Brent S. Murray

Interfacial rheology of food emulsifiers and proteins

Abstract The scientific literature from 1997 (inclusive) to the present on the interfacial rheology of emulsifiers and proteins of relevance to food has been reviewed. Both shear and dilatational rheology of oil–water and air–water interfaces have been covered and the main factors affecting interfacial rheology have been tabulated. Special attention is paid to: the sensitivity of interfacial rheology to film composition and structure; the growing viewpoint of treating proteins films as a two-dimensional gel state; recent theoretical modelling of interfacial rheological effects; those few publications that attempt to relate interfacial rheology to bulk stability. It is concluded that there have been few major advances in the last 4 or 5 years, but the heterogeneity of such adsorbed films seems to be better recognised, both spatially and rheologically, with the challenge remaining to connect this picture to the stability of the corresponding bulk systems.

On the film-forming and emulsion-stabilizing properties of gum arabic: dilution and flocculation aspects

Abstract Physico-chemical factors relevant to the emulsifying and stabilizing properties of gum arabic are discussed. Experimental information is presented relating to (i) the effect of extensive dilution of the aqueous phase on the surface viscosity at the planar oil—water interface and (ii) the effect of non-adsorbing polysaccharide on the emulsion stability. We conclude that the surface rheology of gum arabic films is relatively insensitive to dilution of the aqueous phase, although clear evidence is found for a small but significant reduction in the surface viscosity over a time-scale of the order of 100 h. We observe that addition of dextran to gum arabic emulsions at high ionic strength leads to enhanced instability with respect to creaming; the effect is attributed to depletion flocculation.

Time—dependent surface viscosity of adsorbed films of casein + gelatin at the oil—water interface

Abstract The measured surface viscosity of a mixed protein film adsorbed at the interface between n -hexadecane and a dilute aqueous solution of casein + gelatin is found to depend sensitively on the age of the film and the protein composition of the bulk phase.

Surface activity and emulsifying behaviour of some Acacia gums

Abstract The surface and emulsifying properties of six Acacia gum samples with nitrogen contents in the range 0.09–7.5% have been investigated at neutral pH. Samples which give a relatively rapid lowering of the tension at the n-hexadecane—water interface produce oil-in-water emulsions with small droplets and good stability with respect to creaming. Although the nitrogen content of these gums shows a reasonable correlation with the limiting long-time surface activity, there is no simple relationship between nitrogen content and emulsifying behaviour. It is suggested that the variability in the emulsifying properties of the gum samples from different Acacia species is dependent not only on their total protein (or polypeptide) content but also on the distribution of the protein/peptide between the low- and high-molecular-weight fractions, and on the molecular accessibility of the protein/peptide for adsorption.

Coalescence stability of emulsion-sized droplets at a planar oil–water interface and the relationship to protein film surface rheology

An experimental technique for measuring coalescence times of emulsion sized oil droplets at a planar oil–water interface is described. Distributions of coalescence times as a function of droplet size are presented for systems with n-hexadecane as the oil phase and a protein solution of β-casein, κ-casein or lysozyme as the aqueous phase (10–4 wt% protein, pH 7, ionic strength 0.1 mol dm–3, 25 °C). When the planar interface is aged for only 20 min there is no measurable difference in the distribution of coalescence times for the three proteins. However, when the planar interface is aged for 72 h there is a sizeable fraction of droplets, different for the three proteins, which does not coalesce at all, and the relative efficiencies of the adsorbed proteins in preventing coalescence lie in the order: lysozyme > κ-casein > β-casein. Time-dependent surface shear viscosities at the oil–water interface are reported for adsorbed films of the same three proteins under exactly the same experimental conditions. After 20 min the surface viscosity values for all three proteins are low ( < 1 mN m–1 s), but after 72 h their values are distinctly different: 200 mN m–1 s (lysozyme), 60 mN m–1 s (κ-casein) and < 1 mN m–1 s (β-casein). These results are consistent with the view that there is a positive correlation between coalescence stability and the mechanical strength of protein films adsorbed at the oil–water interface.

Particle-Stabilizing Effects of Flavonoids at the Oil―Water Interface

It has been shown that some common food flavonoids can act as excellent stabilizers of oil-in-water emulsions through their adsorption as water-insoluble particles to the surface of the oil droplets, i.e., Pickering emulsions are formed. Flavonoids covering a wide range of octanol-water partition coefficients (P) were screened for emulsification behavior by low shear mixing of flavonoid+n-tetradecane in a vortex mixer. Most flavonoids with very high or very low P values were not good emulsifiers, although there were exceptions, such as tiliroside, which is very insoluble in water. When a high shear jet homogenizer was used with 20 vol% oil in the presence of 1 mM tiliroside, rutin, or naringin, much finer emulsions were produced: the average droplet sizes (d32) were 16, 6, and 5 μm, respectively. These results may be highly significant with respect to the delivery of such insoluble compounds to the gut, as well as their digestion and absorption.

Mixed layers of sodium caseinate + dextran sulfate: influence of order of addition to oil-water interface.

We report on the interfacial properties of electrostatic complexes of protein (sodium caseinate) with a highly sulfated polysaccharide (dextran sulfate). Two routes were investigated for preparation of adsorbed layers at the n-tetradecane-water interface at pH = 6. Bilayers were made by the layer-by-layer deposition technique whereby polysaccharide was added to a previously established protein-stabilized interface. Mixed layers were made by the conventional one-step method in which soluble protein-polysaccharide complexes were adsorbed directly at the interface. Protein + polysaccharide systems gave a slower decay of interfacial tension and stronger dilatational viscoelastic properties than the protein alone, but there was no significant difference in dilatational properties between mixed layers and bilayers. Conversely, shear rheology experiments exhibited significant differences between the two kinds of interfacial layers, with the mixed system giving much stronger interfacial films than the bilayer system, i.e., shear viscosities and moduli at least an order of magnitude higher. The film shear viscoelasticity was further enhanced by acidification of the biopolymer mixture to pH = 2 prior to interface formation. Taken together, these measurements provide insight into the origin of previously reported differences in stability properties of oil-in-water emulsions made by the bilayer and mixed layer approaches. Addition of a proteolytic enzyme (trypsin) to both types of interfaces led to a significant increase in the elastic modulus of the film, suggesting that the enzyme was adsorbed at the interface via complexation with dextran sulfate. Overall, this study has confirmed the potential of shear rheology as a highly sensitive probe of associative electrostatic interactions and interfacial structure in mixed biopolymer layers.

Interfacial structuring in a phase-separating mixed biopolymer solution containing colloidal particles.

We report confocal microscopy observations of the spatial distribution of monodisperse charge-stabilized colloidal particles (amphoteric polystyrene latex) incorporated within a spinodal-type phase-separating system of mixed biopolymers (gelatin + oxidized starch). Images from samples aged at 40 degrees C demonstrate a strong tendency for the added particles to accumulate at the liquid-liquid interface and to influence the rate of coarsening of the complex bicontinuous microstructure. Large variations in the local curvature of particle-rich interfacial regions are suggestive of a liquid-liquid boundary that is substantially viscoelastic.

Effects of pH on the ability of flavonoids to act as Pickering emulsion stabilizers

The flavonoids tiliroside, rutin and naringin have been investigated as stabilizers of Pickering oil-in-water (O/W) emulsions. The mean droplet size of tetradecane emulsions was considerably smaller at higher pH, especially for rutin. The solubility of flavonoids in the aqueous phase was 4-6 times higher at pH 8 compared to pH 2 for tiliroside and rutin, although all absolute solubilities remained low (<1 mM). This agreed with a slight increase in surface activity of tiliroside and rutin at the O-W interface at pH 8 compared to pH 2. However, improved emulsion stabilization at higher pH is better explained by the significant increase in ζ-potential of the flavonoid particles to more negative values at pH 8, which will improve particle dispersion and increase the charge on the droplets stabilized by them. A buckwheat tea extract, rich in rutin, was also shown to be an effective stabilizer of sunflower O/W emulsions.

On the kinetics of acid sodium caseinate gelation using particle tracking to probe the microrheology

The sol-gel transition of a model dairy system (sodium caseinate solution) which undergoes gelation by acidification has been studied by conventional bulk rheology and particle tracking microrheology, via confocal microscopy. The Brownian diffusion of fluorescent microspheres (0.21, 0.32, 0.5, and 0.89 μm in diameter) with different surface coatings (polyethylene glycol, carboxylate groups and polystyrene) was used to probe spatial mechanical properties of the gels at the scale of microns. The microrheological results are compared with the macroscopic viscoelastic properties (storage and loss shear modulus) measured in a concentric cylinder rheometer (double gap, at shear strain of 0.005 and frequency of 1 Hz). At pH values close to pI of the caseins, where formation of a protein network, i.e., gelation, became obvious from the confocal microscopy and bulk rheological measurements, all the particles had a tendency to adhere to the network. In spite of this, the microrheological values of the moduli were only slightly lower than the macroscopically determined values and the gel points calculated via both techniques tended to be in good agreement. However, the particle tracking method has higher sensitivity and can detect changes in the structuring of the system before these are registered by the bulk rheological measurement.