George Stainsby

Food emulsions and foams

Revue bibliographique concernant les proprietes physiques, chimiques et moleculaires des emulsions alimentaires, en particulier dans le cas de liqueurs a la creme, de solutions de proteines, de boissons rafraichissantes et autres

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.

A comparison of the reactivity of alginate and pectate esters with gelatin

Abstract Aqueous solutions of highly esterified propylene glycol alginate and gelatin interact rapidly in mildly alkaline conditions to form a gel with a very high melting point. The interaction involves the formation of amide bonds between the ester and uncharged amino groups on the protein. Neither high-methoxyl pectin nor highly esterified propylene glycol pectate formed thermostable gels with gelatin, and the lack of reactivity was not due to differences between pectate and alginate in viscosity, rate of depolymerisation or rate of saponification. Pectate esters will react, however, with low molecular weight diamines in anhydrous conditions. It is suggested that the different reactivity of the uronides in water reflects differences in the geometries of their glycosidic links between monomers, and that in alginate it is the mannuronic residues that are involved in these reactions.

Time-dependent surface pressures of adsorbed films of caseinate + gelatin at the oil—water interface

Abstract Using the drop-volume and pendant-drop methods, the change in tension at the n-hexadecane/water interface has been monitored at 25°C during adsorption from aqueous solutions of α S1 - and β-casein, sodium caseinate, gelatin, and mixtures of sodium caseinate + gelatin (protein concentration = 10 −3 wt%, pH = 7.2, ionic strength = 0.005 mol dm −3 ). The two experimental methods give identical results. At short times (⩽ 3 min), for caseinate + gelatin, the derived surface pressure is sensitive to protein concentration but not to protein composition. At longer times (∼ 15 min), adsorption is predominantly controlled by the caseinate, with the surface pressures of the mixtures lying between those for gelatin and caseinate alone at the same bulk protein concentration.

Interfacial and emulsifying behaviour of acetylated field bean protein isolate

Abstract Interfacial and emulsifying properties of modified field bean protein isolate are investigated as a function of the degree of acetylation. Emulsion droplet-size distributions are found to be sensitive to the degree of acetylation, although tensions at the oil—water interface for unmodified and highly acetylated samples are not very different. Field bean protein forms films of high surface viscosity at the oil—water interface, and the viscosity increases with the degree of acetylation (up to a level of about 60%).

Colloid stability of casein-coated polystyrene particles

The stability of casein-coated polystyrene latices with respect to aggregation by sodium chloride has been studied at 20 °C and pH 7.3. Initial aggregation rates have been measured by light scattering as a function of ionic strength (100–1000 mol m–3) for negatively charged particles separately coated with αs1-casein, β-casein, κ-casein and three commercial caseinate mixtures. Casein adsorption reduces the mean electrophoretic mobility of latex particles. At low surface coverage (ca. 10% saturation), derived critical coagulation concentrations are roughly in accord with predictions from DLVO theory based on changes in zeta potential estimated from experimental mobilities. Near saturation coverage, latices are stabilized against aggregation at all ionic strengths up to 1000 mol m–3, presumably by a ‘steric’ mechanism. While each pure casein or caseinate mixture shows the same general behaviour, κ-casein appears quantitatively less effective as a steric stabilizer.

An adsorption effect on the gel strength of dilute gelatin-stabilized oil-in-water emulsions

The incorporation of emulsion oil droplets into a gelatin gel leads to an initial increase in shear modulus at 25 °C for a gelatin concentration of 8 wt % but an initialdecrease for a concentration of 5 wt %. The latter result is consistent with a net lowering of the gelatin concentration available for gelation in the aqueous phase due to adsorption at the oil-water interface.

Behaviour of adsorbed myosin at the oil-water interface

Abstract Interfacial tensions and surface shear viscosities are reported at 7 and 25°C for adsorbed films of myosin at the n-tetradecanebuffer interface (bulk protein concentration 10 −3 wt%, pH 6.5, 0.5 m KCl). Surface behaviour similar to that of other proteins is observed at 25°C, but unique surface behaviour is observed at 8°C. An unusual feature is a surface viscosity which increases with temperature-particularly in young films.