Oxidation refers to a group of chemical reactions that consequence throughout the browning of tea leaves and the manufacturing of style and aroma compounds in accomplished teas. Counting on the type of tea being made, oxidation is each prevented altogether or deliberately initiated, managed after which stopped.

A variety of the oxidation course of revolves spherical polyphenols, notably the enzymes polyphenol oxidase and peroxidase. When the cells inside tea leaves are damaged and the elements inside mix and are uncovered to oxygen,1 a chemical response begins. This response converts the polyphenols commonly known as catechins into flavanoids known as theaflavins and thearubigins (which can be moreover polyphenols).

Theaflavins current tea with its briskness and vibrant type along with its yellow color,2 and thearubigins current tea with depth, physique and its reddish color.3 Chlorophylls are remodeled to pheophytins and pheophorbides (pigments that lend to the black/brown color of dry oxidized tea leaves) all through oxidation, and lipids, amino acids and carotenoids degrade to provide a couple of of tea’s style and aroma compounds. Tea producers use specific methods to impress, halt (restore), and even forestall oxidation with the intention to produce completely totally different flavors in a accomplished tea.

Initiation of Oxidation

Oxidation begins when the cell partitions inside tea leaves are damaged. To comprehend cell hurt, tea producers macerate, roll or tumble tea leaves to intentionally provoke oxidation. Maceration is the quickest path to full oxidation because of the insides of the leaves are immediately uncovered to oxygen, resulting in a better mixture of the chemical substances inside.

Maceration is usually utilized in mass manufacturing methods to create CTC (decrease tear curl) tea or totally different broken leaf teas. The tactic is achieved using a rotorvane or a CTC machine. Rolling ends in a rather a lot slower and gentler oxidation, and it is usually accomplished using a rolling desk or by hand.

Tumbling is a good gentler methodology to impress oxidation, and it is achieved using large cylinders whereby the leaves are tumbled or by hand-shaking the leaves on prime of a shallow bamboo basket. Regardless of the methodology of initiation, good care must be taken up as far as any hurt to the leaves sooner than processing will set off premature oxidation and result in an inconsistently processed accomplished tea.

Administration of Oxidation

Administration over oxidation is maintained by introducing warmth, moist, oxygen-rich air over time. The extent to which oxidation is allowed to occur has an astounding influence on the finished tea. Oxidation occurs most interesting between 80–85°F and is slowed virtually to a halt at 140–150°F.4 Thus, when the tea producer must halt oxidation, they heat the leaves. This heating course of is known as fixing.

Fixing or Halting Oxidation

Fixing works by denaturing polyphenol oxidase and peroxidase – the enzymes primarily accountable for oxidation using heat. Fixing will be commonly known as de-enzyming,5 denaturing or kill-green.

The time interval kill-green is derived from the Chinese language language time interval shaqing (杀青), which interprets to killing the inexperienced. The tea leaves must be heated to roughly 150 ranges Fahrenheit to “halt” oxidation. Oxidation is extra slowed by drying the leaves, but it surely certainly in no way totally stops. At temperatures over 150 ranges Fahrenheit, oxidation continues to occur at an particularly gradual tempo.

The tactic of fixing requires precise administration of the temperature and measurement of heating; each have to be adjusted counting on the size and thickness of the leaves and the amount being processed.

Commonest fixing methods

  • Pan Firing: the place tea leaves are heated in an enormous metallic pan or wok that is heated by gasoline or picket fire6
  • Steaming: the place steam is pressured through the mass of tea leaves7
  • Tumblers: the place a heated tumbler is used to heat the leaves8
  • Baking: the place an oven variety machine is used to heat the leaves9

A lot much less widespread fixing methods

  • Photo voltaic drying: the place the heat of the photo voltaic denatures the enzymes throughout the leaf by dehydration
  • Microwaving: the place electromagnetic waves are used to shortly heat the leaves, seen further in enterprise functions
  • Plunging in boiling water: the place tea leaves are literally plunged into boiling water

When oxidation is prevented altogether, the catechins are left largely intact. The tea leaves keep their inexperienced color and vegetal traits. Think about an apple: as quickly because it’s sliced open it shortly turns brown, nevertheless the apples in apple pie aren’t brown.

It is as a result of the heat used to bake the pie denatured the polyphenol oxidase and peroxidase throughout the apples and prevented enzymatic browning. The an identical browning influence takes place in potatoes, avocados, bananas, and so forth.

When a semi-oxidized tea is being produced, some catechins convert to theaflavins and thearubigins, resulting in a slight browning usually alongside the edges of the leaves and yellower liquor. Lipids, amino acids, and carotenoids moreover begin to interrupt down into style and aroma compounds.

When oxidation is allowed to run its course, the leaves exhibit an aroma and elegance profile totally unrecognizable from a accomplished tea that was exempted from oxidation.

Theaflavins and thearubigins will now outnumber catechins, resulting in a brisk tasting tea with a reddish color throughout the cup. The chlorophylls may have been remodeled to pheophytins and pheophorbides, turning the leaves a coppery brown, and a myriad of latest volatiles may have developed. On this case, the leaves are generally merely dried to halt any still-occurring reactions and to ship the tea to a shelf-stable moisture stage.

It’s a little little bit of a grey house in tea processing because of proper right here, drying might be thought-about a kind of fixing. Heat is getting used and oxidation is being halted. It’s a good occasion of why it is typically essential to view tea processing as further of a continuum moderately than as a particular set of steps. Regardless, there’s further to drying now we have to look into.

Notes and Sources:

  1. Notably when polyphenols throughout the cell’s vacuoles and the peroxidase throughout the cell’s peroxisomes mix with polyphenol oxidase throughout the cell’s cytoplasm. From Michael Harney’s 2008, “The Harney and Sons Data to Tea.”
  2. In Latin, flavus means yellow.
  3. In Latin, rubugin means crimson.
  4. R.P. Basu and M.R. Ullah, “Notes on Tea Fermentation,” Two and a Bud 25, 1 (1978), 7–11.
  5. In semi-oxidized teas, significantly wulongs, a normal descriptor used is “share of oxidation.” Often, this can seemingly be an estimated share besides the tea is produced on a commodity scale the place chemical assessments are carried out to reveal the exact share of oxidation by way of catechin or theaflavin and thearubigin content material materials.
  6. In China, that is called chao qing (炒青), truly, “firing the inexperienced.”
  7. In China, that is called zheng qing (蒸青), truly, “steam inexperienced.”
  8. In China, that is called sha qing ji (杀青机), truly, “kill inexperienced machine.”
  9. In China, that is called hong qing (烘青), truly, “baked inexperienced”