in two endothermic processes (centred at ~60 and ~74 °C), tentatively attributed to thermal dissociation of, respectively, chemically modified and native granule structure. The thermal transitions for both samples are complete by 80 °C, which was therefore chosen as the maximum temperature in the rheological investigations, to minimise loss of granule integrity and release of starch polysaccharides into the xanthan matrix. Both samples, after gelatinisation in water, gave gel-like mechanical spectra at volume fractions below 20%, suggesting association between the swollen granules, in addition to the steric interactions which become dominant at higher concentrations. The magnitude of the increase in moduli observed on progressive addition of starch (1–5 wt%) to solutions of xanthan (0.25 or 0.50 wt%) is far too great to be explained by increased concentration of the xanthan phase on swelling of the starch granules. Comparison with the concentration-dependence of moduli for starch alone, however, suggests that xanthan acts by promoting association between the gelatinised granules, possibly by a depletion flocculation mechanism.

Non-linear viscoelasticity of polysaccharide solutions. 2: Xanthan polysaccharide solutions

Abstract The viscoelastic properties of aqueous solutions of the microbial polysaccharide xanthan have been studied using a wide range of rheological experiments including small deformation oscillatory, steady and transient shear techniques. Part I of this series described the classic ‘random-coil’ polymer response of guar galactomannan solutions under these experimental regimes. The high sensitivity of xanthan solutions to strain and to shear (strain) rate confirm previous observations that its rheological properties are more akin to those of a ‘weak gel’ than of a more conventional entanglement polymer solution, as exemplified by guar. In particular the strain at the peak shear stress γ ∞ (‘overshoot strain’) was found to be around 0.5, and with a minimal peak in the normal stress at γ N ⋍ 1 compared with values of γ ∞ around 2–3 and very high normal stress values of γ N for entanglement coupled semi-dilute solutions (including guar/water) and synthetic polymer melts/solutions. Stress relaxation measurements reveal extremely long (gel like) modes of residual stress in the relaxation spectrum. The observations are consistent with earlier data, and the postulated model of more specific inter-chain couplings between xanthan molecules occurring in solution.

Characterization of the perceived texture of thickened systems by dynamic viscosity measurements

Abstract Previous attempts to correlate perceived thickness in the mouth with shear viscosity (e.g. from rotational measurements) have worked well for ‘true’ solutions (i.e. fluid materials which do not exhibit a so-called ‘yield stress’) but underestimate the oral thickness of ‘weak gels’ (materials such as ketchup and mayonnaise, which show gel-like properties at rest but flow under sufficient stress). We now report that small deformation measurements of dynamic viscosity (η*) under oscillatory shear at a single frequency (~50 rad/s) correlate directly (r2 = 0.95) with panel scores for the perceived thickness of both true solutions and weak gels. Contrary to previous suggestions that perceived ‘sliminess’ decreases with increased ‘shear thinning’, we show that panel scores for ‘sliminess’ (and for ‘stickiness’) are also directly correlated with objective values of η* at ~50 rad/s, irrespective of the extent of shear thinning. The origin of this apparent conflict of evidence is traced to the experimental design used in the earlier studies.

Steric exclusion phenomena in gellan/gelatin systems I. Physical properties of single and binary gels

Abstract The scientific and technological aspects of gellan/gelatin systems were investigated under conditions of low pH and in the presence of a wide range of cations so that the normally important electrostatic interactions between the two polymers should be suppressed. The structural and textural characteristics of mixed gels were monitored using the techniques of mechanical spectroscopy, differential scanning calorimetry and compression testing. Under such conditions where positive interactions between the two components would be minimized, it was found that results could be described on the basis of thermodynamically incompatible polymers which gel independently in their respective phases. Intelligent manipulation of polymer concentration and ionic strength promotes phase inversion from a gellan continuous phase to a system where gelatin forms the supporting matrix. The knowledge gained in this study might assist in developing food products with novel textures.

Non-linear viscoelasticity of polysaccharide solutions. 1: Guar galactomannan solutions

The viscoelastic properties of aqueous solutions of the polysaccharide guar galactomannan have been studied using dynamic oscillatory, steady and transient shear methods. At the concentrations studied, e.g. 1, 2 and 3% w/w, ([η] = 8.8) a typical ‘entanglement’ network response has been observed, with liquid-like behaviour at low frequencies, linear strain dependence up to ∼0.4 and Cox-Merz superposition of η∗(ω) and η(γdot). The evolution of shear stress during start shear experiments (stress overshoot) has been observed and the peak strain (γ∞) found to be independent of shear rate and within experimental error, independent of concentration, having a value of ∼2.1 strain units. For the highest concentration used, (3% w/w), the normal stress overshoot was also observed and the peak strain value found to be ∼20 strain units with significant dependence on shear rate. Stress relaxation measurements have shown measurable residual stress decaying within a few seconds to baseline noise. The effect of mechanical history on sample response was examined by monitoring the stress overshoot ratio after known ‘recovery’ times; this was found to stabilize after times of the order of 102 s, suggesting all of the temporary network structure could reform in this time. The results obtained are discussed in terms of theories for the molecular viscoelasticity of polymer solutions well known to workers in the area of synthetic polymers, but so far applied only to a very limited extent to aqueous polysaccharide solutions.

Mechanical properties of globular proteins gels: 1. Incipient gelation behaviour

Abstract It has long been known that globular protein molecules in concentrated aqueous solution can be converted into a different form by heating or use of denaturing agents. Under certain conditions of pH and ionic strength, elastic gels are formed. This work describes kinetic measurements of viscosity and elasticity close to the gel point (sol-gel transition) in bovine serum albumin solutions. These studies and optical rotation measurements near the gel point lend support to the conclusions of earlier structural studies on this system, that the gel is fibrillar in nature.

Solution rheology of mesquite gum in comparison with gum arabic

Abstract Commercial samples of mesquite gum and food-grade gum arabic were purified by filtration, alcohol precipitation, and extensive dialysis, and their Theological properties were characterised over the full range of concentrations at which solutions could be prepared (up to ~50% w/w). Both gave typical solution-like mechanical spectra, with close Cox-Merz superposition of η (⋗g) and η ∗ (ω) and only slight shear thinning at the highest accessible concentrations, and (ln η rel ) c varied linearly with log c from below 2% w/w to above 50%. The intrinsic viscosity of mesquite gum ([η]≈ 0.11 dl g −1 ) was appreciably lower than that of gum arabic ([η]≈ 0.19 dl g −1 in 0.1 m NaCl at 20 °C), and was independent of ionic strength above I ≈ 0.05, indicating a compact structure capable of only limited contraction. Departures from dilute-solution behaviour (η ~ c 1.4 ) occurred at c [η]≈ 1 for both materials, with a progressive increase in concentration dependence at higher space-occupancy, behaviour typical of soft, deformable particles, rather than of interpenetrating macromolecules. The increase in viscosity with increasing concentration was steeper for mesquite, consistent with evidence from size-exclusion chromatography and dynamic light scattering that the larger (and presumably more deformable) ‘wattle blossom’ component of gum arabic was absent from the mesquite gum sample.

Mechanical spectroscopy of filled gelatin gels

SummarySmall deformation mechanical measurements were performed on the model system of glass filled gelatin gels. By using well characterised samples of glass spheres, rods and irregular pieces of intermediate shape, e.g. plates at varying phase volume of filler, the effect of size, shape and phase volume on the shear storage and loss moduli could be investigated.

Application of polymer blending laws to starch-gelatin composites

Abstract The small-deformation rheology of a range of starch-gelatin composites has been studied, to test the applicability of the isostress and isostrain blending laws to biphasic biopolymer networks. Waxy maize starch (crosslinked and uncrosslinked) was used to minimise release of soluble material into the gelatin phase, and was gelatinised at 80 °C. Phase volumes were determined by measurement of the increase in concentration of the gelatin phase due to swelling of the starch granules. The swelling volumes obtained for the crosslinked and uncrosslinked samples were 9.0 ml/g and 9.65 ml/g, respectively, and were independent of gelatin and starch concentration over the range used (0.88–1.50 wt% gelatin; 1–5 wt% starch). The gelatin-gelatinised starch composites and gelatin-phase samples isolated by centrifugation were gelled by quenching to 5 °C and measured under a controlled time-temperature regime (150 min at 5 °C; heated to 20 °C at 0.25 °/min). The value of the storage modulus (G′) of the gelatinised starch granules, which is the only unknown parameter, was varied by a standard minimisation procedure to obtain the best least-squares fit between observed values of log G′ for the composite gels and those calculated by the polymer blending laws. Good agreement between observed and calculated values was obtained using the isostress model when the trial value of the starch modulus was higher than that of the gelatin phase, and the isostrain model for the converse situation. The fitted moduli for the swollen granules (730 Pa and 55 Pa for the crosslinked and uncrosslinked samples, respectively) are close to those observed experimentally for the same starches gelatinised in water to a packing fraction of ~75%.

The rubber-to-glass transition in high sugar agarose systems

The small and large deformation properties of agarose in the presence of high levels of sugar were investigated. Mixtures can be described as lightly cross-linked rubbers, which undergo vitrification upon cooling. The combined Williams–Landel–Ferry (WLF)/free volume framework was used to derive the glass transition temperature, the fractional free volume, and the thermal expansion coefficient of the glass. Sucrose-rich cosolute crystallizes, but addition of the polymer encourages intermolecular interactions, which transform the mixture into a high viscosity glass. The mechanical properties of glucose syrup, a noncrystalline sugar, follow WLF behavior in the glass transition region and revert to an Arrhenius-type prediction in the glassy state. Measurements on sugar samples and agarose–sugar mixtures were resolved into a basic function of temperature alone and a basic function of frequency (time) alone. The former traces the energetic cost of vitrification, which increases sharply with decreasing temperature. The latter, at long time scales, is governed by the infinite molecular weight of the agarose network. In the region of short times, the effect of free volume is active regardless of the sample composition.