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Author ORCID Identifier


Open Access Dissertation

Document Type


Degree Name

Doctor of Philosophy (PhD)

Degree Program

Food Science

Year Degree Awarded


Month Degree Awarded


First Advisor

David Julian McClements

Second Advisor

Jiakai Lu

Third Advisor

Zhenhua Liu

Fourth Advisor

Lynne A. McLandsborough

Subject Categories

Food Chemistry | Food Processing | Food Science | Other Food Science | Other Nutrition


Multilayer coatings have been proposed as a promising nanotechnology for improving the performance of emulsion-based products in numerous research fields. In foods, these multilayer coatings can be used to improve the encapsulation and protection of bioactive ingredients in delivery systems during storage and passage through the gastrointestinal tract (1, 2). Therefore, there is strong interest in understanding the formation, properties, and performance of these novel coatings. Multilayer coatings are formed by layer-by-layer electrostatic deposition of oppositely charged biopolymers, such as proteins and polysaccharides. A better understanding of the formation and properties of biopolymer multilayer coatings could lead to novel foods with improved performance. The purpose of this research was to improve our understanding of the fabrication and behavior of multilayer nanoemulsions suitable for application in the food industry.

First, the interaction of anionic ɣ-poly-glutamic acid (PGA) and cationic ɛ-poly-L-lysine (PLL) in solution was examined so as to better understand their behavior at interfaces. Electrostatic complexes were formed with a 1:4 mass ratio of polyanion-to-polycation at saturation (pH 7.4). The surface potential and aggregation stability of the complexes was highly dependent on solution pH (2 to 12), which was attributed to alterations in the electrical characteristics of the two polyelectrolytes. In particular, insoluble complexes were formed under pH conditions where there was a strong electrostatic attraction between the two polyelectrolytes, whereas soluble complexes were formed when there was only a weak attraction. The addition of salt (³ 20 mM NaCl) promoted aggregation of the complexes, presumably due to screening of the electrostatic interactions between them. Conversely, temperature (25 to 90 oC) did not have a major impact on the stability of the complexes. These results may be useful for the design of effective oral delivery systems for bioactive agents in foods and other products.

Secondly, the same biopolymers were used to form multilayer coatings around the lipid droplets in oil-in-water nanoemulsions using a sequential layer-by-layer electrostatic deposition approach. Cationic poly-L-lysine (PLL) and anionic poly-glutamic acid (PGA) were used as a pair of oppositely charged polypeptides (pH 4.0). First, a primary emulsion (10% w/w soybean oil-in-water emulsion) was formed consisting of small lipid droplets (d32 = 500 µm) coated by a natural surfactant (0.05% w/w quillaja saponin). Then, cationic PLL was deposited onto the surfaces of the anionic saponin-coated droplets. Lastly, anionic PGA was deposited onto the surfaces of the cationic PLL-saponin-coated droplets. We then assessed the ability of the coatings to protect the lipid droplets from aggregation when the pH (2.0-9.0), ionic strength (0 to 350 mM), or temperature (30-90°C) were altered. The properties of the primary, secondary, and tertiary emulsions were monitored by measuring the mean particle diameter (d32), electrical characteristics (ζ-potential), and microstructure of the lipid droplets. The electrical characteristics of the droplets could be modulated by controlling the number and type of layers used. The primary emulsion had the best resistance to varying environmental conditions, while the secondary emulsion had the worst, suggesting electrostatic deposition should only be used to obtain specific functionalities. Interestingly, PLL detached from the surfaces of the secondary emulsions at high salt concentrations due to electrostatic screening, which improved their salt stability. This phenomenon may be useful for some food applications, e.g., having cationic droplets during food storage, but anionic ones inside the human body.

Thirdly, multilayer coatings were formed from saponins, polypeptides, and polysaccharides using medium chain triglyceride (MCT) lipid droplets as templates (pH 4.0). First, an emulsion containing negatively charged lipid droplets was created using quillaja saponin (QS) as an anionic emulsifier. Second, these anionic droplets were coated with a cationic polypeptide (poly-L-lysine, PLL) to form positively-charged droplets. Finally, these cationic droplets were coated with a negatively-charged polysaccharide, either pectin (PE) or κ-carrageenan (KC), to form anionic droplets. Overall, the 1-layer emulsions had the best resistance to salt, pH, and heat, indicating that quillaja saponins were effective emulsifiers. The 2-layer emulsions had better pH-stability than the 3-layer emulsions, which tended to strongly aggregate under acidic conditions. Conversely, the 3-layer emulsions had better salt-stability than the 2-layer emulsions, which tended to aggregate strongly even at low salt levels (50-100 mM NaCl). All the emulsions were relatively stable to heating (90oC, 30 min).

Fourth, the kinetics of β-carotene degradation in multilayer nanoemulsions was measured. Primary emulsions were formed containing anionic quillaja saponin-coated MCT oil droplets loaded with β-carotene. Secondary emulsions were then formed by depositing cationic polypeptide poly-l-lysine (PLL) onto these anionic droplets. Tertiary emulsions were then formed by depositing anionic poly-glutamic acid (PGA), pectin (PE) or κ-carrageenan (KC) onto these cationic droplets. All the multilayer emulsions were prepared at pH 4.0 to ensure the biopolymers had opposite charges. The kinetics of β-carotene degradation in the different emulsions were then measured when they were incubated at 55°C. In addition, changes in the particle size and ζ-potential of the emulsions were measured using light scattering methods. The chemical stability of the encapsulated carotenoid was highly dependent on the nature the nature of the coating used. The secondary emulsions, which had cationic PLL as an external layer, gave the best protection against color fading, with a final yellowness (b*) of 82% after two weeks. Conversely, the tertiary emulsions, which had anionic polysaccharides or polypeptides as an external layer, gave the worst protection. For instance, when KC was used as the external layer the final yellowness was only 32% after two weeks. These results show that the stability of carotenoids can be improved by controlling the properties of multilayer coatings around oil droplets.