# Effects of Monochromatic and Composite Light Withering on Black Tea Aroma
## 4 Discussion  
In this study, the effect of different monochromatic LED lights and composite light withering on black tea aroma was investigated. Consistent with previous studies [14, 27], appropriate exogenous light exposure during withering altered both the composition and concentration of volatile compounds as well as the major quality components. However, our results demonstrated that single-wavelength LED treatments (BL, RL and YL) tended to reduce the contents of terpenoids, aldehydes, esters and heterocyclic compounds compared to natural light withering (CK). Composite light treatments, particularly red-yellow (RY) light, exhibited more pronounced effects on volatile profiles than monochromatic light, which significantly elevated the level of heterocyclic compounds and some alcohols and aldehydes. The effect of the light spectrum on tea flavor during withering has been the focus for both producers and researchers. It affects the sensory quality and chemical composition of tea with different wavelengths and intensities [14, 32]. To date, various LED lights including white, red, yellow, blue, green, purple, ultraviolet (UV), orange light, and multi-wavelength LED combinations have been applied in the withering process of different kinds of tea [16, 33]. LED light withering has been reported to increase the water extract of tea [32]. However, no significant difference was observed in the water extract content in different groups (Figure 1A). This may be because the CK group received composite spectrum sunlight containing both visible and invisible wavelengths. In other words, it is a compound light. In addition, the effects of different LED light withering conditions on \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Volatile Components**} & \multirow{2}{*}{**Threshold (\(\mu\)g/kg)**} & \multicolumn{2}{c}{**rOAV**} & \multirow{2}{*}{**Odor**} \\ \cline{3-3} \cline{5-5}  & & **RY** & & **CK** & \\ \hline (E)-3,7-dimethylocta-2,6-dienal & 5 & 2.00 & 10.74 & lemon \\
2-phenylethyl acetate & 20 & 0 & 2.82 & flower, honey, rose \\
2-penrylfuran & 4.8 & 9.012 & 23.21 & butter, floral, fruit, green bean \\ heptanal & 31 & 0.78 & 1.60 & citrus, fat, green, nut \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Key volatile components (OAV > 1) screened in RY_vs_CK group. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Volatile Components**} & \multirow{2}{*}{**Threshold (\(\mu\)g/kg)**} & \multicolumn{2}{c}{**rOAV**} & \multirow{2}{*}{**Odor**} \\ \cline{3-3} \cline{5-5}  & & **RB** & & **CK** & \\ \hline (E)-hex-2-enal & 0.25 & 0.00 & 2456.60 & almond, bitter, green, heavy \\
2-methylbutanal & 300 & 1.40 & 3.27 & cabbage, organic, sulfur, wet earth \\
3-methylbutanal & 4.8 & 7.71 & 23.21 & almond, cheese, chocolate, malt \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Key volatile components (rOAV > 1) screened in RB_vs_CK group. the major components of tea vary, with the blue light group showing the lowest TP content and the red light group exhibiting the highest amino acid level [32]. In the present study, the total amount of TP was decreased in the YL and RB groups, potentially attributed to the alteration in the expression of relevant genes by exogenous light [34]. Meanwhile, the YL group displayed the lowest free amino acid level, aligning with a previous study in which yellow light withering significantly reduced amino acid content compared to the red and blue light groups [32]. Moreover, withering with different lights impacts the aroma profiles of tea. It was found that red light reduced the grassy odor but enhanced the floral and fruity fragrance [14]. This is consistent with what we observed in this study that the red light withering group exhibited a strong sweet fragrance. Nevertheless, the quantity and content of volatile components tended to decrease in the single LED light treatment groups (RL, BL, and YL) with respect to the CK group (Figure 2), which was attributed to the fact that the CK group was a composite light.