Eddie G. Pelan

Unique Properties of Bubbles and Foam Films Stabilized by HFBII Hydrophobin

The HFBII hydrophobin is an amphiphilic protein that can irreversibly adsorb at the air/water interface. The formed protein monolayers can reach a state of two-dimensional elastic solid that exhibits a high mechanical strength as compared to adsorption layers of typical amphiphilic proteins. Bubbles formed in HFBII solutions preserve the nonspherical shape they had at the moment of solidification of their surfaces. The stirring of HFBII solutions leads to the formation of many bubbles of micrometer size. Measuring the electrophoretic mobility of such bubbles, the ζ-potential was determined. Upon compression, the HFBII monolayers form periodic wrinkles of wavelength 11.5 μm, which corresponds to bending elasticity k(c) = 1.1 × 10(-19) J. The wrinkled hydrophobin monolayers are close to a tension-free state, which prevents the Ostwald ripening and provides bubble longevity in HFBII stabilized foams. Films formed between two bubbles are studied by experiments in a capillary cell. In the absence of added electrolyte, the films are electrostatically stabilized. The appearance of protein aggregates is enhanced with the increase of the HFBII and electrolyte concentrations and at pH close to the isoelectric point. When the aggregate concentration is not too high (to block the film thinning), the films reach a state with 12 nm uniform thickness, which corresponds to two surface monolayers plus HFBII tetramers sandwiched between them. In water, the HFBII molecules can stick to each other not only by their hydrophobic moieties but also by their hydrophilic parts. The latter leads to the attachment of HFBII aggregates such as dimers, tetramers, and bigger ones to the interfacial adsorption monolayers, which provides additional stabilization of the liquid films.

Novel anisotropic materials from functionalised colloidal cellulose and cellulose derivatives

This feature article describes selected examples of the properties and the methods of preparation of cellulose micro and nano crystallites (whiskers) and derivatives, with aspects related to fabrication of various anisotropic materials. Nanometre sized cellulose crystallites have a variety of novel anisotropic properties markedly different from those of common forms of cellulose. They can be obtained from a variety of native cellulose sources through partial hydrolysis with strong acids or via mechanical defibrillation. We discuss different fabrication techniques and surface modifications of cellulose whiskers which determine their wettability, surface charge and range of applications. Concentrated suspensions of cellulose whiskers of high aspect ratio can form chiral nematic liquid crystalline phases which retain their structure upon evaporation, producing iridescent films. At present, the bulk of the research on cellulose whiskers is focused on creation of composite materials in which they enhance mechanical properties and improve their biodegradability. The high strength of the cellulose nanocrystals has also been utilised in the fabrication of reinforced composite films with applications for anisotropic microcapsule preparation. Microrods and multifunctional microampules from hydrophobised cellulose have recently been recognised as being able to produce super-stable foams with long shelf life and allow the foam structural elements to encapsulate a range of liquid and solid additives.

Interfacial layers from the protein HFBII hydrophobin: Dynamic surface tension, dilatational elasticity and relaxation times

The pendant-drop method (with drop-shape analysis) and Langmuir trough are applied to investigate the characteristic relaxation times and elasticity of interfacial layers from the protein HFBII hydrophobin. Such layers undergo a transition from fluid to elastic solid films. The transition is detected as an increase in the error of the fit of the pendant-drop profile by means of the Laplace equation of capillarity. The relaxation of surface tension after interfacial expansion follows an exponential-decay law, which indicates adsorption kinetics under barrier control. The experimental data for the relaxation time suggest that the adsorption rate is determined by the balance of two opposing factors: (i) the barrier to detachment of protein molecules from bulk aggregates and (ii) the attraction of the detached molecules by the adsorption layer due to the hydrophobic surface force. The hydrophobic attraction can explain why a greater surface coverage leads to a faster adsorption. The relaxation of surface tension after interfacial compression follows a different, square-root law. Such behavior can be attributed to surface diffusion of adsorbed protein molecules that are condensing at the periphery of interfacial protein aggregates. The surface dilatational elasticity, E, is determined in experiments on quick expansion or compression of the interfacial protein layers. At lower surface pressures (<11 mN/m) the experiments on expansion, compression and oscillations give close values of E that are increasing with the rise of surface pressure. At higher surface pressures, E exhibits the opposite tendency and the data are scattered. The latter behavior can be explained with a two-dimensional condensation of adsorbed protein molecules at the higher surface pressures. The results could be important for the understanding and control of dynamic processes in foams and emulsions stabilized by hydrophobins, as well as for the modification of solid surfaces by adsorption of such proteins.

Surface shear rheology of adsorption layers from the protein HFBII hydrophobin: effect of added β-casein.

The surface shear rheology of hydrophobin HFBII adsorption layers is studied in angle-ramp/relaxation regime by means of a rotational rheometer. The behavior of the system is investigated at different shear rates and concentrations of added β-casein. In angle-ramp regime, the experimental data comply with the Maxwell model of viscoelastic behavior. From the fits of the rheological curves with this model, the surface shear elasticity and viscosity, E(sh) and η(sh), are determined at various fixed shear rates. The dependence of η(sh) on the rate of strain obeys the Herschel-Bulkley law. The data indicate an increasing fluidization (softening) of the layers with the rise of the shear rate. The addition of β-casein leads to more rigid adsorption layers, which exhibit a tendency of faster fluidization at increasing shear rates. In relaxation regime, the system obeys a modified Andrades (cubic root) law, with two characteristic relaxation times. The fact that the data comply with the Maxwell model in angle-ramp regime, but follow the modified Andrades low in relaxation regime, can be explained by the different processes occurring in the viscoelastic protein adsorption layer in these two regimes: breakage and restoration of intermolecular bonds at angle-ramp vs solidification of the layer at relaxation.

Surface Pressure and Elasticity of Hydrophobin HFBII Layers on the Air–Water Interface: Rheology Versus Structure Detected by AFM Imaging

Here, we combine experiments with Langmuir trough and atomic force microscopy (AFM) to investigate the reasons for the special properties of layers from the protein HFBII hydrophobin spread on the air-water interface. The hydrophobin interfacial layers possess the highest surface dilatational and shear elastic moduli among all investigated proteins. The AFM images show that the spread HFBII layers are rather inhomogeneous, (i.e., they contain voids, monolayer and multilayer domains). A continuous compression of the layer leads to filling the voids and transformation of a part of the monolayer into a trilayer. The trilayer appears in the form of large surface domains, which can be formed by folding and subduction of parts from the initial monolayer. The trilayer appears also in the form of numerous submicrometer spots, which can be obtained by forcing protein molecules out of the monolayer and their self-assembly into adjacent pimples. Such structures are formed because not only the hydrophobic parts, but also the hydrophilic parts of the HFBII molecules can adhere to each other in the water medium. If a hydrophobin layer is subjected to oscillations, its elasticity considerably increases, up to 500 mN/m, which can be explained with compaction. The relaxation of the layers tension after expansion or compression follows the same relatively simple law, which refers to two-dimensional diffusion of protein aggregates within the layer. The characteristic diffusion time after compression is longer than after expansion, which can be explained with the impedence of diffusion in the more compact interfacial layer. The results shed light on the relation between the mesoscopic structure of hydrophobin interfacial layers and their unique mechanical properties that find applications for the production of foams and emulsions of extraordinary stability; for the immobilization of functional molecules at surfaces, and as coating agents for surface modification.

Hydrophobic Modification of Chitin Whisker and Its Potential Application in Structuring Oil

A facile approach was developed to modify chitin whiskers by reacting them with bromohexadecane, and the potential application of modified whiskers in structuring oil was evaluated. The results of Fourier transform infrared spectra (FT-IR), wide-angle X-ray diffraction (XRD), elemental analysis, solid (13)C NMR, and differential scanning calorimeter (DSC) confirmed that the long alkyl chains were successfully introduced to the chitin whiskers and endowed them with improved hydrophobicity and thermal transition. By hot pressing the modified whiskers, the highly hydrophobic whisker sheets were constructed, showing high contact angles close to 150°. The hydrophobic interaction between the long alkyl chains and chitin backbone induced the crystal alignment with micro-nano structure, leading to the surface roughness and high hydrophobicity of the sheets. Furthermore, the modified whiskers could form a stable dispersion in sunflower oil, displaying a remarkable thickening effect. The viscosity of the oily suspension exhibited temperature dependence and shear-thinning behavior, suggesting great potentials to fabricate oleogel without adding any saturated fat. Furthermore, the intrinsic biocompatibility of α-chitin structure benefits its application in foodstuff, cosmetics, and medical fields.

Sonication–Microfluidics for Fabrication of Nanoparticle-Stabilized Microbubbles

An approach based upon sonication-microfluidics is presented to fabricate nanoparticle-coated microbubbles. The gas-in-liquid slug flow formed in a microchannel is subjected to ultrasound, leading to cavitation at the gas-liquid interface. Therefore, microbubbles are formed and then stabilized by the nanoparticles contained in the liquid. Compared to the conventional sonication method, this sonication-microfluidics continuous flow approach has unlimited gas nuclei for cavitation that yields continuous production of foam with shorter residence time. By controlling the flow rate ratios of the gas to the liquid, this method also achieves a higher production volume, smaller bubble size, and less waste of the nanoparticles needed to stabilize the microbubbles.

Growth of bubbles on a solid surface in response to a pressure reduction.

A diffusion-controlled method is presented to study the growth of bubbles on a solid surface. The bubbles are nucleated spontaneously on a hydrophobic smooth surface in response to a sudden pressure reduction and then grow with an expanding contact line. The evolution of the bubbles in the early stage is found to grow with a constant bubble radius and a decreasing contact angle, while the bubbles continue their growth with a constant contact angle and an increasing bubble radius after the contact angle reaches its equilibrium value. A total variation of about 60° of the contact angle is observed during the growth of the bubbles with the size scale of 10-100 μm in radius. The growing process is described by the diffusion theory with the validation of the growth constant.

A novel hybrid system for the fabrication of a fibrous mesh with micro-inclusions

A novel hybrid system combining microfluidic and co-axial electrospinning techniques has been used to generate different types of fibre structures with varied desirable inclusions using food grade polymers, ethyl cellulose and sodium alginate. The processing conditions in the microfluidic T-junction device, i.e. gas pressure and liquid flow rate were adjusted in order to generate near-monodisperse microbubbles which subsequently serve as a platform for particle generation. These particles exhibit micro-scale diameters and different shapes and some bubbles were incorporated into the fibrous mesh prepared by concurrent electrospinning. The fibre/particle structures obtained with different polymers via this novel method could potentially have many applications in various engineering and biological sectors.

Application of Emulsifiers to Reduce Fat and Enhance Nutritional Quality

Food emulsifiers represent one of the most important classes of food additives in the industrial manufacture of food products. The extensive range of technical functionalities extends beyond emulsification, into aspect such as crystal habit modification, foaming, gelation, complexation with other ingredients, and even antimicrobial function. Accordingly, the range of products, and product applications is equally diverse. Of particular research focus, and in response to current consumer trends, is the role of emulsifiers in the development of foods with enhanced health and wellness properties. Interestingly, various emulsifier systems primarily used for functional roles are being regarded as imparting intrinsic nutritional functionality. However, it is primarily the technical functionality of these materials that can be used to produce products of improved nutritional value. This has been exemplified in recent years in areas such as fat reduction, replacement of fats with poor nutritional value, as well as the use of emulsifiers for protection of bioactive components through encapsulation. This chapter provides pertinent examples of the use of emulsifiers in the structure design of food systems with targeted health and wellness benefits.