# Recent Advances in Volatiles of Teas
## 2 Aromatic Volatile Compounds in Various Kinds of Teas  
### Formation of Tea Aromatic Volatiles  
Volatile compounds in tea products can be formed through many pathways, such as the carotenoid derivatives pathway, the fatty acid derivatives pathway, the terpene derivatives pathway, the phenylpropanolid/benzenoid derivatives pathway, the glycoside hydrolysis pathway and the Maillard reaction pathway. Carotenoids are important precursors of tea volatile compounds, especially the C\({}_{9}\)- to C\({}_{13}\)-aromas. Ionone and damascone are important C\({}_{13}\)-carotenoid-derived compounds (Figure 1) that constitute anessential aroma note in black tea. Testing in a model fermentation system consisting of \(\beta\)-carotene, tea catechins and crude soluble enzymes' preparation extracted from fresh tea leaves showed that \(\beta\)-ionnone was the major volatile product, and \(\beta\)-damascone was the minor product formed during the conversion process by oxidative degradation of \(\beta\)-carotene [2,3]. Concentrations of volatiles, including \(\beta\)-ionone, \(\alpha\)-ionnone, \(\beta\)-damascone and \(\alpha\)-damascone, were significantly increased, and the volatile flavor compound (VFC) index was improved by supplementing carotenoid in tea leaves during black tea fermentation [4]. Lipids, especially fatty acids (FA), contributed greatly to the aroma and flavor volatiles in tea [5]. Glycolipids, which are rich in linolenic acid, are the most abundant lipids in fresh tea leaf. Neutral lipids, which are rich in linoleic, myristic, lauric, stearic, palmitic and oleic acids, are moderately abundant lipids. Phospholipids, which have a high proportion of linoleic, palmitic and oleic acids, are the least abundant lipids in tea [6]. During black tea processing, including withering, rolling and fermentation, the lipids are degraded to produce flavor volatiles by hydrolytic or oxidative action of enzymes on glycolipids and phospholipids [5]. The major fatty acid derivatives include alcohols, aldehydes and lactones. C\({}_{6}\) and C\({}_{9}\) alcohols and aldehydes are key contributors to the \"fresh green\" odor of tea. Methyl jasmonate, an important fatty acid derivative, is a major contributor to the jasmine-like aroma of oolong tea [7]. The aldehydes were synthesized via an intermediate, in which the primary reaction is the dioxygenation of an unsaturated fatty acid catalyzed by a lipoxygenase (LOX). The fatty acid-derived aldehydes could be further transformed to their corresponding alcohols by alcohol dehydrogenase [8]. Geraniol, linalool and linalool oxides are important terpene derivatives in tea, especially in black tea. Geranyl pyrophosphate (GPP), a compound produced in the methylerythritol phosphate (MEP) pathway [9], is the precursor of the terpenoid derivatives geraniol and linalool by geraniol synthase and linalool synthase, respectively [10,11] (Figure 2). The linalool oxidation may be catalyzed by a non-specific enzyme [11]. Linalool is usually present in volatile free form in black tea. However, linalool oxides always occur in the bound forms of primeverosides [12]. Figure 1: Oxidative degradation of \(\beta\)-carotene: (**A**) \(\beta\)-carotene; (**B**) 10°-apo-\(\beta\)-carotenal; (**C**) \(\beta\)-ionone; (**D**) \(\beta\)-damascone. Phenylalanine, a compound produced in the shikimic acid pathway, is the precursor of volatile phenylpropanol/benzenoid derivatives [13]. The volatile phenylpropanol/benzenoid derivatives are important contributors to the fruity and floral smells. Major phenylpropanolids in tea are phenylethanol and phenylacetaldehyde, and major benzenoids are benzyl alcohol and benzaldehyde [13]. The enzymes involving in these biosynthesis pathways in tea remain to be investigated. Many tea volatile compounds are present in glycosidically-bound forms, which are more water soluble and more easily stored, but less volatile than their free aglycone counterparts. Benzyl \(\beta\)-D-glucopyranoside was the first volatile glycoside isolated from tea leaf [14].