*Taveira Magalhaes, (1997a)
*Taveira Magalhaes, (1997a)
rium within 2 hours. Activation energies were calculated using the Arrhenius equation at temperatures ranging from 37 to 100° C. Owing to their pungent and biological properties, gingerols that constitute the major part of ginger oleoresins have been widely studied since 1970.
Modern techniques of extraction and qualitative and quantitative analyses (HPLC, GC/MS as TMS derivatives) provide a powerful identification tool. Their stability according to the medium is an important problem, not only for flavor quality but also for medicinal uses.
Synthesis and Biosynthesis of Pungent Compounds of Ginger Rhizomes
Several syntheses of zingerone, shogaols, and gingerols have been described in the literature (Connell, 1970; Afzal et al. 2001). As an example, synthesis of zingerone and (6)-shogaol from 3-(4-hydroxy-3-methoxy)-2-propenoic acid is reported in Scheme 3.2. The name shogaol comes from "shoga," the Japanese word for ginger.
Zingerone is obtained as colorless needles (mp = 40 to 41° C) with a salicylaldehyde odor (Molyneux, 1971). The first report on the synthesis of (6)-shogaol found in Japanese ginger was by Nomura and Tsurami (1926) by condensation of zingerone and hexanal. The structure of (6)-shogaol was determined using UV, IR, NMR, and MS data (Connell, 1970). The same worker reported the structure of (6)-gingeryl methyl ether using the same spectroscopic methods. The S-trans conformation in the liquid state at room temperature was established. The physical properties of the compound were as follows: crystalline solid, m.p. = 65.5 to 66° C, optical rotation at 23° C = + 28.4° C (chloroform), \max = 282 nm, e = 2,650 (ethanol).
By a solvent extraction procedure, Connell (1970) isolated (6)-paradol (1i) in ginger oleoresin as a pale yellow, optically inactive pungent solid (m.p. = 27 to 29° C), \max = 282 nm, e = 14,500 (ethanol). It was structurally characterized by 1H-NMR, IR, UV, and MS data. It was synthesized from (6)-gingerol (1b) by hydrogenation of the shogaol produced upon dehydration (Scheme 3.3).
Biosynthesis of gingerols (18) from dihydroferulic acid (13) has been reported by Harvey (1981) (Scheme 3.4).
It involves the condensation of dehydroferulic acid (14) first with mevalonic acid and then with a short-chain carboxylic acid such as hexanoic acid to give the intermediate gingerdiones (19). All the compounds have been identified in ginger oleoresin by Connell and Sutherland (1969), Connell (1971), Connell and McLachlan (1972), Masada et al. (1973, 1974a,b), and Raghuveer and Govindarajan (1979).
1-Dehydrogingerdione (21) (Figure 3.3) was isolated and identified as well (Charles et al., 2000). The structures of five-membered ring diaryl heptanoids (22, 23) isolated from purified dichloromethane extracts of ginger were elucidated by spectroscopic (1H-NMR, 13C-NMR, IR, MS) and chemical methods (Kikuzaki and Nakatani, 1996).
Ginger oils are obtained by hydrodistillation or steam distillation, by extraction with supercritical carbon dioxide, or by solvent extraction of dried rhizomes. Yields and chemical composition vary greatly according to the cultivation, areas, countries, experimental, and analytical conditions. Ginger oil is a light yellow to yellow liquid with a oh h^o^
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