Is black tea really 100% oxidized?


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Is black tea really 100% oxidized?

Postby mbanu » Oct 30th, '10, 20:57

I ask because I've recently stumbled across a book on tea processing from the 1950s that suggests that black tea can be fermented too long, which produces "soft, flat or dead tea without pungency, and gives liquors which are dull or brownish. The infused leaf is not bright."

Is this a case of old tea-processing superstition, or were older black teas really only partially oxidized? (Maybe in the 85%+ region?) What about today?
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Re: Is black tea really 100% oxidized?

Postby entropyembrace » Oct 30th, '10, 22:37

Some black teas...many Darjeelings notably are obviously not 100% oxidized...but they´re not made using oolong process either. So a tea can be classified as black without actually having been oxidized completely but I´m not really sure about the details.

But even in other black teas...Yunnans and Keemuns they don´t always seem to be any darker than the darker oolongs such as Oriential Beauty so they might not be fully oxidized either
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Re: Is black tea really 100% oxidized?

Postby Sirwill » Oct 31st, '10, 19:34

Entropye... wrote:Some black teas...many Darjeelings notably are obviously not 100% oxidized...but they´re not made using oolong process either.


Shedding some light on Darjeeling processing and how the leaves are a vast array of colors vs. just dark reds and browns;

This is a direct result of the processing that Darjeeling teas undergo, mostly due to the withering stage.

Withering is a process in which the raw tea leaves lose moisture to make the pliable for rolling and shaping, and it is a time which the chemicals in the tea leaf change. This results in increased levels of caffeine, organic acids and polyphenol oxidase, the enzyme responsible for oxidation.

There are varying levels of withering.

-Soft wither: the leaves lose only about 25-32% of their moisture content, usually employed in the production of CTC and low grown, full bodied teas.

-Medium wither: the leaves lose about 32-40% of their moisture content, used in the production of low grown, full bodied teas, but have a more pronounced aroma than in the soft withered teas.

-Hard wither: the leaves lose about 40-50% of their moisture content, used primarily in the production of high grown teas. There is a noticeable increase in aroma and flavor.

-Very Hard wither: the leaves lose about 70% of their moisture content, used in the production of extreme altitude teas, such as Darjeeling. The result of this extreme moisture loss is the inhibition of oxidation. The enzymes in the cells become too dry to bond with polyphenols when the leaves are rolled. This enzymatic reaction is what turns the leaves to a coppery color we see as black tea. In the case of Darjeeling tea, this process only occurs to a certain degree. In essence, the leaves are fully oxidized, but the variation of color is due to the fact that the chemicals cannot bond to further oxidation levels. This is what gives Darjeeling its leaf appearance, and profound flavor profile.
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Re: Is black tea really 100% oxidized?

Postby Darjeelingteaxp... » Nov 1st, '10, 09:05

Interesting topic indeed, and Sirwill mentioned some of the reasons very well. However, I would add that it is not just withering but the complete set of processes that gives Darjeeling its unique flavour. We have covered some of it here in an article on "What makes Darjeeling Tea so special" - darjeelingteaexpress dot com/darjeeling-tea-gyaan/what-makes-darjeeling-tea-so-special/

Moderator Edit: Link broken per forum rules located in Introductions.

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Re: Is black tea really 100% oxidized?

Postby legend » Nov 3rd, '10, 12:00

It important to make a distinction here about oxidation, fermenation, and the color of the unbrewed leaves.
To use the term oxidation is confusing because black tea, or properly red tea 红茶 hong cha, is processed by allowing the leaves to ferment (发酵fa jiao) in a large vessel, now does it actually reach 100% ? Well this is relative because there is a point that those leaves, because they are completely fermented,will start to spoil very rapidly. Therein lies the skill of crafting genuine red teas. (or any other tea in general)
The black color of the leaves does not come from this fermentation process per se. It is imparted just after when heat in various forms is applied to halt any futher deteriation from fermentation- heating over fire (烘焙 hong pei), in the case of red teas causes the color and you can observe the different ways they manipulate the leaves over the fire by noticing the different areas which are roasted on each leaf individually and each tea variety in general. This is true for red teas and the roasted varieties of wu long including wuyi yan cha 武夷岩茶 and dong fang mei ren
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Re: Is black tea really 100% oxidized?

Postby OchameTaiken » Nov 4th, '10, 03:48

Yes, black tea is really 100% oxidized. And I will vouch for Sirwill's statements regarding the hard withering which Darjeelings can undergo which will result in lower moisture content. Essentially, the tea leaves are still fully oxidized; whether or not we see the full red color of oxidized polyphenols, as when the juices are affixed to the leaf.

You're absolutely right, that a 2nd flush Darjeeling can closely resemble some of the complex characteristics of a Dong Fang Mei Ren. But it's not to say that the Darjeeling isn't fully oxidized, or that it should be categorized as an oolong.

But regarding your original question: does this book mean that some black teas could be less than 100% oxidized?
No; they're simply saying that the fermentation process was held out too long. There is a difference between the natural oxidation occurring withing the leaf, and the human-controlled "fermentation process" (the process of INDUCING that natural oxidation and getting all the juices flowing.) Both withering and fermentation require a delicate balance, and if the process is held out too long (or not long enough), it can ruin the tea. Especially since these stages build up heat in the leaf, which can influence flavors in a bad way.
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Re: Is black tea really 100% oxidized?

Postby beecrofter » Nov 4th, '10, 07:43

Ash is 100% oxidized.
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Re: Is black tea really 100% oxidized?

Postby entropyembrace » Nov 4th, '10, 14:44

If I am understanding correctly you are saying that Darjeeling does have unoxidized polyphenols and that the oxidation reaction is being limited by moisture reduction during the withering process. So the reaction cannot proceed further but the limiting factor is the moisture levels and not that all of the polyphenols present have been oxidized.

In the case of oolong if I understand the process correctly the limiting factor which prevents the oxidation reaction from proceeding is that the application of heat denatures the enzymes involved which also has the effect of limiting the reaction and preventing it from proceeding any further.

So in both cases there are un-oxidized polyphenols present and the only distinction is the method by which the reaction is being limited so that it cannot proceed any further.

btw I am not advocating that Darjeeling be classed as oolong because the limiting method serves as a clear distinction between black and oolong tea...even though the levels of un-oxidized polyphenols does not.
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Re: Is black tea really 100% oxidized?

Postby Sirwill » Nov 5th, '10, 10:11

Entropye... wrote:If I am understanding correctly you are saying that Darjeeling does have unoxidized polyphenols and that the oxidation reaction is being limited by moisture reduction during the withering process. So the reaction cannot proceed further but the limiting factor is the moisture levels and not that all of the polyphenols present have been oxidized.

In the case of oolong if I understand the process correctly the limiting factor which prevents the oxidation reaction from proceeding is that the application of heat denatures the enzymes involved which also has the effect of limiting the reaction and preventing it from proceeding any further.

So in both cases there are un-oxidized polyphenols present and the only distinction is the method by which the reaction is being limited so that it cannot proceed any further.


Exactly correct! =]
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Re: Is black tea really 100% oxidized?

Postby mbanu » Nov 6th, '10, 21:37

Thanks for the replies! Very interesting.

If it helps, here's a fuller transcript of what I was referring to. It comes from the 3rd edition (1955) of Johnson's Note Book for Tea Planters by Reginald J. Johnson:
Leaf which has been put out to ferment changes in colour, gradually acquiring a coppery hue, and developing the aroma associated with fermented leaf.

This change is accompanied by the uptake of oxygren and certain well marked chemical changes in the leaf as stated earlier. The thickness or body of the liquor varies with the quantity of soluble tannin in the leaf. Pungency and strength depend entirely on the tannin content and soluble tannin compounds in the leaf. In this connection it must be borne in mind that the longer the fermentation and the higher the fermenting temperatures, the less is the percentage of soluble substances in the leaf. As fermentation advances, the resulting liquor of the made tea changes from bright green to a very dull, dark-brownish infusion. The liquors become more coloury, eventually becoming thick and muddy, and gradually loose their astringency and pungency according to the length of the fermentation.

Teas made from leaf fermented for short periods of 1/2 to 2 hours give pale, thin liquors, raw or bitter to taste, and light and washy in body. The infused leaf is greenish. Assuming fresh leaf to contain 22 per cent. tannin, such tea may contain 15 per cent.

As fermentation proceeds, rawness gives place to briskness or pungency. The liquors become redder or thicker, and strength develops. Strength is apparently a combination of pungency and thickness. Such tea gives bright, red coloured infused leaf, and on cooling a precipitate, known as the cream, forms. At this stage (about 3 1/2 hours' total time inclusive of rolling) the tea contains about 12 per cent tannin.

Further fermentation produces a soft, flat or dead tea, with no pungency and gives liquors which are dull or brownish in colour. On cooling, a murky or muddy precipitate forms, and the infused leaf, instead of being the bright colour of a new penny, is the dull colour of an old penny. Although this tea contains only slightly less tannin than the strong tea fermented for 3 1/2 hours inclusive of rolling time, certain important chemical changes in the tannin have taken place, the exact nature of which is unknown at present.

In times when flavour is marked it is found that a shorter fermentation period than usual is necessary in order to make the most of this valuable property. Thus some of the liquoring qualities of the tea are sacrificed, but the flavour more than compensates for the loss. The fact that special seasonal flavours develop more quickly than the ordinary tea aroma associated with all black teas has already been mentioned. Indeed, it is considered by some observers that the substances responsible for special flavour are present in fresh leaf and do not require the process of fermentation for their development. Since these substances are very volatile, it is further considered that long wither or ferment especially at high temperatures, reduce flavour.

4. The best period of fermentation should be determined by experiment as this depends to a great extent on local conditions, the type of leaf and wither, and the rolling procedure adopted. The following points, however, should be borne in mind.

(i) A period of 3 to 4 hours including rolling time makes for the best degree of oxidisation of tannin and the fullest development of strength and colour and the natural flavor of the leaf as opposed to seasonal flavour.

(ii) Leaf fermented for too short a period (e.g. 2 1/2 hours including rolling) gives tea with liquors raw or bitter to taste and weak, light, thin or washy in body. The infused leaf is greenish.

(iii) Leaf fermented for longer periods (e.g. more than 4 hours including rolling time) and under conditions of temperature over 75F in a dry atmosphere with hygrometric difference in excess of 3F produce soft, flat or dead tea without pungency, and gives liquors which are dull or brownish. The infused leaf is not bright.

(iv) Correctly fermented leaf gives liquors that are brisk and pungent. The liquors are red and bright and the infused leaf is also bright in colour similar to that of a new penny.


I'm not sure how much of this is considered old-fashioned or out-of-date today... It does seem to be biased towards British-style black tea production. Does anyone have any suggestions for a more up-to-date English reference on tea processing?
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Re: Is black tea really 100% oxidized?

Postby Tead Off » Nov 9th, '10, 09:15

Sirwill wrote:
Entropye... wrote:If I am understanding correctly you are saying that Darjeeling does have unoxidized polyphenols and that the oxidation reaction is being limited by moisture reduction during the withering process. So the reaction cannot proceed further but the limiting factor is the moisture levels and not that all of the polyphenols present have been oxidized.

In the case of oolong if I understand the process correctly the limiting factor which prevents the oxidation reaction from proceeding is that the application of heat denatures the enzymes involved which also has the effect of limiting the reaction and preventing it from proceeding any further.

So in both cases there are un-oxidized polyphenols present and the only distinction is the method by which the reaction is being limited so that it cannot proceed any further.


Exactly correct! =]

I am currently in Darjeeling and asked this question today. The answer given to me is no, it is not 100% oxidized and I believe the above explanation is why.

In general, the term fermented and oxidized are not synonymous. They are different processes producing different results. Don't mean to be anal about it, but, we should use the proper terms if we are going to discuss teas. This was acknowledged today by 2 tea people here in Darj.

It's a lot of fun to see how many different teas this place produces.
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Re: Is black tea really 100% oxidized?

Postby Sirwill » Nov 9th, '10, 11:43

But wouldn't the leaves still be 100% oxidized since after the withering stage they can not be oxidized anymore physically?
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Re: Is black tea really 100% oxidized?

Postby entropyembrace » Nov 9th, '10, 15:35

Sirwill wrote:But wouldn't the leaves still be 100% oxidized since after the withering stage they can not be oxidized anymore physically?


If you think of it that way then green teas are 100% oxidized too.
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Re: Is black tea really 100% oxidized?

Postby Sirwill » Nov 9th, '10, 17:36

Entropye... wrote:
Sirwill wrote:But wouldn't the leaves still be 100% oxidized since after the withering stage they can not be oxidized anymore physically?


If you think of it that way then green teas are 100% oxidized too.


Although, black teas are only exposed to heat after the oxidation process is complete. With green teas, it is stopped before it even starts. There isn't much of a comparison because the processes are meant to produce totally different results.
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Re: Is black tea really 100% oxidized?

Postby entropyembrace » Nov 9th, '10, 21:15

Sirwill wrote:
Entropye... wrote:
Sirwill wrote:But wouldn't the leaves still be 100% oxidized since after the withering stage they can not be oxidized anymore physically?


If you think of it that way then green teas are 100% oxidized too.


Although, black teas are only exposed to heat after the oxidation process is complete. With green teas, it is stopped before it even starts. There isn't much of a comparison because the processes are meant to produce totally different results.


But in green tea the oxidation reaction cannot proceed anymore physically either...if it could then your green tea wouldn´t stay green very long.

The method and timing by which the oxidation reaction is stopped is different but in both cases the reaction is prevented from proceeding.
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