B. Launay

Effects of carrageenan type on the behaviour of carrageenan/milk mixtures

Abstract The apparent hydrodynamic diameter of casein micelles was determined as a function of hydrocolloid concentration at 60°C and during cooling to 20°C. The systems studied contained skim milk 100-fold diluted in permeate in the presence of either lambda-, iota-, kappa-carrageenan or guar gum. These measurements show that all three forms of carrageenan adsorb onto casein micelles, whereas guar gum does not. Lambda-carrageenan (which is always in the coil conformation) adsorbs at all the temperatures studied, whereas the iota and kappa forms adsorb only at temperatures below the onset of the helix–coil transition. The results agree with the idea that adsorption of carrageenan occurs only above a certain minimum charge density. Phase diagrams were established at 60°C for the three carrageenans and the rheological behaviour was followed on cooling and at 25°C. The results suggest that the lambda-carrageenan bridges the casein micelles at low carrageenan concentrations, leading to the sedimentation of carrageenan/milk mixtures at 60°C and structure formation on cooling. At 60°C, iota- and kappa-carrageenan induce depletion flocculation of casein micelles above a critical carrageenan concentration. On cooling, systems containing iota-carrageenan clearly form a network at the helix–coil transition temperature. The network is formed by mixed carrageenan/casein micelle crosslinks. In the presence of excess carrageenan, this network is reinforced by carrageenan/carrageenan crosslinks. For the kappa-carrageenan/casein micelle mixtures the existence of mixed carrageenan/casein micelle crosslinks is less obvious but seems likely.

Concentration dependence of the critical viscoelastic properties of gelatin at the gel point

Gelatin gel properties have been studied through the evolution of the storage [G′(ω)] and the loss [G″(ω)] moduli during gelation or melting near the gel point at several concentrations. The linear viscoelastic properties at the percolation threshold follow a power-law G′(ω)αG″(ω)αωΔ and correspond to the behavior described by a rheological constitutive equation known as the Gel Equation. The critical point is characterized by the relation: tan δ = G″/G′ = cst = tan (Δ · π/2) and it may be precisely located using the variations of tan δ versus the gelation or melting parameter (time or temperature) at several frequencies. The effect of concentration and of time-temperature gel history on its variations has been studied. On gelation, critical temperatures at each concentration were extrapolated to infinite gel times. On melting, critical temperatures were determined by heating step by step after a controlled period of aging. Phase diagrams [T = f(C)] were obtained for gelation and melting and the corresponding enthalpies were calculated using the Ferry-Eldridge relation. A detailed study of the variations of A with concentration and with gel history was carried out. The values of Δ which were generally in the 0.60–0.72 range but could be as low as 0.20–0.30 in some experimental conditions, were compared with published and theoretical values.

Thermal denaturation and heat-induced gelation properties of β-lactoglobulin. Effects of some chemical parameters

Abstract The thermal denaturation of β-lactoglobulin solutions (8.5% protein concentration of which 5.8% was β-lactoglobulin) has been studied by differential scanning calorimetry. For solutions of pH between 1.5 and 8, the denaturation temperature Td was maximum at pH 3.5, and the apparent enthalpy change ΔHapp seemed to be constant from pH 1.5 to 6.6, and decreased significantly at pH 8.0. With addition of N-ethylmaleimide (NEM) to the β-lactoglobulin solution (pH 7) in a molar ratio of up to 1:1, the peak temperature Tp increased slightly and then remained constant, for higher molar ratio, while ΔHapp decreased. With addition of dithiolthreithol (DTT), both Tp and ΔHapp decreased drastically. The kinetic parameters of heat denaturation were determined by the Borchardt and Daniels method as a function of pH. At pH 1.5 and 2.5, the denaturation process presented some degree of reversibility which increased with the scan rate of the first heating of the solution. The process could be described as a second-order reaction at pH The heat-induced gelation properties, the gelation time (heat treatment at 80°C) and gelation temperature (heat treatment from 40 to 90°C at 0.1°C min−1) were studied with an empirical test and an Instron machine, respectively. The addition of NEM (0–20 mM) and DTT (0–32 mM) gave rise to increasing and reducing gelation time (tg) at 80°C, respectively. The onset gelation temperature, determined from the first increase of the apparent Youngs modulus, seemed to be lower than that of the denaturation temperature determined by extrapolation to 0.1°C min−1 of peak temperatures obtained at various scan rates. It was higher than that of the onset denaturation temperature, determined from the increasing edge of the heat flow measured at 5°C min−1 (7.5°C for pH 8 solution). The maximum apparent Youngs modulus Eapp observed with increasing heat treatment displayed a higher value at pH 6.6. The experimental results are compared to published data, and they are discussed in terms of electrostatic and hydrophobic interactions and of SH/S—S interchange reactions.

An investigation into the nature of wheat flour dough adhesive behaviour

Wheat flour dough adhesiveness was measured using a probe test developed previously. Six probes made of the following materials were used in adherence measurements: PMMA (polymethylmethacrylate), SS (stainless steel), PE (polyethylene), PVC (polyvinylchloride), PTFE (polytetrafluoroethylene) and PP (polypropylene). The specific energy of separation (ω, J/m2) was seen to vary with dough water content and probe material. The greatest ω was obtained with the PMMA probe, whereas the lowest one was obtained with the SS, PTFE and PP probes. This result was in accordance with the probe surface tension measurements. The amount of water was seen to have a stronger effect on adhesiveness than probe surface tension due to changes in dough rheology. Adherence measurements conducted with different probe materials and withdrawal rates showed that ω could be factorised into two terms, a constant value tentatively related to the thermodynamic work of adhesion, W0, and a viscoelastic function of rate of separation describing the energy dissipation in dough during debonding. A general model developed for pressure-sensitive adhesives (PSAs) was shown applicable to wheat flour dough adhesiveness. It was also demonstrated that the appropriate measure of dough adhesiveness was not the maximum tensile force, but the energy of separation per unit area of interface, as stated before for PSAs.