Chapter 16
Phospholipid Liposomes: Properties and Potential Use in Flavor Encapsulation
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Norman Weiner College of Pharmacy, University of Michigan, Ann Arbor, MI 48109
Phospholipid liposomes, defined as structures consisting of one or more concentric spheres of lipid bilayers separated by water compartments, show great potential forflavorencapsulation. Since liposomes can be custom-tailored to release their entrapped materials at predetermined rates, they offer unique opportunities to program complex flavor patterns in food products, even in the absence of the oily vehicles generally used to solubilize the flavor components. Since liposomes can now be prepared that are both stable and economical, their commercial application is now realizable. Liposomes have shown great potential as a drug delivery system. An assortment of molecules have been incorporated in liposomes, which can then be used for a variety of purposes. Various amphiphathic molecules have been used to form the liposomes, and the method of preparation can be tailored to control their size and morphology. Molecules can either be encapsulated in the aqueous space or intercalated into the lipid bilayeR.; the exact location of the encapsulated material in the liposome will depend upon its physicochemical characteristics and the composition of the lipids. Whereas liposomes have been used extensively in pharmaceutical and cosmetic products, there is an emerging consensus that they show great potential forflavorencapsulation. A liposome is defined as a structure consisting of one or more concentric spheres of lipid bilayers separated by water or aqueous buffer compartments (Fig. 1). These spherical structures can be prepared with diameters rangingfrom80 nm to 100 um. When wall materials are dispersed in an aqueous phase, hydration of the polar head groups of the lipid results in a heterogeneous mixture of structures, generally referred to as vesicles, most of which contain multiple lipid bilayers forming concentric spherical shells. These were the liposomesfirstdescribed by Bangham (/) and are now referred to as multilamellar vesicles (MLV). Sonication of these lipid dispersions results in size reduction of these liposomes to vesicles containing only a single bilayer with diameters rangingfrom25-50 nm. These structures are referred to as small
0097-6156/95/0610-0210$12.00/0 © 1995 American Chemical Society In Flavor Technology; Ho, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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Phospholipid Liposomes
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unilamellar vesicles (SUV). A number of laboratories have developed single bilayer liposomes which exhibit a size range of 100-500 nm in diameter and these vesicles are referred to as large unilamellar vesicles (LUV). The nomenclature describing liposomes can be confusing (see Table I) since liposomes have been classified as a function of the number of bilayers (e.g., MLV, SUV), or as a function of the method of preparation (e.g., REV, FPV, EIV) or as a function of size (e.g., LUV, SUV).
Table I. Examples of Nomenclature Used to Describe Liposomes
TYPE OF VESICLE
Small, Sonicated Unilamellar Large, Vortexed Multilamellar Large Unilamellar Reverse Phase Evaporation French Press Ether Injection
TERM USED
SUV MLV LUV REV FPV EIV
APPROX. SIZE (um) 0.025-0.05 0.05-10 0.1 0.5 0.05 0.02
The lipids most commonly used to prepare phospholipid- based liposomes are shown in Figure 2. Glycerol containing phospholipids are by far the most commonly used component of liposome formulations. The general chemical structure of these types of lipids is exemplified by phosphatidic acid. The "backbone" of the molecule resides in the glycerol moiety. At position number 3 of the glycerol molecule the hydroxyl is esterified to phosphoric acid (hence the name glycerolphospholipids). The hydroxyls at positions 1 and 2 are usually esterified with long chain fatty acids giving rise to the lipidic nature of these molecules. One of the remaining oxygens of phosphoric acids may be further esterified to a wide range of organic alcohols including glycerol, choline, ethanolamine, serine and inositol. The phosphate moiety of phosphatidic acid carries a double negative charge only at high pH. The pK values for the two oxygens are 3 and about 7. At physiologically relevant pH values this molecule presents more than one net negative charge, but not quite 2. The most abundant glycerol phosphatides in plants and animals are phosphatidylcholine (PC), also called lecithin, and phosphatidylethanolamine (PE), sometimes referred to as cephalin. These two phosphatides constitute the major structural component of most biological membranes. In phosphatidylserine (PS), the phosphoric acid moiety of phosphatidic acid (PA) is esterified to the hydroxyl group of the amino acid L-serine, and in phosphatidylinositol (PI) to one of the hydroxyls of the cyclic sugar alcohol inositol. In the case of phosphatidylglycerol (PG), the alcohol that is esterified to the phosphate moiety is glycerol. Table II shows the fatty acid composition of two common
In Flavor Technology; Ho, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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FLAVOR TECHNOLOGY
MULTILAMELLAR VESICLE
UNILAMELLAR VESICLE
BILAYER Fig. 1. Diagramatic representation of multilamellar and unilamellar vesciles. Source: Ostro, M.J. (1987) Scientific American, 102-11.
In Flavor Technology; Ho, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
16. WEINER
O II R«- C - 0 - C H R
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Phospholipid Liposomes
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