Colloidal-scale self-assembly of microcapsules for food

Microencapsulation is the technique of enclosing valuable or delicate materials in small containers for targeted delivery. These containers may consist of a core and a thin shell surrounding it. The core, with a size ranging from a few to hundreds of micrometers, contains the active material, and serves as template for the microcapsule. The shell, with a thickness of no more than a few hundred nanometers, lends mechanical, physical, or chemical protection, and the means to release the active materials in response to a well defined trigger. Microencapsulation is used in a number of industries, e.g. pharmaceutics and food. In the case of medicines, microencapsulation is important for the delivery of drugs at the correct location, in the right dose, and at the right moment, which otherwise would have to be overdosed to reach the desired therapeutic effects. In the case of food, microencapsulation may mask ingredients that would cause off-taste and protect sensitive or volatile materials like flavors, aromas, and antioxidants. In addition, microencapsulation will enable new product concepts, such as products with new sensory experiences (fizzy products from encapsulated CO2) and, more importantly, healthier foods including nutraceuticals or probiotics. The microencapsulation methods that are used nowadays in industry on a fairly large scale allow limited control over the uniformity of the microcapsule size and geometry, and the thickness of the shell, while the harsh operation conditions pose restrictions on the type of ingredients that can be used. This poses a challenge to improve production methods and materials. Besides, specifically for food, the materials should additionally be food grade and inexpensive. In this thesis, microcapsules (1-20 micron) are constructed via self-assembly; i.e. using driving forces already available in nature. From the routes using self-assembly proposed in literature for microencapsulation two called our special attention, both starting from an oil droplet as template which determines the size of the microcapsule. The first route is that of colloidosome preparation, in which colloidal particles adsorb irreversibly and organize at the oil-water interface, creating a sturdy shell with pores defined by the interstices between the particles. The second route is electrostatic layer-by-layer adsorption: polyelectrolytes of opposite charge are sequentially adsorbed on a charged template, creating a thin film of which the thickness can be controlled with precision in the order of few nanometers. We have combined the two methods by using larger building blocks to construct stronger shells with better defined characteristics, using less adsorption cycles and based on food grade materials. These microcapsules are designed to protect an eventual encapsulated material from low-pH conditions and deliver their contents in response to a change in pH, having in mind applications that would require delivery through the stomach into the small intestine. In Chapter 2, the available techniques to produce colloidosomes are reviewed. Microcapsules can be assembled from both, oil-in-water or water-in-oil emulsions, which translates in the potential ability to encapsulate hydrophilic and hydrophobic materials. Since the size of the microcapsules is defined by the droplet used as template the construction of almost perfectly monodisperse microcapsules is within reach, given the current developments in emulsification technology. The choice of size, geometry, and origin of the colloidal particles to be used to assemble the shell, and the means to lock them together, gives control over the targeting and release behavior of the colloidosomes. The requirement that always needs to be met is that particles should have affinity for the both oil and water at the interface, imposing a restriction on the choice of the to-be-used materials. Further, the methods usually applied to lock the colloidal particles to form the shell (e.g. heat up the microcapsule to sinter particles together) were seen as a drawback for the encapsulation of delicate materials such as foods, for which many active components are heat sensitive or volatile. Without significant sintering of the particles the capsule does not have protective properties. The next chapters are therefore devoted to new microencapsulation techniques that complement the colloidosome and the layer-by-layer adsorption routes. In Chapter 3, the first results are presented on microcapsules prepared by sequential electrostatic adsorption of protein fibrils, with approximate dimensions of 1 µm in length and 4 nm in width, and high methoxyl pectin. The encapsulation procedure was carried out at low pH (3.5), which allowed us to use positively charged oil droplets stabilized with whey protein isolate as templates. Confocal scanning laser microscopy showed that the fibrils adsorbed as an open structure on a layer of pectin in quantities much larger than a monolayer. This was confirmed from the shell thickness determined by scanning electron microscopy. The fibrils remained trapped at the surface after the adsorption of another layer of pectin. It could be shown indirectly that the inclusion of fibrils in the shell had a tremendous impact on the mechanical strength when compared to microcapsules made of layers of protein and pectin only. Since the microcapsules were assembled at low pH, under the action of pH- and ionic strength-dependent electrostatic interactions, we envision that these capsules would survive low pH, and increase their permeability, or totally disorganize, when exposed to a neutral pH. This is further elucidated in the next chapter. In Chapter 4 extensive characterization of the fibril-reinforced microcapsules is reported. Each layer (of fibrils or pectin) added approximately 30 nm to the total thickness, which is considerably more than monolayer coverage, as measured by reflectometry. The microcapsules keep their integrity when exposed to pH below 5.2 (showing slow dissolution at pH 2), but disintegrate at pH 7 or higher. The response is non linear for increasing number of layers, as it was the case for the mechanical strength of the microcapsules. While microcapsules with 7 or less layers had a similar Young modulus, microcapsules with 8 or more layers had twice that strength, around 0.6 GPa, comparable with the strength of polymeric microcapsules that are chemically cross-linked. This was related to the defects present in the shells as observed with scanning electron microscope: capsules with 8 or more layers had smooth and defect-free shells, which resulted in high pH stability. The available food grade materials allowed the definition of a second system (to be assembled also at low pH) presented in Chapter 5. A single layer of charged silica particles was adsorbed on sequential layers of whey protein and high methoxyl pectin, therewith reducing the number of adsorption steps. Those pre-adsorbed layers stabilized the adsorbed hydrophilic silica particles that, otherwise, would make a poor colloidosome structure. In this way a highly porous but strong structure that could be easily loaded was obtained. After loading the pores can be closed through the adsorption of additional layers of protein and pectin. The drawback of the two microencapsulation systems described above is that polyelectrolytes needed to be used in excess, and intermediate washing steps to rinse out the non-adsorbed materials were essential. To overcome this we went a step ahead in Chapter 6 using a microfluidic device to carry out layer-by-layer adsorption of up to ten layers of protein fibrils and pectin in continuous mode. The design of the chip is simple and does not require complex infrastructure around it, since it relies only on the right balance of the hydraulic resistances of different sections of the microfluidic circuit to control the dose of the materials for the microcapsules’ assembly. Although issues like surface modification of the chip for long-run operation, and the scale-up of the process to industrially-interesting volumes are still a challenge, we feel that this is an important step forward toward controlled microcapsule formation. The integration of knowledge on self-assembly, of which some examples can be found in this thesis, combined with the search for new food-grade materials that can act synergistically to assemble a smarter and multifunctional shell, and better design of microfluidics for tight control of this process are key to mature microcapsule formation into a real tool for the food industry. Chapter 7 discusses further requirements for the production of reinforced layer-by-layer microcapsules using microfluidics, and closes this thesis with a general discussion of the results, in the light of possible future developments in the area of microencapsulation. <br/

Microcapsule production by an hybrid colloidosome-layer-by-layer technique

Although many different methods for microencapsulation are known only some of them had been applied at industrial scale, due to complexity, lack of mechanical strength of the resulting capsules, and the costs related to their production. One of such methods is the electrostatic layer-by-layer (LbL) adsorption, which produce shells from oppositely charged polymers. The thickness of those shells can be tuned with nanometric precision, but to build enough strength for practical applications requires the adsorption of an impractical number of layers. We present here a method to produce strong microcapsules combining the assembly of a protein/pectin shell via electrostatic LbL adsorption with the adsorption of bigger charged colloidal particles. Those colloidal particles do not need any pretreatment to modify their wettability, as would be the case for a standard colloidosome route. In this way strong encapsulates with porous walls are obtained, which can be used as easy to load scaffolds. The pores in the walls can be closed through subsequent adsorption of more layers of protein and pectin. Since the assembly scheme occurs at pH 3.5 we expect the produced microcapsules to act as an effective delivery system in food products, protecting their contents from the acidity of the stomach and dissolving later at the small intestine. The proteins and pectins used as basic building blocks are food-grade and inexpensive

Colloidosomes: Versatile microcapsules in perspective

Colloidal particles of different sizes and shapes can organize on suspended particles or emulsion droplets, forming hollow-porous microcapsules called colloidosomes. The potential of the colloidosomes to serve as targeted delivery/controlled release devices has been discussed many times in literature. However, obtaining well-defined colloidosomes at high yields is still an open challenge. We review and compare the different methods reported in literature to produce colloidosomes, not only to show the state of the art and the aspects requiring further development, but also to spot the possible future perspectives of research in this fiel

Mechanical Characterization and pH Response of Fibril-Reinforced Microcapsules Prepared by Layer-by-Layer Adsorption

Despite the fair number of microencapsulation principles that have been developed, the actual protection and targeted delivery of sensitive ingredients remains a challenge in the food industry. A suitable technique should use food-grade and inexpensive materials, and ensure tight control over the capsule size and release trigger mechanism. For example, encapsulates may need to survive the low pH of the stomach to release their contents in the neutral environment of the small intestine. In this work we present layer-by-layer (LbL) microcapsules assembled from whey protein isolate (WPI), high-methoxyl pectin (HMP) and WPI-fibrils. The narrow size distribution of these capsules is determined by the oil-in-water droplets used as templates, and their mechanical properties and pH response can be tuned by the number of layers adsorbed. Capsules with more than eight layers have a mechanical strength comparable to chemically cross-linked polymer capsules, because of the reinforcement by the WPI-fibrils in combination with the shell completion. Typically, capsules with five layers survive pH 2 for more than 2 h, but dissolve within 30 min at pH 7. At higher number of layers, the capsules are even more stable. Contrary to other encapsulates, these capsules can be dried and are suitable for application in dry products

The Use of Exergetic Indicators in the Food Industry – A Review

Sustainability assessment will become more relevant for the food industry in the years to come. Analysis based on exergy including the use of exergetic indicators and Grassmann diagrams is a useful tool for the quantitative and qualitative assessment of the efficiency of industrial food chains. In this paper we review the methodology of exergy analysis and the exergetic indicators that are most appropriate for use in the food industry. The challenges of applying exergy analysis in industrial food chains and the specific features that food processes are also discussed

Exergetic comparison of food waste valorization in industrial bread production

This study compares the thermodynamic performance of three industrial bread production chains: one that generates food waste, one that avoids food waste generation, and one that reworks food waste to produce new bread. The chemical exergy flows were found to be much larger than the physical exergy consumed in all the industrial bread chains studied. The par-baked brown bun production chain had the best thermodynamic performance because of the highest rational exergetic efficiency (71.2%), the lowest specific exergy losses (5.4 MJ/kg brown bun), and the almost lowest cumulative exergy losses (4768 MJ/1000 kg of dough processed). However, recycling of bread waste is also exergetically efficient when the total fermented surplus is utilizable. Clearly, preventing material losses (i.e. utilizing raw materials maximally) improves the exergetic efficiency of industrial bread chains. In addition, most of the physical (non-material related) exergy losses occurred at the baking, cooling and freezing steps. Consequently, any additional improvement in industrial bread production should focus on the design of thermodynamically efficient baking and cooling processes, and on the use of technologies throughout the chain that consume the lowest possible physical exergy

Exergetic comparison of food waste valorization in industrial bread production

This study compares the thermodynamic performance of three industrial bread production chains: one that generates food waste, one that avoids food waste generation, and one that reworks food waste to produce new bread. The chemical exergy flows were found to be much larger than the physical exergy consumed in all the industrial bread chains studied. The par-baked brown bun production chain had the best thermodynamic performance because of the highest rational exergetic efficiency (71.2%), the lowest specific exergy losses (5.4 MJ/kg brown bun), and the almost lowest cumulative exergy losses (4768 MJ/1000 kg of dough processed). However, recycling of bread waste is also exergetically efficient when the total fermented surplus is utilizable. Clearly, preventing material losses (i.e. utilizing raw materials maximally) improves the exergetic efficiency of industrial bread chains. In addition, most of the physical (non-material related) exergy losses occurred at the baking, cooling and freezing steps. Consequently, any additional improvement in industrial bread production should focus on the design of thermodynamically efficient baking and cooling processes, and on the use of technologies throughout the chain that consume the lowest possible physical exergy

Characterisation and use of ß-lactoglobulin fibrils for microencapsulation of lipophilic ingredients and oxidative stability thereof

There is a growing interest in using fibrils from food grade protein, e.g. ß-lactoglobulin, as functional ingredients. In the present study, the functionality of fibrillar ß-lactoglobulin from whey protein isolate (WPI) was compared to native WPI in terms of interfacial dilatational rheology and emulsifying activity at acidic conditions (pH 2.0 and 3.0). We report here for the first time data on microencapsulation of fish oil by spray-drying as well as oxidative stability of the oil in emulsions and microcapsules in dependence of WPI conformation. WPI fibrils exerted a significantly higher elasticity at the oil–water (o/w) interface and a better emulsifying activity at a fixed oil content compared to native WPI. Microencapsulation efficiency was also higher with fibrillar WPI (>95%) compared to native WPI (~90%) at pH 2.0 and a total oil and protein content of 40% and 2.2%, respectively, in the final powder. The oxidative deterioration was lower in emulsions and microcapsules prepared with fibrillar than with native WPI. This was attributed to improved interfacial barrier properties provided by fibrils and antioxidative effects of coexisting unconverted monomers, particularly hydrophilic peptide