Chapter 9 Competitive Adsorption and Protein—Surfactant Interactions in Oil-in-Water Emulsions
Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: December 26, 1991 | doi: 10.1021/bk-1991-0448.ch009
Eric Dickinson Procter Department of Food Science, University of Leeds, Leeds LS2 9JT, United Kingdom
Competitive adsorption in oil-in-water emulsions has been investigated for binary protein/protein and protein/surfactant systems. Analysis of aqueous phase composition in emulsions (10 wt % n-tetradecane, pH 7) indicates fast reversible adsorption for a -casein/β-casein, but slow irreversible adsorption for β-casein/a-lactalbumin, β-casein/β-lactoglobulin and a-lactalbumin/β-lactoglobulin. Surface viscosity data at the planar oil—water interface give further information on milk protein competitive adsorption. Interfacial tensions and viscosities are reported for a mixture of a non-ionic surfactant, octaoxyethylene dodecyl ether (C E ), and sodium caseinate; and stability data are presented for emulsions made with C E + caseinate. Differences between C E and the anionic surfactant, sodium dodecyl sulphate (SDS), are noted. sl
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Most food emulsions are stabilized by proteins, either alone or in combination with low-molecular-weight surfactants (lipids or their derivatives) or high-molecular-weight polymers (polysaccharide hydrocolloids) (1-4). The distribution of these different kinds of molecules between the droplet surface and the bulk phases is an important factor controlling the formation, stability and texture of edible oil-in-water emulsions such as cream liqueurs, salad dressings, or ice-cream. What affects the distribution is (a) competitive adsorption between different proteins and between the proteins and the surfactants and (b) interactions between proteins and the other components at the oil—water interface. It is clear that interfacial protein—polysaccharide interactions can have a significant effect on emulsion stability (3,5), but for reasons of brevity this paper is concerned only with interfacial protein—protein and protein -surfactant interactions. The proteins chosen for study are the milk proteins—a -casein, β-casein, a-lactalbumin and β-lactoglobulin. We report results of experiments designed to quantify the extent to which one pure milk protein A is ab le to displace another pure milk protein B from the s1
0097-6156/91AM48-O114$06.00/0 © 1991 American Chemical Society In Microemulsions and Emulsions in Foods; El-Nokaly, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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Interactions in Oil-in-Water Emulsions
Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: December 26, 1991 | doi: 10.1021/bk-1991-0448.ch009
oil—water interface. An adsorbed film of protein B i s established at the emulsion droplet surface, or at a macroscopic planar oil—water interface, and then protein A i s introduced into the bulk aqueous phase. Time-dependent competitive adsorption i s monitored by following the change i n protein composition of the serum phase ( i n the case of the emulsion) or the change i n surface viscosity of the adsorbed film ( i n the case of the planar interface). Experiments are carried out at neutral pH with n-tetradecane as the o i l phase. Data are also presented for a binary system composed of the proteinaceous food emulsifier, sodium caseinate, and the non-ionic water-soluble surfactant, octaoxyethylene dodecyl ether ( C ^ E g ) . Experimental Materials. Samples of a ^-casein and 3-casein were prepared from whole milk using standard procedures (6,7). The a-lactalbumin and AnalaR-grade n-tetradecane were obtained from Sigma Chemicals (St. Louis, MO). The 6-lactoglobulin and sodium dodecyl sulphate were obtained from BDH Chemicals (Poole, UK). High purity C ^ E g ^s obtained from Nikko Chemicals (Tokyo, Japan). Sodium caseinate was obtained from the Scottish Milk Marketing Board (Renfrewshire, UK). Buffer solutions were made using BDH AnalaR-grade reagents and doubled i s t i l l e d water. w
Emulsion Preparation. Oil-in-water emulsions (20 wt% n-tetradecane, 0.5 wt % pure milk protein or 0.1 wt % sodium caseinate, 20 mM imidazole buffer, pH 7) were prepared at room temperature using a small-scale valve homogenizer (
