Chapter 1: Overview of Bioactive Delivery Systems

There has been growing interest in the design, development, and application of edible delivery systems suitable for application within the modern food industry. As an example, the number of scientific articles published per year with the keywords “delivery system” and “food” increased from 16 in 2000 to over 540 in 2020 (Figure 1.1a). Similarly, the number of citations of papers per year with the same keywords increased from 177 to over 10 800 during the same period (Figure 1.1b). Edible delivery systems can be assembled from many different kinds of food ingredients (including proteins, polysaccharides, lipids, phospholipids, and minerals) using various kinds of processing operations (including homogenization, milling, self-assembly, injection-gelation, and precipitation methods).^1–9  Selection of the most appropriate ingredients and processing methods for a specific application is therefore important. A major factor behind the interest in this area has been the desire to introduce different kinds of active substances into foods so as to improve their physicochemical, stability, sensory, or nutritional attributes.^10,11  These active substances include components to change the aesthetic appeal (colors and flavors), nutritional profile (lipids, proteins, carbohydrates, vitamins, minerals, enzymes, nutraceuticals, and probiotics) or shelf life (antimicrobials and antioxidants) of foods.¹²  There are unique challenges that often restrict the direct incorporation of many of the active substances into commercial foods and beverages, including poor water-solubility, crystallinity, chemical or biochemical instability, off-flavors, low bioavailability, or limited activity.¹³  These challenges can often be overcome by using well-designed delivery systems; however, these systems should also be economically viable, robust in food applications, and capable of being manufactured on a large scale.¹²  Many of the delivery systems reported within scientific publications are not suitable for commercial utilization because they do not meet the latter criteria. As an example, they may be manufactured using ingredients or processing operations which are too costly or unsuitable for use in the food industry, or they consist of particles which are not compatible with the food or beverage matrix that they are intended to be utilized within. In this chapter, the concept of delivery-by-design (DbD) is introduced as a means of rationalizing the formulation of edible delivery systems suitable for application in the food industry.^14,15  The successful implementation of the DbD approach could facilitate the more widespread use of delivery systems within food products.

The DbD approach provides a systematic framework to guide the design, fabrication, and testing of edible delivery systems for particular applications. It can be conveniently divided into a series of stages, which are highlighted schematically in Figure 1.2. A brief discussion of each of these stages is given in this section.^14,15 

In stage 1, the molecular and physicochemical attributes of the active substance that needs to be encapsulated are clearly defined, and any challenges associated with incorporating it into foods or beverages are identified.¹⁴  Each type of bioactive substance has its own unique attributes and challenges, which often means that different delivery systems are required. A number of the most important attributes of active substances that should be defined are highlighted in Table 1.1. As an example, β-carotene is a carotenoid that is used as a natural pigment and nutraceutical that has an extremely low water-solubility, is crystalline at room temperature, tends to degrade under acidic conditions, at high temperatures, and after being exposed to light, and has a very limited oral bioavailability.¹⁶  Consequently, delivery systems are needed to increase its water-dispersibility, stability, and bioavailability. Similarly, curcumin is another natural pigment and nutraceutical that also has a low water-solubility and is crystalline at room temperature, but in this case it degrades under alkaline conditions, at high temperatures, when exposed to light, or when exposed to metabolic enzymes in the human gut.¹⁷  Lipase is a digestive enzyme that is dispersible in water but its activity is reduced if it is exposed to relatively high or low pH values, heating, or stomach fluids.^18,19  These few examples highlight the distinct differences between active substances and the need to fully understand their molecular and physicochemical traits when designing delivery systems.

In stage 2, the desired quality and functional properties of the final food or beverage in which the edible delivery system is going to be utilized should be carefully defined.¹⁴  The quality attributes that are common to most commercial food or beverage products include their optical properties (appearance, including color and opacity), rheology (textural attributes such as viscosity, elastic modulus, and breaking stress), shelf life and resistance to breakdown (stability), and organoleptic properties (mouthfeel and flavor profile). Moreover, many food and beverage products are expected to have other functional attributes, including antimicrobial activity, antioxidant activity, bioavailability, controlled release properties, or physiological effects.¹²  In addition, the range of environmental conditions that may exist within an end-product should be specified as these could affect the performance of the delivery system, including the pH, ionic composition, temperature, mechanical stresses, and ingredient interactions. A number of the most important characteristics of end-products that should be defined and specified are summarized in Table 1.1. It should be noted that many researchers working in this area do not consider the characteristics of the end-product that the delivery systems that they are developing are intended to be used in. As a result, a system that works well under carefully controlled laboratory conditions fails in practice because it is incompatible with the food matrix or environmental conditions. Having said this, some researchers have taken into account the properties of the end-products when designing their delivery systems. For instance, the impact of applying antimicrobial nanoemulsions to the surfaces of fruits and vegetables on their sensory attributes and appearance has been considered.^20,21  The influence of incorporating vitamin-loaded liposomes into fruit juices on their physical and chemical properties has also been investigated.²²  Moreover, the incorporation of vitamin-loaded casein micelles into yogurt on its sensory attributes and physical properties has been elucidated.²³  In future, researchers should take more care in accounting for the physicochemical and sensory characteristics of specific end-products when designing edible delivery systems.

In stage 3, the insights gained from stages 1 and 2 are combined so as to identify the most desirable delivery system attributes required for the incorporation of the active substance into a specific food product.¹⁴  The required attributes are strongly dependent on the characteristics of the active substance and end-product in question, and so they vary considerably depending on the precise nature of the application. A number of important characteristics to consider when specifying a delivery system with the desired attributes are highlighted in Table 1.1. Some of the most important characteristics are the physical state of the delivery system (e.g., liquid, paste, fine powder, or granules), optical attributes (e.g., color and opacity), textural attributes (e.g., viscosity, elastic modulus, and breaking strength), stability (e.g., creaming/sedimentation or aggregation/fragmentation), encapsulation properties (e.g., loading capacity and encapsulation efficiency), and retention/release properties (e.g., environmental factors influencing the location of the active substance in the particles). The importance of defining the desirable characteristics of an edible delivery system can be best appreciated by considering some specific examples. A delivery system that is going to be used to incorporate an omega-3 oil (such as fish, flaxseed, or algae oil) into a clear beverage should be optically transparent, have a low viscosity, have a strong resistance to creaming, flocculation, and coalescence during storage, and provide good protection against lipid oxidation.^24,25  On the other hand, a delivery system intended to incorporate an oil-soluble vitamin into yogurt may be opaque and highly viscous, while not affecting the desirable mouthfeel of the product and providing a high bioavailability of the ingested vitamins.^23,26 

In stage 4, the particle properties that can provide the required physicochemical attributes and functionality in the end-product are defined, which helps to identify a suitable delivery system (Figure 1.1).¹⁴  A number of the most important particle properties that influence the performance of delivery systems are listed in Table 1.1 and shown in Figure 1.3. These include the core and interfacial composition, dimensions, morphology, the physical state (solid, liquid, or semi-solid), concentration, charge, and environmental responsiveness (Figure 1.3). For instance, if the delivery systems should be optically clear, then it is important that the particles it contains do not strongly scatter light waves. This can be achieved by using particles that are relatively small compared to the wavelength of light (d < 50 nm), only using very low particle concentrations (ϕ < 0.1%) and/or using particles that have a relatively low contrast in the refractive index (Δn < 0.01).^27,28  Only a few kinds of edible delivery systems contain particles that are small enough to create optically transparent end-products, including microemulsions, nanoliposomes, nanoemulsions, and biopolymer nanoparticles.¹²  The selection of one of these systems will depend on other factors, such as their compatibility with the food matrix, their resistance to breakdown when exposed to the processing and storage conditions of the food, as well as more practical factors, including ease of handling, economics, regulatory status, and consumer acceptability. Delivery systems containing larger particles that do scatter light strongly (such as emulsions) can be used in end-products where the final food or beverage should be cloudy or opaque.

In stage 5, a suitable manufacturing process for fabricating the delivery system identified in stage 4 should be developed and optimized.¹⁴  Typically, there are a number of different ways of manufacturing a particular kind of edible delivery system. For instance, nanoemulsions can be produced using sonicators, microfluidizers, and low-energy approaches.^6,29,30  Similarly, biopolymer nanoparticles or microparticles can be produced by injection-gelation, emulsion templating, coacervation, and antisolvent precipitation methods.¹²  In addition to generating a delivery system with the appropriate particle characteristics, a manufacturing process should also be chosen based on other factors, including equipment availability, economics, ease of use, robustness, repeatability, reliability, and throughput. After a suitable manufacturing process has been identified, it is then necessary to optimize it. As an example, if a microfluidizer is chosen to produce a nanoemulsion-based delivery system, then the valve design, operating pressure, number of passes, and system composition should all be optimized so as to produce the desired particle properties in the final product.^31,32  Industrially, a manufacturer may compare the performance of a number of manufacturing processes and operating conditions to establish their relative benefits and limitations.

In stage 6 it is necessary to create a rigorous analytical testing regime to make sure the delivery system and the end-product it is incorporated into have the desired structural, physical, and functional properties. Many different analytical instruments and measurement protocols have been established to characterize the properties of the particles in delivery systems, including their number, composition, dimensions, morphology, electrical characteristics, and location.³³  The particle size distribution, mean particle diameter, and polydispersity can be determined using static and/or dynamic light scattering instruments. The electrical characteristics of the particles in a delivery system, such as the variation of the ζ-potential with pH, can be determined using microelectrophoresis instruments. The morphology, aggregation state, and location of the particles can be established using atomic force, electron, light, and/or fluorescence microscopy methods. The chemical stability of the encapsulated components can be monitored by measuring changes in their concentration over time or when exposed to specific environmental stresses. This can be achieved using a range of different analytical methods whose selection depends on the specific characteristics of the active substance being monitored, including colorimetry, chromatography, spectroscopy, and mass spectrometry. A range of analytical instruments are also available to establish the influence of delivery system incorporation on the desired properties of the end-product, including rheometers to characterize textural changes, colorimeters to characterize alterations in color or opacity, chromatography instruments to measure changes in headspace concentrations, and sensory tests to establish alterations in flavor, texture, mouthfeel or appearance.³⁴  The development of a good testing procedure is critical for the development of a successful delivery system for application in foods and beverages, as well as to test the products during manufacture to ensure they meet specifications.

It should be recognized that the development of an effective delivery system for food applications is an iterative process, with the formulation, manufacturing operation, and testing protocol continually being refined and improved (Figure 1.2). As a result, the characteristics of the delivery system and the end-product need to be frequently measured and recorded. The data collected can then be used to optimize the design and manufacturing processes.¹⁴  As a specific example, let us consider the fabrication of a nanoemulsion-based delivery system for encapsulating omega-3 fatty acids and then incorporating them into a fruit drink. This nanoemulsion could be conveniently produced using a microfluidizer. A range of analytical sensors could be employed to measure and record processing parameters throughout the manufacturing operation (such as the homogenization pressure, flow rate, oxygen level, and temperature of the microfluidizer), as well as the characteristics of the omega-3 loaded oil droplets produced (such as their particle size and charge), and the characteristics of the end-product (such as viscosity, appearance, creaming stability, rancidity, and bioavailability). Artificial intelligence and machine learning methods may be particularly advantageous in this stage, as they may be able to find links amongst specific ingredients, processing conditions, and system performance that can be used to optimize the overall process.

As shown in Figure 1.1, there has been a huge increase in the number of publications and citations on the formulation and application of delivery systems suitable for utilization within foods over the past two decades. Nevertheless, much of the research in this area will not find commercial application because the delivery systems proposed are assembled using ingredients and manufacturing processes that are too costly or unreliable, because the delivery systems created are not sufficiently robust to operate effectively in end-products, or because the incorporation of the delivery systems adversely affects the desirable sensory attributes of the end-products. In the future, it will be important to take into account these aspects when developing delivery systems for food applications. The delivery by design approach described here may facilitate the more rapid design, production, and application of delivery systems in food and beverage products.