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Influence of ι-Carrageenan on Droplet Flocculation of β-Lactoglobulin-Stabilized Oil-in-Water Emulsions during Thermal Processing Yeun Suk Gu,* Eric A. Decker, and D. Julian McClements Biopolymer and Colloids Research Laboratory, Department of Food Science, University of Massachusetts, Amherst, Massachusetts 01003 Received June 6, 2004. In Final Form: August 4, 2004 The influence of thermal processing on droplet flocculation in oil-in-water emulsions stabilized by either β-lactoglobulin (primary emulsions) or β-lactoglobulin-ι-carrageenan (secondary emulsions) at pH 6 has been investigated. In the absence of salt, the ζ-potential of the primary emulsion was less negative (-40 mV) than that of the secondary emulsion (-55 mV) due to adsorption of anionic ι-carrageenan to the anionic β-Lg-coated droplet surfaces. The ζ-potential and mean diameter (d43 ≈ 0.3 µm) of droplets in primary and secondary emulsions did not change after storage at temperatures ranging from 30 to 90 °C. In the presence of 150 mM NaCl, the ζ-potential of the primary emulsion was much less negative (-27 mV) than that of the secondary emulsion (-50 mV), suggesting that the latter was less influenced by electrostatic screening effects. The ζ-potential of the primary emulsions did not change after storage at elevated temperatures (30-90 °C). The ζ-potential of the secondary emulsions became less negative, and the aqueous phase ι-carrageenan concentration increased at storage temperatures exceeding 50 °C, indicating ι-carrageenan desorbed from the β-Lg-coated droplets. In the primary emulsions, appreciable droplet flocculation (d43 ≈ 8 µm) occurred at temperatures below the thermal denaturation temperature (Tm) of the adsorbed proteins due to surface denaturation, while more extensive flocculation (d43 > 20 µm) occurred above Tm due to thermal denaturation. In the secondary emulsions, the extent of droplet flocculation below Tm was reduced substantially (d43 ≈ 0.8 µm), which was attributed to the ability of adsorbed carrageenan to increase droplet-droplet repulsion. However, extensive droplet flocculation was observed above Tm because carrageenan desorbed from the droplet surfaces. Differential scanning calorimetry showed that ι-carrageenan and β-Lg interacted strongly in aqueous solutions containing 0 mM NaCl, but not in those containing 150 mM NaCl, presumably because salt weakened the electrostatic attraction between the molecules.
Introduction Electrically charged polysaccharides can adsorb to the surface of oppositely charged emulsifier-coated oil droplets through electrostatic attraction.1,2 Polysaccharide adsorption to droplet surfaces can be either detrimental or beneficial to emulsion stability depending on preparation conditions and system composition.3 Under certain conditions, adsorbed polysaccharides promote droplet flocculation by acting as “bridges” that link two or more droplets together. Under other conditions, polysaccharides interact with the adsorbed emulsifier to form interfacial complexes that improve emulsion stability. In this latter case, the adsorption of polysaccharides to the surface of the emulsifier-coated oil droplets alters the composition, structure, thickness, rheology, and electrical characteristics of the interfacial membrane. These changes in interfacial characteristics alter the sign, magnitude, and range of the colloidal interactions acting between the droplets as well as altering the susceptibility of the interfacial membrane to rupture. For example, the thickness of an emulsifier-polysaccharide membrane is usually much greater than that of an emulsifier membrane, thereby increasing the steric repulsion between the droplets. Emulsion scientists can utilize this interfacial engineering technology to rationally design droplets that * Corresponding author: Tel (413) 545-1009; Fax (413) 545-1262; e-mail
[email protected]. (1) Dickinson, E. Mixed Polymers at Interface in Biopolymer Mixtures; Nottingham University Press: Leicestershire, UK, 1995. (2) Dickinson, E. Food Hydrocolloid 2003, 17, 25. (3) Gu, Y. S.; Decker, E. A.; McClements, D. J. J. Agric. Food Chem. 2004, 52, 3626.
have specific interfacial characteristics known to improve emulsion stability and properties. For example, interfacial engineering could be used to improve the stability of oilin-water emulsions to environmental stresses (such as freezing, drying, thermal processing, pH extremes, or high salt concentrations) or to create emulsions with novel functional properties (such as encapsulation or controlled release). Nevertheless, further work is required to identify the influence of emulsifier type, polysaccharide type, solution composition, and preparation conditions on the properties of the emulsions produced using this technology. In previous studies, we have shown that emulsions stabilized by multilayered interfacial membranes can be produced using a variety of food-grade emulsifiers and biopolymers. Emulsions stabilized by surfactant-chitosan membranes or by surfactant-chitosan-pectin membranes were shown to have good stability to pH, ionic strength, thermal processing, and freezing.4-7 Emulsions stabilized by β-lactoglobulin-pectin membranes were shown to have better stability than those stabilized by β-lactoglobulin membranes at certain pH’s and ionic strengths.8,9 Recently, (4) Ogawa, S.; Decker, E. A.; McClements, D. J. J. Agric. Food Chem. 2003, 51, 2806. (5) Ogawa, S.; Decker, E. A.; McClements, D. J. J. Agric. Food Chem. 2003, 51, 5522. (6) Ogawa, S.; Decker, E. A.; McClements, D. J. J. Agric. Food Chem. 2004, 52, 3595. (7) Aoki, T.; Decker, E. A.; McClements, D. J. J. Agric. Food Chem., in press. (8) Moreau, L.; Kim, H. Y.; Decker, E. A.; McClements, D. J. J. Agric. Food Chem. 2003, 51, 6612. (9) Guzgy, D.; Kim, H. Y.; McClements, D. J. Food Hydrocolloid, in press.
10.1021/la048609r CCC: $27.50 © 2004 American Chemical Society Published on Web 09/21/2004
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we have shown that emulsions stabilized by β-lactoglobulin-ι-carrageenan membranes can be prepared at pH 6 and that these emulsions have better flocculation stability than those stabilized by β-lactoglobulin membranes alone under certain solution conditions.10 In the present study, we examine the influence of thermal processing and NaCl on the stability of emulsions stabilized by β-lactoglobulin-ι-carrageenan membranes, so as to determine whether this polysaccharide can be used to improve the thermal stability of whey protein stabilized emulsions. Experimental Sections Materials. Food-grade ι-carrageenan was kindly donated by FMC BioPolymer (Philadelphia, PA). The manufacturers reported that this sample was in almost pure sodium form with a low amount of contamination from other materials (50 °C), thus confirming the ζ-potential measurements (Figure 1). It should be stressed that the analysis of the emulsions was carried out after they had been cooled to room temperature, so that the desorption of the polysaccharides from the droplet surfaces that occurred at high temperatures seemed to be thermally irreversible (at least on the experimental time scale). This suggests that there was a change in the surface characteristics of the droplets (e.g., due to protein denaturation) and/or the conformation of the polysaccharides (e.g., due to a helixcoil transition) at elevated temperatures that weakened the ι-carrageenan-β-Lg interactions. Differential scanning calorimetry studies were carried out to provide further insight into this phenomenon (see below). The (15) Liu, T. X.; Relkin, R.; Launay, B. Thermochim. Acta 1994, 246, 387.
Gu et al.
Figure 4. Influence of isothermal heat treatment (30-90 °C, 20 min), salt concentration (0 or 150 mM NaCl) on the mean particle diameter (d43) of primary (1°: 5 wt % corn oil, 0.5 wt % β-Lg), and secondary (2°: 5 wt % corn oil, 0.5 wt % β-Lg + 0.1 wt % ι-carrageenan) emulsions at pH 6.
magnitude of the increase in free ι-carrageenan concentration with increasing temperature was considerably higher for the emulsion containing 150 mM NaCl than the one containing 0 mM NaCl. This suggests that the NaCl may have weakened the electrostatic attraction between the β-Lg-coated oil droplets and the ιcarrageenan, thereby facilitating polysaccharide desorption at high temperatures. Droplet Aggregation. The mean particle diameters and microstructure of the primary and secondary emulsions after thermal processing were measured (Figures 4 and 5). In the absence of NaCl, the mean particle diameters in both the primary and secondary emulsions were relatively small and independent of temperature (d43 ) 0.35-0.38 µm). Under these low ionic strength conditions there is a strong electrostatic repulsion between the droplets, which prevents them from coming close enough together to aggregate.16 In the primary emulsions, the mean particle diameters were appreciably larger in the presence of 150 mM NaCl than in the absence of salt, indicating that substantial droplet flocculation had occurred (Figure 4), which was confirmed by optical microscopy (Figure 5). Appreciable droplet flocculation (d43 ≈ 8 µm) occurred at relatively low temperatures (
