萎凋初期光源與相對溼度對茶菁之影響

Abstract

臺灣的部分發酵製作，通常在茶菁採摘後會先進行短暫的日光萎凋，此過程在茶業界與許多研究的結果，認為有助於部分發酵茶在香氣與滋味的表現，但在光質部分研究較少。本研究探討日光、紫外線與紅外線三種光源進行照射對茶菁的影響，並探究照光萎凋時相對溼度，是否會影響光源的效果。照光萎凋者再分為正常萎凋與高濕度萎凋，造成茶菁失水之差異。由於不同光源的照射，加上萎凋相對濕度不同，茶菁在處理結束時含水率已有差異，所以透過延長室內萎凋時間，在茶菁相近含水率時取樣比較。萎凋過程分為三階段，Stage 1為萎凋初期，指照光萎凋結束；Stage 2為萎凋中期，指將茶菁正常萎凋降至目標含水率；Stage 3為萎凋結束，茶菁含水率降至殺菁前之目標含水率，最後製成毛茶樣品為Stage 4。 萎凋初期以紫外光處理之茶菁，在Stage 1總多元酚類與兒茶素類含量較低，萎凋三階段 (Stage 1-3)者總多元酚類與兒茶素類平均含量也較低。萎凋初期照光高濕度處理組，茶菁總游離胺基酸含量高於正常萎凋組。在三個階段比較萎凋初期的光源與相對溼度對茶菁內容物之差異，無一致的趨勢；而以萎凋階段為影響因子，較能看出茶菁內容物變動的趨勢。萎凋過程中茶菁總游離胺基酸、咖啡因與沒食子酸含量會隨萎凋延長而提升，總多元酚類與兒茶素類含量則是隨萎凋延長而下降。此外，當茶菁含水率降低，將導致茶菁細胞膜熱穩定性降低。萎凋初期光源與相對溼度處理，造成萎凋溫度不同，在Stage 1多元酚氧化酶活性與萎凋溫度間回歸關係佳 Y = - 0.339(X-35.612)2 + 19.478 (R2 = 0.93, p < 0.001)，顯示在36.5℃，會有最高之多元酚氧化酶活性。 萎凋初期照光且高濕度萎凋之茶菁，在Stage 3與Stage 4揮發性化合物總相對豐度皆低於正常萎凋之處理；高濕度萎凋處理組茶菁之個別揮發性化合物相對豐度大多低於正常萎凋組者，而照光處理組茶菁之揮發性化合物相對豐度，大部分皆高於室內萎凋組，顯示不照光與照光時高濕度處理，均可能會降低茶菁揮發性物質之含量。

The semi-fermented tea manufacturing process in Taiwan uasully treat the tea leaves a short period of solar-withering after plucking. This step is believed to be helpful to the aroma and taste. This study explores the effect of three light sources including ultraviolet and infrared and solar radiation on tea leaves, and whether the relative humidity is an important factor during light-withering. Therefore, the experiment of light-withering is divided into normal withering and high-humidity withering. High-humidity withering provides the tea leaves a humid environment to reduce the water loss during light-withering. Due to the different light sources and relative humidity, the water contents of tea leaves at the end of the light-withering are different. By extending the indoor withering time, water content of tea leaves will decrease and it becomes possible to sample the tea leaves at similar water content level. The withering process is thus divided into three stages. Stage 1 (early withering): the end of light-withering, Stage 2 (middle withering): down to the target water content, and Stage 3 (end of withering): water content down to the target water content before panning, and the dry tea considered as Stage 4. During the initial withering stage, tea leaves exposed to ultraviolet-A light in Stage 1 exhibited lower overall levels of polyphenols and catechins. Across all three withering stages (Stages 1-3), these compounds were consistently lower in the UVA-treated group. In the early withering stage, under high humidity conditions, the total free amino acid content in tea leaves was higher compared to the normal withering group. Comparing the differences in tea leaves content among the three stages with different light sources and relative humidity, no consistent trend emerged based on this two factors. However, considering the withering stage as a factor revealed clearer trends in tea leaves content changes. Throughout the withering process, the total free amino acid, caffeine, and gallic acid contents increased with prolonged withering, while the levels of total polyphenols and catechins decreased. Additionally, as the moisture content of tea leaves decreased during withering, their cell membrane thermal stability (MTS) decreased. In the early withering stage, variations in light source and relative humidity led to different withering temperatures. The regression relationship between polyphenol oxidase activity (PPO) and withering temperature is good (Y = - 0.339(X-35.612)2 + 19.478, R2 = 0.93, p < 0.001) in Stage 1, indicating that the highest PPO activity occurs at around 36.5°C. The total relative abundances of volatile compounds in Stage 3 and Stage 4 of the tea leaves with high-humidity treatment were lower than those with the normal withering treatment on the early stage of light-withering. In the high-humidity withering group, the relative abundance of individual volatile compounds was mostly lower than in the normal withering group, while the light-withering group exhibited higher relative abundance of volatile compounds compared to indoor-withering, suggesting that both non-light and light-withering group under high humidity conditions could potentially decrease the content of volatile compounds in tea leaves.