Review
Encapsulation of polyphenols – a review

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Research on and the application of polyphenols, have recently attracted great interest in the functional foods, nutraceutical and pharmaceutical industries, due to their potential health benefits to humans. However, the effectiveness of polyphenols depends on preserving the stability, bioactivity and bioavailability of the active ingredients. The unpleasant taste of most phenolic compounds also limits their application. The utilization of encapsulated polyphenols, instead of free compounds, can effectively alleviate these deficiencies. The technologies of encapsulation of polyphenols, including spray drying, coacervation, liposome entrapment, inclusion complexation, cocrystallization, nanoencapsulation, freeze drying, yeast encapsulation and emulsion, are discussed in this review. Current research, developments and trends are also discussed.

Introduction

Microencapsulation, developed approximately 60 years ago, is defined as a technology of packaging solids, liquids, or gaseous materials in miniature, sealed capsules that can release their contents at controlled rates under specific conditions (Desai and Park, 2005, Vilstrup, 2001). The packaged materials can be pure materials or a mixture, which are also called coated material, core material, actives, fill, internal phase or payload. On the other hand, the packaging materials are called coating material, wall material, capsule, membrane, carrier or shell, which can be made of sugars, gums, proteins, natural and modified polysaccharides, lipids and synthetic polymers (Gibbs et al., 1999, Mozafari, 2006)

Microcapsules are small vesicles or particulates that may range from sub-micron to several millimeters in size (Dziezak, 1998). Many morphologies can be produced for encapsulation, but two major morphologies are more commonly seen (Fig. 1): one is mononuclear capsules, which have a single core enveloped by a shell, while the other is aggregates, which have many cores embedded in a matrix (Schrooyen, van der Meer, & De Kruif, 2001). Their specific shapes in different systems are influenced by the process technologies, and by the core and wall materials from which the capsules are made.

Various techniques are used for encapsulation. In general, three steps are involved in the encapsulation of bioactive agents: (i) the formation of the wall around the material to be encapsulated; (ii) ensuring that undesired leakage does not occur; (iii) ensuring that undesired materials are kept out (Gibbs et al., 1999, Mozafari et al., 2008). The current encapsulation techniques include spray drying, spray cooling/chilling, extrusion, fluidized bed coating, coacervation, liposome entrapment, inclusion complexation, centrifugal suspension separation, lyophilization, cocrystallization and emulsion, etc. (Augustin and Hemar, 2009, Desai and Park, 2005, Gibbs et al., 1999).

The main objective of encapsulation is to protect the core material from adverse environmental conditions, such as undesirable effects of light, moisture, and oxygen, thereby contributing to an increase in the shelf life of the product, and promoting a controlled liberation of the encapsulate (Shahidi & Han, 1993). In the food industry, the microencapsulation process can be applied for a variety of reasons, which have been summarized by Desai and Park (2005) as follows: (i) protection of the core material from degradation by reducing its reactivity to its outside environment; (ii) reduction of the evaporation or transfer rate of the core material to the outside environment; (iii) modification of the physical characteristics of the original material to allow easier handling; (iv) tailoring the release of the core material slowly over time, or at a particular time; (v) to mask an unwanted flavor or taste of the core material; (vi) dilution of the core material when only small amounts are required, while achieving uniform dispersion in the host material; (vii) to help separate the components of the mixture that would otherwise react with one another. Food ingredients of acidulants, flavoring agents, sweeteners, colorants, lipids, vitamins and minerals, enzymes and microorganisms, are encapsulated using different technologies (Desai & Park, 2005).

Recently, research and application of polyphenols have been areas of great interest in the functional foods, nutraceutical and pharmaceutical industries (Manach et al., 2004, Scalbert, Manach et al., 2005). Polyphenols constitute one of the most numerous and ubiquitous groups of plant metabolites, and are an integral part of both human and animal diets which possess a high spectrum of biological activities, including antioxidant, anti-inflammatory, antibacterial, and antiviral functions (Bennick, 2002, Haslam, 1996, Quideau and Feldman, 1996). A large body of preclinical research and epidemiological data suggests that plant polyphenols can slow the progression of certain cancers, reduce the risks of cardiovascular disease, neurodegenerative diseases, diabetes, or osteoporosis, suggesting that plant polyphenols might act as potential chemopreventive and anti-cancer agents in humans (Arts and Hollman, 2005, Scalbert, Johnson et al., 2005, Scalbert, Manach et al., 2005, Surh, 2003).

Unfortunately, the concentrations of polyphenols that appear effective in vitro are often of an order of magnitude higher than the levels measured in vivo. The effectiveness of nutraceutical products in preventing diseases depends on preserving the bioavailability of the active ingredients (Bell, 2001). This is a big challenge, as only a small proportion of the molecules remain available following oral administration, due to insufficient gastric residence time, low permeability and/or solubility within the gut, as well as their instability under conditions encountered in food processing and storage (temperature, oxygen, light), or in the gastrointestinal tract (pH, enzymes, presence of other nutrients), all of which limit the activity and potential health benefits of the nutraceutical components, including polyphenols (Bell, 2001). The delivery of these compounds therefore requires product formulators and manufacturers to provide protective mechanisms that can maintain the active molecular form until the time of consumption, and deliver this form to the physiological target within the organism (Chen, Remondetto, & Subirade, 2006). Some physicochemical characteristics and food properties of the major polyphenols from different plant sources are present in Table 1, which shows their limited stability and conditioned solubility. Another unfortunate trait of polypheonls is their potential unpleasant taste, such as astringency (Table 1), which needs to be masked before incorporation into food products (Haslam & Lilley, 1988).

The utilization of encapsulated polyphenols instead of free compounds can overcome the drawbacks of their instability, alleviate unpleasant tastes or flavors, as well as improve the bioavailability and half-life of the compound in vivo and in vitro. There have been a number of recent reviews or mini-reviews on the encapsulation of foods or food ingredients (Augustin and Hemar, 2009, Desai and Park, 2005, de Vos et al., 2010, Flanagan and Singh, 2006, Gouin, 2004, Jafari et al., 2008, Khaled and Jagdish, 2007, McClements et al., 2009, Mozafari, 2005, Mozafari, 2006, Mozafari et al., 2008, Peter and Given, 2009). This review focuses on the encapsulation of the more widely used polyphenols, discussing their effectiveness, variations, developments and trends.

Section snippets

Spray drying

Spray drying encapsulation has been used in the food industry since the late 1950s. Because spray drying is an economical, flexible, continuous operation, and produces particles of good quality, it is the most widely used microencapsulation technique in the food industry and is typically used for the preparation of dry, stable food additives and flavors (Desai & Park, 2005). For encapsulation purposes, modified starch, maltodextrin, gum or other substances are hydrated to be used as the wall

Coacervation

The concept behind coacervation microencapsulation is the phase separation of one or many hydrocolloids from the initial solution and the subsequent deposition of the newly formed coacervate phase around the active ingredient suspended or emulsified in the same reaction media (Gouin, 2004). Coacervation encapsulation can be achieved simply with only one colloidal solute such as gelatin, or through a more complex process, for example, with gelatin and gum acacia. Complex coacervation is usually

Liposomes

Liposomes were first described by Bangham and coworkers in 1965 at Cambridge University (Bangham, Standish, & Watkins, 1965). They are colloidal particles consisting of a membranous system formed by lipid bilayers encapsulating aqueous space(s) (Fig. 2). Owing to the possession of both lipid and aqueous phases, liposomes can be utilized in the entrapment, delivery, and release of water soluble, lipid-soluble, and amphiphilic materials. The underlying mechanism for the formation of liposomes and

Inclusion encapsulation

Molecular inclusion is generally achieved by using cyclodextrins (CDs) as the encapsulating materials. CDs are a group of naturally occurring cyclic oligosaccharides derived from starch, with six, seven or eight glucose residues linked by α (1-4) glycosidic bonds in a cylinder-shaped structure, and denominated as α-, β- and γ-cyclodextrins, in which β- cyclodextrin is commonly applied (Pagington, 1986.) The external part of the cyclodextrin molecules is hydrophilic, whereas the internal part is

Cocrystallization

Co-crystallization is an encapsulation process in which the crystalline structure of sucrose is modified from a perfect to an irregular agglomerated crystal, to provide a porous matrix in which a second active ingredient can be incorporated (Chen, Veiga, & Rizzuto, 1988). Spontaneous crystallization of supersaturated sucrose syrup is achieved at high temperature (above 120 °C) and low moisture (95–97 °Brix). If a second ingredient is added at the same time, the spontaneous crystallization

Nanoencapsulation

Nanoencapsulation involves the formation of active-loaded particles with diameters ranging from 1 to 1000 nm (Reis, Neufeld, Ribeiro, & Veiga, 2006). The term nanoparticle is a collective name for both nanospheres and nanocapsules. Nanospheres have a matrix type of structure. Actives may be absorbed at the sphere surface or encapsulated within the particle. Nanocapsules are vesicular systems in which the active is confined to a cavity consisting of an inner liquid core surrounded by a polymeric

Freeze drying

Freeze drying, also known as lyophilization or cryodesiccation, is a process used for the dehydration of almost all heat-sensitive materials and aromas. Freeze-drying works by freezing the material and then reducing the surrounding pressure and adding enough heat, to allow the frozen water in the material to sublimate directly from the solid phase to the gas phase (Oetjen & Haseley, 2004). Encapsulation by freeze drying is achieved as the core materials homogenize in matrix solutions and then

Yeast encapsulation

Encapsulation of essential oils and flavours by using yeast cells (Saccharomyces cerevisiae) as wall material have proven to to be a low cost, high volume process (Bishop, Nelson, & Lamb, 1998). Yeast encapsulation depends on the yeast cells, which allow the actives to pass freely through the cell wall and membrane, while remaining passively within the cells (Fig. 2). Encapsulation by yeast cells can control the diffusion of actives through the cell wall and membrane, using a defined

Emulsions

Emulsion technology is generally applied for the encapsulation of bioactives in aqueous solutions, which can either be used directly in the liquid state or can be dried to form powders (e.g., by spray, roller, or freeze drying) after emulsification. Therefore it is actually a part of encapsulation process. Basically, an emulsion consists of at least two immiscible liquids, usually as oil and water, with one of the liquids being dispersed as small spherical droplets in the other (Friberg et al.,

Summary and trends

The abundant work on encapsulation of polyphenols is summarized in this paper. The characteristics of capsules produced by the various encapsulation processes are illustrated in Fig. 2, which also shows that the different morphologies can be achieved by these techniques. All of the work reported and summarized in this paper, has been undertaken since the year 2000 (Table 2), with the research and related reporting indicating the current worldwide interest in the subject. From the literature, it

Acknowledgement

This work is supported by the Postdoctoral Research Fellowship of The University of Queensland. The authors thank Dr John Schiller for his professional proof reading.

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