Tea Aroma Analysis Based on Solvent-Assisted Flavor Evaporation Enrichment

Summary

Presented here is a method for enriching and analyzing the volatile components of tea extracts using solvent-assisted flavor evaporation and solvent extraction followed by gas chromatography-mass spectrometry, which can be applied to all types of tea samples.

Abstract

Tea aroma is an important factor in tea quality, but it is challenging to analyze due to the complexity, low concentration, diversity, and lability of the volatile components of tea extract. This study presents a method for obtaining and analyzing the volatile components of tea extract with odor preservation using solvent-assisted flavor evaporation (SAFE) and solvent extraction followed by gas chromatography-mass spectrometry (GC-MS). SAFE is a high-vacuum distillation technique that can isolate volatile compounds from complex food matrices without any non-volatile interference. A complete step-by-step procedure for tea aroma analysis is presented in this article, including the tea infusion preparation, solvent extraction, SAFE distillation, extract concentration, and analysis by GC-MS. This procedure was applied to two tea samples (green tea and black tea), and qualitative as well as quantitative results on the volatile composition of the tea samples were obtained. This method can not only be used for the aroma analysis of various types of tea samples but also for molecular sensory studies on them.

Introduction

Tea is a preferred beverage of many people all over the world^1,2. The aroma of the tea is a quality criterion as well as a price-determining factor for tea leaves^3,4. Thus, the analysis of the aroma composition and content of tea is of great significance for molecular sensory studies and the quality control of tea. As a result, aroma composition analysis has been an important topic in tea research in recent years^5,6,7.

The content of aroma components in tea is very low, as they generally only account for 0.01%-0.05% of the dry weight of the tea leaves⁸. Furthermore, the large amount of non-volatile components in the sample matrix significantly interferes with analysis by gas chromatography^9,10. Therefore, a sample preparation procedure is essential to isolate the volatile compounds in tea. The key consideration for the isolation and enrichment method is minimizing the matrix interference and, at the same time, maximizing the preservation of the original odor profile of the sample.

Solvent-assisted flavor evaporation (SAFE), originally developed by Engel, Bahr, and Schieberle, is an improved high-vacuum distillation technique used to isolate volatile compounds from complex food matrixes^11,12. A compact glass assembly connected to a high-vacuum pump (under a typical operating pressure of 5 x 10⁻³ Pa) can efficiently collect volatile compounds from solvent extracts, oily foods, and aqueous samples.

This article described a method that combines the SAFE technique with solvent extraction to isolate volatile substances from a black tea infusion, followed by analysis using GC-MS.

Protocol

1. Preparation of the internal standard and tea infusion

1. Stock solution: Dissolve 10.0 mg of paraxylene-d10 (see Table of Materials) in 10.0 mL of anhydrous ethanol to prepare a 1,000 ppm stock solution of the internal standard.
2. Working solution: Dilute 1 mL of the stock solution (step 1.1) to 100 mL with pure water to prepare a 10 ppm working solution of the internal standard.
   NOTE: The working solution must be prepared on the same day as the analysis.
3. Place 3 g of tea leaves (both for green tea and black tea, see Table of Materials) into an Erlenmeyer flask, and add 150 mL boiling water. Cover the flask with a glass stopper.
4. After 5 min, quickly filter out the tea infusion through a 300-mesh sieve.
5. Wash the spent tea leaves twice with 30 mL of water, and combine the wash solution with the tea infusion.
6. Cool the tea infusion to room temperature quickly in an ice water bath.
7. Add 1.00 mL of working solution (step 1.2) into the tea infusion, and mix them well.

2. Distillation of the tea infusion by SAFE and liquid-liquid extraction of the distillate

1. Prepare the SAFE assembly following the steps below.
   1. Install the SAFE assembly (Figure 1), and connect the distillation bottle at the lower left (Figure 1[3]) and the collection bottle at the lower right (Figure 1[4]). Connect the circulating water tube at the rear of the SAFE glass assembly. Install the cold trap (Figure 1[5]), and connect the tube to the vacuum pump (see Table of Materials) at the upper right of the glass assembly.
      NOTE: Check the connection of the circulating water tube; ensure that the inlet enters the top and the outlet exits from the bottom. Use deionized water for the circulation to prevent the scale from blocking the white tube in the SAFE assembly, which would result in poor circulation of the circulating water and the eventual explosion of the SAFE assembly. The distillation bottom (Figure 1[3]) can be stirred by a stirring bar to facilitate the evaporation of the sample.
   2. Set the temperature of the circulating water to 50 °C and that of the water bath for the sample flask to 40 °C. Close the vacuum valve (Figure 1[2]).
2. Perform the vacuum pump operation.
   1. Power on the vacuum pump.
   2. Gradually increase the speed to the maximum speed of 100%.
      NOTE: If the speed does not reach 100%, check whether the system is airtight and whether there is solvent residue inside the system.
   3. After reaching a high vacuum (preferably 10^-3 Pa)
      NOTE: The vacuum will improve when the liquid nitrogen is added to the cold trap.
3. Perform sample distillation.
   1. Start the water circulation.
   2. Add liquid nitrogen to the cold trap to cover the outside of the collecting bottle.
   3. Pour the tea infusion into the sample funnel at the top left (Figure 1[1]), and then cover it with a glass stopper.
   4. Introduce the sample into the distillation flask dropwise. Control the sample drop speed so that the vacuum is kept in the proper range of around 10⁻³ Pa.
      NOTE: Add liquid nitrogen during the process to ensure that the right collecting bottle is always submerged in liquid nitrogen. Try to avoid condensate formation in the cold trap.
4. Turn off the vacuum pump after the distillation is completed.
   1. Press the power switch. When "STOP" flashes, press the Enter key to confirm.
   2. Unplug the power cord when the speed of the molecular pump decreases to "0".
      NOTE: Restart only when the speed decreases to "0".
5. Restore the system to atmospheric pressure.
   1. Remove the grinding plug above the sampling bottle.
   2. Unscrew the knob of the vacuum valve slowly to restore the system to atmospheric pressure.
6. Take down the collecting bottle with the sample.
   1. Remove the liquid nitrogen outside the collection bottle after recovering the system to atmospheric pressure.
   2. Unscrew the collection bottle slowly. Take down the collecting bottle with the sample carefully.
   3. Close the circulating water.
7. Perform liquid-liquid extraction of the SAFE distillate.
   1. Let the SAFE distillate in the bottle warm to room temperature.
   2. Extract the SAFE distillate thrice with 50 mL of dichloromethane (see Table of Materials).
   3. Combine the dichloromethane layers. Dry the extract with anhydrous sodium sulfate (see the Table of Materials).
      NOTE: The anhydrous sodium sulfate in the solvent is considered dry enough when it is no longer cemented and can flow freely.
   4. Concentrate the extract to about 2 mL using a gentle nitrogen stream.
   5. Transfer to a sample vial of 1-2 mL, and further concentrate to 200 µL using a gentle nitrogen stream.

3. GC-MS analysis and data processing

1. Analyze the aroma concentrates prepared in protocol section 2 using a GC-MS system (Figure 2) equipped with fused silica capillary columns (see the Table of Materials).
2. Use helium as carrier gas with a linear velocity of 40 cm/s.
3. Inject 3 µL of the concentrate in splitless injection mode.
4. Set the GC oven temperature program: (1) hold at 40 °C for 5 min; (2) increase to 200 °C at 5 °C/min; (3) increase to 280 °C at 10 °C/min; (4) hold at 280 °C for 10 min.
5. Operate the mass selective detector in positive EI mode¹³ with a mass scan range from 30 m/z to 350 m/z at 70 eV.
6. Deconvolute the GC-MS data using the Automated Mass Spectral Deconvolution and Identification System (AMDIS, see the Table of Materials).
7. Match and qualify the data after deconvolution using the NIST (National Institute of Standards and Technology) 17 mass spectrometer search program³.
8. Calculate the retention index of the compounds¹⁴ based on the result of a set of n-alkanes (C5-C25, see the Table of Materials) under the same GC conditions.
9. Identify the GC peaks using the NIST mass spectrometry library and the retention index database based on the simultaneous matching of the mass and retention indexes.
10. Calculate the concentration of each volatile component in the SAFE sample relative to the internal standard using the TIC (total ion chromatography) peak area.
11. Repeat the analysis three times, starting from the tea infusion preparation.

Results

The analytical procedure described above is illustrated in this section using the example of the aroma analysis of black tea and green tea samples.

A representative GC-MS chromatogram is shown in Figure 3. Figure 3A shows a set of n-alkanes, and Figure 3B shows the profile of an internal standard. The evaluation results for the extracts from the green tea and black tea samples are shown in

Discussion

This article describes an efficient method for analyzing volatile compounds in tea infusions using SAFE and GC-MS analysis.

Tea infusions have a complex matrix with a high content of non-volatile components. Several methods have been described in the literature for isolating the volatile components from tea infusions. A common method is simultaneous distillation extraction (SDE)^15,16. However, it is not suitable for the analysis of tea...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Alkane mix (C10-C25)	ANPEL	CDAA-M-690035
Alkane mix (C5-C10)	ANPEL	CDAA-M-690037
AMDIS	National Institute of Standards and Technology	version 2.72	Gaithersburg, MD
Analytical balance	OHAUS	EX125DH
Anhydrous ethanol	Sinopharm
Anhydrous sodium sulfate	aladdin
Black tea	Qianhe Tea		Huangshan, Anhui province, China
Concentrator	Biotage	TurboVap
Data processor	Agilent	MassHunter
Dichloromethane	TEDIA
GC	Agilent	7890B
GC column	Agilent	DB-5MS
Green tea	Qianhe Tea		Huangshan, Anhui province, China
MS	Agilent	5977B
p-Xylene-d10	Sigma-Aldrich
SAFE	Glasbläserei Bahr
Ultra-pure deionized water	Milipore	Milli-Q
Vacuum pump	Edwards	T-Station 85H