Chemistry of Color and Taste of Made Tea

The infusion obtained from made tea differs significantly in color, taste, and flavor from that of fresh tea flushes. These characteristics are acquired during the manufacturing process that follows tea flush harvesting. It is the process of manufacturing that creates the differences in color, taste, and flavor among various teas.

1. Color

Tea color is an important factor in evaluating tea, along with aroma and taste. The green color in tea is determined by chlorophyll content and the ratio of chlorophyll A and B. Yellow color comes from TFs, flavonols, and glycosides. Yellowish-brown color may appear in old or over-brewed green tea due to xanthophyll and carotenoids. Red color is the main shade in black tea and is produced by enzymatic oxidation and condensation of catechins during fermentation. Black color in black tea comes from decomposed products of chlorophyll, protein, pectin, sugar, and phenolic compounds. However, inappropriate polymerization between phenolic compounds and catechins can cause the infused leaf to appear dull. Excessive amounts of theafulvin can also contribute to this effect.

Color of green tea

Green tea has a greenish or yellowish-green color without any red or brown color. The chlorophyll content mainly determines the shade of green in the made tea, and the ratio of chlorophyll A to B creates different shades. The degradation of chlorophyll can make the color darker under high temperature and humid storage conditions. The greenish color in the infusion comes from the exposed chlorophyll due to the degradation of protein in the chloroplast plastid during the manufacturing process. Flavonols, flavones, and their glycosides contribute to the yellow color in the infusion. The autooxidation of catechins can cause browning and make the infusion turn reddish brown after several hours.

Color of black tea

Black tea undergoes a rolling process that disrupts the leaf cell and mixes cellular catechins and PPO. The resulting enzymatic oxidation and condensation of catechins during the fermentation process produce colored TFs and TRs, which are responsible for the different shades of red color in black tea. TFs are generally yellow and TRs are generally red, and the different ratios of these compounds contribute to the color of the tea infusion. Theaflavins are responsible for the briskness of taste, coppery color of infused leaf, and the “body,” “depth of color,” “richness,” and “fullness” of black tea infusion. TRs also contribute to the color of black tea infusion. The CTC method of manufacture produces higher amounts of TF and TR type soluble pigments than the orthodox method due to less disruption of the leaf and increased enzymatic oxidation of green leaf catechins. The degradative rate of chlorophyll determines the tendency of black tea to appear black or brown. Other phenolic compounds, including isotheaflavin, neotheaflavin, TF-3-gallate, TF3′-gallate, TF-3,3′-digallate, theaflavic acid, and epitheaflavic acid, also contribute to the color formation of tea infusion.

Color of oolong tea

Oolong tea is a type of tea that is partially fermented, with varying degrees of fermentation leading to different shades of color in the tea. Green tea has no oxidation, while light-fermented oolong has about 25%, heavy-fermented oolong has 40-45%, and black tea has more than 50%. The shade of color in oolong tea ranges from dark green to light brown black to brown black depending on the level of fermentation. Light-fermented oolong has a dark green color due to partially oxidized catechins and partially degraded chlorophyll, while moderate- to heavy-fermented oolong has a reddish-brown infusion color due to TRs and their oxidized polymers. Oolong tea also contains high levels of pheophorbide A and B, which contribute to its black bloom color, as well as carotenoids and a higher pheophytin/TR ratio compared to black tea. The color-determining compounds in oolong tea include flavonols, flavones, TFs, TRs, and homobisflavan compounds such as Oolonghomobisflavan A, B, Theasinensin D, E, F, G, and Oolongtheanin. The color of the tea leaves in oolong tea often shows a “red-rimmed green leaf” due to the rotating process that disrupts the cellular tissue at the edge of the leaves during the withering process.

2. Taste

The taste of food is made up of five basic sensations: sweetness, astringency, sourness, bitterness, and umami. A delicious cup of tea infusion has a balanced combination of these sensations. Astringency is a drying, puckering sensation that affects the whole tongue. Bitterness is mostly tasted at the back and sometimes the sides of the tongue. Umami is a Japanese term that describes a “meaty” or “brothy” taste.

Taste of green tea

Green tea is characterized by strong astringency and bitterness, medium umami, sweetness, and slight sourness. The major taste sensations in green tea are astringency and bitterness. The astringency and bitterness of green tea infusion are mainly determined by the contents of catechins and other phenolic compounds. Free type catechins are less bitter and astringent than gallate type catechins. The threshold value of caffeine is approximately 20 mg/100 ml in water. Amino acids such as theanine and serine contribute to the umami taste of green tea infusion, while sugars contribute to its sweetness. Flavanols cause 70-75% of the bitterness and astringency, and amino acids cause 70% of the umami taste of green tea. Saponin compounds from tea leaves are related to the bitterness of green tea.

Taste of black tea

Good quality black tea infusion has a bright reddish brown color, a strong and brisk taste, and a rich flavor. Astringency in black tea can be categorized into two types: tangy and non-tangy.

Tangy astringency has a sharp and puckering action with little aftertaste effect, while non-tangy astringency is tasteless, mouth-drying, and mouth-coating with a lingering aftertaste effect. The tangy astringency of black tea is related to bitterness and is caused by compounds with a galloyl group, TFs, and caffeine.

Caffeine together with black tea polyphenols is necessary for the expression of reasonable amounts of tangy astringency, but caffeine does not contribute to non-tangy astringency. The decaffeination process can change the nature of astringency from tangy to non-tangy.

Polyphenolic compounds, such as gallated tea flavonols, are related to astringency and bitterness, while non-gallated tea flavonols are only slightly related to astringency. TFs in solution are very astringent, but in tea, their astringency is reduced due to an interaction with bitter caffeine.

TRs are also closely related to the taste of black tea, with TR1 contributing significantly to the dull color of infusion and being negatively related to briskness, while TR3 is positively related to briskness. Caffeine contributes to the briskness of black tea.

The total TF content is not the sole determinant factor of market price, as other factors also affect the valuation of black tea. In tea with lower TF levels, the correlation between TF content and market price is generally positive but with statistically non-significant correlation coefficients, suggesting that other factors affect the valuation of black tea.

In contrast, a good correlation between tasters’ evaluation and TF contents was established in Malawi and northeast India. Green leaf (-)-ECG, (-)-EGCG, and caffeine are potential quality indicators before the processing of tea leaves.

Taste of oolong tea

Good quality oolong tea is characterized by its mellow and sweet taste and special aroma. The taste of oolong tea is unusual and depends on the degree of fermentation. The content of TFs is very low or absent, even in heavy-fermented oolong tea. Instead, TRs content is formed via the oxidation of EGC and its gallate, while secondary polyphenolic compounds such as oolonghomobis-flavane, theasinensin, and oolongtheanine are related to the infusion taste. The mellowness and sweetness of oolong tea come from the integrated taste of non-oxidized catechins, TRs, some secondary oxidative polyphenolic compounds, caffeine, free amino acids, and related sugars. Oolong tea has lower astringency taste and stronger sweetness taste than green tea, but the compounds responsible still need clarification.

3. Tea Cream and Tea Scum

Tea cream is a complex formed from theaflavin, TF-gallate, EGCG, ECG, TRs, caffeine, caffeic acid, gallic acid, ellagic acid, chlorophyll, and bisflavanol A, B. It is soluble in hot water and indicates the strength and briskness of black tea. The amount of tea cream is higher in black teas with TFs content greater than 0.4%, which produce a bright reddish brown infusion. However, this cream-down phenomenon may not occur in some black teas made from microphyll cultivars. Tea scum is an unsightly film that forms on the surface of tea infusions in hard water areas. It is amorphous, high molecular weight organic material together with calcium carbonate, and contains approximately 45 carbon atoms with hydroxyl groups, carbonyls, and some unsaturated groups. Calcium and sodium originate from water, while potassium, manganese, and aluminum come almost entirely from the tea leaf, and magnesium is provided in comparable amounts by both sources. Tea scum formation involves the aerial oxidation of tea solubles, probably the polyphenolic components such as catechins, theaflavins, and thearubigins.