Withering is a crucial process that determines the quality of white tea (WT). Solar withering (SW) is reported to contribute to the aroma quality of WT. However, the mechanism by which aroma is formed in WT subjected to SW remains unclear. In this study, through headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC–MS) and transcriptomics, we found that 13 key genes enriched in the mevalonic acid and methylerythritol phosphate pathways, such as those of 1-deoxy-D-xylulose-5-phosphate synthase and terpineol synthase, were significantly upregulated, promoting the accumulation of α-terpinolene, geraniol, and nerolidol, which imparted floral and fruity odors to WT subjected to SW. Additionally, the significant upregulation of lipoxygenases enriched in the lipoxygenase pathway promoting the accumulation of hexanol, 1-octen-3-ol, (E, Z)-3,6-nonadien-1-ol, and nonanal, which contributed to the green and fresh odor in WT subjected to SW. This study provided the first comprehensive insight into the effect mechanism of SW on aroma formation in WT.

Colorant adulteration is a common problem in tea safety control; thus, a rapid identification method is required. In this study, we optimized the fabrication parameters of various sensors to enhance their performance. R6G was used as a probe molecule, demonstrating that the sensnor remained stable for 120 days. Based on surface-enhanced Raman spectroscopy, the optimized sensors were used to identify and quantify mixed colorants (sunset yellow, lemon yellow, carmine, and erythrosine). Partial least squares prediction models were developed for each colorant (0.5-300 μg/mL), with R2 > 0.900 and RPD > 2.27; these indicated the accuracy of the sensors. The results also revealed a model recovery range of 95.9 % to 116 %, with RSD < 3.94 %, indicating the universality of our proposed method. Overall, the proposed method enables the detection of mixed-colorant adulteration in black tea within 3 min, thereby representing a novel method for the assessment of tea quality.

Withering leads to the formation of high-quality flavor in black tea. This study investigated the effects of natural withering (NW), sun withering (SW), and warm-air withering (WW) on the flavor compounds of withered leaves by using target metabolomic and transcriptomic approaches. The results revealed that 4282, 4770, and 1246 differentially expressed genes (DEGs) were identified in the NW, SW, and WW samples, respectively. The expression of DEGs involved in volatile compound and flavonoid biosynthesis was the highest in the WW sample, which may be the main reason for the significant increase in volatile compound and catechin contents in the WW sample. The withering methods mainly affected the contents of non-proteinaceous amino acids, such as theanine. Withering methods significantly affect the expression levels of DEGs involved in metabolic pathways; this provides a systematic explanation for flavor formation in black tea.

Maillard reactions occurred during the roasting process of large-leaf yellow tea (LYT) were the main reason for the formation of their unique flavor. However, the contribution of different amino acids and soluble sugars to the formation of heterocyclic compounds in the Maillard reaction is still unclear. First, this study found that trimethylpyrazine, 2-ethyl-3,5-dimethylpyrazine, 3-ethyl-2,5-dimethylpyrazine and 2,5-diethylpyrazine, which had higher odor activity values (OAV), were significantly increased with the increase of roasting temperature. In contrast, only five amino acids (l-theanine, glutamic acid, lysine, glutamine, histidine) and three monosaccharides (d-glucose, d-fructose, and l-fucose) decreased significantly with increasing roasting temperature, which were significantly and positively correlated with each other. This indicates that these five amino acids and three monosaccharides were involved in the Maillard reaction during the roasting process. In addition, the model thermal reaction results showed that lysine contributed significantly to the formation of pyrazine especially with d-glucose to methylpyrazine, 2,5-dimethylpyrazine, trimethylpyrazine and 3-ethyl-2,5-dimethylpyrazine, while l-theanine contributed significantly to the formation of 1-ethylpyrrole-2-carbaldehyde. The d-glucose and d-fructose were the main contributors to the Maillard reaction among soluble sugars. This study lays the foundation for future studies on the relationship between volatile compounds and Maillard reactions.

The mechanism through which solar withering (SW) affects the quality of white tea is unclear. To address this gap in the literature, in this study, we used metabolomics and transcriptomics to investigate the effect of SW on the quality of WT. WT that underwent SW was slightly more bitter and astringent than WT that underwent natural withering (control group). Specifically, SW considerably increased the concentration of astringent flavonoids and flavone glycosides in WT. This increase was mainly attributed to the upregulated expression of key genes in the shikimic acid, phenylpropanoid, and flavonoid biosynthesis pathways, such as shikimate kinase, chalcone synthase, and flavonol synthase. In addition, SW experienced considerable heat and light stress. The levels of glycerophosphatidylcholine and carbohydrates increased in response to the stress, which also affected the taste of WT. The results of this study indicate the mechanism through which SW affects the quality of WT.