Extraction and Analytical Method for Tea Aroma

Sample preparation is necessary before any analysis can be done. The methods used to extract flavor components can greatly affect the final product’s composition. The choice of method depends on several factors such as the concentration of the substance and its properties. The best method should be quick, accurate, use minimal solvent, and be cost-effective.

1. Steam Distillation

Steam distillation is a common method used to extract volatile components in tea analysis. It can be done at atmospheric or reduced pressure. Yamanishi et al. (1972) identified 57 compounds using steam distillation. However, steam distillation at normal pressure can produce off-flavors due to the thermal degradation or hydrolysis of some components. Thus, the final concentration of analytes may not correspond to that in tea. For instance, (Z)-3-hexenal is isomerized to (E)-2-hexenal when steam distillation is used, leading to the former’s undetectability (Hatanaka and Harada, 1973).

2. Simultaneous Distillation Extraction (SDE)

Simultaneous Distillation Extraction (SDE) is a method used for analyzing volatile compounds in various kinds of tea. It was first reported in 1964 and has since been modified by several researchers. SDE can be conducted at atmospheric pressure or under vacuum, and it can highly concentrate the volatile compounds of interest from a dilute solution using only small volumes of solvent within a short time. However, some researchers have observed artifact formation and serious decomposition of volatile compounds in green tea infusion using this method. Kawakami (1982) reported that glycosides can easily be hydrolyzed, lactone structures can easily be opened, and ketones can be formed as degradation products from carotene by heating in aqueous media.

3. Solvent Extraction

Solvent extraction is a method for extracting volatile compounds from tea. It is different from steam distillation because lower temperatures are used, which preserves the natural composition of the volatiles. Solvent extraction is particularly effective for isolating highly water-soluble flavor constituents, which are not easily recovered by other methods. However, a disadvantage of this method is that non-volatile materials like fats and polyphenols can also be extracted, which may interfere with the analysis. To overcome this, a modified method involves passing the brewed extract through a Porapak Q column to adsorb aroma compounds, which are then eluted with solvent. The best solvent for this method is diethyl ether.

4. Headspace Analysis

Headspace analysis typically involves two primary techniques: static and dynamic. In static analysis, the vapor phase is in equilibrium with the solute, while in dynamic analysis, this equilibrium is constantly changing.

Static

The static headspace analysis is a direct approach to sensory analysis since it captures the volatiles that are easily released from tea infusion and reach the human nose. This method involves injecting a specific volume of headspace directly onto a gas chromatographic column with or without concentration. The advantages of this method are that it requires no sample preparation, is simple and fast, avoids changes in volatile composition due to chemical reactions, and provides accurate results on the composition of perceived odor. However, it is limited to products with high concentrations of volatile materials and cannot detect odorants in low concentrations. Therefore, a concentration process is needed to detect such odorants. Heins et al. (1966) used this technique for identifying aroma components in dry tea leaves.

Dynamic

The dynamic method of headspace analysis involves sweeping the volatiles into an adsorbent trap using an inert gas. The trap collects most of the organic constituents, which are then recovered through thermal desorption. This method detects different volatile fractions than the static method because yields depend on gas velocity and adsorption/desorption selectivity. A microprocessor-based purge and trap-concentrator is commonly used for dynamic headspace analysis. Samples are purged with an inert gas and trapped on an adsorbent, while water vapor and purged gas pass through to the GC column. This technique has been used to analyze various volatile components in tea, such as enantiomers of volatile compounds in black tea, volatile acids in tea, and volatile components in green tea.

5. Supercritical Fluid Extraction (SFE)

This method uses gas above its critical point, where it acts like a liquid solvent. CO2 is a popular choice because of its moderate critical parameters. Extraction can be done using either static or dynamic methods. The first application of this method for tea aroma research was done by Vitzthum et al. (1975) who extracted volatile components of black tea using supercritical CO2. However, SFE is less effective than solvent extraction, as it is more selective and extracts fewer compounds.

6. Solid Phase Extraction (SPE)

Solid Phase Extraction (SPE) is a quick and sensitive technique used for sample preparation and concentration. It is often used to isolate small amounts of chemicals from complex mixtures. The process involves passing the sample through a solid phase material, which binds the compounds of interest while impurities are washed away. The compounds of interest are then eluted using a strong solvent. SPE is more efficient and less labor-intensive than traditional liquid/liquid extraction, and it provides better recoveries while reducing sample and solvent consumption. For example, SPE has been used with GC/MS analysis to detect volatile organic compounds in tea samples.

7. Identification and Quantification

Tea aroma compounds are often separated and detected using gas-liquid chromatography (GLC) with a flame ionisation detector (FID). However, specific detectors are used for more detailed information on certain compounds. Mass spectrometry is a powerful tool for identifying and confirming the structure of aroma compounds, and additional information can be obtained using NMR, UV, and IR spectroscopy. Tea aroma components are complex, so quantification is usually based on the relative amounts of volatiles using an internal standard like ethyl decanoate.