揮發性化合物組成與包種茶炒菁時機之關係

Abstract

包種茶為輕度部分發酵茶，特色為優雅的花香，製作時需仰賴製茶師以嗅聞監控茶菁氣味變化，來決定每個步驟的動作時機。但是，決定停止氧化、進入炒菁步驟前的氣味特徵，各製茶師敘述不同。本研究以菁味程度，標定炒菁前的氣味特徵，分析炒菁前、後及茶葉的揮發性化合物組成與茶葉品質內容物，以瞭解炒菁前氣味與茶葉品質之關聯，作為後續製茶研究的基礎。 試驗以包種茶製程攪拌次數及炒菁前氣味的菁味程度作為主要處理因子。試驗一於2018年冬季、2019年春季製作包種茶，製程攪拌次數分別為3、4、5次，炒菁前菁味程度分為高、中、低三個程度。試驗二於2019年的冬季製作包種茶，製作時，攪拌次數為3次與4次，菁味程度則分為高、低兩種程度，並同時以專業師傅及電子鼻判定菁味程度。分別於炒菁前、後和粗製茶隨機取樣3重複，並以氣相層析串聯質譜儀分析揮發性化合物，以高效能液相層析儀分析兒茶素異構物、咖啡因、沒食子酸，粗製茶則增加分析總多元酚含量、總游離胺基酸含量、總還原醣含量。 炒菁前茶菁的揮發性化合物組成，以具有青草氣味的醛類化合物為主，以(E)-2 -己烯醛((E)-2-Hexenal)為代表，炒菁時菁味程度低的茶菁，含有更多具有花果氣味的萜類化合物，如香葉醇(Geraniol)、芳樟醇(Linalool)、吲哚(Indole)，及帶甜香的酯類，如(Z)-3-己酸己烯酯((Z)-3-Hexenyl hexanoate)；而攪拌次數多的茶菁揮發性化合物種類較多，且有較高比例的花果香氣化合物。炒菁後茶菁的揮發性化合物大量逸散，幾乎無具有綠葉氣味的化合物，以吲哚的相對豐度最高，組成比例在攪拌次數較多、菁味程度高的處理組合中含量比例較高，菁味程度中等的處理組合則有較高比例的萜醇類。吲哚及苯乙腈(Benzyl nitrile)在炒菁前後可較為穩定辨識出，且在炒菁前後具中或高相關性的化合物， 具作為炒菁前氣味指標化合物的潛力。 經沖泡後的茶葉葉底，揮發性化合物種類豐富，以主成分分析可依揮發性化合物組成類型，區別不同攪拌次數處理，並能指示不同菁味程度。不同處理間，兒茶素異構物及總兒茶素含量幾無顯著差異。在高菁味程度的茶葉中，有更多多元酚類及游離胺基酸類，而攪拌次數增加時，還原醣類含量增加，游離胺基酸類含量減少，結果顯示菁味程度及攪拌次數會影響茶湯內容物比例及滋味，在菁味程度低時炒菁能減少澀味，增加甜味，進而獲得更加優良的包種茶品質。本研究解析包種茶炒菁前氣味的菁味表現對茶葉品質的影響，說明適宜炒菁時機的重要性。

Paochung oolong tea is a light fermentation tea which features for the elegant floral aroma. The manufacturing process depends on tea masters monitoring the odour changes of tea leaves by sniffing to determine the optimum processing time of each step. However, the odour profile of tea leaves considered suitable to stop the oxidation and pan-firing vary by tea masters. The study targets the odour profile before pan-firing by the intensity of green odour. The volatile organic compounds (VOCs) of fresh tea leaves before and after pan-firing are analysed, and assessed the VOCs and constitute of tea infusion, to understand the effects of odour profile of fresh tea leaves before pan-firing on tea quality. Aiming to provide an odour index for the future works of the tea industry. The main experimental factors are times of shaking and green odour intensity before and-firing. In 1st experiment, Paochung oolong tea was made in the 2018 winter and 2019 spring with 3,4,5 times of shaking and high, medium, low green odour intensity respectively. In 2nd experiment, Paochung oolong tea was made in the 2019 winter, with 3,4 times of shaking and high, low green odour intensity respectively. Besides, green odour intensity is assessed by tea masters and gas sensors. The triplicates are conducted in each treatment. Gas chromatography-mass spectrometry and high performance liquid chromatography were used to analysed VOCs, catechin epi-isomers, caffeine and gallic acid. The total polyphenol content, total free amino content and total reduced sugar content of tea infusion were also analysed. The results showed that the VOCs of fresh tea leaves before pan-firing are mainly constituted of aldehydes such as (E)-2-hexenal, which gives green grass aroma. The treatment of low green odour intensity contains more terpenoids which give floral or fruity aroma, such as geraniol, linalool and indole, and esters which give sweet aroma, such as (Z)-3-hexenyl hexanoate. The varieties of VOCs and floral and fruity aroma compounds increased with times of shaking in the fresh tea leaves. Most of VOCs dispatches after pan-firing, no green odour VOCs are detected. Indole is the main constitutes and shows a higher ratio in high green odour intensity treatment; the terpene alcohols showed a higher ratio in medium green odour intensity treatment. Indole and benzyl nitrile are detected steadily and show medium or high correlation in fresh tea leaves before and after pan-firing. It could be a potential odour index for fresh tea leaves before pan-firing. The VOCs varieties are more abundant in infused tea leaf compared to the fresh tea leaves after pan-firing and could be clustering by times of shaking and green odour intensity by principal component analysis. Nearly no significant difference is suggested in catechin epi-isomer and total catechin content between treatments. There are more total polyphenol and free amino acid in high green odour intensity treatment, more reduced sugar and less free amino acid in the treatment of fewer times of shaking. The results showed that the green odour intensity and times of shaking have effects on tea infusion constitute and taste. A better tea quality could be acquired by pan-firing the fresh tea leaves of low green odour intensity, which shows fewer astringent compounds and more sweet compounds. The research study the effects of the green odour of fresh tea leaves before pan-firing on the tea quality indicate the importance of pan-firing timing.