Bettina Wolf

Phase-separated biopolymer mixture rheology: Prediction using a viscoelastic emulsion model

The relationship between the morphology and rheology of phase-separated biopolymer mixtures is investigated. Biopolymer mixtures, which are utilized in the food industry for their textural and structuring properties, often phase separate and demix to form water-in-water emulsions. Controlling the morphology of biopolymer mixtures during flow processing and inducing gelation of one or both phases lead to products with novel microstructures and material properties [B. Wolf et al., Food Hydrocolloids 14, 217–225 (2000)]. An emulsion model [J. F. Palierne, Rheol. Acta 29, 204–214 (1990)], commonly used for the prediction of the linear viscoelastic properties of polymer blends, is used here to relate the rheology to the morphology of water-in-water emulsions. The system under investigation is a gelatin–maltodextrin mixture which phase separates at 60 °C for particular concentrations, characterized by a binodal curve, into a gelatin-rich and maltodextrin–rich phase. Emulsions with phase volumes of 10% and 30% w...

Influence of gelation on particle shape in sheared biopolymer blends

The morphology of biopolymer droplets gelled under shear as the dispersed phase of a two-phase mixture of biopolymers in aqueous solution (or “biopolymer blend”) was investigated experimentally. The process involves shearing the liquid biopolymer blend either at a steady stress till gelation, for creation of ellipsoidal gelled particles, or a stress step-up process for creation of high aspect ratio particles. The resulting particle shapes are analyzed and compared against models for affine deformation conditions [Janssen (1997)] and steady shear conditions [Maffetone and Minale (1998)]. Although they are only strictly valid for Newtonian fluid phases, application of these models to a situation where one phase is gelling, has provided insight into the morphological changes occurring in the shear-gelation process.The morphology of biopolymer droplets gelled under shear as the dispersed phase of a two-phase mixture of biopolymers in aqueous solution (or “biopolymer blend”) was investigated experimentally. The process involves shearing the liquid biopolymer blend either at a steady stress till gelation, for creation of ellipsoidal gelled particles, or a stress step-up process for creation of high aspect ratio particles. The resulting particle shapes are analyzed and compared against models for affine deformation conditions [Janssen (1997)] and steady shear conditions [Maffetone and Minale (1998)]. Although they are only strictly valid for Newtonian fluid phases, application of these models to a situation where one phase is gelling, has provided insight into the morphological changes occurring in the shear-gelation process.

String phase formation in biopolymer aqueous solution blends

In previous publications it has been shown that the flow-morphology relationships that apply to two-phase polymer blends or solutions, and to immiscible emulsions, also apply to two-phase aqueous solutions of mixed biopolymers, (referred to here as “biopolymer blends”). The relationships in question are classical theories of single drop deformation in shear flow, fibril breakup upon cessation of steady shear flow, and the Palierne model. In this paper, we report the experimental investigation of string phase formation in steady shear as another structural phenomenon which has been observed in polymer blends or polymer solutions, but has not been previously reported for biopolymer blends. The rheological and morphological behavior of gelatin-dextran mixtures was analyzed using conventional rotational rheometry and shear-microscopy techniques. It was found that the development of a string phase depends on the applied shear rate, the viscosity ratio and the phase volume of the minor phase. A rheological model by Kume and Hashimoto (1995) predicting string phase morphology was successfully applied for viscosity ratios not too dissimilar from unity. The application of a further model by Jeon and Hobbie (2001), however, gave ambiguous results with regard to the prediction of string phase formation.In previous publications it has been shown that the flow-morphology relationships that apply to two-phase polymer blends or solutions, and to immiscible emulsions, also apply to two-phase aqueous solutions of mixed biopolymers, (referred to here as “biopolymer blends”). The relationships in question are classical theories of single drop deformation in shear flow, fibril breakup upon cessation of steady shear flow, and the Palierne model. In this paper, we report the experimental investigation of string phase formation in steady shear as another structural phenomenon which has been observed in polymer blends or polymer solutions, but has not been previously reported for biopolymer blends. The rheological and morphological behavior of gelatin-dextran mixtures was analyzed using conventional rotational rheometry and shear-microscopy techniques. It was found that the development of a string phase depends on the applied shear rate, the viscosity ratio and the phase volume of the minor phase. A rheological mode...

Shear thickening of an emulsion stabilized with hydrophilic silica particles

The flow behavior of particle stabilized oil-in-water emulsions with different dispersed volume fractions was analyzed in steady shear on a rotational rheometer employing a coaxial cylinder geometry. The dispersed phase of the emulsion was a mixture of equal volumes of a polar oil, isopropyl myristate, and a nonpolar oil, dodecane. The continuous phase was an aqueous suspension of hydrophilic colloidal silica particles of 8nm diameter with the pH adjusted to pH2 in order to stabilize the emulsion [Binks and Whitby, Colloids Surf., A 253, 105–115 (1995)]. Droplet diameters were of the order of a few micrometers, and droplet surfaces apparently show dense particle coverage. We show that the markedly different interfacial structure in particle stabilized emulsions when compared to surfactant stabilized emulsions is reflected in the rheological behavior. To illustrate these differences, the rheological behavior of a comparable surfactant stabilized emulsion with the particles in the aqueous phase replaced by ...

Solution interactions of diclofenac sodium and meclofenamic acid sodium with hydroxypropyl methylcellulose (HPMC)

Many pharmaceutical agents require formulation in order to facilitate their efficacious delivery. However, the interaction between the active species and the formulation additives has the potential to significantly influence the pharmocokinetics of the active. In this study, the solution interactions between hydroxypropyl methylcellulose (HPMC) with two non-steroidal anti-inflammatories - the sodium salts of diclofenac and meclofenamate - were investigated using tensiometric, rheological, NMR, neutron scattering and turbidimetric techniques. The two drugs behaved very differently-meclofenamate addition to HPMC solutions led to substantial increases in viscosity, a depression of the gel point and a marked reduction in the self-diffusion coefficient of the drug, whereas diclofenac did not induce these changes. Collectively, these observations are evidence of meclofenamate forming self-assembled aggregates on the HPMC, a phenomenon not observed with diclofenac Na. Any process that leads to aggregation on a nonionic polymer will not be strongly favoured when the aggregating species is charged. Thus, it is hypothesised that the distinction between the two drugs arises as a consequence of the tautomerism present in meclofenamate that builds electron density on the carbonyl group that is further stabilised by hydrogen bonding to the HPMC. This mechanism is absent in the diclofenac case and thus no interaction is observed. These studies propose for the first time a molecular basis for the observed often-unexpected, concentration-dependant changes in HPMC solution properties when co-formulated with different NSAIDs, and underline the importance of characterising such fundamental interactions that have the potential to influence drug release in solid HPMC-based dosage forms.

Experimental study of the break-up of starch suspension droplets in step-up shear flow

The deformation and break-up of a droplet of suspension of swollen-in-water starch granules placed in an immiscible fluid, silicon oil, are investigated. The study was carried out on a physically modified waxy maize starch suspension with a theoretical granule volume fraction of 100%. Granules were swollen to their maximum; they were highly deformable. Measurements were carried out using a counter rotating shear cell. The starch suspension was shear thinning above a yield stress of about 90–100 Pa showing elasticity (first normal stress difference) above 300 Pa. The rheo-optical experiments were carried out by stepping up the stress from zero to a constant value and deformation of suspension droplets of various sizes was observed with time. Critical break-up capillary numbers Ca∗ were calculated and correlated with droplet-to-matrix viscosity ratio p. At low p values Ca∗ were found to be smaller or close to those determined for Newtonian fluids. No break-up was observed for viscosity ratios above 0.1, a l...

Programmed emulsions for sodium reduction in emulsion based foods.

In this research a microstructure approach to reduce sodium levels in emulsion based foods is presented. If successful, this strategy will enable reduction of sodium without affecting consumer satisfaction with regard to salty taste. The microstructure approach comprised of entrapment of sodium in the internal aqueous phase of water-in-oil-in-water emulsions. These were designed to destabilise during oral processing when in contact with the salivary enzyme amylase in combination with the mechanical manipulation of the emulsion between the tongue and palate. Oral destabilisation was achieved through breakdown of the emulsion that was stabilised with a commercially modified octenyl succinic anhydride (OSA)-starch. Microstructure breakdown and salt release was evaluated utilising in vitro, in vivo and sensory methods. For control emulsions, stabilised with orally inert proteins, no loss of structure and no release of sodium from the internal aqueous phase was found. The OSA-starch microstructure breakdown took the initial form of oil droplet coalescence. It is hypothesised that during this coalescence process sodium from the internalised aqueous phase is partially released and is therefore available for perception. Indeed, programmed emulsions showed an enhancement in saltiness perception; a 23.7% reduction in sodium could be achieved without compromise in salty taste (p < 0.05; 120 consumers). This study shows a promising new approach for sodium reduction in liquid and semi-liquid emulsion based foods.

Enhancing saltiness in emulsion based foods

BackgroundThe concept of enhancing saltiness perception in emulsions and a liquid food formulated with the emulsions (ambient vegetable soup) through increasing salt concentration in the continuous phase while retaining the fat content of the (aqueous continuous) product was evaluated. This was accomplished by increasing the droplet phase volume using duplex emulsion technology. Viscosity and droplet size distribution was measured. Saltiness evaluation was based on simple paired comparison testing (2-Alternate Forced Choice tests, BS ISO 5495:2007).ResultsSingle and duplex emulsions and emulsion-based products had comparable mean oil droplet diameters (25 to 30 μm); however, viscosity of the duplex emulsion systems was considerably higher. Sensory assessment of saltiness of emulsion pairs (2AFC) indicated duplex technology enhanced saltiness perception compared to a single emulsion product at the same salt content (6.3 g/100 g) in both simple emulsions and the formulated food product (P = 0.0596 and 0.0004 respectively) although assessors noted the increased viscosity of the duplex systems. The formulated food product also contained pea starch particles which may have aided product mixing with saliva and thus accelerated tastant transport to the taste buds. Lowering salt content in the duplex systems (to levels of aqueous phase salt concentration similar to the level in the single systems) resulted in duplex systems being perceived as less salty than the single system. It appears that the higher viscosity of the duplex systems could not be “overruled” by enhanced mixing through increased droplet phase volume at lowered salt content.ConclusionsThe results showed that salt reduction may be possible despite the added technology of duplex systems increasing the overall measured viscosity of the product. The changes in viscosity behavior impact mouthfeel, which may be exploitable in addition to the contribution towards salt reduction. With a view to applying this technology to real processed foods, it needs to be tested for the product in question but it should be considered as part of a salt reduction tool box.

Morphology and shear viscosity of aqueous two-phase biopolymer-surfactant mixtures

The relation between composition, rheology, and morphology in phase separated pullulan-sodium dodecyl sulphate systems containing sodium chloride has been investigated using rheo-optical methods. The rheological measurements showed that the apparent viscosity of these aqueous two-phase systems depends on the chemical composition of each phase, their volumetric composition, and the viscosity ratio of the separated phases. Optical observations revealed a droplet like morphology over a wide range of shear rates at low to moderate volume fractions of pullulan-rich phase. In some mixtures, string phases were observed at higher volume fractions of pullulan-rich phase and shear rates. Simultaneous analysis of the rheological data and observed structures at different shear rates/compositions indicates a close link between rheology and morphology of aqueous two-phase systems and provides a simple tool for predicting morphology on the basis of rheological data.

Pickering Particles Prepared from Food Waste

In this paper, we demonstrate the functionality and functionalisation of waste particles as an emulsifier for oil-in-water (o/w) and water-in-oil (w/o) emulsions. Ground coffee waste was chosen as a candidate waste material due to its naturally high content of lignin, a chemical component imparting emulsifying ability. The waste coffee particles readily stabilised o/w emulsions and following hydrothermal treatment adapted from the bioenergy field they also stabilised w/o emulsions. The hydrothermal treatment relocated the lignin component of the cell walls within the coffee particles onto the particle surface thereby increasing the surface hydrophobicity of the particles as demonstrated by an emulsion assay. Emulsion droplet sizes were comparable to those found in processed foods in the case of hydrophilic waste coffee particles stabilizing o/w emulsions. These emulsions were stable against coalescence for at least 12 weeks, flocculated but stable against coalescence in shear and stable to pasteurisation conditions (10 min at 80 °C). Emulsion droplet size was also insensitive to pH of the aqueous phase during preparation (pH 3–pH 9). Stable against coalescence, the water droplets in w/o emulsions prepared with hydrothermally treated waste coffee particles were considerably larger and microscopic examination showed evidence of arrested coalescence indicative of particle jamming at the surface of the emulsion droplets. Refinement of the hydrothermal treatment and broadening out to other lignin-rich plant or plant based food waste material are promising routes to bring closer the development of commercially relevant lignin based food Pickering particles applicable to emulsion based processed foods ranging from fat continuous spreads and fillings to salad dressings.