# Effects of Monochromatic and Composite Light Withering on Black Tea Aroma
## 1 Introduction  
Black tea, the most widely consumed tea globally, is highly valued for its distinctive and charming flavor [1]. The processing of black tea includes withering, rolling, fermentation, and drying [2]. Of these, withering is the initial step in the processing of black tea and a key stage that affects the quality of black tea [3, 4]. During withering, fresh tea leaves diminished moisture under controlled temperature and humidity [5]. Simultaneously, macromolecular compounds in the tea leaves are hydrolyzed and the flavor of tea develops with the changes in volatile compounds [6, 7, 8].  
The formation of black tea aroma is a complex process influenced by multiple factors, which involves all stages of black tea processing [9]. Withering is a crucial process that affects the aroma of black tea [10, 11]. In this process, low-boiling-point aroma substances such as cis-3-Hexen-1-ol gradually decrease, while both the variety and the total content of small-molecule volatiles like alcohols and aldehydes enhance with the increase in hydrolytic enzyme activity [3]. These compounds mainly present a floral and fruity fragrance. Studies have found that withering method, as well as parameters like withering time, temperature, humidity, and light quality, can affect the aroma of black tea [2, 12, 13].  
Light is an important factor that influences the metabolism of tea leaves, and similarly, light quality during withering can easily affect the volatile profile of black tea [14]. It was found that red light withering improved the floral and sweet aroma of black tea and white tea while strengthening the chestnut-like fragrance of green tea [15; 16]. Blue and red light withering reduced the grassy aroma and enhanced floral-fruity characteristics [14]. In addition, yellow light withering enhanced the fruity aroma of tea by promoting terpenoid accumulation [17]. These results indicate that monochromatic LED light withering is effective in improving both the composition and content of volatile components. Notably, sunlight withering also improves the flavor of tea [3] and is widely applied in production. In this method, tea leaves were exposed to sunlight to enhance the floral and fruity aroma by spurring non-enzymatic reactions, enhancing the activity of related enzymes, or upregulating the expression of these enzymes [18]. It is well known that sunlight contains a lot of visible and invisible light. Based on this, we hypothesized that the mixture of different monochromatic lights could also improve the aroma of tea. However, little research has been performed on the effect of mixed light on the aroma of black tea.  
Therefore, in this study, different monochromatic lights (red, blue, and yellow light) and composite light (red-blue and red-yellow composite light) were applied in the withering of black tea, and the effects of withering with different light qualities on the volatile components of black tea were investigated by using sensory evaluation and headspace solid-phase microextraction combined with gas chromatography-mass spectrometry (HS-SPME-GC-MS). The findings will provide a theoretical foundation and technical guidance for optimizing exogenous light withering in black tea processing and improving black tea quality.