Formation of Tea Aroma

Tea is one of the most popular beverages in the world and is enjoyed for its unique aroma and flavor. The characteristic aroma of tea can be classified into several categories, including green odor, floral aroma, roasted and nutty aroma, off-flavors, stale flavor, photo-induced flavor, and retort smell. Each of these categories has its unique chemical makeup and can be influenced by various factors such as processing, storage, and exposure to light. In this article, we will explore each of these categories of tea aroma in detail and understand what gives tea its unique flavor and aroma.

Green Odor

The green odor is the scent of fresh grass or plants. Researchers have studied this odor in tea plants and found eight compounds that make up the scent, including (E)-2-hexenal, (Z)-3-hexenol, and n-hexanol. These compounds are produced in the leaves from fatty acids during tea processing. Additionally, aldehydes formed from amino acids by polyphenol oxidase and unsaturated fatty acids released during processing contribute to the aroma of tea.

Floral Aroma

The terpenes are responsible for the characteristic odor of flowers. Monoterpenes, such as citronellol, geraniol, linalool, nerol, and a-terpineol, are the most important components responsible for the floral odor of tea. These terpenes have a 5-carbon isoprene unit in common as their skeletal building block and can be linked together in multiples of two (monoterpenes), three (sesquiterpenes), four, six, and nine, and even higher.

Sesquiterpenes have a less pronounced flavor value compared to monoterpenes. In addition to the free, odor-producing forms of monoterpene alcohols, glycosidically bound monoterpene alcohols are present in tea, and their occurrence is not naturally present but produced during the withering or fermentation stage of black tea production.

The content of the flavor components released from the bound forms increases greatly after the addition of crude enzyme extracted from fresh tea leaves. The varietal and seasonal changes in both free and bound monoterpene alcohols in the clones cultured in the Qimen (Keemun) area in China have been investigated, and geraniol was found to be the highest among the monoterpene alcohols.

The aroma precursors were abundant in young leaves and decreased as the leaf aged. Glycosidase activity also decreased as leaves aged but was high in stems. In green made tea, the free forms of monoterpene alcohols are connected not only with the content of the bound forms of monoterpene alcohols, but also with ß-glucosidase activity in the corresponding fresh leaves of cultivars.

In black tea, the liberation of free monoterpene alcohols in made tea mainly depends on the corresponding content of bound forms of monoterpene alcohols.

Roasted and Nutty Aroma

The roasted and nutty aroma in tea is mainly caused by pyrazines. Pyrazines are produced through a process called non-enzymatic browning, which occurs when a reducing sugar’s carbonyl group combines with a protein amino group at temperatures of 100°C or higher. Pyrazines are also synthesized by plants and microorganisms and are present in many fermented foods and beverages.

The pyrazine structure can be highly substituted, resulting in a large family of compounds that produce unique flavors at very low concentrations. The Maillard reaction also produces furans, pyrroles, thiophenes, and other heterocyclic compounds, contributing to the complex array of volatile components in tea.

L-theanine, the most abundant amino acid in tea, produces pyrazines and other compounds when heated to high temperatures. Pyrazines can be produced in tea only when the firing temperature is over 100°C and depend on heating time and the amount of tea leaves.

Off-flavor

Off-flavor in food refers to any flavor that is not typically associated with that particular food product. This can be an unacceptable flavor or simply a flavor that is not commonly found in that type of food. There are various types of tea, each with their unique flavors and tastes. Therefore, an acceptable flavor in one type of tea may be considered an off-flavor in another type of tea. The off-flavor in tea varies from tea to tea due to the wide range of characteristic flavors found in different teas.

Stale flavor

Tea is usually stored for a period of time before consumption, but this can lead to chemical changes that cause a decrease in quality, especially in aroma quality. This is known as the stale or stored flavor.

Different types of tea have unique flavors, so what may be acceptable in one type of tea may be considered an off-flavor in another. For example, in black tea, stale flavor is associated with the presence of certain compounds such as n-hexanal and benzyldehyde.

In contrast, green tea can experience an increase in volatiles content during storage due to the auto-oxidation of fatty acids. To reduce the aged flavor, tea should be stored in moisture-proof packaging in an inert atmosphere of nitrogen, and green tea can benefit from a refiring process.

Photo-induced flavor

Exposure to light can cause an off-flavor in tea known as photo-induced flavor. This off-flavor can be detected in green tea after just one day of light exposure, with a strong off-flavor noted after four days. Light has a marked negative effect on the aroma and taste of tea, with little impact on its appearance or infusion color. Certain volatile components in green tea, such as pentanal and butanol, increase significantly when exposed to light, while others such as dimethyl sulfide, which contributes to the fresh flavor of green tea, decrease significantly. Bovolide is the most evident compound formed even under low light conditions and could be used as an indicator of light exposure in tea.

Retort smell

Kinugasa and Takeo (1989) studied the flavor deterioration of canned tea drink during retort-sterilization. They found an increase in the contents of several compounds, including 4-vinylphenol, which is known as the major compound of the retort smell. This off-flavor can be avoided by adding a suitable amount of ß-cyclodextrin during manufacturing. Furthermore, the content of precursors of volatile compounds in tea leaves is higher in young leaves than in matured leaves. The precursors in tea leaves are easily hydrolyzed during the firing process of Kamairicha (a type of roasted green tea) at 200–250°C, resulting in a lower content of precursors of volatile compounds and weaker off-flavor in the drinks of Sencha made from matured leaves and the drink of Kamairicha.

Smoky-burnt odor

Roasted green tea often has smoky and burnt odors caused by pyrazines, pyrroles, guaiacol, naphthalene, and indene. Burnt odor is usually detected during the drying process, while the smoky odor can occur during all heat treatment steps. Non-volatile compounds like polyphenols, caffeine, fatty acids, and chlorophylls can also contribute to the off-flavors of tea.