Hans Tromp - Academia.edu (original) (raw)
Papers by Hans Tromp
Food Biophysics, 2014
The disruption of casein micelles, as found in cows' milk, was investigated at pressures up to 30... more The disruption of casein micelles, as found in cows' milk, was investigated at pressures up to 300 MPa with small angle neutron scattering (SANS). From the decrease of the overall level of scattering, the expected disruption of the micelles was concluded. This disruption was incomplete, and stable at 300 MPa, independent of pressure history. At intermediate pressures, pressure history, (step-wise increase from ambient, or stepwise decrease from 300 MPa) had a strong effect on the scattering patterns. At high pressures, where scattering patterns were sufficiently structured, sizes of scattering entities could be deduced. The scattering entities at high pressure showed a bimodal size distribution. One size was outside the window of quantitative observation and may be similar to that of the native micelles or residual fat globules, the other smaller size, found under high pressure is in the 10-20 nm range and may correspond clusters of about 10 separate casein molecules. The pressure-induced disruption of casein micelles is partially reversed after return to ambient pressure, but small scattering entities, absent before pressure treatment, remain.
Annual review of food science and technology, Jan 10, 2015
The sensory perception of texture is an important contributor of our general appreciation of food... more The sensory perception of texture is an important contributor of our general appreciation of foods. Food texture is mainly described in terms of mouthfeel and afterfeel attributes. The role of oral processing in the perception of texture and the role of microstructure therein have been reviewed regularly over recent years ( Chen & Engelen 2012 , Foegeding et al. 2011 , Stieger & van de Velde 2013 ) and are not, therefore, addressed in this review. The scope of this review relates to the molecules that underlay the texture of foods. Protein, carbohydrate, and fat are the major structuring components in foods. In this review we focus on the physical interactions between proteins and polysaccharides that form the basis for the microstructure and texture of these foods. In general, food products are classified in four categories by their sensory and rheological properties: liquids, semisolids, soft solids, and hard solids ( van Vliet et al. 2009 ). These four categories provide a useful...
Journal of the Science of Food and Agriculture, 2002
ABSTRACT
Carbohydrate Research, 2005
This paper describes the effect of the j/i-ratio on the physical properties of j/i-hybrid carrage... more This paper describes the effect of the j/i-ratio on the physical properties of j/i-hybrid carrageenans (synonyms: kappa-2, j-2, weak kappa, weak gelling kappa). To this end, a series of j/i-hybrid carrageenans ranging from almost homopolymeric j-carrageenan (98 mol-% j-units) to almost homopolymeric-carrageenan (99 mol-% i-units) have been extracted from selected species of marine red algae (Rhodophyta). The j/i-ratio of these j/i-hybrids was determined by NMR spectroscopy. Their rheological properties were determined by small deformation oscillatory rheology. The gel strength (storage modulus, G 0) of the j/i-hybrids decreases with decreasing j-content. On the other hand, the gelation temperature of the j-rich j/i-hybrids is independent of their composition. This allows one to control the gel strength independent of the gelation or melting temperature. The conformational order-disorder transition of the j/i-hybrids was studied using optical rotation and high-sensitivity differential scanning calorimetry. High-sensitivity DSC showed that the total transition enthalpy of the j/i-hybrids goes through a minimum at 60 mol-% j-units, whereas for the mixture of j-and i-carrageenan, the total transition enthalpy is a linear function of the composition. With respect to the ordering capability, the j/i-hybrid carrageenans seem to behave as random block copolymers with length sequence distributions truncated from the side of the small lengths. Intrinsic thermodynamic properties (e.g., transition temperature and enthalpy) of j-and i-sequences in these copolymers are close to those of their parent homopolymers. The critical sequence length for j-sequences is 2-fold of that for i-sequences.
Biopolymers, 2002
A series of iota-carrageenans containing different amounts of nu-carrageenan (0-23 monomer %) hav... more A series of iota-carrageenans containing different amounts of nu-carrageenan (0-23 monomer %) have been prepared from neutrally extracted carrageenan of Eucheuma denticulatum. nu-Carrageenan is the biochemical precursor of iota-carrageenan. The conformational order-disorder transition and rheological properties of these carrageenans were studied using optical rotation, rheometry, size exclusion chromatography coupled to multiangle laser light scattering, and high-sensitivity differential scanning calorimetry. The helix forming capacity of iota-carrageenan turns out to decrease monotonously with increasing amount of nu-units. In contrast, the rheological properties of iota-carrageenan are remarkably enhanced by the presence of a small amount of nu-units, yielding a maximum twofold increase in G' at 3% nu-units. It is concluded that the structure-forming capacity of iota-carrageenan, containing a small amount of nu-carrageenan, is significantly higher than that of pure iota-carrageenan. This phenomenon is explained in terms of the balance between the helical content and the number of cross-links between chains, taking into consideration the fact that nu-units introduce "kinks" in the chain conformation enabling neighboring chains to connect. Increasing amounts of nu-units increase the number of cross-links in the network, resulting in increased gel strength. On the other hand, a reduced length of the helical strands weakens the cross-links between the different chains and, consequently, the gel.
Food Research International, 2001
Demonstrations are given of the application of confocal scanning light microscopy (CSLM) on mixtu... more Demonstrations are given of the application of confocal scanning light microscopy (CSLM) on mixtures of gelatine with other food biopolymers. The various aspects of phase separation are visualised in two and three dimensional images. Contrast through fluorescence is obtained either by covalent labelling or non-covalent labelling. There is special focus on the system gelatine/dextran, which is shown to be a very suitable mixture for the study of the morphology of phase separated biopolymer mixtures.
Molecular Physics
The microscopic structure and dynamics of droplet dispersions formed by phase separation of two N... more The microscopic structure and dynamics of droplet dispersions formed by phase separation of two Newtonian polymer solutions were observed under simple shear. The phases had equal volumes. Under certain conditions, these dispersions show long, wormlike, extended structures after several minutes of shearing. The development of these structures was recorded in microscopic movies. These were interpreted by calculating two-point grey value correlation functions. The divergence of the correlation in the flow direction was taken as a signature of the onset of the extended structures. It was demonstrated that a viscosity ratio far from the unity of the two phases is necessary for the formation of the long shear-stabilised structures. At a viscosity ratio close to unity, the systems maintain isolated droplets in a dynamic equilibrium of coalescence and break-up. The structure formation is the result of shear-induced coalescence. The need for a difference in viscosity between the phases suggests a relation with shear banding seen in shear-thinning systems. GRAPHICAL ABSTRACT
Food Hydrocolloids
Abstract To predict in-mouth perception of semi-solid foods like yogurt, their lubricating behavi... more Abstract To predict in-mouth perception of semi-solid foods like yogurt, their lubricating behavior needs to be understood. In food industrial applications of oral tribology, it is preferable and more efficient to use a material with a hydrophilic surface to mimic the tongue in the in vitro setup. We investigated and compared whey protein isolate (WPI) gel and polydimethylsiloxane (PDMS), based on their physicochemical characteristics and their ability to discriminate the friction behavior of a set of commercial yogurts known to elicit different mouthfeels. Stiffness and wettability of WPI gel were more similar to the human tongue than PDMS. The friction factors of Newtonian fluids measured on WPI gel allowed to construct a master Stribeck curve; this was not the case for PDMS. The yogurts could be clearly discriminated when measured on WPI gel. Their friction factors in the mixed regime correlated to the fat content and to perceived creaminess. Yogurts’ discrimination was not optimal on PDMS. The measurements on WPI gel, done in the speed range 1–0.001 m/s, were compared to a second speed range (0.2–0.002 m/s), believed to be more relevant for in-mouth conditions. Yet, the first speed range gave the best discrimination, also between samples with small differences in fat content (semi vs full-fat). Statistical analyses were performed on the datasets obtained using WPI gel, for a detailed characterization of its discriminative predictive power, demonstrating that the average friction in the mixed regime, and the slope in the mixed regime are the most discriminating features for tribological behavior.
Physical Review Letters
Using fluorescence microscopy we study the adsorption of single latex microparticles at a water/w... more Using fluorescence microscopy we study the adsorption of single latex microparticles at a water/water interface between demixing aqueous solutions of polymers, generally known as a water-in-water emulsion. Similar microparticles at the interface between molecular liquids have exhibited an extremely slow relaxation preventing the observation of expected equilibrium states. This phenomenon has been attributed to "long-lived" metastable states caused by significant energy barriers ∆F ∼ γA d kBT induced by high interfacial tension (γ ∼ 10 −2 N/m) and nanoscale surface defects with characteristic areas A d 10-30 nm 2. For the studied water/water interface with ultra-low surface tension (γ ∼ 10 −4 N/m) we are able to characterize the entire adsorption process and observe equilibrium states prescribed by a single equilibrium contact angle independent of the particle size. Notably, we observe crossovers from fast initial dynamics to slower kinetic regimes analytically predicted for large surface defects (A d 500 nm 2). Moreover, particle trajectories reveal a position-independent damping coefficient that is unexpected given the large viscosity contrast between phases. These observations are attributed to the remarkably diffuse nature of the water/water interface and the adsorption and entanglement of polymer chains in the semidilute solutions. This work offers some first insights on the adsorption dynamics/kinetics of microparticles at water/water interfaces in bio-colloidal systems.
Food Hydrocolloids, 2016
We synthesize colloidal zein particles using the anti-solvent precipitation method and study part... more We synthesize colloidal zein particles using the anti-solvent precipitation method and study particle behavior at water-water interfaces. When added to phase-separating aqueous mixtures of fish gelatin and dextran, particles accumulate at the interface. In order to explain the mechanism of particle accumulation at the water-water interface, we investigate how zein particles interact with polymers (i.e. fish gelatin and dextran). We show that both polymers adsorb similarly on particle surface, which can explain why particles form contact angles close to 90°. Moreover, we show that particle accumulation is accompanied by aggregation. Those aggregates are able to arrest the late stage of the demixing process of the emulsion by the formation of a stable particle-rich layer at the water-water interface. This layer is referred as a ‘foam-like layer’ due to its morphology similar to that of a wet (non-drained) foam, and contains droplets of one phase, surrounded by particle-stabilized lamellae of the other phase.
Macromolecules, 2015
Upon demixing, an aqueous solution of a polyelectrolyte and an incompatible neutral polymer yield... more Upon demixing, an aqueous solution of a polyelectrolyte and an incompatible neutral polymer yields two phases separated by an interface with an ultralow tension. Here, both in theory and experiment, we study this interfacial tension in detail: how it scales with the concentrations of the polymers in the two phases and how it is affected by the interfacial difference in the electrical potential. Experiments are performed on an aqueous model system of uncharged dextran and charged nongelling gelatin. The experimental tension scales to the power ∼3 with the tie-line length in the phase diagram of demixing, in agreement with mean-field theory where space is filled with a binary mixture of polymer blobs. The interfacial electrical potential difference is experimentally found to decrease the interfacial tension in a way that is consistent with Poisson–Boltzmann theory inspired from Frenkel and Verwey–Overbeek.
The Journal of Physical Chemistry
ABSTRACT
Gums and Stabilisers for the Food Industry 12, 2004
Soft matter, Jan 4, 2015
Stabilization of gas bubbles in water by applying solid particles is a promising technique to ens... more Stabilization of gas bubbles in water by applying solid particles is a promising technique to ensure long-term stability of the dispersion against coarsening. However, the production of large quantities of particle stabilized bubbles is challenging. The delivery of particles to the interface must occur rapidly compared to the typical time scale of coarsening during production. Furthermore, the production route must be able to overcome the energy barriers for interfacial adsorption of particles. Here we demonstrate that ultrasound can be applied to agitate a colloidal dispersion and supply sufficient energy to ensure particle adsorption onto the air-water interface. With this technique we are able to produce micron-sized bubbles, solely stabilized by particles. The interface of these bubbles is characterized by a colloidal shell, a monolayer of particles which adopt a hexagonal packing. The particles are anchored to the interface owing to partial wetting and experience lateral compre...
Food Hydrocolloids, 2015
This study looks at the influence of reduced levels of hydration as a driving force for transitio... more This study looks at the influence of reduced levels of hydration as a driving force for transitions in the secondary structure of hydrated proteins. A simple protein-water system was used to study the conditions of typical protein-rich dairy food systems at a fixed pH level, salt content, and temperature. Freezedried beta-lactoglobulin (Type A) from bovine milk was dissolved directly into two different buffer systems over a wide range of concentrations between 1 mg/ml (~54 mM) and 200 mg/ml (~0.01 M) but at a fixed pH level, pH 3. Circular dichroism (CD), attenuated total reflectance Fourier transform infrared spectroscopy (ATR FTIR), and thioflavin T (ThT) Assay fluorescence spectroscopy were used to measure changes in the secondary structure with respect to protein solution concentration at 20 C. The findings of all of the techniques indicate that the majority of the secondary structure changes occur within the low protein concentration regime (i.e. <50 mg/ml) before a critical aggregation threshold. Dimerisation, formed by besheet cross-linking, is the likeliest mechanism of aggregation. The formation of dimers however counters the current assumption that at pH 3 only monomers exist; rather it seems there is a gradual evolution of the monomeric unfolded state with increasing concentration occurs yielding a bsheet rich refolded aggregate. Most interesting is the low concentration region (i.e. between 1 mg/ml and 40 mg/ml) where most secondary structural alterations are found to occur; before physical crowding effects are possible. The results indicate that BLG has a limited solubility even in a low concentration regime.
Carbohydrate …, 2001
The coil-to-helix transition and temperature dependence of the viscosity of commercial kappa/iota... more The coil-to-helix transition and temperature dependence of the viscosity of commercial kappa/iota-hybrid carrageenans produced by the red algae Sarcothalia crispata, Mazaella laminarioides, and Chondrus crispus were studied using rheometry and optical rotation. The structure of these kappa/iota-hybrid carrageenans was determined by 1H and 13C NMR spectroscopy combined with monosaccharide composition analysis. The coil-to-helix transitions, measured by polarimetry and rheometry, of the kappa/iota-hybrid carrageenans are significantly different from those of pure kappa- and iota-carrageenan, and from hand-made mixtures thereof. This provides evidence that the kappa/iota-hybrid carrageenans are mixed chains, containing both kappa- and iota-repeating units.
Macromolecules, 1995
ABSTRACT
The Journal of Chemical Physics, 1992
ABSTRACT
Food Biophysics, 2014
The disruption of casein micelles, as found in cows' milk, was investigated at pressures up to 30... more The disruption of casein micelles, as found in cows' milk, was investigated at pressures up to 300 MPa with small angle neutron scattering (SANS). From the decrease of the overall level of scattering, the expected disruption of the micelles was concluded. This disruption was incomplete, and stable at 300 MPa, independent of pressure history. At intermediate pressures, pressure history, (step-wise increase from ambient, or stepwise decrease from 300 MPa) had a strong effect on the scattering patterns. At high pressures, where scattering patterns were sufficiently structured, sizes of scattering entities could be deduced. The scattering entities at high pressure showed a bimodal size distribution. One size was outside the window of quantitative observation and may be similar to that of the native micelles or residual fat globules, the other smaller size, found under high pressure is in the 10-20 nm range and may correspond clusters of about 10 separate casein molecules. The pressure-induced disruption of casein micelles is partially reversed after return to ambient pressure, but small scattering entities, absent before pressure treatment, remain.
Annual review of food science and technology, Jan 10, 2015
The sensory perception of texture is an important contributor of our general appreciation of food... more The sensory perception of texture is an important contributor of our general appreciation of foods. Food texture is mainly described in terms of mouthfeel and afterfeel attributes. The role of oral processing in the perception of texture and the role of microstructure therein have been reviewed regularly over recent years ( Chen & Engelen 2012 , Foegeding et al. 2011 , Stieger & van de Velde 2013 ) and are not, therefore, addressed in this review. The scope of this review relates to the molecules that underlay the texture of foods. Protein, carbohydrate, and fat are the major structuring components in foods. In this review we focus on the physical interactions between proteins and polysaccharides that form the basis for the microstructure and texture of these foods. In general, food products are classified in four categories by their sensory and rheological properties: liquids, semisolids, soft solids, and hard solids ( van Vliet et al. 2009 ). These four categories provide a useful...
Journal of the Science of Food and Agriculture, 2002
ABSTRACT
Carbohydrate Research, 2005
This paper describes the effect of the j/i-ratio on the physical properties of j/i-hybrid carrage... more This paper describes the effect of the j/i-ratio on the physical properties of j/i-hybrid carrageenans (synonyms: kappa-2, j-2, weak kappa, weak gelling kappa). To this end, a series of j/i-hybrid carrageenans ranging from almost homopolymeric j-carrageenan (98 mol-% j-units) to almost homopolymeric-carrageenan (99 mol-% i-units) have been extracted from selected species of marine red algae (Rhodophyta). The j/i-ratio of these j/i-hybrids was determined by NMR spectroscopy. Their rheological properties were determined by small deformation oscillatory rheology. The gel strength (storage modulus, G 0) of the j/i-hybrids decreases with decreasing j-content. On the other hand, the gelation temperature of the j-rich j/i-hybrids is independent of their composition. This allows one to control the gel strength independent of the gelation or melting temperature. The conformational order-disorder transition of the j/i-hybrids was studied using optical rotation and high-sensitivity differential scanning calorimetry. High-sensitivity DSC showed that the total transition enthalpy of the j/i-hybrids goes through a minimum at 60 mol-% j-units, whereas for the mixture of j-and i-carrageenan, the total transition enthalpy is a linear function of the composition. With respect to the ordering capability, the j/i-hybrid carrageenans seem to behave as random block copolymers with length sequence distributions truncated from the side of the small lengths. Intrinsic thermodynamic properties (e.g., transition temperature and enthalpy) of j-and i-sequences in these copolymers are close to those of their parent homopolymers. The critical sequence length for j-sequences is 2-fold of that for i-sequences.
Biopolymers, 2002
A series of iota-carrageenans containing different amounts of nu-carrageenan (0-23 monomer %) hav... more A series of iota-carrageenans containing different amounts of nu-carrageenan (0-23 monomer %) have been prepared from neutrally extracted carrageenan of Eucheuma denticulatum. nu-Carrageenan is the biochemical precursor of iota-carrageenan. The conformational order-disorder transition and rheological properties of these carrageenans were studied using optical rotation, rheometry, size exclusion chromatography coupled to multiangle laser light scattering, and high-sensitivity differential scanning calorimetry. The helix forming capacity of iota-carrageenan turns out to decrease monotonously with increasing amount of nu-units. In contrast, the rheological properties of iota-carrageenan are remarkably enhanced by the presence of a small amount of nu-units, yielding a maximum twofold increase in G&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; at 3% nu-units. It is concluded that the structure-forming capacity of iota-carrageenan, containing a small amount of nu-carrageenan, is significantly higher than that of pure iota-carrageenan. This phenomenon is explained in terms of the balance between the helical content and the number of cross-links between chains, taking into consideration the fact that nu-units introduce &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;kinks&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; in the chain conformation enabling neighboring chains to connect. Increasing amounts of nu-units increase the number of cross-links in the network, resulting in increased gel strength. On the other hand, a reduced length of the helical strands weakens the cross-links between the different chains and, consequently, the gel.
Food Research International, 2001
Demonstrations are given of the application of confocal scanning light microscopy (CSLM) on mixtu... more Demonstrations are given of the application of confocal scanning light microscopy (CSLM) on mixtures of gelatine with other food biopolymers. The various aspects of phase separation are visualised in two and three dimensional images. Contrast through fluorescence is obtained either by covalent labelling or non-covalent labelling. There is special focus on the system gelatine/dextran, which is shown to be a very suitable mixture for the study of the morphology of phase separated biopolymer mixtures.
Molecular Physics
The microscopic structure and dynamics of droplet dispersions formed by phase separation of two N... more The microscopic structure and dynamics of droplet dispersions formed by phase separation of two Newtonian polymer solutions were observed under simple shear. The phases had equal volumes. Under certain conditions, these dispersions show long, wormlike, extended structures after several minutes of shearing. The development of these structures was recorded in microscopic movies. These were interpreted by calculating two-point grey value correlation functions. The divergence of the correlation in the flow direction was taken as a signature of the onset of the extended structures. It was demonstrated that a viscosity ratio far from the unity of the two phases is necessary for the formation of the long shear-stabilised structures. At a viscosity ratio close to unity, the systems maintain isolated droplets in a dynamic equilibrium of coalescence and break-up. The structure formation is the result of shear-induced coalescence. The need for a difference in viscosity between the phases suggests a relation with shear banding seen in shear-thinning systems. GRAPHICAL ABSTRACT
Food Hydrocolloids
Abstract To predict in-mouth perception of semi-solid foods like yogurt, their lubricating behavi... more Abstract To predict in-mouth perception of semi-solid foods like yogurt, their lubricating behavior needs to be understood. In food industrial applications of oral tribology, it is preferable and more efficient to use a material with a hydrophilic surface to mimic the tongue in the in vitro setup. We investigated and compared whey protein isolate (WPI) gel and polydimethylsiloxane (PDMS), based on their physicochemical characteristics and their ability to discriminate the friction behavior of a set of commercial yogurts known to elicit different mouthfeels. Stiffness and wettability of WPI gel were more similar to the human tongue than PDMS. The friction factors of Newtonian fluids measured on WPI gel allowed to construct a master Stribeck curve; this was not the case for PDMS. The yogurts could be clearly discriminated when measured on WPI gel. Their friction factors in the mixed regime correlated to the fat content and to perceived creaminess. Yogurts’ discrimination was not optimal on PDMS. The measurements on WPI gel, done in the speed range 1–0.001 m/s, were compared to a second speed range (0.2–0.002 m/s), believed to be more relevant for in-mouth conditions. Yet, the first speed range gave the best discrimination, also between samples with small differences in fat content (semi vs full-fat). Statistical analyses were performed on the datasets obtained using WPI gel, for a detailed characterization of its discriminative predictive power, demonstrating that the average friction in the mixed regime, and the slope in the mixed regime are the most discriminating features for tribological behavior.
Physical Review Letters
Using fluorescence microscopy we study the adsorption of single latex microparticles at a water/w... more Using fluorescence microscopy we study the adsorption of single latex microparticles at a water/water interface between demixing aqueous solutions of polymers, generally known as a water-in-water emulsion. Similar microparticles at the interface between molecular liquids have exhibited an extremely slow relaxation preventing the observation of expected equilibrium states. This phenomenon has been attributed to "long-lived" metastable states caused by significant energy barriers ∆F ∼ γA d kBT induced by high interfacial tension (γ ∼ 10 −2 N/m) and nanoscale surface defects with characteristic areas A d 10-30 nm 2. For the studied water/water interface with ultra-low surface tension (γ ∼ 10 −4 N/m) we are able to characterize the entire adsorption process and observe equilibrium states prescribed by a single equilibrium contact angle independent of the particle size. Notably, we observe crossovers from fast initial dynamics to slower kinetic regimes analytically predicted for large surface defects (A d 500 nm 2). Moreover, particle trajectories reveal a position-independent damping coefficient that is unexpected given the large viscosity contrast between phases. These observations are attributed to the remarkably diffuse nature of the water/water interface and the adsorption and entanglement of polymer chains in the semidilute solutions. This work offers some first insights on the adsorption dynamics/kinetics of microparticles at water/water interfaces in bio-colloidal systems.
Food Hydrocolloids, 2016
We synthesize colloidal zein particles using the anti-solvent precipitation method and study part... more We synthesize colloidal zein particles using the anti-solvent precipitation method and study particle behavior at water-water interfaces. When added to phase-separating aqueous mixtures of fish gelatin and dextran, particles accumulate at the interface. In order to explain the mechanism of particle accumulation at the water-water interface, we investigate how zein particles interact with polymers (i.e. fish gelatin and dextran). We show that both polymers adsorb similarly on particle surface, which can explain why particles form contact angles close to 90°. Moreover, we show that particle accumulation is accompanied by aggregation. Those aggregates are able to arrest the late stage of the demixing process of the emulsion by the formation of a stable particle-rich layer at the water-water interface. This layer is referred as a ‘foam-like layer’ due to its morphology similar to that of a wet (non-drained) foam, and contains droplets of one phase, surrounded by particle-stabilized lamellae of the other phase.
Macromolecules, 2015
Upon demixing, an aqueous solution of a polyelectrolyte and an incompatible neutral polymer yield... more Upon demixing, an aqueous solution of a polyelectrolyte and an incompatible neutral polymer yields two phases separated by an interface with an ultralow tension. Here, both in theory and experiment, we study this interfacial tension in detail: how it scales with the concentrations of the polymers in the two phases and how it is affected by the interfacial difference in the electrical potential. Experiments are performed on an aqueous model system of uncharged dextran and charged nongelling gelatin. The experimental tension scales to the power ∼3 with the tie-line length in the phase diagram of demixing, in agreement with mean-field theory where space is filled with a binary mixture of polymer blobs. The interfacial electrical potential difference is experimentally found to decrease the interfacial tension in a way that is consistent with Poisson–Boltzmann theory inspired from Frenkel and Verwey–Overbeek.
The Journal of Physical Chemistry
ABSTRACT
Gums and Stabilisers for the Food Industry 12, 2004
Soft matter, Jan 4, 2015
Stabilization of gas bubbles in water by applying solid particles is a promising technique to ens... more Stabilization of gas bubbles in water by applying solid particles is a promising technique to ensure long-term stability of the dispersion against coarsening. However, the production of large quantities of particle stabilized bubbles is challenging. The delivery of particles to the interface must occur rapidly compared to the typical time scale of coarsening during production. Furthermore, the production route must be able to overcome the energy barriers for interfacial adsorption of particles. Here we demonstrate that ultrasound can be applied to agitate a colloidal dispersion and supply sufficient energy to ensure particle adsorption onto the air-water interface. With this technique we are able to produce micron-sized bubbles, solely stabilized by particles. The interface of these bubbles is characterized by a colloidal shell, a monolayer of particles which adopt a hexagonal packing. The particles are anchored to the interface owing to partial wetting and experience lateral compre...
Food Hydrocolloids, 2015
This study looks at the influence of reduced levels of hydration as a driving force for transitio... more This study looks at the influence of reduced levels of hydration as a driving force for transitions in the secondary structure of hydrated proteins. A simple protein-water system was used to study the conditions of typical protein-rich dairy food systems at a fixed pH level, salt content, and temperature. Freezedried beta-lactoglobulin (Type A) from bovine milk was dissolved directly into two different buffer systems over a wide range of concentrations between 1 mg/ml (~54 mM) and 200 mg/ml (~0.01 M) but at a fixed pH level, pH 3. Circular dichroism (CD), attenuated total reflectance Fourier transform infrared spectroscopy (ATR FTIR), and thioflavin T (ThT) Assay fluorescence spectroscopy were used to measure changes in the secondary structure with respect to protein solution concentration at 20 C. The findings of all of the techniques indicate that the majority of the secondary structure changes occur within the low protein concentration regime (i.e. <50 mg/ml) before a critical aggregation threshold. Dimerisation, formed by besheet cross-linking, is the likeliest mechanism of aggregation. The formation of dimers however counters the current assumption that at pH 3 only monomers exist; rather it seems there is a gradual evolution of the monomeric unfolded state with increasing concentration occurs yielding a bsheet rich refolded aggregate. Most interesting is the low concentration region (i.e. between 1 mg/ml and 40 mg/ml) where most secondary structural alterations are found to occur; before physical crowding effects are possible. The results indicate that BLG has a limited solubility even in a low concentration regime.
Carbohydrate …, 2001
The coil-to-helix transition and temperature dependence of the viscosity of commercial kappa/iota... more The coil-to-helix transition and temperature dependence of the viscosity of commercial kappa/iota-hybrid carrageenans produced by the red algae Sarcothalia crispata, Mazaella laminarioides, and Chondrus crispus were studied using rheometry and optical rotation. The structure of these kappa/iota-hybrid carrageenans was determined by 1H and 13C NMR spectroscopy combined with monosaccharide composition analysis. The coil-to-helix transitions, measured by polarimetry and rheometry, of the kappa/iota-hybrid carrageenans are significantly different from those of pure kappa- and iota-carrageenan, and from hand-made mixtures thereof. This provides evidence that the kappa/iota-hybrid carrageenans are mixed chains, containing both kappa- and iota-repeating units.
Macromolecules, 1995
ABSTRACT
The Journal of Chemical Physics, 1992
ABSTRACT