Ioanna Zafeiri and Bettina Wolf of the University of Birmingham review the applications of Pickering particles in emulsion stabilisation and discuss recent results concerning the use of lipid particles and natural co-products from food manufacturing as Pickering emulsion stabilisers.
Much of our everyday nutrition is based on foods that are emulsions or have been emulsified at a certain stage during their processing. Emulsions are formed when at least two immiscible fluids, such as oil and water, are mixed. The generated temporarily stable mixture consists of fine droplets of one liquid phase dispersed into the other. Oil-in-water (e.g. milk, salad dressings, mayonnaise) and water-in-oil (e.g. butter, margarine) emulsions are the two most common types of food emulsions. Due to the above mentioned instability, surfactants are traditionally used as a physical barrier to prevent droplets from coming together. However, in the last two decades, solid particles of micro- or sub-micron-dimensions have been gaining prominence as emulsion stabilising agents. Via the so-called Pickering mechanism, these colloidal particles can adsorb irreversibly at the oil-water interface, to sterically (i.e. mechanically) hinder typical destabilisation pathways, such as flocculation, coalescence or Ostwald ripening.
A diagram illustrating a particle and a surfactant-stabilised droplet of an oil-in-water (o/w) emulsion is presented in Figure 1, including two scanning electron microscope (SEM) images showing the surface of a droplet covered densely with particles. The particles in this instance are spherical silica particles that stabilise emulsion droplets in the presence of a trivalent salt. While this is not a system relevant to foods, it is an excellent example of the fact that once the particles have adsorbed at the interface, they will remain adsorbed. This is demonstrated in the second SEM image (Figure 1D) showing a droplet of the same emulsion following removal of the trivalent salt. Similarly, and relevant to food formulations, changing pH or salt concentration in Pickering emulsions will not compromise the surface decoration of the emulsion droplets with particles and thus, unless otherwise compromised, emulsion stability is retained. Due to their scope of added functionalities, Pickering emulsions have accrued a great deal of both theoretical and commercial interest.
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Figure 1. Diagram (top) of (A) a Pickering-stabilised and (B) a small molecular weight surfactant-stabilised emulsion droplet. The freeze fracture SEM images (bottom) show (C) an emulsion stabilised by 0.23 μm spherical silica particles in the presence of a trivalent salt that retains its surface microstructure following removal of the salt (D)[1]. |
The food industry has not been able to adopt Pickering stabilisation strategies on a large scale, mainly due to the lack of a reservoir of edible structures that exhibit a Pickering functionality.
The appeal of Pickering systems
Although the concept of stabilisation by an interfacially adsorbed particulate material has long been applied in foods, e.g. casein micelles in homogenised milk, ice crystals in ice cream or fat crystals in fatty spreads[2], the ‘secret life’ of Pickering emulsions has begun to be unravelled only over the past two decades. What is particularly appealing about these systems is the potential to limit completely, or to an extent, the need to use surfactants, which may be desirable to address sustainability issues or for economic reasons. In recent years, a shift towards an on-the-go lifestyle, coupled with an increasing demand for benign ingredients for diet supplementation, has evolved to become an – almost – mainstream attitude on the part of consumers globally. Consumers are seeking healthier, natural and/or minimally processed foods, while they scrutinise nutrition labels for unadulterated ingredients. As a consequence, food and beverage companies are racing to develop new products, or re-formulate existing ones, to render them more natural and ‘clean-label’. The trend for label-friendly ingredients and therefore, healthier/safe foods necessitates the sourcing or constructing of food grade particles for Pickering stabilised systems, preferably from readily accessible and inexpensive resources. Yet the food industry has not been able to adopt Pickering stabilisation strategies on a large scale, mainly due to the lack of a reservoir of edible structures that exhibit a Pickering functionality.
Once surface-active particles reach the oil/water interface, as aforenoted, they are irreversibly adsorbed with huge detachment energies. Therefore, emulsions stabilised by such particles exhibit excellent long-term stability against coalescence; for instance, emulsions stabilised by spore microparticles (derived from the ground pine plant) have been found to be ‘completely stable’ for more than one year after their preparation[3]. This feature is of vital importance in the food industry as certain food emulsions need to be stable for a few days/months after being opened, while others might have to remain stable for years prior to consumption (e.g. sauces, dressings, cream liqueurs). Following the Pickering stabilisation route, emulsion-based food products can be designed to have an extended shelf-life although microbial stability should be taken into account.
The fact that these colloidal particles have the potential to reside permanently at the interface, makes the interface more rigid and robust. In turn, particle-laden interfaces are more tolerant to further processing after emulsion formation, and can act as a means to control lipid oxidation. Additionally, due to the thickness of the compact interfacial films or ‘shells’, they can be suitable for holding in place or protecting any active species, making them ideal candidates for the encapsulation and delivery of drugs/bioactive compounds. Depending on their composition, particles for Pickering stabilisation can be tailored to respond to triggered release of the encapsulated compounds, which could be specific flavours in the mouth or nutrients in the gastrointestinal tract. Furthermore, they have shown great promise in the formation and control of more complex emulsion microstructures (e.g. double emulsions), which could have applications in lowering fat content in emulsion-based foods. The benefits of using Pickering emulsions for food applications are explained in more detail in two very enlightening recent review papers[4,5].
Which materials can be used as candidates for Pickering approaches?
Much of the Pickering emulsion research to-date has been concerned with the study of model colloidal systems of inorganic origin and the most successful one is undoubtedly silica. A plethora of studies in the literature have been conducted utilising silica particles which has enabled the scientific community to gain deeper and valuable insights into the mechanisms involved in the stabilisation of Pickering emulsions. When it comes to food systems, particles that can be used for emulsion stabilisation must be sourced from biological or biocompatible materials. Edible ingredients that have been screened and employed for forming and stabilising emulsions are proteins and hydrocolloids, such as starch, cellulose and cellulose derivatives, chitin and chitosan[4,5]. Using these materials as the building blocks of particulate stabilisers could be tremendously advantageous as they are food-grade, readily available and cheap, and they also have the potential to undergo mass production[6]. Regardless of the origin, it is important to note that the successful performance of a particle in stabilising via a Pickering mechanism is determined by its size (average size should be at least an order of magnitude smaller than the emulsion droplet size) and wettability characteristics (i.e. how the particle is wetted from the two liquid emulsion phases).
It is also common practice that particles with a Pickering functionality act in situ at the oil/aqueous interface to enable stable emulsions to be formed. In lieu of this approach, there is huge potential in designing and constructing particles that can be added into a process (e.g. emulsification), allowing well-controlled and uniform experimental conditions to be established. Such a pathway could be applied by employing particles based on edible lipids or, alternatively, derived from co-products of the food manufacturing industry. Lipid-based particles (i.e. fat crystals), being hydrophobic (fat-loving), are more effective in stabilising water-in-oil (w/o) emulsions but currently there are a limited number of studies focusing on lipid particles for the stabilisation of oil-in-water (o/w) emulsions. On the other hand, there has been substantial hype around co-products from the food processing industries. Among the interesting paradigms, flavonoids (e.g. rutin hydrate) that are often wasted from the wine/juice processing industries, have demonstrated an ability to serve as stabilisers of simple o/w emulsions[7] and complex water-in-oil-in-water (w/o/w) Pickering emulsions[8]. As another co-product based approach, lignin-rich food particle systems have been described to stabilise simple o/w and w/o emulsions. These two separate classes of lipid and lignin-rich particles have featured in the recent research of the authors and are therefore discussed in more detail below for their potential to act as Pickering stabilisers.
Flavonoids (e.g. rutin hydrate) that are often wasted from the wine/juice processing industries, have demonstrated an ability to serve as stabilisers of simple o/w emulsions.
Lipid particles as Pickering emulsion stabilisers
Particles made of lipids have recently emerged as a very promising route in the development of edible particle-stabilised emulsions. The potential stems from their superior biocompatibility coupled with the versatility/functionality that their composition offers. The latter characteristic can pave the way for a precise control over particles’ encapsulation or interfacial properties when it comes to their use as either delivery vehicles or Pickering stabilisers. It is not due to chance that a number of commercial foods are in part, or wholly, stabilised by surface-active crystallised lipids that position themselves at the oil/water interface (e.g. spreads, ice cream, whipped cream, margarine).
Naturally occurring organic compounds, such as triglycerides (triacylglycerols, i.e. the principal components of fats and oils) and waxes, were chosen as the lipid-based materials. The approach was to form a simple micro/sub-micron emulsion in which the micro/sub-micron droplets were eventually used as templates for the particles. More specifically, the lipid underwent heating above its melting point and was then emulsified with the aqueous phase that contained very small amounts of surfactant, using an ultrasound probe. The surfactant can be either of a small (Tween 80) or a high molecular weight (sodium caseinate, i.e. protein found in milk) and its role was to aid the breakup of droplets into small sizes during homogenisation while at the same time, minimising coalescence. The generated hot o/w emulsion was then cooled in order to solidify the droplets and form solid lipid particles dispersed in water.
Results showed that sub-micron crystalline lipid particles could be produced and their characteristics, such as size and size distribution, melting profile and interfacial behaviour, could be controlled by adjustments to surfactant type and concentration (Figure 2A), as well as lipid structure. Particles’ stability upon storage was driven by the surfactants’ and lipid materials’ physicochemical properties (e.g. polymorphism)[9]. The dispersions of solid lipid particles were subsequently used for the stabilisation of sunflower oil-in-water emulsions. Stable to coalescence (up to ~4 weeks) 20 wt% o/w emulsions with droplets of ~ 6-30 μm formed in the presence of solid lipid particles were produced and the ‘decorated’ oil droplets were visualised (Figure 2B)[10]. As seen in the micrograph, lipid particles (red) were mostly located around the oil droplets adsorbing at the interface; this provided evidence of a Pickering-type stabilisation.
Importantly, the study additionally demonstrated for the first time that lipid particles have the potential to be dried (using the industrially applicable freeze-drying method) and rehydrated, with minimal loss to their Pickering functionality. Experiments revealed that dried and rehydrated lipid particles produced using sodium caseinate could withstand the harsh dehydration process without any use of conventional drying aids (cryoprotectants) and they regained their microstructural properties upon reconstitution. Not only that, they also showed an ability to generate the same droplet size emulsions as the ones that were formed when employing particles that had not undergone a drying stage (Figure 2C). Having a food ingredient in a powder form (or isolated from its aqueous environment) is highly desirable from both the industry’s and the consumer’s viewpoints, given the ease of transportation and storage, as well as the potential convenience (e.g. instant food products that can be reconstituted simply by addition of water).
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Figure 2. (A) Particle size distribution of solid wax particles formed in the absence and presence (2 wt%) of a low molecular weight surfactant[9]. (B) Confocal micrograph of an o/w emulsion stabilised by wax particles; red colour represents fluorescence signal from the dyed colloidal wax particles[10]. (C) Droplet size distribution of o/w emulsions formed with non-dried (before FD) and dried/rehydrated (after FD) solid wax particles in the presence of sodium caseinate. |
Particles derived from food manufacturing co-products as Pickering emulsion stabilisers
The research on plant-based co-products of the food manufacturing industry as feedstocks for particles with a Pickering functionality was triggered by the observation that cocoa particles, as a commercially available food ingredient, readily stabilised o/w emulsions[11]. This property was attributed to the presence of lignin as a natural cell wall component in these particles, while the naturally present lipids were demonstrated to not be a contributing factor[12]. Lignin is present in vascular plants, usually within the cell walls and also between cells. It provides structural support and plays a crucial role in conducting water. After cellulose, lignin is the second most abundant natural polymer in the world. It is an aromatic polymer that is less hydrophilic than cellulose and describes a class of material rather than a particular polymer structure. There are applications of plant biomass where it is desirable to remove lignin, for example extracting celluloses as feedstocks for producing bioethanol fuel, and hydrothermal processing followed by solvent extraction is a suitable method to achieve this.
During the hydrothermal processing (i.e. heating the biomass dispersed in aqueous solvent to a temperature above the liquefaction temperature of lignin, in practice between around 180 and 250°C), the lignin is liberated from the cell wall and ‘extruded’ towards the surface of the biomass particles. As lignin is largely repelled by water, it deposits onto the surface appearing as microscopic small surface-adhered droplets. An example is shown in Figure 3. Coffee grounds collected from coffee outlets were milled and then hydrothermally treated for 1 hour at increasing temperature. The lowest temperature selected was below the lignin liquefaction temperature and the particle surface (B) appears very similar to the untreated particle (A). It is evident that the treatment was more efficient with increasing temperature (C-F), although it appears that the surface-adhered droplets fused together following treatment at the highest temperature (F). The untreated particles stabilised o/w emulsions, whilst the treated particles stabilised w/o emulsions, although the w/o emulsions had a coarser microstructure (larger droplets). The minimum processing temperature required to impart w/o emulsion stabilisation functionality varies between feedstocks due to their differing biological make up. Hydrothermal treatment time also has an impact, for example for Brewer’s spent grain we have observed that processing longer at a slightly lower temperature, but still above 150°C, e.g. 2 hours as opposed to 1 hour, was an alternative process parameter combination to achieve w/o emulsion stability.
As an alternative approach, the surface-adhered droplets may be extracted with ethanol, for example, and via antisolvent precipitation they can be transformed into sub-micron particles. This has been demonstrated for woody biomass[14] but is equally applicable to co-products of the food and drink manufacturing industry, e.g. cocoa shell particles. As evidenced above (Figure 3), their hydrothermal processing creates these characteristic surface-adhered droplets. Extracting and dripping the ethanol-dissolved extract into agitated water led to the formation of sub-micron particles that stabilise o/w emulsions. This development is still in its infancy with the preliminary results showing promise for future developments, not only towards a new food ingredient, but also towards valorisation of waste and the creation of a circular economy.
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Figure 3. Scanning electron microscope images of (A) dried coffee particle; (B) dried and milled coffee particle and (C-F) following hydrothermal treatment at (C) 150oC; (D) 200oC; (E) 250oC; and (F) 275oC for 1 hour. Scale bar represents 10 μm[13]. |
Conclusions and future perspectives
Pickering stabilisation of emulsions, as well as foams, is clearly an attractive proposition. The interfaces created during processing are practically irreversibly stabilised, unless a de-stabilisation mechanism (e.g. for the delivery of a bioactive) is programmed in. Such Pickering-stabilised systems therefore tend to be robust against additional ingredients which may destabilise interfaces in surfactant-stabilised emulsions and foams. However, they may still need to be protected against visual separation, e.g. by the addition of a thickener. Application in manufacturing requires the provision of suitable ingredients providing this functionality and these, to the best of the authors’ knowledge, are not commercially available for the food and drink industry at present. Future developments are necessary to overcome this hurdle, ensuring the development of clean-label ingredients to satisfy consumers’ demands. There is huge opportunity in the valorisation of industry co-products, which coincides with current industry strategy towards a sustainable circular economy and limiting environmental impact.
Ioanna Zafeiri, Research Fellow and Bettina Wolf, Professor in Microstructure Engineering
Microstructure Engineering Research Group, School of Chemical Engineering, University of Birmingham. Edgbaston Campus, B15 2TT, UK
Email:I.Zafeiri@bham.ac.uk
Acknowledgements
IZ acknowledges support by Cargill, Innovate UK and the Engineering and Physical Sciences Research Council (EPSRC) [grant number EP/G036713/1]. BW acknowledges support by the EPSRC [grant number EP/K030957/1], the Biotechnology and Biological Sciences Research Council [grant number BB/I532602/1], Nestlé PTC York, Campden BRI and Mondelez. BW thanks Joanne Gould, James Huscroft and Holly Cuthill for their research.
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