Alan Parker

How does xanthan stabilise salad dressing

Abstract The creaming behaviour of model pourable salad dressings has been studied over a wide range of xanthan and oil phase concentrations. They were produced by mixing a colloidally stable emulsion with xanthan solutions. The model dressings were completely flocculated by depletion at all xanthan concentrations >0.1 g/l. Creaming behaviour was in three phases: an initial delay phase, followed by more or less linear creaming and finally cream compression. Delayed creaming has not been reported previously. It is of great practical interest, because a complete absence of creaming is the aim of successful dressing formulation. The length of the delay phase was proportional to the emulsion concentration, inversely proportional to the oil/aqueous phase density difference and scaled over more than four orders of magnitude with the xanthan concentration to powers between 3 and 4. Traditionally, the stabilisation of salad dressing by xanthan is explained by the yield stress of its solutions. It is shown that they do not have one, and we offer an alternative explanation. It is suggested that the depletion flocculation of the emulsion droplets, induced by the addition of xanthan, leads to the formation of a particle network and that it is the time-dependent yield stress of this network which stabilises the dressing.

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.

A novel method for determining the dissolution kinetics of hydrocolloid powders

Food hydrocolloids are sold as powders so they must be dissolved before use. In modern manufacturing practice, rapid dissolution can be an important specification. However, quantifying dissolution kinetics is a problem, because when lumps form the process becomes irreproducible, partly because lumps tend to stick to the stirrer. This paper presents a simple method for determining the dissolution kinetics of soluble polymer powders which solves these problems. The dissolution kinetics were followed using a modified rotational viscometer. The key modification is that when the powder is added, there is a vortex below the axis of the rotor, so that the powder cannot stick to it. The amount of hydrocolloid dissolved is followed via the torque on the viscometer. Measurements were performed under both dispersing and non-dispersing conditions. Under dispersing conditions each powder grain dissolves individually, and it is shown that the dissolution rate is proportional to the amount of powder surface. Under non-dispersing conditions, grains can stick together, forming lumps. The reproducibility of the method under both conditions is demonstrated using pectin. It was used to quantify the empirical observation that reducing the powder grain size increases the dissolution rate, but it also greatly increases lump formation. q 1999 Elsevier Science Ltd. All rights reserved.

Rheology and fracture of mixed ι- and κ-carrageenan gels : two-step gelation

Abstract It is shown that under certain circumstances, on cooling mixed ι- and κ-carrageenan solutions, the two forms gel separately at different temperatures, with the ι form gelling first. This ‘two-step gelation’ was only observed when both sodium and potassium ions were present, with a sodium/potassium mole ratio of between 1 and 100. For such mixed gels, a κ fraction as low as 2·5% of the total carrageenan has significant effects on their rheology, both at low deformation and fracture. In these systems, the κ form, gelling in the presence of an existing ι gel, produces measurable rheological effects at much lower concentrations than if it were alone. This behaviour can be used as a sensitive test of the ‘rheological purity’ of samples of ι-carrageenan.

Effect of the Addition of High Methoxy Pectin on the Rheology and Colloidal Stability of Acid Milk Drinks

The effect of high methoxy pectin on the behaviour of model acid milk drinks has been investigated using viscometry, laser diffraction size measurement and microelectrophoresis. The results of these different tests are interpreted using the theory of colloidal stability. With increasing pectin concentration, the behaviour changes from flocculated with few particles below 0.2µm diameter and shear thinning, time-dependent rheology to non-flocculated, many sub 0.2µm particles and low viscosity with Newtonian rheology. With even higher pectin concentrations, the acid milk drink remains non- flocculated, but with increasing viscosity and close to Newtonian rheology.

Experimental and theoretical study of polymer flow in porous media

In this paper, an extensive study is presented on the single-phase flow of xanthum/tracer slugs in a consolidated sandstone. The phenomena studied include polymer/tracer dispersion, excluded/inaccessible-volume effects, polymer adsorption, and viscous fingering. In some floods, there is also evidence of nonequilibrium effects. Macroscopic flow equations are derived that include terms to model all the behaviors listed above. A microscopic approach is also developed that describes certain features of polymer flow in porous media semiquantitatively.

Interfacial viscoelasticity controls buckling, wrinkling and arrest in emulsion drops undergoing mass transfer

Contrary to the notion that ‘oil and water do not mix’, many oils possess a residual diffusive mobility through water, causing the drop sizes in oil-in-water emulsions to slowly evolve with time. Liquid interfaces are therefore typically stabilized with polymeric or particulate emulsifiers. Upon adsorption, these may induce strong, localized viscoelasticity in the interfacial region. Here, we show that shrinkage of oil drops due to bulk mass transfer may render such adsorption layers mechanically unstable, causing them to buckle, crumple and, finally, to attain a stationary shape and size. We demonstrate using two types of model interfaces that this only occurs if the adsorption layer has a high interfacial shear elasticity. This is typically the case for adsorbed layers that are cross-linked or ‘jammed’. Conversely, interfacial compression elasticity alone is a poor predictor of interface buckling or arrest. These results provide a new perspective on the role of interfacial rheology for compositional ripening in emulsions. Moreover, they directly affect a variety of applications, including the rapid screening of amphiphilic biopolymers such as the Acacia gum or the octenyl succinic anhydride modified starch used here, the interpretation of light scattering data for size measurements of emulsion drops, or the formulation of delivery systems for encapsulation and release of drugs and volatiles.

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).

Viscoelasticity of anionic wormlike micelles: effects of ionic strength and small hydrophobic molecules

The impact of NaCl and small hydrophobic molecules, used in perfumery, on the viscoelastic properties of aqueous solutions of sodium lauryl ether sulphate is studied. As the salt concentration increases, the viscosity passes through a maximum. Empirically, this behaviour is well known and is referred to as the ‘salt curve’. Dynamic rheological measurements revealed a detailed picture of how salt affects the different length scales in the entangled wormlike micelles. Adding hydrophobic molecules does not change the shape of the salt curve, but they can shift the maximum to lower salt concentrations and alter the peak viscosity. The shift in the salt curve can generally be understood as due to either flattening of the interface, caused by insertion of amphiphilic molecules into the surfactant layer, or micellar swelling, caused by solubilization of very hydrophobic molecules in the micellar core. The viscosity decrease at the salt curve maximum is inversely proportional to the hydrophobicity of the added molecules, so less hydrophobic compounds cause larger decreases. It is hypothesized that these molecules interact with the surfactant headgroups and thereby soften the interfacial film. Our analysis suggests that the addition of hydrophobic additives reduces the persistence length and results in an effective shortening of the micelles.

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.