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  Section: Molecular Biology of Plant Pathways » Metabolic Engineering of Plant Allyl/Propenyl Phenol and Lignin
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Current Sources/Markets for Specialty Allyl/Propenyl Phenols

 
     
 
The major current source of allyl/propenyl phenols is clove oil, which generally contains eugenol (33, 75–85%), eugenyl acetate (35, 8–15%), and other minor components. The oil is a product of the clove tree, Syzygium aromaticum [Eugenia caryophyllata], and is mainly produced in Madagascar, Tanzania (Zanzibar and Pemba Islands), as well as in Indonesia. Worldwide production of this oil approximates 2000 ton year-1 (Bauer et al., 2001), with a gross market value of approximately US$ 30–US$ 70 million per annum through its applications in flavors, fragrances, and antibacterials alone (George Uhe Company, 2006). This market size is, however, largely dictated by (1) the labor-intensive gathering of cloves, (2) the costs associated with obtaining the oil, and (3) limited growth/production capabilities. Therefore, a potential exists for the expansion of the source(s) of these molecules through the use of new (bio)technologies.

Eugenol (33) and its natural derivative, eugenyl acetate (35), are widely used in the perfumery and flavor industries, being considered by the Food and Drug Administration (FDA) as Generally Recognized As Safe (‘‘GRAS’’) for use as food additives. For example, eugenol (33) is responsible for the majority of clove-like flavors in beverages, ice cream, baked goods, and candy (Maralhas et al., 2006), whereas methyleugenol (34) is a flavoring ingredient in ice cream, candy, and cola soft drinks (Smith et al., 2001; U.S. Environmental Protection Agency, 2006a). Eugenol (33) is also present in kretek (clove) cigarettes (usually as 40% ground cloves, 60% tobacco) and is used as an industrial source of isoeugenol (39) (by alkaline isomerization) and methyleugenol (34). Because of its excellent analgesic and antibacterial properties, eugenol (33) has long been employed in dentistry in combination with zinc oxide forming a polymerized eugenol cement, which is then used for surgical dressings, temporary fillings, pulp capping agents, and cavity liners (Skinner, 1940; Weinberg et al., 1972). Isoeugenol (39) is widely used as a flavor and fragrance additive in baked goods, beverages, chewing gum, and personal products such as perfumes and soaps (Badger et al., 2002). Isoeugenol (39) also used to be the main source for the industrial production of vanillin (40, Fig. 13.9), but the latter is now more commonly derived either from the petrochemical guaiacol (41) or from by-products of the pulp/paper (lignin) industry (Hocking, 1997).


Chemically related products, anethole (38) and methylchavicol (32), on the other hand, are currently isolated as minor by-products (1–2%) of crude sulfate turpentine (CST) processing (The Flavor and Fragrance High Production Volume, 2002; 2005). This requires a lengthy isolation process of fractional distillation starting from CST, and fractional crystallization from the semipurified terpenoid/phenolic mixtures thus produced.Most of themethylchavicol (32) obtained in this way is then subjected to alkaline isomerization to generate
FIGURE 13.9 Structures of vanillin (40), guaiacol (41), and anisole (42).
FIGURE 13.9 Structures of vanillin (40), guaiacol
(41), and anisole (42).
more anethole (38), although chemical synthesis from anisole (42, Fig. 13.9) using propionyl chloride or propionaldehyde can also be used. Although CST accounted for 97% of total turpentine production in the United States in 1990, steadily increasing from 27.4% in 1950, the total production of turpentine declined to approximately 20.7 million gallons in 1999 (Haneke, 2002) [containing an estimated 680 and 740 tons methylchavicol (32) and anethole (38), respectively]. As wages increased, the labor-intensive production of turpentine oil became increasingly less competitive economically, and thus much of the original botanical-derived turpentine was substituted by cheaper petroleumbased solvents in the intervening years.Anethole (38) finds application as a perfume in detergents, soaps, and shampoos, and as a flavoring agent in licorice, ice cream, baked goods, and alcoholic beverages (Newberne et al., 1999), while methylchavicol (32) is added as a component of root beer and anise-type flavors, as well as condiments and meat seasonings (The Flavor and Fragrance High Production Volume Consortia, 2005).

Some allyl/propenyl phenols have also recently been approved by the Environmental Protection Agency (EPA) for use as insecticides, insect repellents, and/ or insect lures, perhaps reflecting the original roles/functions of these natural products in planta. Applied as unmodified natural products, they have more specific and less persistent insecticidal activities and therefore pose reduced environmental risk relative to the commonly used organophosphates and pyrethroids. Interestingly, eugenol (33) is used as an insecticide not only for various crop plants and ornamentals but also for pets (U.S. Environmental Protection Agency, 2006b); furthermore, it can be used as an attractant in insect traps specific for Japanese beetles (U.S. Environmental Protection Agency, 2006b,c). Anise oil, on the other hand, is approved for use on lawns and ornamentals as a dog and cat repellant (U.S. Environmental Protection Agency, 2006d), whereas methylchavicol (32), when applied to trees, repels bark beetles (for example, the southern pine beetle) thereby limiting their aggregation and reproduction abilities (U.S. Environmental Protection Agency, 2006e). Additionally, methyleugenol (34) is perhaps the most economically important allyl/propenyl phenol insect attractant, being in widespread use in insect traps for certain fruit flies, including the Oriental fruit fly, Mediterranean fruit fly, and solanaceous fly (Jang et al., 2003; U.S. Environmental Protection Agency, 2006a). (Fruit flies are one of the most destructive pests to Hawaii’s agricultural industry, where these low-cost traps are one of the few environment-friendly recommended suppression techniques.)
 
     
 
 
     



     
 
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