Krassimir P. Velikov

Colloidal delivery systems for micronutrients and nutraceuticals

The formulation of micronutrients and nutraceuticals in the design of functional foods brings enormous technological challenges. The incorporation of micronutrients and/or nutraceuticals can compromise the product functionality. Issues often encountered are related to unwanted changes in the product physico-chemical stability, appearance, texture, flavour, taste and bioavailability due to inherited instability or interactions with other ingredients. This review intends to present the general strategies in using colloidal dispersions as delivery systems for micronutrients and nutraceuticals. Some illustrative examples will be given on how colloidal delivery systems can be utilised in the design of novel functional foods.

Sodium caseinate stabilized zein colloidal particles.

The present work deals with the preparation and stabilization of zein colloidal particles using sodium caseinate as electrosteric stabilizer. Colloidal particles with well-defined size range (120-150 nm) and negative surface potential (-29 to -47 mV) were obtained using a simple antisolvent precipitation method. Due to the presence of caseinate, the stabilized colloidal particles showed a shift of isoelectric point (IEP) from 6.0 to around pH 5.0 and thus prevent the aggregation of zein near its native IEP (pH 6.2). The particles also showed good stability to varying ionic strength (15 mM-1.5 M NaCl). Furthermore, stabilized particles retained the property of redispersibility after drying. In vitro protein hydrolysis study confirmed that the presence of caseinate did not alter the digestibility of zein. Such colloidal particles could potentially serve as all-natural delivery systems for bioactive molecules in food, pharmaceutical, and agricultural formulations.

Oil-in-water Pickering emulsions stabilized by colloidal particles from the water-insoluble protein zein

Few fully natural and biocompatible materials are available for the effective particle-stabilization of emulsions since strict requirements, such as insolubility in both fluid phases and intermediate wettability, need to be met. In this paper, we demonstrate the first use of water-insoluble proteins, employing the corn protein zein as a representative of this family, as effective particle-stabilizers of oil-in-water emulsions of natural oils and water. For this purpose, we synthesized zein colloidal particles through an anti-solvent precipitation procedure and demonstrated their use in the formation of stable oil-in-water Pickering emulsions as a function of particle concentration, pH and ionic strength. We confirmed that the wetting properties of zein, studied as a function of pH and ionic strength, strongly favor interfacial particle adsorption with oil-in-water three-phase contact angles θow close to 90°. We found that unmodified zein colloidal particles can produce stable, surfactant-free o/w emulsions with droplet sizes in the range 10–200 μm under experimental mixing conditions (2 min with Ultra Turrax homogenizer at 13500 rpm) at pH above and below the isoelectric point of zein, for low to moderate ionic strengths (1–10 mM). Under conditions where the particle volume fraction is low (<0.2 wt%) or at low pH, the resulting emulsions are not stable against coalescence. At a higher ionic strength, the zein particles have a tendency to aggregate and the resulting emulsions flocculate, forming an emulsion–gel phase.

Photonic crystals of core-shell colloidal particles

We report on the fabrication and optical transmission studies of thin three-dimensional photonic crystals of high-dielectric ZnS-core and low-dielectric SiO2-shell colloidal particles. These samples were fabricated using a vertical controlled drying method. The spectral position and width of a stopgap depend on the core-to-shell ratio, in a manner consistent with numerical calculations. Both experiments and calculations show that the relative L-stopgap width in the case of high-index core low-index shell particles can be larger in comparison to the case of homogeneous particles of either material. The core-shell morphology gives additional control over the photonic stopgap characteristics.

Quercetin loaded biopolymeric colloidal particles prepared by simultaneous precipitation of quercetin with hydrophobic protein in aqueous medium

Quercetin loaded biopolymeric colloidal particles were prepared by precipitating quercetin (water insoluble polyphenol) and zein (hydrophobic protein), simultaneously, by adding their hydro-alcoholic solution to aqueous solution in presence of sodium caseinate as an electrosteric stabiliser. The presence of protein resulted in altering the shape of quercetin precipitates from needle-like to spherical shape at higher zein proportions, as confirmed by transmission electron microscopy. The average particle size of zein:quercetin composite particles was below 200 nm (130-161 nm) with negative surface charge (-30 to -41 mV), as confirmed by dynamic light scattering and electrophoretic mobility data. Solid state characterisation (X-ray diffraction) and spectroscopic measurements (UV-Vis and IR spectroscopy) confirmed characteristic changes in quercetin due to the entrapment in the biopolymeric matrix of colloidal particles. Results from anti-oxidant study demonstrated the advantage of entrapping quercetin in the colloidal particles in terms of the chemical stability in the alkaline pH and against photodegradation under UV-light irradiation.

Photonic crystals of shape-anisotropic colloidal particles

Spherical silica (SiO2), zinc sulfide (ZnS), and core-shell particles of these materials undergo substantial anisotropic plastic deformation under high-energy ion irradiation. Individual particles can be turned into oblate or prolate ellipsoids with exact control over the aspect ratio. In this letter, we report on the fabrication and optical characterization of thin three-dimensional photonic crystals of spherical particles, which have been anisotropically deformed into spheroidal oblates by means of ion irradiation. As a result of the collective deformation process, both the unit cell symmetry and the particle form factor have been changed leading to appreciable tunability in the optical properties of the photonic crystal.

The Polyphenol EGCG Inhibits Amyloid Formation Less Efficiently at Phospholipid Interfaces than in Bulk Solution

Age-related diseases, like Alzheimers disease and type 2 diabetes mellitus, are characterized by protein misfolding and the subsequent pathological deposition of fibrillized protein, also called amyloid. Several classes of amyloid-inhibitors have recently been tested, traditionally under bulk conditions. However, it has become apparent that amyloid fibrils and oligomers assemble and exert their cytotoxic effect at cellular membranes, rather than in bulk solution. Knowledge is therefore required of inhibitor activity specifically at the phospholipid membrane interface. Here we show, using surface-specific sum-frequency generation (SFG) spectroscopy and atomic force microscopy (AFM), that the commonly used (-)-epigallocatechin gallate (EGCG) is a much less efficient amyloid inhibitor at a phospholipid interface than in bulk solution. Moreover, EGCG is not able to disaggregate existing amyloid fibrils at a phospholipid interface, in contrast to its behavior in bulk. Our results show that interfaces significantly affect the efficiency of inhibition by EGCG inhibitors and should therefore be considered during the design and testing of amyloid inhibitors.

Label-Free Imaging of Lipophilic Bioactive Molecules during Lipid Digestion by Multiplex Coherent Anti-Stokes Raman Scattering Microspectroscopy

The digestion and absorption of lipophilic, bioactive molecules such as lipids, physiologically active nutrients (nutraceuticals), and drugs play a crucial role in human development and health. These molecules are often delivered in lipid droplets. Currently, the kinetics of digestion of these lipid droplets is followed by in vitro models that simulate gastrointestinal conditions, while phase changes within the lipid droplets are observed by light or electron microscopy. However real-time, spatially resolved information about the local chemical composition and phase behavior inside the oil droplet is not accessible from these approaches. This information is essential as the surface and phase behavior determine the local distribution of molecules in the oil droplets and thus may influence the rate of uptake, for example, by impairing the effective transfer of bioactive molecules to intestinal cells. We demonstrate the capability of multiplex coherent anti-Stokes Raman scattering (CARS) microspectroscopy to image the digestion process non-invasively, with submicrometer resolution, millimolar sensitivity, and without the need for labeling. The lipolysis of glyceryl trioleate emulsion droplets by porcine pancreatic lipase is imaged, and the undigested oil and the crystalline lipolytic products are distinguished by their different vibrational signatures. The digestion of droplets containing the phytosterol analogue ergosterol is also probed, and the crystals are observed to dissolve into the lipolytic products. The lipophilic drug progesterone and Vitamin D(3) are dissolved in glyceryl trioctanoate emulsion droplets, and the local concentration is mapped with millimolar sensitivity. The bioactive molecules are observed to concentrate within the droplets as the oil is hydrolyzed. This observation is ascribed to the low solubility of these molecules in the lipolytic products for this system. Neither the type of bioactive molecule nor the initial radius of the emulsion droplet had a large effect upon the rate of digestion under these conditions; lipolysis of the triglyceride by pancreatic lipase appears insensitive to the type of bioactive molecule in solution. These findings shed important new light on lipid digestion and open new possibilities for the chemical visualization of lipid digestion and phase changes in lipid droplets containing bioactive molecules, which in combination with other existing techniques will provide a full picture of this complex physicochemical process.

Magnetically Responsive Pickering Foams

We introduce a new class of Pickering foams which can be manipulated using a magnetic field. These foams are stabilized by a mixture of magnetic and nonmagnetic particles. They exhibit excellent stability in the absence of a magnetic field, but can be rapidly destroyed on demand with the application of a threshold field. We characterize their stability in the absence of a magnetic field by measuring the rate of water drainage from the foam as a function of time. We also correlate their collapse behavior under a magnetic field to the foam liquid fraction, as well as the concentration of magnetic particles in the foam. This novel system can be used to study the properties of Pickering foams, and has potential applications in noncontact defoaming processes.

Morphology and Persistence Length of Amyloid Fibrils Are Correlated to Peptide Molecular Structure

The formation of amyloid fibrils is a self-assembly process of peptides or proteins. The superior mechanical properties of these fibrils make them interesting for materials science but constitute a problem in amyloid-related diseases. Amyloid structures tend to be polymorphic, and their structure depends on growth conditions. To understand and control the assembly process, insights into the relation between the mechanical properties and molecular structure are essential. We prepared long, straight as well as short, worm-like β-lactoglobulin amyloid fibrils and determined their morphology and persistence length by atomic force microscopy (AFM) and the molecular conformation using vibrational sum-frequency generation (VSFG) spectroscopy. We show that long fibrils with near-100% β-sheet content have a 40-times higher persistence length than short, worm-like fibrils with β-sheet contents below 80%.