Combined physical and oxidative stability of food Pickering emulsions

Many food products contain lipid droplets dispersed in an aqueous phase (e.g., milk, mayonnaise), thus are oil-in-water (O/W) emulsions. Food emulsions may be subjected to destabilization, both from a physical and a chemical perspective. Physical destabilization is generally prevented by the use of conventional emulsifiers such as surfactants and proteins. Chemical destabilization, in particular lipid oxidation, is a major concern in food products, especially when healthy polyunsaturated fatty acids are present, and this degradation is usually mitigated by the use of synthetic antioxidants, often in large amounts.

The use of alternative ingredients for the formulation of food emulsions has been emerging, for example solid particles (so-called Pickering particles, that are very popular nowadays) that irreversibly adsorb to the interface and therewith provide high physical stability; or natural antioxidants such as tocopherols and rosemary extracts, which are attractive in the current clean-label trend to prevent lipid oxidation. The efficiency of these natural antioxidants is unfortunately often not optimal, which can be explained by their tendency to locate into the oil or water phase, whereas lipid oxidation is initiated at the oil-water interface, and thus is the place where antioxidants should be located to optimally exert their protective effect.

The objective of this project was to develop food emulsions with a new and controlled architecture directed at yielding both excellent physical and oxidative stability. In these emulsions the oil droplets were covered by food-grade Pickering particles that exert a double role: they act as physical stabilizers, and as a reservoir for antioxidant molecules located close to the oil-water interface, therewith preventing the first lipid oxidation events, which is expected to drastically enhance antioxidant activity.

The first part of this thesis focused on the preparation and characterization of a new food-grade lipid-based Pickering particles, referred to as colloidal lipid particles (CLPs). We prepared both surfactant-covered and protein-covered CLPs, and found that the type of emulsifier largely determined their morphology: protein-covered CLPs were roughly spherical, whereas surfactant-covered CLPs looked more lath-like (Chapters 3 and 6). We also showed that the lipid material alters the crystal polymorphism and subsequent CLP structure, which consequently influenced their performance as emulsion stabilizers (Chapter 3). For instance, surfactant-covered CLPs containing only high melting point lipids showed highly ordered crystalline structures, and formed jammed, cohesive interfacial layers once adsorbed onto oil droplets, whereas the ones containing a fraction of low melting point lipids showed less ordered crystalline structures and formed thin and bridged layers.

Since protein-covered CLPs were particularly resilient to subsequent emulsification processes, these particles were used to study the formation of emulsion droplets in a microfluidic device and their stability to short-term coalescence (Chapter 4). We found a non-monotonic dependency of the droplet stability on the particle concentration: at low surface coverage, CLPs had a destabilizing effect as incompletely covered surfaces led to droplet-droplet bridging and subsequent coalescence, whereas at higher surface coverage, particles formed an effective barrier against droplet coalescence, resulting in physically stable emulsions over the time scales probed.

As a next step, we investigated lipid oxidation in Pickering emulsions stabilized by protein-based CLPs that did not contain antioxidants (Chapter 5). We showed that these Pickering emulsions had a similar oxidative stability as conventional protein-stabilized emulsions for a similar composition of the oil droplets. Yet, when in both emulsions the same amount of solid lipids was present (either as stabilizing CLPs, or within the oil droplet core), a Pickering emulsion had a higher physicochemical stability. This shows that the location of crystallizable lipids influences lipid oxidation in O/W emulsions, and thus needs to be carefully considered in emulsion design.

CLPs that did contain the lipophilic antioxidant α-tocopherol are presented in Chapter 6. The chemical stability of α-tocopherol was negatively influenced by lipid crystallization that probably promoted the localization of α-tocopherol close to the particle surface, which was further enhanced by emulsifiers that actively induce lipid crystallization. When applied as Pickering stabilizers in O/W emulsions (Chapter 7), lipid oxidation was reduced compared to control emulsions with the same composition and structure, but where the antioxidant was present in the core of the oil droplets. This confirmed that the interfacial localization of the antioxidant is crucial to prevent lipid oxidation in emulsions, and that the two main instability issues (i.e., physical and chemical instability) of emulsions can be mitigated through one single approach.

After establishing the proof of concept with the CLPs, we used biobased particles (that may contain antioxidants) from various natural sources to stabilize O/W emulsions (Chapter 8). Emulsions stabilized by matcha tea powder or spinach leaf powder were both highly physically and oxidatively stable, which shows that the double functionality that we achieved using purposely built particles (CLPs) can also be achieved with naturally occurring particles.

In the general discussion of the thesis (Chapter 9) we describe that the dual functionality of CLPs can also be reached using other food components, which makes this approach a generic one. We expect that the system could be further improved, for example, by increasing the residence time of antioxidants at the interface. To do so, we probably need to link the time scale at which the relevant oxidation events occur with those during which the antioxidant actually resides at the interface. Follow-up research on entrapment of antioxidants within particles is needed to reach long residence times at the interface while not compromising the ability of antioxidants to exert their chemical activity. To conclude: through our approach the highly-stable food emulsions of the future may come within reach.