ReviewMicroencapsulation: industrial appraisal of existing technologies and trends
Introduction
The food industry expects increasingly complex properties from food ingredients and such complex properties can oftentimes only be provided by microencapsulation. Microencapsulation has been used in the past to mask the unpleasant taste of certain ingredients and also to simply convert liquids to solids. However, in recent years, the concept of controlled release of the encapsulated ingredient at the right place and the right time has become more and more interesting. Controlled release of the ingredients can improve the effectiveness of food additives, broaden the application range of food ingredients and ensure optimal dosage. With carefully fine-tuned controlled release properties, microencapsulation is not just an added value, but is also the source of totally new ingredients with matchless properties. The growing interest by food technologists in the enormous potential of microencapsulation is demonstrated by the exponential increase in the number of publications (non-scientific and scientific articles and patents) published over the years since the mid 1950s, as illustrated by Fig. 1. Liposome entrapment and spinning disk, as well as coacervation to as lesser extent, have experienced the most rapid growth in interest from researchers and technologists.
Sophisticated shell materials and technologies have been developed and an extremely wide variety of functionalities can now be achieved through microencapsulation. Any kind of trigger can be used to prompt the release of the encapsulated ingredient, such as pH change (enteric and anti-enteric coating), mechanical stress, temperature, enzymatic activity, time, osmotic force, etc. However, cost considerations in the food industry are much more stringent than in, for instance, the pharmaceutical or cosmetic industries. Therefore, the cost-in-use (see Popplewell, 2001, for an excellent theoretical treatment of the cost-in-use concept) of the encapsulated product must be tolerable in the final foodstuff. Some microencapsulation technologies, however scientifically impressive they are, might not be appropriate for all, if any, applications. When microencapsulation is used to prevent excessive degradation of a sensitive ingredient or to reduce flashing off of volatile flavors during baking, in order to save on an expensive ingredient, the cost-in-use must in fact be lower than the non-encapsulated ingredient. However, if microencapsulation provides the ingredient with a unique property that is not achievable without encapsulation, then the cost-in-use can be slightly higher than the non-encapsulated ingredient. As a rule of thumb, the customer will accept a price increase of €0.1 per portion for a new product. Considering that functional ingredients are used at low levels in foodstuffs (1–5%), a maximum cost for a microencapsulation process in the food industry can be roughly estimated at €0.1/kg.
There are a number of excellent review papers on microencapsulation technologies, shell materials and applications of microencapsulated ingredients in the food industry (Benita, 1998, Garcia-Anton et al., 1997, Heintz et al., 2001, Jackson & Lee, 1991, Manekar & Joshi, 1998, Kondo, 2001, Omanakutty & Matthew, 1985, Shahidi & Han, 1993, Sparks et al., 2001, Sparks et al., 1999, Sparks & Jacobs, 199). The latest review papers include Gibbs, Kermasha, Alli, and Mulligan (1999), Brazel (1999) and Augustin, Sansuansri, Margetts, and Young (2001). The aim of this paper is not to describe technologies and survey applications, but rather to give a critical industrial perspective on the microencapsulation technologies, their advantages, flaws and variations, as well as to review interesting emerging technologies and trends.
Section snippets
Spray drying
Spray drying encapsulation has been used in the food industry since the late 1950s to provide flavor oils with some protection against degradation/oxidation and to convert liquids to powders. Thorough reviews of the technology (Cho et al., 2000, Hecht & King, 2002, King, 1990, Langrish & Fletcher, 2001, Re, 1998, Rosenberg & Sheu, 1988), of the wall material properties (Buffo & Reineccius, 2000, Faldt & Bergenstahl, 1996, Gascon et al., 1999, Hogan et al., 2001, Kim et al., 1996, McNamee et
Spray cooling/chilling
Spray cooling/chilling is the least expensive encapsulation technology and is routinely used for the encapsulation of a number of organic and inorganic salts as well as textural ingredients, enzymes, flavors and other functional ingredients to improve heat stability, delay release in wet environments, and/or convert liquid hydrophilic ingredient into free flowing powders. Spray cooling/chilling is typically referred to as ‘matrix’ encapsulation in the literature because the particles are more
Spinning disk and centrifugal coextrusion
Spinning disk and centrifugal coextrusion are similar processes in that they are both atomisation methods that can be used in modified spray cooling/chilling encapsulation. Spinning disk involves the formation of a suspension of core particles in the coating liquid and the passage of this suspension over a rotating disk under conditions that result in a film of the coating much thinner than the core particle size (see Sparks, Jacobs, & Mason 1993). Atomization of the mixture at the edge of the
Extrusion
Extrusion microencapsulation has been used almost exclusively for the encapsulation of volatile and unstable flavors in glassy carbohydrate matrices (Bencezdi & Blake, 1999, Bencezdi & Bouquerand, 2001, Blake, 1994, Gunning et al., 1999, Qi & Xu, 1999, Reineccius, 1991, Saleeb, 1999). The main advantage of this process is the very long shelf life imparted to normally oxidation-prone flavor compounds, such as citrus oils, because atmosphere gases diffuse very slowly through the hydrophilic
Fluidized bed
Fluidized bed technology is a very efficient way to apply a uniform layer of shell material onto solid particles. Interestingly, fluidized bed technology is one of the few advanced technologies capable of coating particles with basically any kind of shell material (polysaccharides, proteins, emulsifiers, fats, complex formulations, enteric coating, powder coatings, yeast cell extract, etc.). Therefore, the controlled release possibilities are considerably more versatile with the fluidized bed
Coacervation
Coacervation is a unique and promising microencapsulation technology (see Fig. 3) because of the very high payloads achievable (up to 99%) and the controlled release possibilities based on mechanical stress, temperature or sustained release. Coacervation is typically used to encapsulate flavor oils (Arneodo, 1996, Bakker et al., 1999, Chalupa & Calzolari, 1997, Korus, 2001, Korus et al., 2003, Porzio & Madsen, 1996, Soper, 1995), but can also be adapted for the encapsulation of fish oils (
Alginate beads
Alginate beads have been used extensively in microencapsulation because they are extremely easy to prepare on a lab-scale, the process is very mild, it can be conducted in sterile environments and virtually any ingredient can be encapsulated, whether it is hydrophobic or hydrophilic, sensitive to temperature, a thin liquid or a viscous oil, a solid, etc. However, two major drawbacks limit the use of such microcapsules in the food industry. First, as easy as it is to make small batch using a
Liposomes
Liposome microencapsulation has been used mostly in pharmaceutical applications to achieve, for example, targeted delivery of paramagnetic contrast agents for cancer cell detection by MRI or in cosmetic applications for the stabilization of skin nutrients in cosmetic cream products. However, the technology has evolved in recent years to the point that it is now conceivable for liposome encapsulation to become a routine process in the food industry (Gregoriadis, 1987, Kim & Baianu, 1991, Kirby &
RESS/SAS
Supercritical fluids exist above the critical point and exhibit properties intermediate between those of liquids and gases: low viscosity, low density, high solvating power, high diffusivities and high mass transfer rates. Basically, supercritical fluid can be regarded as dense, solvating gases or a low-viscosity, low density liquids. A number of compounds can be brought to a supercritical state, such as carbon dioxide, water, propane, nitrogen, etc. However, keeping in mind the particular
Inclusion encapsulation
Inclusion encapsulation generally refers to the supra-molecular association of a ligand (the ‘encapsulated’ ingredient) into a cavity-bearing substrate (‘shell’ material). The encapsulated unit is kept within the cavity by hydrogen bonding, VDW forces or by the entropy-driven hydrophobic effect. In the food industry, commercially-available molecular entities having suitable molecular-level cavities are uncommon. Examples include the six, seven, or eight-membered cyclic glucose molecules called
Trends
Fig. 4 summarizes the pros/cons, the distinctive features and the similarities of the major microencopsulation technologies discussed in the previous sections. New microencapsulation technologies are relentlessly devised and invented by academics and industrial researchers: in 2002, over 1000 patents were filed concerning various microencapsulation processes and their applications and over 300 of these patents were directly related to food ingredient encapsulation. Some of these new processes
Conclusion
Despite the wide range of encapsulated products that have been developed, manufactured and successfully marketed in the pharmaceutical and cosmetic industries, microencapsulation has found a comparatively much smaller market niche in the food industry. The technology is still far from being fully developed and has yet to become a conventional tool in the food technologist's repertoire for several reasons. First of all, the development time is rather long and involving requires multidisciplinary
Acknowledgements
Dr. Kathryn Tse is gratefully acknowledged for stimulating discussions and help in assembling the manuscript. The skilful technical assistance of Carsten Bjørn Hansen was indispensable for the preparation of the microcapsules. Support from Dr. Lars Høegh and Ms. Susanne Kjærgaard Olesen, without which this paper would not have been possible, is especially appreciated. Mr. Ejvind Kringelum, who has raised the awareness and relentlessly inculcated the author with crucial commercial perspectives,
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