ReviewRecent advances in microencapsulation of probiotics for industrial applications and targeted delivery
Introduction
Probiotics have been defined in several ways, depending on our understanding of the mechanisms of action of their effects on the health and well-being of humans. The most commonly used definition is that of Fuller (1989): probiotics are live microbial feed supplements that beneficially affect the host by improving its intestinal microbial balance. Recently Food and Agriculture Organization (FAO) of the United Nations and the World Health Organization (WHO) define probiotics as “Live microorganisms (bacteria or yeasts), which when ingested or locally applied in sufficient numbers confer one or more specified demonstrated health benefits for the host” (FAO/WHO, 2001). The beneficial effects of probiotics on the human gut flora include antagonistic effects and immune effects. The use of probiotic bacterial cultures stimulates the growth of preferred microorganisms, crowds out potentially harmful bacteria and reinforces the body's natural defense mechanisms (Dunne, 2001, Gismondo et al., 1999). The mechanism of anti-pathogenic effect may be through decreasing the luminal pH by the production of short chain fatty acids such as acetic acid, lactic acid or propionic acid, rendering vital nutrients unavailable to pathogens, altering the redox potential of the environment, producing hydrogen peroxide or producing bacteriocins or other inhibitory substances (Kailasapathy & Chin, 2000).
Probiotics may cause cell-mediated immune responses, including activation of the reticulo-endothelial system, augmentation of cytokine pathways and stimulation of pro-inflammatory pathways such as tumour necrosis factors and interleukin regulation, without being a target of the host immune system (Gill et al., 2001, Isolauri, 2000, Isolauri et al., 2000). Probiotics may even activate macrophages directly (Tejada-Simon, Ustunol, & Pestka, 1999). Recently, probiotics have been proposed for various treatments of human intestinal barrier dysfunctions such as lactose intolerance, acute gastroenteritis, food allergy, atopic dermatitis, Crohn's disease, rheumatoid arthritis, and colon cancer (Kalliomaki et al., 2003, Lee et al., 2003, Rinkinen et al., 2003, Salminen et al., 1998).
Lactic acid bacteria (LAB) are the most important probiotic microorganisms typically associated with the human gastrointestinal tract. These bacteria are Gram-positive, rod-shaped, non-spore-forming, catalase-negative organisms that are devoid of cytochromes and are of non-aerobic habit but are aero-tolerant, fastidious, acid-tolerant and strictly fermentative; lactic acid is the major end-product of sugar fermentation (Axelsson, 1993). A few of the known LAB that are used as probiotics are Lactobacillus acidophilus, Lactobacillus amylovorous, Lactobacillus casei, Lactobacillus crispatus, Lactobacillus delbrueckii, Lactobacillus gasseri, Lactobacillus johnsonoo, Lactobacillus paracasei, Lactobacillus plantarum, Lactobacillus reuteri, Lactobacillus rhamnosus etc. (Mäkinen & Bigret, 1993).
Other common probiotic microorganisms are the bifidobacteria. Bifidobacteria are also Gram-positive and rod-shaped but are strictly anaerobic. These bacteria can grow at pH in the range 4.5–8.5. Bifidobacteria actively ferment carbohydrates, producing mainly acetic acid and lactic acid in a molar ratio of 3:2 (v/v), but not carbon dioxide, butyric acid or propionic acid. The most recognized species of bifidobacteria that are used as probiotic organisms are Bifidobacterium adolescentis, Bifidobacterium animalis, Bifidobacterium bifidum, Bifidobacterium breve, Bifidobacterium infantis, Bifidobacterium lactis and Bifidobacterium longum. Other than these bacteria, Bacillus cereus var. toyoi, Escherichia coli strain nissle, Propioniobacterium freudenreichii, and some types of yeasts, e.g. Saccharomyces cerevisiae and Saccharomyces boulardii have also been identified as having probiotic effects (Holzapfel, Haberer, Geisen, Bjorkroth, & Schillinger, 2001).
Because of their perceived health benefits, probiotic bacteria have been increasingly included in fermented dairy products, including yoghurts, soft-, semi-hard and hard cheeses, ice cream and frozen fermented dairy desserts (Desmond et al., 2005, Dinakar and Mistry, 1994, Stanton et al., 2003, Stanton et al., 2001, Stanton et al., 2005).
The ability of probiotic microorganisms to survive and multiply in the host strongly influences their probiotic benefits. The bacteria should be metabolically stable and active in the product, survive passage through the upper digestive tract in large numbers and have beneficial effects when in the intestine of the host (Gilliland, 1989). The standard for any food sold with health claims from the addition of probiotics is that it must contain per gram at least 106–107 cfu of viable probiotic bacteria (FAO/WHO, 2001). However, there are still several problems with respect to the low viability of probiotic bacteria in dairy foods. Several factors have been reported to affect the viability of probiotics in fermented dairy products, including titratable acidity, pH, hydrogen peroxide, dissolved oxygen content, storage temperature, species and strains of associative fermented dairy product organisms, concentration of lactic and acetic acids and even whey protein concentration (Dave and Shah, 1997, Kailasapathy and Supriadi, 1996, Lankaputhra et al., 1996). Survival is, of course, essential for organisms targeted to populate the human gut, one of the most important issues in health benefit provision by probiotic bacteria.
Different approaches that increase the resistance of these sensitive microorganisms against adverse conditions have been proposed, including appropriate selection of acid- and bile-resistant strains, use of oxygen-impermeable containers, two-step fermentation, stress adaptation, incorporation of micronutrients such as peptides and amino acids, and microencapsulation (Gismondo et al., 1999).
Section snippets
Microencapsulation technology
Microencapsulation 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 the influences of specific conditions (Anal and Stevens, 2005, Anal et al., 2006, Kailasapathy and Masondole, 2005). A microcapsule consists of a semi-permeable, spherical, thin, and strong membrane surrounding a solid/liquid core, with a diameter varying from a few microns to 1 mm. A brief description of
Spray- and freeze-dried probiotic products
Probiotic cultures for food applications are frequently supplied in frozen or dried form, as either freeze-dried or spray-dried powders (Holzapfel et al., 2001). The successful spray drying of Lactobacilli and Bifidobacteria has previously been reported for a number of different strains, including L. paracasei (Desmond et al., 2002, Gardiner et al., 2000), Lactobacillus curvatus (Mauriello, Aponte, Andolfi, Moschetti, & Villani, 1999), L. acidophilus (Prajapati, Shah, & Dave, 1987), L. rhamnosus
Encapsulation of probiotics in polymer systems
Encapsulation of probiotics in a biodegradable polymer matrix has a number of advantages. Once entrapped/encapsulated in matrix beads or in microcapsules, the cells are easier to handle than in a suspension or in slurry. The number of cells in beads or microparticles can be quantified, allowing the dosage to be readily controlled. Cryo- and osmo-protective components can be incorporated into the matrix, enhancing the survival of cells during processing and storage. Finally, once the matrix
Conclusions and future directions
Sophisticated shell materials and technologies have been developed and an extremely wide variety of functionalities can now be achieved through microencapsulation. Any type of triggers can be used to prompt the release of the encapsulated ingredients, such as pH changes, mechanical stress, temperature, enzymatic activity, time, osmotic force, etc. Encapsulated probiotic bacteria can be used in many fermented dairy products, such as yoghurt, cheese, cultured cream and frozen dairy desserts, and
Acknowledgement
The authors would like to thank Fonterra Co-operative Group Limited, New Zealand for financial support of this work.
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