Jean-Louis Doublier

Viscoelastic properties of maize starch/hydrocolloid pastes and gels

Abstract Maize starch aqueous suspensions (concentration: 4%) have been pasted in the presence of another polysaccharide (guar gum, locust bean gum or xanthan gum) at a concentration ranging from 0.1 to 0.5%. The viscoelastic behaviour of these mixed systems has been investigated by performing harmonic measurements in three steps: (i) a mechanical spectrum at 60°C, (ii) a gel-cure experiment at 25°C and (iii) a mechanical spectrum of the gel at 25°C. The starch dispersion being described as a composite, its viscoelastic properties in the pasted and in the gelled state, as well, are primarily governed by the volume fraction occupied by the swollen particles which accounts for two-thirds of the total volume. The continuous phase makes an additional contribution due to its own viscoelastic properties. The added polysaccharide is concentrated within the continuous phase which makes it play an increasing role in the viscoelastic behaviour of the composite as the hydrocolloid content is increasing. More spectacular effects are experienced with xanthan gum than with the galactoman-nans in relation to the ‘weak’ gel properties of xanthan systems. Furthermore, the mechanical spectra of the mixed systems indicate that they are less elastic than with starch alone. Complex modifications in the starch dispersion are thus brought about by the hydrocolloid; these can be ascribed to phase separation processes in relation to incompatibility phenomena between unlike polysaccharides.

A rheological investigation of cereal starch pastes and gels. Effect of pasting procedures

Abstract Wheat and maize starches have been pasted using four different pasting procedures by varying the heating and stirring rates. The flow behaviour of hot starch pastes was investigated over a wide range of concentrations (∼ 3 to 10·5%). For each type of starch each pasting procedure resulted in a specific flow behaviour. Flow curves were tentatively interpreted on the basis of swelling-solubility values, and it was demonstrated that the overall viscosity of starch pastes is primarily governed by a combination of the volume fraction of the disperse phase and the concentration and composition of the continuous phase. For the highest concentration, deformability of swollen particles seems to play a prevailing role. Concentrated starch gels also exhibited different elastic properties, depending upon pasting procedure and type of starch. Wheat starch gels were stiffer than maize gels and it was suggested that the main structural parameters involved are the deformability of the swollen particles and the amylose concentration of the continuous network.

Physico-chemical characterisation of sago starch

The physico-chemical characteristics of various sago starch samples from South East Asia were determined and compared to starches from other sources. X-ray diffraction studies showed that all the sago starches exhibited a C-type diffraction pattern. Scanning electron microscopy showed that they consist of oval granules with an average diameter around 30 μm. Proximate composition studies showed that the moisture content in the sago samples varied between 10.6% and 20.0%, ash between 0.06% and 0.43%, crude fat between 0.10% and 0.13%, fiber between 0.26% and 0.32% and crude protein between 0.19% and 0.25%. The amylose content varied between 24% and 31%. The percentage of amylose obtained by colourimetric determination agreed well with the values obtained by fractionation procedures and potentiometric titration. Intrinsic viscosities and weight average molecular weight were determined in 1M KOH. Intrinsic viscosity for amylose from sago starches varied between 310 and 460 ml/g while for amylopectin the values varied between 210 and 250 ml/g. The molecular weight for amylose was found to be in the range of 1.41×106 to 2.23×106 while for amylopectin it was in the range of 6.70×106 to 9.23×106. The gelatinisation temperature for the sago starches studied varied between 69.4°C and 70.1°C. The exponent ‘a’ in the Mark–Houwink equation and the exponent ‘α’ in the equation Rg=kMα was found to be 0.80 and 0.58, respectively for amylose separated from sago starch and these are indicative of a random coil conformation. Two types of pasting properties were observed. The first was characterised by a maximum consistency immediately followed by sharp decrease in consistency while the second type was characterised by a plateau when the maximum consistency was reached.

Micellar-casein-κ-carrageenan mixtures. I. Phase separation and ultrastructure

Abstract The phase behaviour of micellar-casein (MC)–κ-carrageenan mixtures was investigated using different ionic forms of the carrageenan (Na + , K + ) in different ionic conditions and at two temperatures. At 50°C, that is above the conformational transition temperature of the carrageenan, a macroscopic phase separation was clearly evidenced. Accordingly, ternary phase diagrams were established as a function of the biopolymer concentration for each system. By analogy with other particulate systems, it was assumed that this phase separation originated from a depletion–flocculation mechanism. The ultrastructure of the MC–carrageenan mixtures is described at 20°C using confocal laser scanning microscopy and phase contrast microscopy. All observations showed that phase separation has taken place. A casein-rich phase was dispersed in a carrageenan-rich phase at a low casein content (below 3%); above this casein concentration in the mixtures, the phase separation process yielded a casein continuous network within the carrageenan network. It was suggested that the phase separation process initiated at 50°C was almost immediately inhibited by gelation upon cooling, resulting in a biphasic system the structure of which depended upon the initial composition.

A rheological characterization of kappa-carrageenan/galactomannan mixed gels: A comparison of locust bean gum samples

Mixed gels of kappa-carrageenan and locust bean gum (LBG) obtained from different varieties of Portuguese carob trees and commercial gums were compared. The viscoelastic properties of the gels were measured using dynamic parallel-plate geometry. Mixed gels at 1·0% of total polysaccharide concentration without addition of KCl showed, whatever the LBG sample, a synergistic maximum when the ratio of kappa-carrageenan to LBG was 8020. The amplitude of this maximum varied with the LBG sample. The gels prepared at 0·3% total concentration with KCl added, showed a synergistic maximum at the same mixing ratio and the amplitude varied in a similar manner. Each sample was fractionated into the fraction soluble at 25°C and the fraction soluble at 90°C. Mixed gels of kappa-carrageenan with cold-water-soluble and hot-water-soluble fractions, and also with tara gum and guar gum were prepared at the 8020 ratio. It was found that the synergistic maxima were related to the intrinsic viscosity and the M/G ratio. A linear relationship between the storage modulus G′max at the synergistic maximum and the product of the intrinsic viscosity and the square of the mannose to galactose ratio was found, suggesting that the synergistic mechanism can be ascribed to both the unsubstituted (galactose-free) regions of the galactomannan and the molecular weight.

Effect of thermomechanical treatment on the rheological properties of crosslinked waxy corn starch

Abstract Cross-linked waxy maize starch (CWMS) are currently used as thickening or gelling agents in the food industries under high heating temperature and high shear conditions such as occurs, for example, in homogenizers. The rheological properties of CWMS dispersed in water have been studied in relation to its swelling behaviour. The main parameters that have been dealt with are starch concentration (2–4%), pasting temperature (90–130 °C) and shear conditions (∼10 5 s −1 ). The swelling behaviour was assessed by means of swelling experiments and particle size analysis. The rheological study was performed by means of steady shear measurements (viscometry) and in oscillatory shear (viscoelasticity). The rheological properties of CWMS suspensions were strongly dependent on the concentration regime of the suspension. A yield volume fraction of ∼0.4 was found to be critical. Beyond this value, starch dispersions exhibited the behaviour of suspensions of swollen particles, that means shear-thinning behaviour with a yield stress and measurable viscoelastic properties, typical of a gel-like system. All these properties strongly depended on the pasting temperature with an optimum around 124 °C for which the starch suspension was considered as “well-cooked”. Sensitivity of starch granules to shear forces was also strongly dependent on the pasting temperature. When starch granules were undercooked, their swelling properties, and hence their rheological properties were reinforced by additional high shearing. In contrast, when the starch granules were well-cooked or overcooked the rheological properties were depressed by shearing as a result of their high fragility. These overall results allowed evaluation of the role of the combined effect of heating temperature (above 100 °C) and shear conditions on the swelling behaviour and rheological properties of suspensions of CWMS in the formulation of starchy products.

Viscoelastic properties of xanthangalactomannan mixtures: comparison of guar gum with locust bean gum

The rheological behaviour of xanthanguar gum systems has been investigated and compared to that of xanthanlocust bean gum mixtures using oscillatory shear and creep-recovery measurements. The total polysaccharide concentration was kept constant at 0.5% w/w, the xanthangalactomannan ratio ranged from 199 to 9010 and three ionic strengths were studied. As for xanthanlocust bean gum mixtures, strong synergistic phenomena were exhibited by xanthanguar gum systems. Clearly, the addition of xanthan gum even at a very low level to a guar gum solution induced a transition of the system from a macromolecular solution to a structured system displaying gel-like properties. The comparison between three guar gum samples with different molecular weights evidenced a strong effect of the molecular weight: the higher this parameter, the stronger the synergistic interaction. At a low xanthangalactomannan ratio (< 1090), xanthanguar gum mixtures, xanthanlocust bean gum mixtures (with comparable galactomannan molecular weights) resulted in similar viscoelastic behaviour. In contrast, at a higher xanthangalactomannan ratio, a stronger synergism was exhibited with locust bean gum, as expected. The rheological properties were greatly influenced by the presence of electrolyte. In the case of xanthanguar gum systems, the storage (G′) and the loss (G″) moduli were increased when electrolyte was present, in contrast to xanthanlocust bean gum mixtures which exhibited a reverse tendency. These results provide evidence that xanthan gum plays a major role in the rheological behaviour of the xanthangalactomannan systems even at a low concentration. Overall, similar results were found with xanthanguar or xanthanlocust bean gum mixtures. However, differences in the mechanism may exist according to the mannosegalactose ratio of the galactomannan and also to the xanthangalactomannan ratio and the ionic strength.

A study of the viscosity of cellulose derivatives in aqueous solutions

Abstract The flow behaviour of two types of cellulose derivatives (carboxymethylcellulose (CMC) and hydroxyethylcellulose (HEC)) in aqueous solution was investigated at 25°C, pH 7·0 and 0·1 m (NaCl) ionic strength, within a wide concentration range (0·05–20 g litre−1) using two coaxial cylinder viscometers covering shear rate range 0·01–700 s−1. Each flow curve was characterized by the limiting viscosity at zero shear rate (η0) and, above a critical concentration cs, by a relaxation time (τ) corresponding to the shear rate above which the apparent viscosity became shear rate dependent (shear-thinning behaviour). Variations of the specific viscosity at zero shear rate, ηsp0, as a function of c[η] led to typical curves as for other polysaccharides. The curves for two CMC samples differing in their molecular weights closely super-imposed and were in agreement with the relationships previously found for alginate and λ-carrageenan, while data for HEC gave a different curve. Each of these curves exhibited two critical concentrations, c∗ and c∗∗, the significance of which is discussed. A master flow curve could be obtained for each polysaccharide using the reduced coordinates η r = η η 0 and γ r = τ × γ and described by a reduced Soong and Shen model with two parameters. The relaxation time τ was shown to vary as c4·3–4·4 for c > ce, ce being intermediate between c∗ and c∗∗.

Phase separation in dextran/locust bean gum mixtures

Abstract Dextran/locust bean gum (LBG) mixtures have been prepared and investigated with respect to their phase separation behaviour. These systems exhibited phase separation at 20 °C, the upper phase, itself biphasic, being enriched with locust bean gum but also containing dextran, whereas the lower phase contained only dextran. This lower phase was a liquid. The upper phase, which did not flow, was characterized by means of rheological dynamic measurements. Clearly, its behaviour was typical of a gel, the three-dimensional structure of which can be ascribed to self-association of LBG chains owing to the very high concentration of the galactomannan in this upper phase. The self-association of the galactomannan was confirmed by fluorescence microscopy carried out on mixtures containing fluorescein isothiocyanate (FITC)-labelled dextran. The rheological behaviour of a concentrated LBG solution was also investigated as a function of time, clearly showing progressive formation of a weak gel structure.

Effect of galactomannan addition on the thermal behaviour of κ-carrageenan gels

Abstract Gelation/melting cycles of κ-carrageenan/galactomannan (guar, tara and locust bean gums) binary systems have been studied by measuring dynamic rheological parameters. Two experimental conditions were used, (i) the total polysaccharide concentration was kept at 1% and the κ-carrageenan/galactomannan ratio fixed at 4:1 and (ii) the κ-carrageenan concentration was fixed at 0·75% and the galactomannan content varied from 0% to 1·2%. A thermal hysteresis was observed for all mixed systems and was found to depend on the galactomannan used. From a comparison of the gelation temperature (Tg) and melting temperature (Tm) to values obtained with κ-carrageenan alone, it was suggested that galactomannan interferes with gel structure by the formation of a secondary network provided that the M/G ratio is high enough.