Margaret M. Robins

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.

TECHNIQUE TO MEASURE EMULSION CREAMING BY VELOCITY OF ULTRASOUND

ABSTRACT A new non-intrusive, Instantaneous technique to monitor the creaming/sedimentation of colloidal dispersions is reported. Here the technique is used to determine the detailed concentration profiles of water-continuous emulsions as a function of height and time. The emulsions consist of hexadecane stabilised by nonionic surfactant (Brij 35) In water In the presence and absence of a polysaccharide thickener (hydroxyethylcellulose). The technique involves the measurement of the velocity of ultrasound through the emulsion, which depends simply on the velocity through the individual phases and the bulk composition of the emulsion (provided the individual droplets are smaller than the wavelength of the ultrasound). Applications of the technique may include In-line monitoring of composition during the processing of dispersions and predictive testing of dispersion stability (shelf-life).

Gravitational destabilization of emulsions flocculated by non-adsorbed xanthan

Abstract The anionic polysaccharide xanthan, widely used as a stabilizer in food dispersions, induces flocculation by a depletion mechanism. The effect of flocculation, induced by the presence of xanthan, on the gravitational destabilization (creaming and compaction) of 20% oil-in-water emulsions is presented. The detailed gravitational destabilization processes are studied by following changes in the concentration profile with time. In the absence of flocculant the polydisperse droplets rise as a diffuse meniscus and form a cream of ~70% oil at the top of the sample. Such behaviour is consistent with creaming in a dispersion of non-interacting polydisperse particles. In contrast the flocculated droplets form a sharp meniscus and cream rapidly. At the top of the sample the floes form a less concentrated cream which undergoes compaction under gravity. The compaction is characterized by a concentration-dependent bulk modulus.

Determination of gravitational separation in dispersions from concentration profiles

Abstract A non-intrusive technique is used to investigate the gravitational sedimentation/creaming of particles in a dispersion. The velocity of ultrasound is measured over a series of heights in the dispersion, and the data are converted to concentration profiles which are collected over a period of time. Often the method detects sedimentation/creaming at a very early stage and can therefore predict long-term stability. Data are presented on a model suspension, and compared with theoretical predictions. In a polydisperse emulsion the creaming behaviour gives a good estimate of the particle size distribution. In an emulsion containing a depletion flocculant, coexistence of two phases is detected, and the compaction of the flocculated cream layer gives a measure of the strength of the network.

Stability of oil-in-water emulsions. The effect of dispersed phase and polysaccharide on creaming

Abstract The creaming of concentrated alkane-in-water emulsions stabilised by a non-ionic surfactant has been measured in the presence of hydroxyethylcellulose polysaccharide. Creaming rates under gravity were determined by visual observation. In the emulsions containing polysaccharide the creaming was much more rapid than expected from consideration of the oil droplet size, density difference, the rheological properties of the continuous phase and the current oil concentration. Flocculation effects would appear to be the primary mechanism for the anomalous behaviour, although other effects cannot be entirely ruled out at this stage. The final oil concentrations in the creamed emulsion phases have also been measured, and an interpretation is given in terms of the pressure in the cream, the droplet size and the interfacial tension.

Use of concentration profiles in creaming emulsions to determine phase coexistence

Abstract Concentration profiles are presented for 20% alkane-in-water emulsions in the presence of hydroxyethylcellulose (HEC). The non-adsorbing, high molecular weight HEC (Natrosol 250HR) is present in the continuous phase at up to 0.04% w/w. At ≤0.02% HEC a diffuse lower meniscus is apparent during creaming, consistent with the movement of individual polydisperse droplets. At 0.04% HEC a sharp lower meniscus forms and rises rapidly, indicating that the droplets are flocculated. At 0.03% HEC both sharp and diffuse lower menisci are observed, corresponding to the coexistence of ∼ 5% oil as unflocculated droplets with ∼ 15% oil as aggregates. The results are thought to be consistent with phase separation induced by a polymer depletion mechanism.

Creaming in Flocculated Oil-in-Water Emulsions

Publisher Summary Many foods are emulsions during or after manufacture. In oil-in-water emulsions, the dispersed oil droplets generally possess a lower density than the continuous aqueous phase. Unless the droplets are very small or the emulsion is very concentrated, the density difference leads to the gradual accumulation of the droplets at the top of the container (creaming) with consequent loss of perceived quality. Polysaccharide stabilizers are frequently used to reduce the rate of creaming as well as to impart the required mouth-feel properties to a food product. This chapter presents a paper that presents creaming results for n-alkane-in-water emulsions containing the polysaccharide hydroxyethylcellulose. The analysis of the data in the paper draws heavily on the work of A. S. Michaels and J. C. Boiger, who investigated the sedimentation behavior of flocculated kaolin suspensions. Their treatment includes general descriptions of flocculated systems and it has been found to be a useful starting point for the preliminary analysis of the results presented in the paper.

USE OF THE VELOCITY OF ULTRASOUND TO DETERMINE THE PHASE VOLUME OF DISPERSIONS

A technique to determine concentration profiles in non-aerated dispersions from the velocity of ultrasound V is described. A pulse-overlap method is used to obtain the values of V at each height over the dispersion. A concentration profile is easily obtained from the velocity values, and its variation with time allows the detailed study of sedimentation/creaming processes. We present concentration profiles for an electrostaticallystabilised latex dispersion and for alkane-in-water emulsions in the presence and absence of non-adsorbing polymer (which induces flocculation). The data are compared with the theoretical profiles for a dispersion of non-interacting, monodisperse hard spheres. Changes in concentration profile are detected before they are visually apparent and the technique has industrial applications in product development and quality control.