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ar-Curcumene, a-zingiberene

aEach compound has been quantified using naphthalene as an internal standard.

aEach compound has been quantified using naphthalene as an internal standard.

H2O)+, (M + NH4)+, and (M + NH3 + NH4)+, respectively. NCI (OH—) is particularly suitable for the identification of aliphatic and terpenic esters of essential oils that give rise to the formation of RCOO— ions. In the case of alcohols, the RO— ions is the base peak. In the NCI technique, correct analysis of unsaturated monoterpenes is complicated because of the secondary reactions between (M — H)— ions and nitrous oxide. On the other hand, molecular ions have low intensity because protons are difficult to abstract. Saturated terpenes cannot be ionized by this technique.

Miscellaneous Methods

More and more sophisticated spectroscopic methods such as UV, high resolution infrared (HRIR), 'H-NMR, and 13C-NMR are usually employed for the identification of unknown compounds previously extracted from a complex mixture. But these methods are very time consuming and expensive.

IR spectra as a film on a KBr disk of some naturally occurring sesquiterpene hydrocarbons (P-sesquiphellandrene, and zingiberene) have been reported by Wenninger et al. (1967). IR spectra of zingiberene have also been published by Herout et al. (1953) and Pliva et al. (1960). But the assigned structure was that of P-zingiberene instead of the a-isomer. Before 1970, a summary of these works was reported by Connell (1970). Van Beek (1991) claimed that IR spectroscopy can distinguish between various types of ginger oils by comparing the intensity of peaks at 3470 cm—1 (alcohols), 1743 cm—1 (esters), and 1680 cm—1 for conjugated aldehydes. Addition of large quantities of extraneous materials can also be detected. Coupling GC/HPIR (high performance infrared) constitutes another interesting method of identification, but it has limited use in the case of complex mixtures.

UV spectroscopy at 375 nm has been used as a method for determination of (6)-gingerol in ginger oil consignments from China (Li, 1995).

13C-NMR of ginger oils is very complex and the spectra are difficult to interpret because of the great number of peaks. On the other hand, compounds occurring below 0.2% are not measurable. The method can be used in addition to the usual technique based on retention indices and mass spectra. 'H-NMR and 13C-NMR are extremely valuable techniques for elucidation of the structure of an unknown isolated compound.

Atomic emission spectroscopy methods have also been shown to be effective and accurate in determining the components of food spices and essential oils from Zingib-eraceae and other families (Zaidi et al., 1992). A new fluorimetric assay at 481 to 486 nm for the determination of the pungency of fresh ginger from China (gingerols) was found to be better than the standard TLC/densitometric method (Variyar et al., 2000).

Oleoresins: Gingerols, Shogaols, and Related Compounds

Oleoresins contain the nonvolatile pungent principles of ginger in addition to some essential oils and other nonvolatile compounds such as carbohydrates and fatty acids (Connell, 1970). Several reviews have been devoted to the chemistry and properties of gingerols and shogaols as pungent compounds of ginger rhizomes that are considered responsible for its medicinal properties (Nakatani, 1995; Kikuzaki, 2000) (see Table 3.11). Oleoresins are obtained by solvent extraction (acetone, ethanol, dichloromethane, dichloroethane, and trichloroethane). Yields range from 3 to 11% and sometimes can

Table 3.11 Summary of several works on ginger oleoresins

Origin

Methods/compounds

Author(s)

Australia

Solvent extraction (acetone) (6)-, (8)-, (10)-

Connell and Sutherland

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