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Langmuir 2006, 22, 7480-7486
Irreversible Thermal Denaturation of β-Lactoglobulin Retards Adsorption of Carrageenan onto β-Lactoglobulin-Coated Droplets 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 April 14, 2006. In Final Form: June 29, 2006 The influence of isothermal heat treatments on the adsorption of anionic carrageenan molecules to the surfaces of anionic β-lactoglobulin-coated droplets has been investigated. The ζ-potential, mean particle diameter, microstructure, and creaming stability of emulsions containing β-lactoglobulin-coated droplets and/or carrageenan molecules that had previously been heat treated at temperatures ranging from 30 to 90 °C for 20 min were measured (pH 6.0, 150 mM NaCl). Three different heat treatments were used to establish the physicochemical origin of the influence of thermal history on the adsorption of carrageenan molecules to the protein coated droplets: (i) droplets and carrageenan were mixed at room temperature, then heated together; (ii) droplets were heated, cooled to room temperature, then mixed with carrageenan; (iii) carrageenan was heated, cooled to room temperature, then mixed with droplets. For treatments i and ii appreciably more carrageenan adsorbed to the protein-coated droplet surfaces at temperatures e 60 °C than at higher temperatures. For treatment iii, carrageenan adsorbed to the droplet surfaces across the whole temperature range. These results suggest that an irreversible thermal denaturation of the adsorbed β-lactoglobulin molecules inhibited the adsorption of carrageenan molecules to the droplet surfaces. We postulate that there is a patch of positive charge on the surface of the native globular protein molecules which becomes more diffuse upon thermal denaturation. We found that the carrageenan molecules were unable to protect the β-lactoglobulin-coated droplets at high temperatures (T > 60 °C) because they desorbed from the droplet surfaces. Nevertheless, adsorption of ι-carrageenan was capable of protecting the droplets against flocculation caused by surface denaturation of the adsorbed proteins at lower temperatures (T e 50 °C).
Introduction Electrically charged polysaccharides can adsorb to the surface of oppositely charged emulsifier-coated oil droplets through electrostatic attraction.1-3 Polysaccharide adsorption to droplet surfaces can be either detrimental or beneficial to emulsion stability depending on preparation conditions and system composition. Under certain conditions, adsorbed polysaccharides promote droplet flocculation by acting as “bridges” that link two or more droplets together.2,4-8 Under other conditions, polysaccharides interact with the adsorbed emulsifier to form interfacial complexes that improve emulsion stability.3,7-15 In the latter * To whom correspondence should be addressed. Phone: (413) 5451009. Fax: (413) 545-1262. E-mail:
[email protected]. (1) Benichou, A.; Aserin, A.; Garti, N. J. Dispersion Sci. Technol. 2002, 23, 93. (2) Dickinson, E. Food Hydrocolloids 2003, 17, 25. (3) Gu, Y. S.; Decker, A. E.; McClements, D. J. Langmuir 2005, 21, 5752. (4) Dickinson, E.; Semenova, M. G.; Antipova, A. S.; Pelan, E. G. Food Hydrocolloids 1998, 12, 425. (5) Dickinson, E.; Pawlowsky, K. Food Hydrocolloids 1998, 12, 417. (6) Galazka, V. B.; Smith, D.; Ledward, D. A.; Dickinson, E. Food Hydrocolloids 1999, 13, 81. (7) Gu, Y. S.; Decker, E. A.; McClements, D. J. Langmuir 2004, 20, 9565. (8) Moreau, L.; Kim, H. J.; Decker, E. A.; McClements, D. J. J. Agric. Food Chem. 2003, 51, 6612. (9) Gu, Y. S.; Decker, E. A.; McClements, D. J. J. Agric. Food Chem. 2004, 52, 3626. (10) Gu, Y. S.; Regnier, L.; McClements, D. J. J. Colloid Interface Sci. 2005, 286, 551. (11) Gu, Y. S.; Decker, E. A.; McClements, D. J. Food Hydrocolloids 2005, 19, 83. (12) Gu, Y. S.; Corradini, M. G.; McClements, D. J.; DesRochers, J. J. Agric. Food Chem. 2006, 54, 417. (13) Guzey, D.; Kim, H. J.; McClements, D. J. Food Hydrocolloids 2004, 18, 967. (14) Klinkesorn, U.; Sophanodora, P.; Chinachoti, P.; Decker, E. A.; McClements, D. J. Food Hydrocolloids 2005, 19, 1044. (15) Klinkesorn, U.; Sophanodora, P.; Chinachoti, P.; McClements, D. J.; Decker, E. A. J. Agric. Food Chem. 2005, 53, 8365.
case, adsorption of polysaccharides to the surface of the emulsifiercoated oil droplets alters the composition, structure, thickness, rheology, and electrical characteristics of the interfacial membrane.16-20 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. Emulsion scientists can utilize this interfacial engineering technology to rationally design droplets that have specific interfacial characteristics known to improve emulsion stability and properties. For example, interfacial engineering could be used to improve the stability of oil-inwater emulsions to environmental stresses (such as freezing, drying, thermal processing, pH extremes, or high salt concentrations) or create emulsions with novel functional properties (such as encapsulation, protection, and/or controlled release).10,20-23 Recently, we have shown that emulsions containing droplets coated by β-lactoglobulin-carrageenan interfaces can be prepared at pH 6 and that these emulsions have better flocculation stability than those stabilized by β-lactoglobulin interfaces alone under certain solution conditions.3,7,9-11 As part of these studies we examined the influence of heating on the ability of ι-carrageenan to adsorb to the surfaces of β-lactoglobulin-coated droplets.7 We (16) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3427. (17) Caruso, F.; Furlong, D. N.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1998, 14, 4559. (18) Caruso, F.; Mohwald, H. J. Am. Chem. Soc. 1999, 121, 6039. (19) Decher, G.; Essler, F.; Hong, J. D.; Lowack, K.; Schmitt, J.; Lvov, Y. Abstr. Pap.sAm. Chem. Soc. 1993, 205, 334. (20) Decher, G.; Schlenoff, J. B. Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Wiley-VCH: Weinheim; 2003. (21) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (22) Caruso, F.; Wang, D.; Liang, Z. J.; Yu, A. Abstr. Pap.sAm. Chem. Soc. 2002, 223, U379. (23) Surh, J.; Gu, Y. S.; Decker, E. A.; McClements, D. J. J. Agric. Food Chem. 2005, 53, 4236.
10.1021/la061021v CCC: $33.50 © 2006 American Chemical Society Published on Web 08/05/2006
Adsorption of Carrageenan
found that carrageenan molecules adsorbed to the droplet surfaces at room temperature and that they remained adsorbed after the emulsions had been heated to temperatures ranging from 30 to 60 °C but that they became desorbed after the emulsions had been heated to higher temperatures. We also found that the stability of the emulsions to droplet flocculation was improved at temperatures where the carrageenan molecules remained adsorbed to the droplet surfaces. In our previous studies we were not able to identify the physicochemical origin of desorption of carrageenan molecules from the droplet surfaces at temperatures exceeding 60 °C. It was suggested that this may have been caused by (i) a conformational change of the adsorbed proteins (thermal denaturation) and (ii) a conformational change of the carrageenan molecules (helix-to-coil transition). The purpose of the present study is to provide a deeper understanding of the physicochemical origin of this desorption phenomenon. In the present study we examine the influence of carrageenan type (ι or λ), thermal holding temperature (30-90 °C), and thermal history of the protein-coated droplets and/or carrageenan molecules on polysaccharide adsorption and emulsion stability. These two types of carrageenan were selected because they have different molecular characteristics.24,25 The ι- and λ-carrageenan molecules are linear polysaccharides that have two and three sulfate groups per disaccharide, respectively. The ι-carrageenan undergoes a reversible coil-to-helix transition in aqueous solution around 85 °C,7 while the λ-carrageenan has a flat ribbonlike conformation.24 β-Lactoglobulin (β-Lg) is a compact globular protein (molecular mass ) 18.3 kDa) obtained from the whey fraction of cow’s milk. It has an isoelectric point of around 4.75.2 and undergoes an irreversible thermal transition around 75 °C in solution7 and in emulsions.26,27 Experimental Sections Materials. Food-grade ι- and λ-carrageenan were 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 minerals (
