Bernard Launay

Viscosity of locust bean, guar and xanthan gum solutions in the Newtonian domain: a critical examination of the log (ηsp)o-log c[η]o master curves

The viscosity in the low shear rate Newtonian domain of three biopolymers, locust bean gum, guar gum and xanthan gum was studied as a function of temperature and of polymer concentration in various aqueous solvents. The intrinsic viscosities [η]o of both galactomannans are not modified in the presence of 10 or 40% sucrose. In this case, a master curve relating the Newtonian specific viscosity (ηsp)o, to the reduced concentration c[η]o is obtained and allows (in good agreement with theoretical conjectures), two critical concentrations C∗ and C∗∗ to be defined, from which the value of the expansion coefficient α may be estimated. For xanthan, as expected for a polyelectrolyte, [η]o depends strongly on salt concentration and on added sucrose and the results did not obey the above-mentioned master curve. However, it is shown that (ηsp)o depends only on xanthan concentration when C > C∗∗, and then it is assumed that chain dimensions have attained their unperturbed values whatever the solvent. Considering that both types of chains, random coils (galactomannans) and semi-rigid (xanthan) should give the same (ηsp)o-C[η]o master curve for C > C∗∗ when [η]o is replaced by its unperturbed counterpart [η]θ, a method for estimating [η]θ for the xanthan sample is proposed. In conclusion, the numerous exceptions to the widely accepted (ηsp)o vs C[η]o “universal” behaviour are mainly ascribed to significant differences in expansion coefficient values which depend on both the polymer and the solvent.

Xanthan/locust bean gum interactions at room temperature

Abstract Rheological and ultracentrifugation studies have been conducted on heated and unheated mixtures of xanthan with whole locust bean gum, and temperature fractions of the latter possessing different mannose: galactose ratios. The results suggest that xanthan and galactomannans may interact by two distinct mechanisms. One takes place at room temperature, gives weak elastic gels, and has little dependence upon the galactose content of the galactomannan, whilst the second requires significant heating of the polysaccharide mixture, gives stronger gels, and is highly dependent upon galactomannan composition. The results are discussed with reference to existing models proposed for the xanthan/galactomannan interaction.

Gelation and flocculation of casein micelle/carrageenan mixtures

The specific attractive interaction between carrageenan and casein micelles is widely exploited in the dairy industry to tailor the texture of dairy desserts. However, in industrial practice it has been observed that the addition of carrageenan can cause casein micelles to sediment rapidly when the mixture is above the gelation temperature. To better understand this undesirable phenomenon, carrageenan/skim milk mixtures have been observed over a wide range of compositions at 65 and 25°C. Interactions occurred for iota, kappa and lambda carrageenan at both temperatures. At 65°C sedimentation was observed when the carrageenan concentration was above 0.2%. At 25°C either phase separation or gelation could occur. Depletion flocculation is suggested as a partial explanation for these observations. The effects of casein micelles on the viscoelastic properties of iota carrageenan have been measured in the absence of flocculation. Their presence decreases carrageenan s gelation concentration and increases both the gelation temperature and the elastic modulus.

Study of the compatibility/incompatibility of gelatin/iota-carrageenan/water mixtures

The incompatibility of acid gelatin/iota-carrageenan mixtures has been studied. Both these biopolymers undergo a conformational coil-helix transition under suitable conditions of temperature and salt. The aim of this work was to study the concentration at which mixtures are incompatible and the influence of pH, salt and temperature on the phase diagram. Incompatibility occurred over a wide range of concentrations for mixtures prepared in deionized water. Compatibility was increased by increasing the pH or the salt concentration. Temperature did not greatly influence the size of the incompatible region. This is in agreement with the hypothesis that attractive electrostatic interactions lead to associative phase separation (traditionally called complex coacervation).

Iota-carrageenan/casein micelles interactions: evidence at different scales

Abstract The effect of temperature on the behaviour of iota-carrageenan (CI) 0.1 wt.%/casein micelles (CM) 0–5 wt.% mixtures has been studied using three techniques: confocal laser scanning microscopy (CLSM), differential scanning calorimetry (DSC) and spectrophotometry. The microscopy clearly shows that those mixed systems separate in two phases, one being enriched in CM. It has been shown that the CM concentration seems to have an effect on the extent of the phase separation phenomenon. The DSC experiments show that addition of CM modifies the helix to coil transition temperature of carrageenan. The enthalpy of melting of helices decreases as the CM concentration increases, and the peak is shifted towards higher temperature. Local electrostatic interactions between carrageenan chains and CM have been studied by a spectrophotometric method using methylene blue (MB) properties of absorption. The absorption spectra of MB in presence of CI and CM were compared with the one of MB in presence of carrageenan alone at temperatures above and below the carrageenan coil to helix transition. The modifications of the spectrum by addition of CM are discussed in terms of interactions and rigidification of the carrageenan chains.

Viscoelastic properties of ι-carrageenan/gelatin mixtures

Abstract The storage ( G ′) and loss ( G ′) moduli of transparent mixtures of ι-carrageenan and gelatin have been measured during gelation and melting and compared with those of gelatin and ι-carrageenan systems alone. The moduli of the mixtures were always higher than the sum of those of their components alone. Gelatin and ι-carrageenan seem to reinforce each other. Three specific temperature domains can be distinguished, delimited by the melting and helix-coil transition temperatures of gelatin and ι-carrageenan. An interpretation based on conformational changes and an attractive electrostatic interaction between the two biopolymers is suggested. The temperature dependence of G ′ and G ″ for mixtures has been studied as a function of thermal history (time and temperature of gel ageing), sodium chloride concentration and molecular weight of the ι-carrageenan.

Gelatin/iota-carrageenan interactions in non-gelling conditions

The negatively charged polysaccharide, iota-carrageenan has been mixed with two types of gelatin (PS and LH) which differ by their pI, about 9 and 4.5, respectively. At pH 7, gelatin PS chains have a net positive charge and gelatin LH chains have a net negative charge. Phase diagrams in non-gelling conditions (60°C) and pH 7 have been established in both 25 and 100 mM NaCl. The iota-carrageenan/gelatin PS phase diagrams are typical of systems involving attractive interactions leading to the formation of complexes: both polymers are mostly in the lower phase and increasing salt concentration leads to a reduction of the phase separated domain. The iota-carrageenan/gelatin LH mixture shows a very different behavior. No macroscopic bulk phase separation is observed. However, an increase in the turbidity measured by spectrophotometry is observed for certain compositions which would indicate microscopic phase separation. From the limit between the turbid and transparent concentration domains, the binodal may be drawn. Considering the negative net charge of gelatin LH chains at pH 7, repulsive interactions were expected to govern the phase separation. However, in this case too, by increasing salt concentration the phase-separated domain is reduced, where an increase would have been expected for thermodynamic incompatibility. By following the evolution of the absorption spectrum of a methylene blue/iota-carrageenan/gelatin LH mixture as a function of the salt concentration, it has been shown that gelatin LH chains associate with carrageenan ones in low salt conditions up to 200 mM NaCl.

Shear softening and thixotropic properties of wheat flour doughs in dynamic testing at high shear strain

Shear softening and thixotropic properties of wheat flour doughs are demonstrated in dynamic testing with a constant stress rheometer. This behaviour appears beyond the strictly linear domain (strain amplitude γ0 ≥ 0.2%),G′,G″ and |η*| decreasing with γ0, the strain response to a sine stress wave yet retaining a sinusoidal shape. It is also shown thatG′ recovers progressively in function of rest time. In this domain, as well as in the strictly linear domain, the Cox-Merz rule did not apply butη(γ) and |η*(ω))| may be superimposed by using a shift factor, its value decreasing in the former domain when γ0 increases. Beyond a strain amplitude of about 10–20%, the strain response is progressively distorted and the shear softening effects become irreversible following rest.