Damascenone Fig and ionone Fig

β-Damascenone (Fig. 3) and β-ionone (Fig. 4a) are two representative aromas formed from carotenoid degradation. β-damascenone has an apple-like flavor and has an extremely low threshold in water (0.002ppb). It was first identified in Bulgarian rose oil in 1970 [9] and is an essential odor in black tea infusion [10–12]. It comes from the enzymatic oxidation of neoxanthin (Fig. 3). The first step is the cleavage of neoxanthin by dioxygenases between the C-9 and C-10 double bond, yielding grasshopper ketone. Next, this ketone is enzymatically reduced to allenic triol, which is known as a progenitor of β-damascenone. The last step is acid-catalyzed dehydration to odoriferous β-damascenone [13]. In addition, it can directly originate from neoxanthin in non-enzymatic reactions, such as thermal degradation or oxidation, under acidic conditions during the tea manufacturing process [14]. β-Ionone (Fig. 4a) is a significant contributor to the flavor of green and black tea and has a low odor threshold (0.007ppb). It can be produced either by enzymatic reactions during fermentation or thermal degradation during the green tea manufacturing process (Fig. 4a) [15]. It comes from the primary oxidation of β-carotene. Fermentation and heat-drying steps are both needed to generate the final product β-ionone. β-ionone can be further oxidized to 5,6-epoxy-β-ionone. After two reduction steps, it is converted to a saturated triol that undergoes an intramolecular cyclization followed by an oxidation reaction generating dihydroactinidiolide and theaspirone, which are viewed as critical aromas in determining the characters of black tea (Fig. 4b) [8,16,17]. Table 1 lists tea aromas generated by primary and secondary enzymatic oxidations from their carotenoid precursors [8,18]. Non-enzymatic degradation of cox pathway includes photo-oxidation (solar withering and solar drying), auto-oxidation, and thermal degradation (steaming, pan-firing, rolling, and drying). As an example, photo-oxidation of β-carotene under UV light results in 5,6-epoxy- β-ionone, 3,3-dimethyl-2,7-octanedione, 2,6,6-trimethyl-2-hydroxycyclohexanoe, dihydroactinidiolide, and β-ionone. The first step is the epoxidation of the double β-ionone bond on cyclohexene, followed by the cleavage of epoxides, C-9 and C-10 double bond, or C-7 and C-8 double bond, producing cyclic and straight chain aromas (Fig. 5a). Another example is the formation of oolong tea aromas nerolidol, α-farnesene, and geranylacetone from photo-oxidation of phytofluene (Fig. 5b) [18]. Table 2 lists the main carotenoid-derived tea aromas in three types of tea. Aromas yield higher concentrations during the fermentation step due to enzymatic oxidation. Indoor withering is another essential enzymatic oxidation that generates substantial aroma in oolong tea. Solar drying and thermal degradation are non-enzymatic ways for the formation of green tea aromas. β-ionone, 5,6-epoxy-β-ionone, nerolidol, and dihydroactinidiolide account for the high concentration of flavors in green tea. Dihydroactinidiolide, theaspirone, nerolidol, and safranal are highly concentrated flavors in black tea. Oolong tea combines both aromas in green and black tea with different concentrations depending on its fermentation degree [8,18,19].
Lipids as precursors Unsaturated fatty acids, such as α-linolenic acid, linoleic acid, oleic acid, and palmitoleic acid, are precursors for six to ten carbon aroma compounds, such as (E)-2-hexanal (leafy), (E)-2-hexanol, and (Z)-3-hexanol (leafy), which contribute fresh and greenish odors in tea infusion (Fig. 6) [20]. Formation of these volatile aromas from the oxidation of tea lipids is usually associated with two main pathways. The first pathway is an oxidation reaction initiated by free radicals, such as autoxidation, photo-oxidation, and thermal oxidation. The rate of lipid oxidation increases with the unsaturation degree of lipids. The second pathway is called lipoxygenase-mediated lipid oxidation, which is also the main pathway contributing to the flavor of tea.