ABSTRACT
Baking is the last, and yet most essential, step in the formation of the final distinct flavour of most black tea products. In this study, the influence of baking on the volatile components of Dianhong black tea, which is one of the most popular black teas in China, was investigated. As a very fast, effective, and simple method for collecting volatiles, headspace solid-phase microextraction (HS-SPME) with a 65 µm PDMS/DVB fibre was used to extract the volatile compounds of Dianhong black tea, and the volatile components were further separated and identified by gas chromatography-mass spectrometry (GC-MS). The results showed that the peak areas of alcohols, aldehydes, esters, and ketones were increased, but that those of some alkanes, acids, and nitrogen compounds did not demonstrate an obvious increase after baking. The differences generated by baking process actually result in the distinct flavour of most black teas and can be used as a model for analysing the chemical composition of special flavours, and for quality improvement and process control.
Introduction
Approximately 78% of the global tea production is attributable to black tea, thus making it the most widely consumed beverage in the world. One of the many contributory factors to its high acceptability is its flavour.[Citation1,Citation2] Dianhong tea is one of the most well-known black teas from China, and is produced by using two leaves and a bud from the Yunnan broad-leaf tea variety as raw materials. It is plucked, withered (partially desiccated), rolled, fermented, dried, and, finally, baked.[Citation3] Dianhong black tea is known globally for its unique “mellow strong” flavour, beautiful shape, and bright colour. Indeed, for more than 70 years, Dianhong tea has been exported to over 30 countries and regions.[Citation4]
The baking process is the last, and yet most essential, step in the formation of the final distinct flavour of most black tea products. Under the action of thermal effects, such as Maillard’s reaction, various tea components undergo thermal physical and chemical changes. Additionally, aromatic substances may spread to the surface, thereby further forming and improving the tea flavour. Baking usually adopts different temperature gradients and duration to comprehensively revise the colour, aroma, and taste of tea. Furthermore, the baking processes used for different types and grades of tea are not identical. Generally, the sensory quality first increases with an increase in the extension of the roasting time and then subsequently subsides. However, the temperature should not be too high because it is easy to induce a burnt flavour. These processes are also utilized to make tea taste mellow and delicate and to give the tea and its infusion a golden colour.[Citation5,Citation6] In recent years, black tea production has risen rapidly, and consumer quality requirements are increasing. Improving the overall aroma and taste quality has been major topic in the study of black tea. Fermentation and baking have played very important roles in the formation of tea aroma. Wang et al.[Citation7] found that the fermentation processes altered the profiles of volatile compounds in tea. The total concentration of five volatile compounds, (E)-2-hexenal, benzaldehyde, methyl-5-hepten-2-one, methyl salicylate, and indole, was shown to be able to clearly discriminate unfermented and fermented teas. Wu et al.[Citation8] found that the content of some flavour compounds varied significantly according to differently fermented teas, which indicated that these active constituents may discriminate fermentation degrees. The sensory score first increased and then decreased during the baking treatment. Chemical analysis indicated that the contents of compounds with flowery scents, such as linalool, farnesene, and nerolidol, decreased but that the compounds with an overbaking scent, such as pyrroles, increased. The smell of overbaking manifested after a long period of baking. The total content of aroma compounds changed minimally during the baking treatment, but the sensorial scores were different. These results indicated that the relative content of each aroma compound could greatly affect the aroma quality. Kumazawa et al.[Citation1] used the volatile fraction obtained before and after the heat processing of the black tea samples and detected 10 odour-active peaks for which the aroma dilution factor changed. These odourants were the most important components involved in changing the black tea odour during heat processing.
A number of published works have described the effects of different production methods on black tea quality.[Citation9–Citation11] However, information concerning the effect of baking is limited. Therefore, a detailed investigation into the change in the aroma components in Dianhong black tea before and after baking is essential. Headspace solid-phase microextraction (HS-SPME), which is widely used to extract the volatile components of tea, is a rapid, simple, sensitive, and solvent-free method.[Citation12–Citation15] The objective of this study was to investigate the effects of baking on the peak area of volatile components and volatile oil extraction yield in Dianhong black tea. This study is expected to improve the knowledge of the effect of baking on black tea. Thus, the information from this study could assist manufacturers for quality improvement and control and could help consumers and merchants better understand the special and fascinating flavour of black tea.
Materials and methods
Materials
Three different batches of black tea were provided by the Yunnan Dianhong group, a limited liability company. All tea leaves were picked in the autumn of 2014 with two-bud leaves. Approximately 50 years of these tea trees (Yunnan big-leaf species tea), and these teas were processed in March 2015. The process of Dianhong black tea manufacturing before and after baking is shown in .
Figure 1. The production process of Dianhong black tea before and after baking.
Extraction of volatile oil
Tea samples were crushed and screened through a mesh of size of 80. A 30 g ground tea sample was weighed, placed in a 500 mL triangular flask, and then infused with 300 mL of methylene chloride. After resting for 4 h, the liquid from the mix was treated with an ultrasonic wave for 30 min. The liquid was then filtered four to six times through a vacuum suction filter. The filtrate was then concentrated under vacuum relief conditions with a water bath temperature of 40°C. This process was stopped when the filtrate boiled away, and the final solid concentrate was obtained. The volatile oil extraction yield was calculated using the following formula:
Extraction yield = weight of final solid concentrate/weight of tea sample × 100%
HS-SPME method
The HS-SPME method was used according to our previous study, using 65 µm polydimethylsiloxane/divinylbenzene fibre coatings (Supelco, USA).[Citation16] A 2.0 g ground tea sample was placed in a 20 mL sealed headspace vial and infused with 5 mL boiling water. HS-SPME conditions were established with an autosampler (Varian Pal Autosampler, Switzerland). The tea sample was continuously stirred at 250 rpm for 60 min at 80°C. After extraction, the fibre was removed from the vial and immediately inserted into a gas chromatography-mass spectrometry (GC-MS) injector for absorbance (250°C for 3.5 min) and further analyses. Each tea batch was repeated three times.
GC-MS analysis
A 7890A GC and 5975C MSD combined system (Agilent Technologies, USA) was used for GC-MS analysis. A HP-5MS chromatographic column (30 m × 0.25 mm × 0.25 µm film thicknesses) equipped with a split-less injector was established. High-purity helium was used as the gas carrier at a flow rate of 1 mL/min. The injector temperature was 250°C. The temperature was programmed from 50°C (held for 1 min), increased to 210°C at a rate of 3°C per minute (held for 3 min), and then increased to 230°C at a rate of 15°C per minute. The MS ion source temperature was 230°C, and the electron energy was 70 eV. The scan range was 35–500 amu, and the solvent delay time was 2.8 min.
Data processing
Chromatographic peaks were recognized and identified using the NIST08.L MS data library and retention indices (RI) method, as described in the literature.[Citation17–Citation19] The retention index of each compound was obtained with a 1 µL n-alkanes mixture (C8-C40; Sigma-Aldrich, USA) under the same GC-MS experimental conditions. The relative percentage content of volatiles was calculated as follows:
Relative percentage content (%) = single constituent area/total area × 100%
For an obvious expression of the effect of baking on the volatile components of black tea, the peak area changes of the components before and after baking were compared. The peak area normalization method was used for quantitative analysis, rather than the internal and external standard methods. The volatile components of tea form a highly complex system, and each type of tea may contain hundreds of compounds. Therefore, it is unlikely to use hundreds of these compounds at the same time to carry out accurate quantitative, and some volatile components are missing in the market. Some studies[Citation20,Citation21] have used an internal standard method to quantitatively evaluate the volatile components of tea; however, this is limited to the solvent extraction method. We previously attempted to use ethyl decylate as an internal standard for qualification analysis. However, our results showed that the peak area had poor reproducibility and stability. Similarly, it was reported that the presence of tea particles caused an adsorption of the internal standard in SPME-GC/MS analysis (tea leaf itself could also absorb odourant) when 1 µL of 0.03% ethyl heptanoate was used as an internal standard.[Citation7] For these reasons, internal and external standard methods were not used in this work.
Results and discussion
Sensory evaluation results
Throughout the manufacturing process, we performed strict sensory evaluations to obtain adequate products, which also ensured that the samples from each batch had good representation. These methods utilized the national standard test methods (GB/T 23778-2009). The results of the sensory test of black and green teas are shown in .
Table 1. Sensory evaluation of black tea samples.
Volatile oil extraction yield of three batches of Dianhong black tea before and after baking treatment
For the total ion current (TIC) of tested Dianhong black teas, the identification of peaks was performed by NIST searching. A total of 53 volatile compounds were identified, and most of them had previously been identified by other authors.[Citation4,Citation22,Citation23] shows typical GC-MS chromatograms from the Batch 1 samples with and without baking. Volatile compounds are listed according to different categories, including their chemical names and the change in the peak area with and without baking, in . The relative area percentages and odour description note in the literature for all of the volatile compounds identified in the Dianhong black teas are listed in .
Table 2. Volatile compounds and change in peak area in Dianhong black tea before and after baking.
Table 3. Volatile compounds and their relative contents in Dianhong black tea.
Figure 2. TIC of Dianhong black tea from Batch 1 (A, before baking; B, after baking) and the abundance change of linalool in Dianhong black tea from Batch 1 (A, before baking; B, after baking).
Alcohols were the most abundant chemical group, with a total of 19 identified, and represented more than 70% of the total percentage content of identified compounds in the samples. Alcohol compounds were dominated by terpene alcohols and their derivatives. shows that after baking, the peak areas of most alcohol compounds were increased, with the exceptions of α-cadinol, 3,7,11,15-tetramethyl-2-hexadecen-1-ol, isophytol, and phytol. Alcohol compounds without peak area increases had relatively higher boiling points, showed almost no odour activity, and made little contribution to flavour. The levels of monoterpene alcohols and their derivatives, including the linalool oxides, linalool, and geraniol, were high (). Linalool (floral) and geraniol (rose-like) had a flavour dilution factor of 625, which indicated that these odourants contribute more actively to the aroma profile of Dianhong black tea.[Citation4] These two compounds have been reported to have relatively low threshold values of 0.4–0.8 and 1 mg/m3 in air, respectively.[Citation29] The abundance change of linalool (Batch 1) before and after baking is shown in . Furthermore, the peak area of linalool shows an obvious increase after baking, which indicates that it may make a large contribution to the aroma of the baked tea.
Seven aldehydes were identified in the samples, and the peak areas for all identified aldehyde compounds were increased after baking. Aldehydes are generally considered to be the oxidized products of unsaturated fatty acids.[Citation24] Wang et al.[Citation23] reported that the main aldehyde constituents in black tea were hexanal, (E)-2-hexenal, benzaldehyde, and benzeneacetaldehyde. These compounds have also been reported as important aroma constituents in Lapsang Souchong black tea.[Citation33] Aldehydes, which contribute green, malty, fatty, sweet, floral, and fruity aromas to foods,[Citation22] may make the flavour of black tea more prominent after baking.
A total of 14 hydrocarbons were identified. As shown in , alkanes and aromatics are presented as the compounds richest in hydrocarbons. In contrast, alkanes and aromatics had a moderate effect on aroma characteristics because they contributed relatively few odours. Terpene compounds may play an important role in tea aroma, but their levels were relatively low in our samples. The peak areas of many alkanes were not obviously increased and mostly decreased after baking. This result suggests that these compounds make little contribution to the flavour of baked Dianhong black tea. Pang et al.[Citation4] used dynamic headspace dilution analysis combined with gas chromatography-olfactometry (GC-O) to identify odour-active compounds in Dianhong black tea infusion. It was also found that β-myrcene, d-limonene, and (E)-3,7-dimethyl-1,3,6-octatriene, with a flavour dilution factor of 5, made a contribution to aroma. The peak area of these compounds was also increased after baking in our study, which suggests that these sesquiterpene compounds contribute to the special fragrance of baked Dianhong black tea.
Cis-jasmone, α-ionone, geranyl acetone, β-ionone, and 6,10,14-trimethyl-2-pentadecanone were detected as major ketones in our samples. The peak areas of these compounds were all increased after baking, except that of 6,10,14-trimethyl-2-pentadecanone. These ketones were possibly formed by thermal degradation, the oxidation of fat, degradation of amino acids, and Maillard reaction.[Citation34] Although detected at low relative area percentages, ketones may make a considerable contribution to the flavour of Dianhong black tea because each ketone delivers unique odours.
Four esters were identified in our samples, of which methyl salicylate had a higher level and a peak area increase after baking. Wang et al.[Citation7] found that methyl salicylate was an important compound that must be considered to account for the degrees of fermentation found in commercial teas. Hexadecanoic acid methyl ester, methyl linoleate, and methyl linolenate did not show an increasing trend; the levels of these compounds were low in fragrance activity, thus contributing little to aroma. Caffeine was the sole nitrogen compound detected, and hexadecanoic acid and linolenic acid were the major acids detected. These high-boiling-point compounds generally have no odour activity, and their peak areas did not increase after baking, which suggests that these compounds have a limited effect on the aroma characteristics of Dianhong black tea.
Peak area change trend of black tea with and without baking
The peak area change trend of volatile compounds for the batches of black tea with and without baking based on their chemical class is shown in . This figure shows that the peak areas of alcohols, aldehydes, esters, and ketones all increased after baking. Most of the identified compounds in these classes had odour activity. After high-temperature baking, they released a further characteristic odour, thus making the aroma durable and outstanding. However, the peak areas of some compounds with high boiling points or no odour activity, such as alkanes, acids, and nitrogen compounds, did not increase obviously after baking. These observations suggest that the contribution of these compounds to the overall aroma of baked black tea is minor.
Figure 3. The peak area change of the volatile compounds in black tea with and without baking based on their chemical class.
Baking technology is the most effective and economical way to shape flavour and remove unpleasant aromas in black tea. Baking not only improves the deterioration in quality caused by long-term storage, but it can also prevent further moisture loss, thus making it an important method for effectively extending shelf life and improving quality. Consumers have differing preferences for baked tea, and diverse baking methods are often used in tea production. Our study examined the influence of baking only on the aroma of black tea. However, tea quality is also influenced by other factors, such as taste, tea leaf shape, and colour. Baking may influence the taste substances of black tea, such as catechins, amino acids, caffeine, and flavones, and further research exploring the influence of baking on these components is indicated. Additionally, baking temperature and time are very important to tea quality. The flavour types and grades of tea, baking temperature, and time are often not the same. Therefore, how to choose the most suitable baking time and temperature for different teas must be studied in future work. Overall, the differences generated during the baking process actually result in the distinct flavour of most black teas and can be used as a model for analysing the chemical composition of special flavours and for quality improvement and process control.
Conclusion
Overall, a total of 53 volatile components were identified in six Dianhong black teas (19 alcohols, 14 hydrocarbons, 7 aldehydes, 5 ketones, 4 esters, 3 acids, and 1 nitrogen compound). Alcohols were the most abundant aroma components, especially terpene alcohols (linalool and its oxides, e.g., geraniol and nerolidol). After baking, the extraction yield was higher, and the peak areas of alcohols, aldehydes, esters, and ketones were increased. The peak areas of some compounds with high boiling points or no odour activity, such as alkanes, acids, and nitrogen compounds, did not show an overall increase. The total peak areas of all of the identified aroma components were increased after baking, which suggests that baking improved the aroma components of black tea, thus making its flavour more prominent and persistent.
Funding
This work was supported by the National Natural Science Foundation of China (number 31460228) and scientific research funds in Yunnan Province department of education (number 2014Y089).
Additional information
Funding
References
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