Evaluation of Roasting Effect on Selected Green Tea Volatile Flavor Compound and Pyrazine Content by HS-SPME GC-MS
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applied sciences Article Evaluation of Roasting Effect on Selected Green Tea Volatile Flavor Compound and Pyrazine Content by HS-SPME GC-MS Roberto Gotti 1 , Alberto Leoni 1 and Jessica Fiori 2,* 1 Department of Pharmacy and Biotechnology, University of Bologna, Via Belmeloro 6, 40126 Bologna, Italy; [email protected] (R.G.); [email protected] (A.L.) 2 Department of Chemistry “G. Ciamician”, University of Bologna, Via Selmi 2, 40126 Bologna, Italy * Correspondence: Jessica.fi[email protected] Featured Application: The characterization of the variation of the volatile flavour compound (VCF) absolute content can be an important contribution in the evaluation of roasted green tea quality, named Hojicha tea, especially with respect to the roasting conditions such as temperature and time. This is a very important aspect in the Hojicha tea trade, to obtain the typical “roasted” aroma and, at the same time, to maintain the widely reported beneficial properties of green tea. Abstract: The present study aims at the development and validation of a quali-quantitative headspace solid-phase microextraction gas chromatography–mass spectrometry (HS-SPME GC-MS) method for the analysis of odorants in different Chinese and Japanese green teas. A 65 µm PDMS/DVB fiber was used, and GC-MS was performed on a ZB-5 (30 m × 0.25 mm ID, 0.25 µm film) column. The main odorants in green tea samples were identified and the method was fully validated. Linearity (r2 0.981–0.999), sensitivity (LOQ 0.005–0.04 µg/mL), reproducibility (CV% 3.12–10.37), accuracy (recovery% 79.08–99.17) and matrix effect (ME% −9.5 to +4.5) were determined. Quantitation of Citation: Gotti, R.; Leoni, A.; Fiori, J. 2,4-heptadienal, β-damascenone, β-ionone, linalool, indole, 2-ethyl-6-methyl-pyrazine, 2-ethyl-5- Evaluation of Roasting Effect on methyl-pyrazine, 2-ethyl-3,6-dimethylpirazine, 2-ethyl-3,5-dimethyl-pyrazine and 2-acetyl-pyrazine Selected Green Tea Volatile Flavor was carried out in the presence of isotopically labeled compounds as the internal standards. The Compound and Pyrazine Content by proposed method was applied to the comparison of the profile of the volatile flavor compounds HS-SPME GC-MS. Appl. Sci. 2021, 11, (VFCs) of green tea subjected to roasting treatment at three different temperatures (160, 180 and 8217. https://doi.org/10.3390/ 200 ◦C for 30 min). In particular, the roasting process was monitored by following the quantitative app11178217 variations of the selected odorant content, considered as the most important contributory components ◦ Academic Editor: Nuno Rodrigues to the Hojicha or black tea (roasted tea) flavor. A temperature of 160 C was found to be the best roasting temperature. Received: 11 August 2021 Accepted: 1 September 2021 Keywords: green tea; hojicha tea; volatile flavor compounds; pyrazines; HS-SPME; GC-MS; Published: 4 September 2021 roasting process Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- 1. Introduction iations. Green tea, the most widely consumed beverage in Asia, has recently become more widespread in the West where black tea is traditionally consumed. Green tea is made solely with the leaves of Camellia Sinensis that have undergone minimal oxidation under processing. The popularity of green tea is due to many factors. Firstly, the biological value Copyright: © 2021 by the authors. related to the high content of catechins with recognized antioxidant properties. Indeed, Licensee MDPI, Basel, Switzerland. a number of epidemiological studies have shown that regular tea drinkers have a lower This article is an open access article chance of developing cardiovascular disease, degenerative disorders and cancers [1–4]. The distributed under the terms and manufacturing process of green tea includes steaming, characteristic of most of the Japanese conditions of the Creative Commons teas, like Sencha, or pan-firing, typical of most of the Chinese teas, i.e., Bancha. During Attribution (CC BY) license (https:// these processes, the oxidase enzyme, present in tea leaves, is inactivated, thus preserving creativecommons.org/licenses/by/ the green color of the leaves and the catechin composition. Together with the claimed 4.0/). Appl. Sci. 2021, 11, 8217. https://doi.org/10.3390/app11178217 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 12 Appl. Sci. 2021, 11, 8217 2 of 12 During these processes, the oxidase enzyme, present in tea leaves, is inactivated, thus pre- serving the green color of the leaves and the catechin composition. Together with the claimed health-promoting effects, another contributory factor to green tea’s popularity is itshealth-promoting flavor. The pleasant effects, smell another of green contributory tea depends factor primarily to green on tea’s the popularity composition is its of flavor. the The pleasant smell of green tea depends primarily on the composition of the odorants odorants and their transformation due to different treatments, including roasting, steam- and their transformation due to different treatments, including roasting, steaming, baking, ing, baking, sun-drying or storage conditions (Figure 1) [5,6]. For example, Hojicha is the sun-drying or storage conditions (Figure1)[ 5,6]. For example, Hojicha is the name given name given to some green tea varieties that are subjected to a roasting process at a high to some green tea varieties that are subjected to a roasting process at a high temperature temperature (from 160 to 200 °C), altering the leaf color from green to reddish-brown and (from 160 to 200 ◦C), altering the leaf color from green to reddish-brown and the aroma. the aroma. Hojicha is often made from Bancha, tea from the last harvest of the season, or Hojicha is often made from Bancha, tea from the last harvest of the season, or from other from other varieties, including Sencha. varieties, including Sencha. Figure 1. Different tea production processes. Figure 1. Different tea production processes. The desirable flavor of Hojicha, mainly defined as “roasty”, is due to the formation of pyrazinesThe desirable [5,7,8], however,flavor of Hojicha, the changes mainly in the de odorantfined as composition“roasty”, is due also to contribute the formation to the ofcharacteristic pyrazines [5,7,8], aroma. however, Pyrazines the and changes acrylamide in the are odorant commonly composition generated also via contribute the Maillard to thereaction characteristic during the aroma. roasting Pyrazines process, and giving acrylamide roasted are green commonly tea a strong generated odor [9]. via On the the Mail- other lardhand, reaction the high during demand the roasting for green process, tea is also giving due roasted to the presence green tea of a bioactivestrong odor substances, [9]. On themainly othercatechins, hand, the hencehigh demand the Hojicha for green pan-firing tea is processalso due should to the bepresence optimized of bioactive to avoid substances,catechin degradation mainly catechins, [10,11] and hence high the levels Hoji ofcha acrylamide, pan-firing whichprocess is should probably be carcinogenic optimized toto avoid humans catechin [12], without degradation compromising [10,11] and the quality.high levels Since of commercial acrylamide, Hojicha which teais probably contains a carcinogenicnon-negligible to amounthumans of[12], acrylamide, without Mizukamicompromising et al. the have quality. suggested Since roasting commercial conditions Ho- ◦ ◦ jicha(160 teaC contains for 30 min, a non-negligible or 180 C for amount 15 min) of which acrylamide, minimize Mizukami the formation et al. have of acrylamidesuggested roastingbut at the conditions same time (160 ensure °C for the 30 min, formation or 180of °C pyrazines for 15 min) which which contribute minimize tothe the formation pleasant ofaroma acrylamide [8]. In but general, at the comparingsame time ensure the composition the formation of Sencha of pyrazines Ariake which tea, considered contribute as to a thetypical pleasant reference aroma sample, [8]. In general, Bancha Hojichacomparin teag showsthe composition lower levels of ofSencha catechins, Ariake the tea, absence con- sideredof both as theanine a typical enantiomers reference sample, and a relativelyBancha Hojicha high level tea shows of the distomerlower levels (−)-catechin of catechins, and theof (absence−)-gallocatechin of both theanine gallate. Thisenantiomers suggested and that a relatively thermal epimerization high level of the of ( −distomer)-epicatechin (−)- catechinand (−)-epigallocatechin and of (−)-gallocatechin gallate gallate. occurs This [2]. suggested that thermal epimerization of (−)- Herein, an investigation was conducted on the variation in the composition of the epicatechin and (−)-epigallocatechin gallate occurs [2]. main aromas of selected Bancha and Sencha tea and the formation of pyrazines following Herein, an investigation was conducted on the variation in the composition of the roasting. The roasting process was carried out at different temperatures (160, 180 and main aromas of selected Bancha and Sencha tea and the formation of pyrazines following 200 ◦C) for 30 min in order to assess the impact on the composition of the tea. The roasted roasting. The roasting process was carried out at different temperatures (160, 180 and 200 conditions were selected according to Mizukami et al. [8]. °C) for 30 min in order to assess the impact on the composition of the tea. The roasted To this end, qualitative analysis of the composition of 22 green teas (including Bancha conditions were selected according to Mizukami et al. [8]. and Sencha) and two black teas and quantitative analysis on the main odorants from tea infusion was performed by headspace–solid-phase microextraction gas chromatography (HS-SPME GC-MS) in total ion current (TIC) and in single ion monitoring (SIM) modes. Appl. Sci. 2021, 11, 8217 3 of 12 According to our qualitative results and to previous studies carried out by aroma extract dilution analysis (AEDA) [8,13,14], gas chromatography–olfactometry [15] and headspace–solid-phase gas chromatography–mass spectrometry (HS-SPME-GC-MS) [16,17], some of the most characteristic green tea odorants were selected and quantitatively deter- mined.