# Characterization of Volatile Substances in Pu-erh Tea (Raw Tea) at Different Storage Times
## 1 Introduction  
Pu-erh tea is a unique geographical indication product from Yunnan, valued by consumers for its unique flavor characteristics and exceptional health benefits [1, 2]. It is made from the fresh leaves of Yunnan big-leaf tea trees (_Camelia sinensis_ var. _assamica_ (_Mast._) _Kitamura_) grown within the scope of the Protected Geographical Indication area [3]. Based on processing technology, Pu-erh tea is divided into two types: Pu-erh tea (raw tea) and Pu-erh tea (ripe tea). Pu-erh tea (ripe tea) undergoes a unique fermentation process, resulting in a bright red color and a richer mellow flavor [4]. Unlike Pu-erh tea (ripe tea), Pu-erh tea (raw tea) leaves go through the processes such as spreading, killing, twisting, drying in sunlight, refining and steam molding, etc. The resulting Pu-erh tea (raw tea)is yellow and bright, with a strong taste. In the Pu-erh tea market, it is often said that \"the older the better\", meaning that Pu-erh tea with a longer storage time has a higher market value than newer tea. During storage, the changes in Pu-erh tea (raw tea) are more pronounced compared to Pu-erh tea (ripe tea) which has undergone fermentation. After storage, Pu-erh tea (raw tea) exhibits a deeper color, a more pronounced flavor, and a richer, long-lasting aroma [5]. The transformations in Pu-erh tea (raw tea) are characterized by a high degree of unpredictability and complexity, making the study of its quality changes during the storage process of significant importance. Aroma is one of the most important characteristics in evaluating the quality of tea, and studies have shown that its contribution to the quality of tea reaches 25-40%. It can be used as an important indicator for judging the advantages and disadvantages of tea quality characteristics [6; 7]. Generally, the characteristics of aroma in various dark teas are unique. Tibetan tea shows an oily aroma, Pu-erh tea emphasizes a stale aroma, and Liupao tea is characterized by stale, woody, and betelnut aromas [8]. Prolonged storage leads to significant changes in the aroma of the tea. The aroma characteristics of compressed white tea after storage evolve from initially sweet, fruity, and floral notes to more pronounced stale, woody, and herbal characteristics [9]. Related studies showed that the storage process has the most significant effect on the aroma of Pu-erh tea (raw tea); after a long time of storage, Pu-erh tea (raw tea) can be transformed into aged tea, and the longer the time, the better the quality [10; 11]. Compared with Pu-erh tea (ripe tea), the aroma of Pu-erh tea (raw tea) contains a variety of complex, volatile components, mainly fruity/floral aroma [12]. The characteristic volatiles in Pu-erh tea (raw tea) are linalool, tridecane, caffeine, dihydroactinidiolactone, \(\beta\)-violetone, 6,10,14-trimethyl-2-pentadecanone, and dodecane, etc. With long-term aging, Pu-erh tea (raw tea) undergoes complex chemical changes, resulting in a decrease in the levels of low-boiling-point alcohols and hydrocarbons and an increase in some high-boiling-point acids and methoxyphenolic compounds [13]. Therefore, Pu-erh tea (raw tea) stored for a long period should have the quality characteristics of pure aroma, mellow and sweet. Previous studies on the aroma of Pu-erh tea (raw tea) have focused on processing technology [14], varieties [15; 16], grades of raw materials [17], different regions [18; 19; 20], etc. Changes in volatile components related to the aroma of Pu-erh tea during long-term scientific storage have not yet been fully revealed. With the improvement in testing instruments and methods, a variety of methods are available for the determination of aroma, and GC-MS, which is capable of qualitatively and quantitatively characterizing volatile compounds, is the most frequently used analytical technique for the detection and analysis of aroma compounds in Pu-erh tea [21].