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Wednesday Wisdom with Michelle & KC : Week 3 : Withering & the development of flavour in tea

Welcome tea lovers to our next episode of Wednesday Wisdom.  
What's it all about?  You can read more on the background to this blog in our first post but in short every week I come together for an hour of discussion with KC from Ambootia and then distill the points that interest me into a blog for you to read & enjoy.  I always start this blog by saying that in the tea world and indeed the wider world we are all students.  My aim in sharing these interests is not to be 'right' indeed in many cases I expect to return to these topics and add more details - it is merely to ignite thought, conversation and interest in these fascinating topics that sit behind the cup of tea we all enjoy.  Feel feel free to add, comment and get involved!  I hope you enjoy the journey!
Today takes us from the tea fields into the factory as we explore processing with a focus on withering & the development of flavour in tea.  I hope you enjoy!

Withering & the development of flavour in tea

KC : 'withering is the most important part of tea processing.  It sets the quality of the tea and is a CQP - a critical quality point - one that cannot be reversed.  If a mistake is made then it cannot be undone so careful consideration is needed in the approach'.  We join the leaf as it has been plucked - it is essential that the time from the garden itself to the withering beds is as short as possible - [in the case of Ambootia a maximum limit of 20 minutes is set from even the furthest reaches of the garden]  Handling of the leaf in these early stages is critically important : this 1995 paper focused on Malawi but looking at 'Leaf Handling' concluded : leaves should be not be compressed at any stage during handling and should never be allowed to heat up to 40°C. Acceptable tea quality should result if leaves reach the factory within 5 h of plucking and their temperature remains below 35° at all times.  While standards will be different in different factories and in different parts of the tea world this highlights the importance of careful leaf handling.  Indeed this same study concludes that 25% of the market value of tea can be lost before green leaf arrives at the factory door if it is poorly handled whilst in the field and during transit.  We will return to this topic at the end of this blog.  

After this step we enter the withering rooms and place the leaves in the withering beds.  But hang on - we cannot simply put all the leaves into the withering beds - where the leaves have come from must be taken into consideration.  Back to KC 'you cannot take leaves from a Light pruned [LP] tea bush and put these together with leaves from a unpruned tea tree.  Why?  Well a light pruned tea bush will have larger, more fleshy and moisture-full leaves.  The leaves from the unpruned bush will have small leaves and the juices will be more concentrated.  You want even-ness in the wither and if the material in the withering trough is not even then this is impossible to achieve'.  So as KC shares with me 'there is much talk about 'fine' plucking but it is actually 'even' plucking that is the biggest catch point.  Unequal plucking has an impact right down the line and will ultimately end up with an uneven tea.  Pluck skillfully and evenly' says KC.  Let us also dwell a little on the previous topic of leaf handling - any damage or mis-handling of leaves that has occurred at these early stages in leaves may cause premature critical changes and also lead to uneven withering

So what is withering?

I like this definition from this paper which I will cite in the next stage of this blog 
'Withering or partial desiccation is the first important step for improving the quality of final tea......[..]....The freshly harvested leaves are conditioned physically and chemically for the next operation During this period, the shoots partially lose moisture and the turgid shoots become flaccid. The rigid leaf becomes flexible and the shoots can now be twisted or rolled without breaking or becoming damaged into pieces...[...]... Ultimately, this activity catalyzes the oxidative reaction, which is mainly responsible for the quality of the product [...] Withering brings about biochemical changes in tea leaves. To acquire the improved aroma, flavor, and other benefits in black tea, the proper withering of leaves is exceedingly important [...] 

This requires a lot of space and withering rooms are very large with long troughs stretching from one end of the room to the other often with large fans underneath [you can also have open air withering although in that case the controls that we go on to mention below will not be able to be applied] Tea is then spread thinly on these 'trays' to allow free air access to the leaves.  

'Tea is the art of managing AIR' : KC

'Tea is the art of managing Air' shares KC.  'People think that air is free but in the world of tea it is one of the most costly materials' 'You need ambient dry air to pick up moisture.  As a general rule you need for every square foot you need 40 cubic feet of air per minute - so the tea maker must make complex calculations based on the size of the troughs and amount of leaf.  Holding capacity of troughs do vary – in Darjeeling it is customary to accommodate 1kg GL/ sqft of trough area while in Assam its often 1.5 kg for making Orthodox and 2kg for making CTC' 

Over time all the tea makers I know tell me that these decisions just become second nature but of course only after a huge amount of study, trial and error and dedication.  Tea makers are always the most humble of people.  
We need to remember that tea growing areas have a lot of moisture in the air - these will further moisten the dry leaves.  So you need to mix dry air with the incoming ambient air to create an environment which will facilitate the take up of moisture from the leaves.  In closed building, air is circulated by fans; drawing air from bulking chambers. Humidity is reduced by mixing with hot air - 'but in the case of warm air' KC tells me 'the temperature of the combined bulk and ambient air must be below 80F - you don't want to lose too much moisture while preserving essential oils in tea leaves that contribute to aroma and flavour developments' We are starting to see how, far from simply placing leaves in a trough, withering is an art

Let us consider the two types of withering that are occurring :-

1. Physical Withering : physical changes

The primary aim of withering is to reduce the moisture content of the leaf so physical withering is as you may expect : the loss of moisture from the leaves.  Remember - the next step after withering is rolling so the leaf needs to lose its turgidity and become more pliable, rubbery or flaccid in order to move into this part of the process. As KC explains this is the point at which the sap in the cell leaf is becoming concentrated.  Time, temperature and humidity are all important factors.  

2. Chemical Withering - chemical changes

This may be less familiar to us than the 'physical withering' but chemical withering or 'chemical changes in the composition of the leaf' is a process that starts directly after plucking.  If you walk into a withering room you will often pick up a distinct aroma - this is because the biochemical process which results in the tea aroma starts at withering [see later].  In fact some tea makers will use their sense of smell to determine when a tea is reaching its optimal withering time.  Chemical withering is independent of of the rate of moisture loss and therefore may take place at a different rate.  The tea maker therefore has to make a calculation about how much time to allow for the physical wither in order that the optimum chemical wither can be achieved.    Chemical withering is a complex topic with many biochemical changes best summed up by this short extract - plenty more reading here through the linked papers if you are interested to pursue one line or another.  These are topics that KC and I will delve into more in the future

Biochemical changes during chemical withering

Chemical withering plays a vital role in increasing the level of caffeine content (Sanderson, 1964), polyphenol oxidase activity (Ullah and Roy, 1982), amino acids, and sugars (Owuor and Orchard, 1989); Tomlins and Mashingaidze, 1997). Chemical withering results in the reduction of chlorophyll content (Wickremasinghe, 1975) and leads to the formation of different kinds of volatile flavor compounds (Mahanta and Baruah, 1989; Ravichandran and Parthiban, 1998). The breakdown of proteins into amino acids during chemical withering is one of the most important biochemical changes occurring during the process (Hussain et al., 2006).


In conclusion this is a critical step in black tea making and has a strong relationship with temperature & time two words you will commonly hear tea makers refer to - we will return to this at the end of the blog.  

How much Withering?

You may have heard the term 'hard wither'.  In Darjeeling during the first flush you will often hear tea makers refer to a 'hard wither'.  In KCs words 'you need more moisture loss and the leaf can take it' : this concentrates the aroma.  Lets pause to explore this.  We have previously discussed how as you go higher into the hills the leaves become smaller - there is less moisture in the soil the plant has to put the moisture it has somewhere in order to conserve it.  Therefore you have smaller thicker leaves.  Compare this to a tropical region like Assam where moisture is all around and the plant will have fewer larger leaves - here moisture retention is less of an issue.  'To bring this to life a little more consider that 1 bush in Darjeeling may have 1000 small leaves compare to a bush in Assam with 400 larger leaves' shares KC.    So, back to withering - the small thick leaves from the first flush in Darjeeling can take a harder wither due to the moisture being trapped or stored deep inside the thicker leaves.  Later in the season during the monsoon moisture is high and the wither must be adjusted.  In Assam a different approach is taken - leaves are larger and thinner and the wither must be adjusted accordingly.

Getting the withering just right is critical.  For example under-wither [with regards to the physical withering] and the presence of too much moisture can result in the wet leaves clogging machinery or being thrown out of the rollers.  'Too much moisture will result rapid and uncontrolled oxidation wich eventually results blackish teas with watery cups' explains KC.  Over wither and the hard leaves can blunt the roller with the knock on effect of too much fibre in the leaf after rolling.  This must be finely balanced with the chemical wither which is all about increasing quality :  a longer wither allows the development of more aroma and flavor compounds in the leaves. However too long and the leaf will become dehydrated and important chemical activity will cease - the potential of the tea lost.  

Ready to Roll? 

So the withering has prepared the leaves for rolling by making them flaccid and permeable to juices.  Their moisture content has dropped to about 70% and they are able to be rolled without damage to the delicate leaf shoots.   'To give you something to visualise if you start with 100kg of dry leaf then after withering you will have around 32kg left' - shares KC.  'As you roll the cell leaves you 'lacerate' the cell walls and the juices ooze out and envelope the tea leaf - the tea leaf is essentially engulfed by the juice and it is these juices that will get oxidised in the fermentation' stage : how well this is expressed will depend on the condition of the leaf after withering'

There are a number of variables in rolling : you can vary the pressure from light to hard and decisions are made by the tea maker based on a number of factors including but not exclusively the type of leaf, type and quality of machinery, temperature, wither and of course the type of tea that is desired.

Fermentation [known to us as oxidation]

A note on Fermentation : you will often see the term 'fermentation' used when talking about the black tea industry in India.  As we share at the Tea House this is more accurately described as oxidation and refers to 'enzymatic oxidation of the polyphenols present in the leaves'  This paper describes very well much of what we are to talk about next especially in terms of the development of flavour in tea

'Young tea shoots are rich in different kinds of polyphenols of which flavonols (catechins) are the most abundant.  During rolling the cellular integrities of the leaf is disrupted and the catechins and the enzymes (e.g., polyphenoloxidase and peroxidase) that present in different compartments within the cell are mixed up. This results in a series of biochemical reactions that lead to the formation of oxidized polyphenolic compounds such as theaflavins and thearubigins. So catechins along with their oxidation products are responsible for the sensory characteristics of black tea' paper

Theaflavins & Thearubigins

'Theaflavins are responsible for the astringency, brightness, colour (yellowish) and briskness of the black tea. Thearubigins (TR) formed during fermentation contribute to the mouthfeel (thickness) and colour (reddish brown) of the tea Therefore fermentation time is very important in maintaining the TF/TR ratio' paper

So during fermentation some of the most important characteristics of tea develop.  Turning to the topic of TF & TR KC explained to me how 'If one goes up the other one drops, you have to balance them.  If you allow extended fermentation you start to produce a little alcohol and this can manifest as a fruity over fermented profile.  If you under-ferment then the flavours are not fully developed and you can get quite a watery tea'   This can be explained by the fact that many of the qualities of tea are derived from the same group of chemical compounds
'excessive production of one property will naturally take place at the expense of another. Briskness, quality, strength and colour change with time and temperature during fermentation and each character is at its best at different times.  Only optimum fermentation [beyond which the quality begins to decline] ensures strength, brightness, briskness and quality of the liquor'

It is most common to discuss the subject of the tea flavanols TF and TR at the fermentation stage of tea processing but we can also briefly visit the topic at the point of withering.  You remember that we discussed aroma and its relationship with chemical withering earlier - as we know during withering when moisture is removed enzymic activity increases and consequently oxidative reactions take place just as they do during 'fermentation': catechin content decreases as they get converted to theaflavins and further to thearubigins exactly as we discussed above.  Therefore theaflavins (TF) and thearubigins (TR) are also formed at this stage.  

Knowing that these key flavour compounds are being formed so early on in the processing brings us full circle back to the fact that the profile of your final cup starts to be determined way before this latter stages of processing .  We discussed leaf handling at the start of this blog and if we now link this to flavour compounds we start to gain a deeper understanding.  For example there is evidence to suggest that TF converts rapidly to TR at high temperatures.  Consider then, right at the start of the process when leaf is handled  and packed.  Studies have show that 'compacting the leaf at this stage during handling and packing increases the leaf temperature and this appears to be followed by significant decrease in TF content. After plucking, if the leaves are kept in a basket for a long period, the degradation of TF to TR increase'  Taking all of the above into consideration and having an understanding that TF is responsible for 'astringency, brightness, colour (yellowish) and briskness of the black tea' it therefore makes sense that these actions right at the start of the tea processing  may have adverse effects in these areas.  
There is so much more to explore on this topic and I know many of you may be left wondering what the optimal ratio of TF:TR is in order to deliver the perfect cup of black tea.  Rest assured much work has been done around this and much still remains unknown.  There is some interesting reading in this paper but I think this may be a topic for another day!  Ill leave you to ponder the figure below from this paper in advance of further discussions!  Happy Wednesday all!

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