Plant volatiles, isolation methods

In other isolation methods, where the ccmpound(s) was removed from the donor plants, the plant material was either dried or macerated prior to cold and hot water treatment. Soxhlet-type extraction was employed when organic solvents were used. Leaves and stems from the intact plants were extracted to collect the suspected volatile substances and those chemicals likely to be released by rain, mist... 

This chapter discusses some more recent variations of methods for isolation of volatiles fiom food and plant materials. For particular problems there are advantages to each of the three main types of isolation methods, direct extraction, steam distillation and dynamic headspace. Direct solvent extraction is the only method which is reasonably efficient in isolating components of both high and low water solubility. Because food and plant volatiles are usually water soluble at their ppm concentrations their isolation by steam distillation does not fit the theory s required non-miscible conditions and this may be better considered a type of dynamic headspace isolation. By atkpting ideas fiom a recently published direct solvent extraction metiiod, which used excess sodiiun fate to bind all water in aqueous foods, the authors discovered an effective dynamic headspace meAod for isolating Furaneol and other water soluble volatiles. 

BUTTERY UNG Methods for Isolating Food and Plant Volatiles... 

Buttery, R. G. Ling, L. C. Methods for Isolating Food and Plant Volatiles. In Biotechnology for Improved Foods and Flavors Takeoka, G. R., Teranishi, R., Williams, R J., Kobayashi, A., Eds. ACS Symposium Series 637 American Chemical Society Washington, DC, 1996 pp 240—248. 

R. G. Buttery and L. C. Ling, Methods for isolating food and plant volatiles. Biotechnology for Improved Foods and Havors (G. R. Takeoka, R. Teranishi, P. J. Williams, and A. Kobayashi, eds.), American Chemical Society, Washington, DC, 1996, p. 240. 

Nepeta (Lamiaceae) is a genus of perennial or annual herbs found in Asia, Europe and North Africa. About 250 species of Nepeta are reported of which, 67 species are present in Iran. Some species of this genus are important medicinal plants and their extracts have been used for medicinal purposes. Aerial parts of Nepeta sintenisii Bornm. was subjected to hydrodistillation and the chemical composition of isolated essential oil has been analyzed by GC/MS method for first time. Identification of components of the volatile oil was based on retention indices relative to n-alkanes and computer matching with the Wiley275.L library, as well as by comparison of the fragmentation patterns of the mass spectra with those reported in the literature. 

The isolation of atropine, scopolamine, and cocaine occurred long before the development of modern analytical techniques. Gas chromatography was the first instrumental technique available in the field of separation science and thus it is not surprising that these alkaloids were firstly analyzed by GC despite their low volatility. With the advent of capillary columns and the proliferation of various sample introduction and detection methods, GC has evolved as the dominant analytical technique for screening, identification, and quantitation of tropane alkaloids of plant origin as well as in biological fluids. The state-of-the-art of GC analysis of tropane alkaloids has been the subject of two comprehensive reviews [45,58]. We shall therefore mainly focus on publications which have appeared since 2002. 

In the beginning of the nineteenth century, analytics of plant matter samples started with that of plant ashes. In addition, no methods were available then which could have enabled intact biological materials to be digested for complete, no-Ioss analyses without burning them before. Hence, volatile elements then could not be detected, let alone quantified in biomass. Elements then found in plant ashes (Fe, Na, K, Ca, etc.) were both abundant and had been discovered in other sources before. As, e.g., no spectroscopic methods whatsoever were at hand earlier than about 1860, technical prospects for trace analysis then were dim at best (there are very few instances of elements detected in environmental samples/spectra prior to their isolation on Earth helium (in 1868) and techne-tinm (in 1952) were found in stellar spectra before being isolated from or detected in terrestrial minerals... 

Extraction of Essential Oils from Plants. Essential oils are aromatic substances widely used in the perfume industry, the pharmaceutical sector, and the food and human nutrition field. They are mixtures of more than 200 compounds that can be grouped basically into two fractions a volatile fraction, which constitutes 90-95% of the whole oil, and a nonvolatile residue, which constitutes the remaining 5-10%. The isolation, concentration, and purification of essential oils have been important processes for many years, as a consequence of the widespread use of these compounds. The common methods used are mainly based on solvent extraction and steam distillation. SFE has been used for the extraction of essential oils from plants, in an attempt to avoid the drawbacks linked to conventional techniques (57). Such is the case with the extraction of flavor and fragrance compounds, such as those from rose (58), rosemary (59), peppermint (60), eucalyptus (61), and guajava (62). The on-line coupling of the extraction and separation ietermi-nation steps (by SFE-GC-FID) has been proposed successfully for the analysis of herbs (63) and for vetiver essential oil (64). 

As reported in the structural determination of BL, CS, DL, and typhasterol, MS is an essential technique for BRs isolated in pure form. However, in most cases, isolation of BRs in pure form is time-consuming and tedious work because of their very low concentration in plant materials. BRs are highly polar and involatile compounds. Therefore, conversion of BRs into volatile derivatives in gas phase makes it easy to characterize BRs in a partially purified bioactive fraction by GC/MS or GC/selected ion monitoring (SIM), which are analytical techniques most frequently used in natural products chemistry. The desired derivatives of BRs are BMBs or MB-TMSs. Another convenient and useful technique is HPLC. HPLC has now been routinely and effectively employed in the purification of natural BRs. Microanalysis of BRs by HPLC has recently been developed, which involves transformation of BRs into derivatives with a fluorophore or an electrophore by use of pre-labeling reagents. Immunoassay techniques to analyze plant hormones have recently advanced and are readily accessible by plant physiologists. RIA for BRs has also been developed. In this section, micro-analytical methods of BRs using GC/MS (SIM), HPLC, and RIA are described. 

An advantage of the direct extraction method (3-6) is that volatiles of both low and high water solubility are isolated in one operation. Other commonly used methods such as steam distillation or dynamic headspace are not effective in isolating highly water soluble compounds such as Furaneol or maltol fiom mostly aqueous food and plant materials whereas these compounds can often be isolated efficiently by direct solvent extraction. 

Static headspace isolation normally involves taking a sample of the equilibrium headspace (a few ml) immediately above the food. This can be directly injected onto the GC column or more usually first concentrated on an adsorbent trap. The GC analysis of this small sample can give useful information such as the detection of rancidity in a food by measuring hexanal concentration (20). Static headspace can also be useful for the analysis of very volatile compounds such as acetaldehyde and dimethyl sulfide. However, in order to get enough material into the headspace, the sample frequently has to be heated to 60-100° C which, in some cases, could give an unrealistic picture of the volatiles of the food or plant material. Static headspace is a very rapid method, but it does not give a comprehensive analysis of the volatiles, and in the case of foods, may miss the most important. 

Analysis of plants normally involves a sample preparation stage such as extraction or distillation followed by analysis with gas chromatography or liquid chromatography. The common methods used currently for the isolation of essential oils from natural products are steam distillation and solvent extraction (Ozel Kaymaz, 2004). Losses of some volatile compounds, low extraction efficiency, degradation of xmsaturated compounds through thermal or hydrolytic effects, and toxic solvent residue in the extract may be encountered with these extraction methods. Recently, more efficient extraction methods, such as supercritical fluid extraction (SFE) (Simandi et al., 1998) and accelerated solvent extraction (ASE) (Schafer, 1998) have been used for the isolation of organic compounds from various plants. Subcritical or superheated water extraction (SWE) is non-toxic, readily available, cheap, safe, non-flammable and is a recyclable option. 

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