The multifaceted role of propolis particles in aqueous dispersions and in oil-in-water emulsions

Chourmouziadi Laleni, Nelli ORCID: 0000-0001-8544-3190 (2023). The multifaceted role of propolis particles in aqueous dispersions and in oil-in-water emulsions. University of Birmingham. Ph.D.

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Abstract

Consumer preferences towards formulated products that utilise natural active species over synthetic, has been generating significant industrial activity. As a result, a range of natural species possessing a multitude of functionalities has been investigated. One such natural component is propolis which is made by honeybees for healing and antiseptic purposes and has been associated with antibacterial, antifungal and antioxidant functionalities, among others. However, propolis has limited aqueous solubility which restricts its use in industrial formulations. On the other hand, it has been also established that common carrier solvents used in literature to dissolve propolis (e.g. ethanol and DMSO) can ‘misrepresent’ the active’s functionality by themselves exhibiting a range of underlying effects (e.g. antibacterial).

The present study investigated the enhancement to propolis’ performance in a purely aqueous environment, by dispersing propolis extracts into nanoparticles via direct ultrasonication. The hypothesis put forward was that sonication will enhance the functionality by decreasing the size of propolis particles (thus increasing the overall active surface area), and by promoting the dissolution of individual active species (present in the extracts) into the carrier (aqueous) phase. The effect of sonication time on the dispersions’ particle size, polydispersity, zeta potential, chemical functionality and antibacterial activity were monitored. The aqueous carrier phase was isolated to determine the potential (further) dissolution of active species following sonication. The study indeed confirmed this hypothesis and demonstrated that the aqueous propolis particle formulations exhibited antioxidant and antibacterial activity, while sonication was shown to encourage the extraction/dissolution of functional compounds into the carrier phase.

Τhe functionality of the propolis particles was then ‘transferred’ into an oil-in-water (O/W) emulsion setting, by using the propolis particle aqueous dispersions as the continuous phase. Propolis particles alone were not able to produce stable emulsions, but co-stabilisation in the presence of surfactant or protein species proved more advantageous. Emulsion stability was investigated, for different oil and surfactant/protein fractions as well as for different oil droplet sizes. The role of the propolis particles in the interfacial stabilisation of the emulsions was studied by dynamic interfacial tensiometry and contact angle measurements and imaged using fluorescent microscopy. It was revealed that propolis particles exhibit some affinity for the emulsion interface (Pickering-like functionality) and thus can provide stabilisation in tandem with low concentrations of a co-stabiliser, even though some displacement of the particles was observed. Emulsions exhibited adequate stability over the course of two months, dependant on the storage temperature, pH environment and co-stabiliser used. Ultimately the choice of co-stabiliser was shown to be crucial in terms of emulsion stability as well as for the preservation of the antibacterial activity provided to the systems by the presence of the propolis particles.

In recent years, Pickering particles have been a popular research topic for the stabilisation of O/W emulsions. However, beyond their capacity for stabilisation, little research has been conducted on the ability of Pickering particles to offer further functionality to an emulsion microstructure, such as to provide or enhance their antibacterial or antioxidant character. One approach to develop such multifunctional colloidal structures is to exploit the emulsion stabilisation (Pickering functionality) of species that already exhibit specific/desired (non-Pickering) physicochemical characteristics. Within the spirit of such an approach, the present work finally assesses the ability of propolis particles to enhance the oxidative stability and antibacterial activity of O/W emulsions. Firstly, control (non-propolis) emulsions were assessed with respect to lipid oxidation, showing that although oil content and storage temperature influence the rate of hyperoxide production, oil droplet size did not cause a significant effect. Subsequently propolis emulsions were formulated and their lipid oxidation was measured by altering key parameters such as droplet size, propolis and oil content as well as pH. It was established that propolis significantly supresses lipid oxidation in emulsion systems compared to the controls tested. Propolis emulsions also exhibited efficient antibacterial activity, eliminating the bacteria population, although their rate of action was slower than when simply present in aqueous dispersions. Finally, it was concluded that incorporation of propolis particles increased the viscosity of the emulsions.

In conclusion, the present work demonstrates that propolis’ activity can be enhanced in an aqueous environment via ultrasound sonication. Propolis particles were also shown to exhibit Pickering-like functionality in the emulsion systems as well as oxidative stability and antibacterial properties. The ultimate goal of this research is to utilise these propolis colloidal species as natural ingredients offering both stability and antibacterial activity to industrially relevant emulsion formulations.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):