Designing Novel Emulsion Performance by Controlled Hetero-Aggregation of Mixed Biopolymer Systems
Date of Award
Open Access Dissertation
Doctor of Philosophy (PhD)
D. Julian Mcclements
The increase in obesity and overweight in many countries has led to an upsurge of interest in the development of reduced fat food products. However, the development of these products is challenging because of the many roles that fat droplets normally plays in these food products, including contributing to flavor, texture, appearance, and bioactivity. The goal of this research was to develop novel reduced-fat emulsions based on hetero-aggregation of oppositely charged food-grade colloidal particles or polymers.
Initially, lactoferrin (LF) and β-lactoglobulin (β-Lg) were selected as emulsifiers to form protein-coated fat droplets (d43 ≈ 0.38 μm) with opposite charges at neutral pH: pKaβ-Lg ≈ 5 < pH 7 < pKaLF ≈ 8.5. Droplet aggregation occurred when these two emulsions were mixed together due to electrostatic attraction. The structural organization of the droplets in these mixed emulsions depended on the positive-to-negative particle ratio, particle concentration, pH, ionic strength, and temperature. The nature of the structures formed influenced the rheology, stability, and appearance of the mixed emulsions, which enabled some control over emulsion functionality. The largest microclusters were formed at particle ratios of 40% LF-coated and 60% β-Lg-coated fat droplets, which led to mixed emulsions with the highest apparent viscosity or gel strength. At low total particle concentrations (0.1%), there was a relatively large distance between microclusters and the mixed emulsions were fluid. At high particle concentrations (>20%), a three-dimensional network of aggregated droplets formed that led to gel-like or paste-like properties. The influence of environmental stresses on the physicochemical stability of the microclusters formed by hetero-aggregation was investigated: pH (2-9); ionic strength (0-400 mM NaCl); and temperature (30-90 ºC). Large microclusters were obtained at pH 7 (d43 ≈ 10 μm) with the absence of salt at room temperature. More acidic (< pH 6) or alkaline (> pH 8.5) solutions resulted in smaller aggregates by minimizing the electrostatic attraction between the protein-coated fat droplets. Microclusters dissociated upon addition of intermediate levels of salt, which was attributed to screening of attractive electrostatic interactions. Heating the microclusters above the thermal denaturation temperature of the proteins led to an increase in gel-strength, which was attributed to increased hydrophobic attraction.
The influence of hetero-aggregation of lipid droplets on their potential biological fate was studied using a simulated gastrointestinal tract (GIT). Results showed that the mixed emulsions had high viscosity in the simulated oral environment but exhibited similar rheological properties and particle characteristics as single-protein emulsions in the simulated gastric and small intestinal tract regions. The mixed emulsions also had similar lipid digestion rates in the simulated small intestine as single-protein emulsions suggesting that they could be used as delivery systems for bioactive lipophilic compounds in reduced fat food products.
The possibility of using more practical food ingredients to promote heteroaggregation system was also examined. Whey protein isolate (positive) and modified starch (negative) were selected as building blocks due to their opposite charges at pH 3.5. The largest aggregates and highest viscosities occurred at a particle ratio of 70% MS and 30% WPI, which was attributed to strong electrostatic attraction between the oppositely charged droplets. Particle aggregation and viscosity decreased when the pH was changed to reduce the electrostatic attraction between the droplets.
Finally, the influence of interfacial properties on the chemical stability of bioactive components in emulsion-based delivery systems containing mixed proteins was studied. Lactoferrin (LF: pI ≈ 8) and β-lactoglobulin (β-Lg: pI ≈ 5) were selected to engineer the interfacial properties. Interfaces with different structures were formed: LF only; β-Lg only; LF-β-Lg (laminated); β-Lg -LF (laminated); β-Lg /LF (mixed). The influence of pH, ionic strength, and temperature on the physical stability of β-caroteneenriched emulsions was then investigated. LF- emulsions were stable to the pH change from 2 to 9 but the aggregation was occurred in intermediate pH for other emulsions. β- Lg- emulsions aggregated at low salt concentration (≥ 50mM NaCl), however other emulsions were stable (0 - 300mM NaCl). β-Lg /LF (mixed) emulsions were unstable to heating (≥ 60 ºC), but all other emulsions were stable (30 to 90 ºC). Color fading due to β-carotene degradation occurred relatively quickly in β-Lg-emulsions (37 ºC), but was considerably lower in all other emulsions, which was attributed to the ability of LF to bind iron or interact with β-carotene.
Overall, this study shows that hetero-aggregation may be a viable method of creating novel structures and rheological properties that could be used in the food industry.
Mao, Yingyi, "Designing Novel Emulsion Performance by Controlled Hetero-Aggregation of Mixed Biopolymer Systems" (2013). Open Access Dissertations. 826.