Getting to Know Your Tea Terms: Fermentation

The debate over whether black teas and oolongs are oxidized or fermented came up the other day. We had looked at the term “oxidation” awhile back here, so I thought that it was time to address the term “fermentation” and see if it can shed some light on that debate or not.
Actually, I see fermentation mis-applied to oolong teas and other non-fermented teas. While there may be a minor amount of fermentation occurring, in my humble opinion, oxidation is the primary process involved here. I think there is a big difference here and these terms are used too interchangeably. But I’m open-minded and also wanted to see how others defined these terms.
Part of the confusion is probably due to Westerners calling fully oxidized tea by the term “black tea” instead of what the Asians call it (“red tea”). What we call “fermented teas” are the true black teas.
The best known fermented teas, according to Wikipedia:
  • Dark tea (Chinese: 黑茶; pinyin: hēi chá)
  • Pu-erh tea (Chinese: 雲南普洱茶; pinyin: yúnnán pǔ’ěr chá)
  • Liu’an tea (Chinese: 安徽六安籃茶; pinyin: ānhùi lìu’ān lán chá)
  • Liubao tea (Chinese: 廣西六堡茶; pinyin: guǎnxī lìubǎo chá)
  • Hunan dark tea (Chinese: 湖南黑茶; pinyin: húnán hēi chá)
  • Laobian tea (Chinese: 湖北佬扁茶; pinyin: húběi lǎobiǎn chá)
  • Kangzhuan tea (Chinese: 四川康砖茶; pinyin: sìchuān kangzhuan chá)
  • Bian tea (Chinese: 四川邊茶; pinyin: sìchuān biān chá)
  • Pha tea (Chinese: 发茶; pinyin: fā chá)
So, what is “fermentation”? Some input from a recent discussion on Facebook:
  • “…fermentation had deeper meaning to the process which has not changed with the changed nomenclature…we never heard this percentage system and how was this calibrated… Fermentation also happens naturally as oxidation and tea processing is never oxidation alone…I can bet my life on it…”
  • “… ‘oxidation’ is the natural breakdown of the leaf, whereas ‘fermentation’ requires external elements (microbes). However, someone recently pointed out that natural oxidation requires microbes, that a tea leaf would not degrade at all if it were in an entirely sterile environment.”
  • “…Taiwan is also processing some tea with minimal oxidation and rolling but calling it “green tea” though technically oolong.” [This comment introduces the further issue of when do you cross that oxidation/fermentation threshold from green tea to oolong.]
I scoured the internet for what others were saying, sorted the wheat from the chaff (a lot of both) as the saying goes, and ended up with these:
  • “Fermentation is the process during which the Polyphenols in the tea leaf are oxidized in [the] presence of the enzymes and subsequently condensed to form Colored compounds contributing to the quality attributes of tea.”
  • “…The term fermentation when applied to tea is something of a misnomer, as the term actually refers to how much a tea is allowed to undergo enzymatic oxidation by allowing the freshly picked tea leaves to dry.”
  • “Tea fermentation is the process in which tea leaves are allowed to dry and thereby undergo enzymatic oxidation.”
  • “Fermented tea, often called kombucha tea, is common in countries like China, Russia, Korea and Germany where it is known for medicinal purposes.”
  • “Oolong or wulong tea fermentation is a process of oxidation. Oxidation is a more appropriate term, since fermentation suggests a lack of oxygen. Black tea is fully oxidized. Green tea is un-oxidized. Fermenting oolong tea involves exposing tea leaves to oxygen and pan-roasting them.”
  • “The green tea fermentation actually begins as soon as the Camellia sinensis leaves are plucked. This is because tea leaves start to brown, wilt and oxidize once they have been picked as its necessary chlorophyll diminishes and the tea leaves release their tannins. … The process of fermenting tea is actually called enzymatic oxidation, as it is an aerobic process, meaning that the leaves require air in order to ‘ferment.’”
  • “Often incorrectly referred to as ‘Fermentation.’ Oxidation is a chemical reaction that occurs in the presence of an oxidizing agent such as the oxygen in our atmosphere. (Fermentation, on the other hand, is an anaerobic metabolic process which results in the production of ethanol. This process DOES NOT occur in the production of tea.)”
  • “Black tea fermentation is essentially an oxidation process. After the plucked tea leaves are treated by series of processes called withering (removal of moisture by air flow), pre-conditioning and CTC (essentially maceration and cutting of leaves), the leaves are subjected to the process of fermentation by exposing them to air by laying the cut tea leaves on floor, trough or moving conveyor under controlled temperature, humidity and air-flow conditions.”
  • “Fermentation of black tea is the series of chemical changes that happen under the assistance of enzyme during making process, mainly refers to the oxidization of polyphenols. Fermentation is the key process determining black tea’s quality. It promotes the oxidization of polyphenol in the tea leaf with the help of enzyme; meanwhile other chemical substance will change, too, making the green tea leaves into red color. The unique aroma and flavor of black tea will then be formed.”
  • “Tea is often described as either fermented, semi-fermented or not-fermented. The term when applied to tea refers to oxidation, not conversion of sugar to alcohol as in wine. Wine and beer are fermented through the addition of bacteria or yeast to grapes or hops. Tea fermentation occurs when the leaves are exposed to air.”
  • “Fermentation in teas is somewhat different than the typical fermentation process that is substances such as wine undergo. The tea fermentation process allows the leaves to transform through enzymatic oxidation while drying.”
Based on the above, it seems that “fermentation” is being used as an equivalent to “oxidation,” possibly due to decades or even centuries of processors using the former over the latter. It’s a lot like the word “liquor” which is misnomer since there is no alcohol in tea (and therefore no real tea “drunkenness”).
One site lists 5 factors involved in successful tea leaf “fermentation” (reworded here for brevity and clarity) that sounds more like “oxidation”:
  • A good temperature for fermentation is 2 to 6°C above normal room temperature, with 30°C being ideal (meaning that the room temperature should be at about 25°C).
  • High humidity (at or above 95%) is better for fermentation, assuring more complete and even darkening of the leaves.
  • Ventilate the fermentation room to assure sufficient oxygen for the chemical changes and to remove the carbon dioxide produced during fermentation. An exhaust fan on the wall of the fermentation room is one method, or you can open the doors and/or windows (you may not want to leave them open all the time but open and close periodically).
  • Lay out the leaves in bamboo trays (8 to 10 centimeters thickness is typical), and set them in the fermentation room. If layered too thickly, the leaves will not get enough fresh air and will warm too quickly; if laid too thin, they will cool too easily.
  • Time is critical since fermentation begins from rolling. How long it takes from there depends on the season – 3 to 5 hours in Spring or when the outside temperature is lower and 2 to 3 hours in summer and autumn or when the outside temperature is hotter.
I would have to conclude that the term “fermentation,” other than when applied to processing pu-erhs, kombucha, and the few others listed at the beginning of this article, is being used where “oxidation” is meant. In fermentation of tea leaves, they are exposed to microflora, humidity, and oxygen in the air which may also produce some reactivated oxidative enzymes in the leaves. This process alters the aroma and flavor of the tea, mellowing it and turning them from being astringent or bitter into ones with pleasant mouthfeels and aftertastes.
Hopefully, this article has shed some light on the subject rather than further confusing the issue.

About Janice and Stephen Shelton

Purveyors of Premium Teas
This entry was posted in Tea Info for Newbies and Up and tagged , , . Bookmark the permalink.

One Response to Getting to Know Your Tea Terms: Fermentation

  1. Pingback: 5 Observations About Aged Teas | Fine Tea Focus

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