Progress 09/01/02 to 08/31/06
Outputs
Overview: During the course of this award, interfacial phenomena associated with the adsorption of beta-casein were studied using several techniques, interfacial rheology, atomic force microscopy (AFM), infrared reflectance absorbance spectroscopy (IRRAS), and zeta potential measurements. Interfacial rheology of beta-casein: A magnetic rod interfacial shear rheometer was used to measure the properties of beta-casein adsorbed at the air/solution interface as a function of aging time. Over a wide range of concentrations the initial rheology of the adsorbed surface layer was dominated by a viscous response of the interface. Interfacial gelation was observed after about 15 h of aging, long after the surface tension had stabilized. Although both components of the complex interfacial shear modulus (i.e. the storage and loss moduli) gradually increased with aging time, the ratio of the loss to the storage modulus - the loss tangent - decreased and dropped below unity. The
frequency dependence of the shear modulus was consistent with sol-gel transitions observed in bulk systems and described within the context of percolation theory. Structural evolution of beta-casein at the air-water interface: Adsorbed layers of beta-casein were transferred to solid supports and imaged by AFM. The aging of the layer was accompanied by the formation of distinct disk-shaped protein nanoparticles (~20 nm in diameter). Under conditions where a gelled layer was expected we observed ordering of the particles and the formation of elongated aggregates or linear rows. Brewster angle microscopy images were also obtained during the adsorption and gelation processes and during the degradation of the protein layer following addition of the surfactant sodium dodecyl sulfate (SDS). If SDS was added prior to interfacial protein gelation, the layer developed a foam-like morphology consistent with a fluid interfacial protein layer. However, if SDS was added after gelation, the protein
layer was observed to fracture, consistent with the behavior of a solid phase. Interfacial interactions of beta-casein with calcium ions: In the absence of calcium, a weak interfacial gel was found to form after about 2.5 h. Also in the absence of calcium, the adsorbed beta-casein film exhibited some degree of both intra- and intermolecular structural organization. For example, IRRAS spectra showed a measurable amount of ?-helix content; and AFM images indicated the presence of interfacial aggregates with a characteristic lateral length scale of 20-30 nm, which we interpret as hemi-micelles. At Ca:beta-casein molar ratios above ~5:1, a stronger interfacial gel formed twice as quickly. Also under these conditions there was little evidence of structural organization. On the basis of these findings, we hypothesize that calcium binding destabilizes the coupled intra- and intermolecular structural organization, and that the loss of organization permits more rapid interfacial gelation.
These phenomena are characteristic of the air-water interface; they are not accompanied by analogous structural changes in bulk solution.
Impacts
The detailed mechanistic understanding of food protein interfacial and colloidal phenomena will ultimately lead to new applications for these biomolecules. In particular, the characterization and control of the ultra-thin gel layer that forms spontaneously at the surface of protein solutions will suggest novel ways in which these food components can be used to stabilize other food products or even non-food emulsions such as paints or coatings.
Publications
- Bantchev, G.B. and Schwartz, D.K. Structure of beta-casein layers at the air/solution interface: Atomic Force Microscopy studies of transferred layers, Langmuir, 20:11692-11697 (2004).
- Bantchev, G.B. and Schwartz, D.K. Surface rheology of beta-casein layers at the air/solution interface: Formation of a two-dimensional physical gel. Langmuir 19:2673-2682 (2003).
- Vessely, C.R., Carpenter, J.F. and Schwartz, D.K. Calcium-Induced Changes to Molecular Conformation and Aggregate Structure of beta-Casein at the Air-Water Interface, Biomacromolecules, 6, 3334-3344 (2005).
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Progress 10/01/04 to 09/30/05
Outputs
The influence of calcium on interactions of beta-casein at the air-water interface was studied by several techniques, including interfacial rheology, atomic force microscopy (AFM), infrared reflectance absorbance spectroscopy (IRRAS), and zeta potential measurements. In the absence of calcium, a weak interfacial gel was found to form after about 2.5 h. Also in the absence of calcium, the adsorbed beta-casein film exhibited some degree of both intra- and intermolecular structural organization. For example, IRRAS spectra showed a measurable amount of alpha-helix content; and AFM images indicated the presence of interfacial aggregates with a characteristic lateral length scale of 20-30 nm, which we interpret as hemi-micelles. Upon the addition of calcium, particularly at Ca:beta-casein molar ratios above 5:1, a stronger interfacial gel formed more quickly; for example the interfacial shear moduli increased twice as rapidly. Also under these conditions (5:1 Ca:beta-casein
ratio) there was little evidence of structural organization, i.e. the alpha-helix peaks were very weak and AFM images showed a disordered, but continuous film, without distinct hemi-micelles. On the basis of these findings, we hypothesize that calcium binding destabilizes the coupled intra- and intermolecular structural organization, and that the loss of organization permits more rapid interfacial gelation. These phenomena are characteristic of the air-water interface; they are not accompanied by analogous structural changes in bulk solution.
Impacts
The detailed mechanistic understanding of food protein interfacial and colloidal phenomena will ultimately lead to new applications for these biomolecules. In particular, the characterization and control of the ultra-thin gel layer that forms spontaneously at the surface of protein solutions will suggest novel ways in which these food components can be used to stabilize other food products or even non-food emulsions such as paints or coatings.
Publications
- Bantchev, G.B. and Schwartz, D.K. "Surface rheology of beta-casein layers at the air/solution interface: Formation of a two-dimensional physical gel" Langmuir 19:2673-2682 (2003).
- Bantchev, G.B. and Schwartz, D.K. "Structure of beta-casein layers at the air/solution interface: Atomic Force Microscopy studies of transferred layers" Langmuir, 20:11692-11697 (2004)
- Vessely, C.R., Carpenter, J.F. and Schwartz, D.K. "Calcium-Induced Changes to Molecular Conformation and Aggregate Structure of beta-Casein at the Air-Water Interface" Biomacromolecules, 6, 3334-3344 (2005)
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Progress 10/01/03 to 09/30/04
Outputs
Adsorbed layers of beta-casein were transferred to solid supports and imaged by atomic force microscopy. The aging of the layer was accompanied by the formation of distinct disk-shaped protein nanoparticles (20 nm in diameter). Under conditions where a gelled layer was expected (from previous interfacial rheology experiments also supported by this award) we observed ordering of the particles and the formation of elongated aggregates or linear rows. Brewster angle microscopy images were also obtained during the adsorption and gelation processes and during the degradation of the protein layer following addition of the surfactant sodium dodecyl sulfate (SDS). If SDS was added prior to interfacial protein gelation, the layer developed a foam-like morphology consistent with a fluid interfacial protein layer. However, if SDS was added after gelation, the protein layer was observed to fracture, consistent with the behavior of a solid phase. These results provide insight into
the fundamental nanoscale mechanisms of interfacial gelation for this important dairy protein.
Impacts
The detailed mechanistic understanding of food protein interfacial and colloidal phenomena will ultimately lead to new applications for these biomolecules. In particular, the characterization and control of the ultra-thin gel layer that forms spontaneously at the surface of protein solutions will suggest novel ways in which these food components can be used to stabilize other food products or even non-food emulsions such as paints or coatings.
Publications
- Bantchev, G.B. and Schwartz, D.K. Structure of beta-casein layers at the air/solution interface: Atomic Force Microscopy studies of transferred layers Langmuir, 20:11692-11697 (2004)
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Progress 10/01/02 to 09/30/03
Outputs
A magnetic rod interfacial shear rheometer was used to measure the properties of beta-casein adsorbed at the air/solution interface as a function of aging time. Over a wide range of concentrations the initial rheology of the adsorbed surface layer was dominated by a viscous response of the interface. Interfacial gelation was observed after about 15 h of aging, long after the surface tension had stabilized. In particular, although both components of the complex interfacial shear modulus (i.e. the storage and loss moduli) gradually increased with aging time, the ratio of the loss to the storage modulus - the loss tangent - decreased and dropped below unity. The frequency dependence of the shear modulus wass consistent with sol-gel transitions observed in bulk systems and described within the context of percolation theory. The nanostructural changes associated with the gelation were also investigated. Adsorbed layers were transferred to solid supports and imaged by
atomic force microscopy. Several transfer methods were investigated. Our data suggested that the process of protein adsorption or incorporation in the layer continued even after 48 h of aging. The aging of the layer was accompanied by the formation of protein nanoparticles (about 20 nm in diameter). Under conditions where a gelled layer was expected (from the rheological experiments) we observed ordering of the particles and the formation of elongated aggregates or linear rows. Brewster angle microscopy images were obtained during the adsorption and gelation processes and during the degradation of the protein layer following addition of the surfactant sodium dodecyl sulfate (SDS). If SDS was added prior to interfacial protein gelation, the layer developed a foam-like morphology consistent with a fluid interfacial protein layer. However, if SDS was added after gelation, the protein layer was observed to fracture, consistent with the behavior of a solid phase.
Impacts
The detailed mechanistic understanding of food protein interfacial and colloidal phenomena will ultimately lead to new applications for these biomolecules. In particular, the characterization and control of the ultra-thin gel layer that forms spontaneously at the surface of protein solutions will suggest novel ways in which these food components can be used to stabilize other food products or even non-food emulsions such as paints or coatings.
Publications
- Bantchev, G.B. and Schwartz, D.K. Surface rheology of beta-casein layers at the air/solution interface: Formation of a two-dimensional physical gel. Langmuir 19:2673-2682 (2003).
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