R.H.W. Wientjes

Rheological properties of whey protein/gum arabic coacervates

Complex coacervation in whey protein (WP)/gum arabic (GA) mixtures occurred in a specific pH window between 2.5 and 4.8. After phase separation, a concentrated polymer (also called coacervate) phase was obtained, whose viscoelastic properties were studied at various pH values. The viscosity of the WP/GA coacervate exhibited a surprisingly low shear-rate dependence, especially at pH 4.0, from 0.3 to 30 s−1 and a shear thinning above 30 s−1, indicating a structural change. Hysteresis in the flow curve was measured at the pH values at which the electrostatic interactions were the strongest. Hysteresis was due to a slow structural rearrangement of the coacervate phase and, with time, the initial viscosity was completely recovered, showing that structural changes were reversible. In frequency sweep experiments, the values of the viscous modulus G″ were up to ten times higher than the values of the elastic modulus G′, indicating the highly viscous character of the coacervates. pH 4.0 appeared to be the pH at wh...

Enzymatic modifications of pectins and the impact on their rheological properties

The interaction of new pectin-modifying enzymes (Novo) with three commercial pectins and their chemically de-O-acetylated and de-esterified counterparts is investigated. The study focuses not only on the ability of these pectins to act as substrates, but also on the consequences of modifications on their rheology and molecular weight. We present results that demonstrate that highly specific modifications of pectins are attainable. Enzymatic alterations of the side chain regions yielded polymers having a significantly lower viscosity, in the absence of calcium. In contrast, the viscosity of calcium-free pectin samples was increased after de-esterification of the backbone with pectin methylesterase. If the rheological properties of de-methoxylated pectin were studied in the presence of calcium, G′-values were increased 35-fold and the gel-like properties were markedly enhanced.These preliminary experiments emphasise the high application potential of enzyme-treated pectins. However, further investigations are required to custom-tailor pectins for utilisation in various food or other products.

A new transient network model for associative polymer networks

A new model for the linear viscoelastic behavior of polymer networks is developed. In this model the polymer system is described as a network of spring segments connected via sticky points (as in the Lodge model). [Lodge, A. S., “A network theory of flow birefringence and stress in concentrated polymer solutions,” Trans. Faraday Soc. 52, 120–130 (1956).] An important extension (with respect to previous models) is that chain connectivity is taken into account. All segments that are located in between connected stickers are supposed to carry stress. The attachment and detachment of stickers is described with kinetic equations in which activation energies play a role. Simultaneous transitions involving groups of stickers are allowed. The model shows a strong dependence upon the number of segments per chain. Broad relaxation spectra can be obtained. The storage modulus can have more than one plateau corresponding with the fact that stress relaxation may need the breakup of several bonds.

The Transition Zone for Gelatin Gels as Measured by High-Frequency Rheology

AbstractTlie high-frequency (100 Hz-200 kHz) linear viscoelastic behaviour of gelatin in the transition zone was investigated. During the early stages of ageing, the dynamic shear modulus for a 4 wt% gelatin gel at the gel point at 25 µC was found to vary according to a power law G′ ∝G″ α ωn with n ≈ 0.75. Within the framework of a recent model by Groot et al. [J. Chem. Phys., 104,9202 (1996)], this value is consistent with a result derived earlier from large deformation rheology, suggesting that the network bonds in a gelatin gel are more stretched than an ideally swollen polymer chain. However, ageing effects observed in large deformation experiments are not observed for the small deformation high-frequency data.

A new transient network model applied to guar gum

Guar gum is a linear polymer with associative sites which can connect and disconnect reversibly. Experimental visco-elastic moduli showed two storage modulus plateaus, a broad relaxation spectrum and partial time temperature superposition.