# Characterization of Volatile Substances in Pu-erh Tea (Raw Tea) at Different Storage Times
## 3 Results and Analysis
### Differences and Characterization of Volatile Substances in Different Groups
Combined with the results of the sensory evaluation, Group A4 was characterized by a clear aroma, so these substances are the key volatile for the formation of a fragrance aroma. Through a comprehensive evaluation of these four groups of key volatile compounds, three substances are present in all four groups: trans(\(-\))-5-methyl-3-(1-methylphenyl)-Cyclohexene, L-\(\alpha\)-Terpineol, and benzaldehyde. These compounds may play a role in coordinating the floral, fruity, fresh, and smoky aromas of Pu-erh tea (raw tea). During storage, Trans-(\(-\))-5-methyl-3-(1-methylphenyl)-Cyclohexene increased in the 11 th year of storage, and its content was relatively stable between the 12 th and 15 th years of storage. The contents of L-\(\alpha\)-Terpineol and benzaldehyde showed an overall increasing trend. The formation of \(\alpha\)-terpineol is dependent on post-fermented microbial activities [31]. The amino acid-derived volatile benzaldehyde spike may be due to the oxidation of toluene. This key aromatic compound is consistent with previous research findings, and the trend of its changes during storage shows similarity. This further suggests that the variations of these volatile compounds during storage play a crucial role in the transformation of the aroma characteristics [43]. The unique substances in Group A1 were fenchene, (E)-1,2,3-trimethyl-4-propenyl-naphthalene, (+/\(-\))-theaspirane (honey), and decanal (fruity, floral), which were the key substances for the fruity/floral aroma types. Decanal was considered to be the most important aroma component due to its ROAV \(\geq\) 100 in Pu-erh tea, which is consistent with previous conclusions [44]. The unique substances in Group A2 were 2-ethyl-Furan (burnt), camphene (floral), 1-methyl-4-(1-methylphenyl)-Benzene (woody), and cis-\(\beta\)-Ocimene (fruity, floral), which are the key aroma-presenting substances for the smoke aroma type. At the same time, all four of these substances have ROAV \(\geq\) 1, so they are the key aroma substances of the smoky aroma type. Compared with Groups A1 and A2, the unique substances in Group A3 were 1-methyl-4-(1-methylylidene)-Cyclohexene (woody), \(\alpha\)-Terpinene (fruity, floral), trans-\(\beta\)-Ocimene (fruity), \(\gamma\)-Terpinene (fruity), (E,E)-2,4-Heptadienal (fragrance), and octanal (fragrance). Among them, 1-methyl-4-(1-methylylidene)-Cyclohexene, \(\alpha\)-Terpinene, trans-\(\beta\)-Ocimene, (E,E)-2,4-Heptadiena, and octanal have ROAV \(\geq\) 1. This indicates that these are the key aroma-presenting substances for the fragrance aroma type. Octanol was also identified in Group A4, and its ROAV value was greater than 100. This indicates that octanol is a key component in the formation of aroma types and plays a significant role in influencing the richness of the aroma. MA's research shows that octanal is considered the characteristic active aroma compound of Pu-erh tea with a green flavor [45]. The unique substances of Group A4 were 2,5-Dimethoxyethylbenzene, 2,4-Dimethylanisole, 1,2,3-Trimethoxybenzene, 3,4-dimethoxyytoluene, which interacted with other substances to affect the persistence and richness of the aroma. O-methylation promotes simple phenols into methoxyphenols, which contribute to fragrance changes during storage [46]. In the samples of Group A4, the contents of alcohols and methyl groups were higher than those of other samples, especially in the 21 st year of storage; the methyl groups increased sharply. The methyl group is the key component of the aging flavor [47; 48; 49; 50], which indicates that the samples are in the process of transformation from fresh flavor to aged flavor. With further storage, it was hypothesized that the aroma of the samples should be gradually transformed into an aging aroma.