What is Astringency?
Astringency is often confused with tastes like bitterness when in fact it is actually a feeling rather than a taste (although as we will see some tastebuds may be involved in stimulating astringency).
Astringency comes from the Latin 'adstringere' meaning “to bind fast” and refers to any chemical which causes body tissues to shrink.
In the mouth, astringency is that drying, puckering sensation that comes from all kinds of food and drink, from Tea to wine and fruits to dark chocolate.
Is Astringency in tea a bad thing?
No! Astringency is a key part of the enjoyment of tea. It adds structure and physicality to the experience. A tea without any astringency ends in a short way, it reduces the three-dimensional experience of tea drinking to a 2D taste.
Tea drinking is about incorporating smell, aroma, taste, temperature and physical sensation into a complete experience. Without any astringency, the sense of physicality of the tea stops after swallowing. With astringency, the physical experience continues many minutes after your last sip and that dryness motivates you to take another sip. Astringency adds dynamism to tea drinking, it gives the experience change, shape and movement like any good piece of music or filmmaking.
Of course, people have different preferences about the level and types of astringency that they enjoy but it seems to be a pretty much universal trend that the more tea you enjoy the more you appreciate astringency.
How do we sense Astringency?
This is where things get a little complicated and the truth is that the science behind astringency has not been completely revealed.
On commonly told misconception in both the tea and the wine world is that astringency is solely caused by mechanically making your saliva less lubricating. The theory goes that the polyphenols in tea, wine or any other foods bind with proline-rich proteins in your saliva making insoluble compounds and reducing the slipperiness of your saliva.
This is probably not the truth or at most, is a small part of the cause of astringency. Studies have shown that some chemicals that we all find astringent actually do not bind to saliva proteins and most people agree that you can feel astringency even if you do not move your tongue around your mouth. So this mechanical explanation of astringency is incomplete.
Studies on astringency have revealed that it is more likely that astringency comes from chemical receptors in the mouth which are part of the trigeminal system. This is the system which detects physical sensations like touch and temperature. The likelihood is that astringent chemicals like polyphenols activate G-coupling protein receptors on the cell membranes of the mouth which trigger G Proteins to turn on and send messages via the trigeminal nerve which the brain interprets as astringency. The most likely receptors to create the feeling of astringency are TRPV1 which is a receptor associated with experiencing extreme heat like chilli or perhaps TRPA1 which is associated with extreme cold like mints and is more often felt like a catch in the throat when you have some olive oil or a particularly cooling Hui Gan PuErh.
So why do chillis and mints not give us a sensation of astringency, well some scientists believe that the requirement for astringency is that these trigeminal receptors have to be stimulated at the same time as certain taste buds – specifically tastes like bitter which is why the words ‘bitter and astringent’ often go together in tasting notes.
Therefore, the feeling of astringency is probably created by a combination of systems – chemical signalling via your trigeminal system and taste buds and perhaps a reduction in lubrication in the saliva.
While we do not understand everything about astringency, the take away point here is that it is not sensed by a single off/on switch but a multitude of switch combinations. I believe that this is possibly why there are noticeably different types of astringency in tea appreciation in terms of where in the mouth you feel it, how long it lasts and if it is associated with a flavour.
Which chemicals cause astringency?
In general, the group of compounds which mostly cause astringency in food is Polyphenols. These are compounds produced by plants (and some animals). They are used by the plants to protect themselves from UV radiation, provide pigmentation, prevent microbial infections, control growth as well as other important processes.
Tannins are a subset of Polyphenols and are often mistakenly talked about in tea. Tannin comes from German word ‘tanna’ which refers to an Oak or Fir tree, and tannins originally refers to the use of wood pigment polyphenols to tan leather. The tannins in wine come in part from the seeds and stems but are added to by ageing in wooden casks.
Tea leaves actually contain very few true tannins compared with the main polyphenol compounds in tea called Catechins. These are classed as pseudo tannins. Another misconception is that tea contains tannic acid (a specific type of tannin extracted from a few plants which is often used in the wine industry).
In tea, there are many types of catechins in varying amounts depending on a variety of factors. As a tea oxidises the Catechins are converted into Theaflavins and Thearubigins. These have an orange to red pigmentation and contribute to the colour of tea.
Astringency studies show that the Catechins are more astringent than Theaflavins and Thearubigins. To add further depth, the different types of Catechins have varying levels of astringency so, for example, EGCG and EGC are more astringent than C or CG.
You don’t have to learn all of this but the take-away points here are:
1. Astringency is mostly caused by Polyphenols
2. Catechins in lighter teas are more astringent than the Theaflavins and Thearubigins in more oxidised tea.
3. There are many types of Catechins and Theaflavins which have varying levels of astringency so the levels of each compound in each sip of tea will determine the level and type of astringency you experience.
Why do we feel astringency?
I am fascinated by the evolutionary reasons for taste like bitterness and I wanted to learn more about the reasons for astringency. Unfortunately, there is not a lot of research out there so this is my very unproven reasoning.
Polyphenols are produced by the plant to protect itself and assist its survival. It does not want you to consume them and therefore it makes them astringent so as to be undesirable to eat by any animals including us. An unripe fruit is intentionally astringent because the seed is not mature enough to create another plant and the plant wants us all to wait before eating. Once the seed is ready the plant reduces the astringency and the fruit becomes more palatable.
Therefore, it might be that astringency is a plant’s warning signals in the same way as bitterness to avoid being eaten. There is an evolutionary advantage for us to sense astringency and be cautious as we have learnt that these astringent compounds are very biologically active and are not intended by the plant to be eaten. Therefore caution should be taken because these compounds could be either a medicine or a poison (or both).
A baby will never willingly eat bitter or astringent foods but as we grow we learn which bitter or astringent foods are medicinal and safe and over the years we evolve a love for the taste and sensation of things like wine, cocoa, beer, coffee and tea. In other words, we sense astringency and bitterness in order to alert early caution that these are biologically active compounds and plants use these sensation to try to avoid being eaten!
Ten Factors which affect Astringency
These are ten factors which probably contribute to the astringency of your tea. Of course there are others such as soil, weather, ageing etc but these are the main factors for you to consider.
As we said earlier, the main contributing compounds to astringency in tea are polyphenols and the ones which are most astringent are Catechins. Therefore, contrary to the popular misinformation out there, unoxidised teas like Green tea are more astringent than darker teas like oolongs and black tea.
I think that the reputation for black tea being the most astringent largely comes from the fact that most people have been brewing it ‘wrong’ in the form of small particles and for extra long brewing times. If you take the same weight and leaf size of a green and black tea and brew with the same parameters you will find that green tea is more astringent than black.
One important note though is that, as we know, astringency is caused by a combination of receptors working together. Black tea will more likely stimulate the ‘hotter’ receptors with its cocoa bitter notes and therefore the experience of astringency will be different than that of Green tea.
Aside from the oxidation phase, the level of heating or roasting is traditionally used to control astringency. The heavier a tea is heated or roasted the lower the Catechin content and therefore this traditional knowledge is backed up scientifically.
CONCLUSION: In general the level of astringency is closely linked to the level of oxidation. The darker the tea the less astringent it will be when brewed in the same way. The flavours in the tea will affect our perception of the astringency. Heating and roasting tea reduces astringency through the lowering of polyphenol levels.
2. LEAF SIZE
The smaller the leaf size, the more surface area for extraction which means a stronger brewed tea. This means that more polyphenols are released which will increase the astringency. Whole leaf tea is more controllable as you can work with other parameters to dial in the perfect level of astringency whereas broken leaves or tea bags will extract much more quickly giving you less control of astringency and flavour.
CONCLUSION: Smaller and broken leaves will have more astringency than larger and whole leaves.
The temperature of the water is key to controlling astringency. The different compounds in the tea leaves extract at different speeds at different temperatures. Catechins, for example, extract slowly at cool temperatures but when the water is boiling hot they will extract very quickly. This is why you are always advised to brew Green tea at lower temperatures – to control bitterness and astringency.
CONCLUSION: As you increase water temperature you are increasing the level of astringency in the cup. Control temperature to find the perfect level of astringency for you.
Different tea varieties have a different balance and level of polyphenols. There has not been enough research in the multitude of different varieties. In general, popular opinion is that Assamica tea has more astringency than Sinensis small leaf varieties, although I believe that the type of astringency experience is different between these two main varieties (I think Assamica has more of a dry, transforming to juicy astringency compared with a more simple astringency in Sinensis but this is based on experience).
CONCLUSION: Tea variety has an effect on astringency although there is not enough data comparing different varieties grown and processed identically to make any further statements.
Very sparse data shows a correlation between altitude and astringency. Higher altitude Ceylon tea has been shown to have higher astringency than lower altitude. This is what we would expect because higher altitude means slower growing and more requirement for protection from the suns UV and so we would expect higher polyphenol levels.
CONCLUSION: Not enough data but there may be a correlation between altitude and astringency.
6. PICKING SEASON
Not enough data but a study did show that Darjeeling had slightly higher levels of astringency in first flush followed by autumn flush and lowest was second flush. The differences were not much at all.
CONCLUSION: Not enough data to assess.
7. PICKING PROTOCOL
There are no studies on the astringency levels of buds vs small leaves vs larger leaves. Studies have shown that the total Catechin content in tea is highest in the young leaves and reduces as the leaf ages which makes sense as the tender young leaves are the ones needing most protection. Therefore, I think it is probable that younger leaves have more astringency than larger leaves and this is certainly my experience.
Buds are fairly protected by multiple layers and so it would be my guess that they have lower levels of Catechins than the young leaves.
CONCLUSION: Tea made of exclusively young leaves (like Green and Black tea) are more likely to have higher astringency than those made with larger leaves (Oolongs) or exclusively buds (like some White tea).
When you aerate a tea after brewing you are definitely changing the chemical compounds in the tea. It is likely that the unoxidised catechins in the brew will become oxidised through the aerating process and therefore will reduce astringency.
A very common technique in the sub-continent of Asia is to brew a tea ultra-strong and then ‘pull’ the Black tea between two jugs. Black tea will have some Catechin levels although not as much as Green tea and the oxidisation of these Catechins will result in a less astringent tea.
You can easily test this at home by brewing a Green tea and splitting the liquor in half. Aerate one half by pouring between two cups for a couple of minutes and then compare the colour of the tea with the un-aerated tea – it will be darker with lower astringency.
This is commonly used as an explanation for slurping tea but I do not think that there is enough opportunity for significant polyphenol oxidation during slurping to make any noticeable difference to astringency.
CONCLUSION: Aeration of brewed tea will reduce astringency but will change the taste.
An often used trick when brewing very strong black tea for Hong Kong milk tea is to brew the tea with an eggshell. This makes a less astringent tea.
The explanation is that the eggshell increases the pH making the brewing process happen in more alkaline water. While it is true that the calcium in eggshell will make the water more alkaline, I have found no studies showing that this is the reason for a reduced astringency. Indeed, some alkaline water sellers claim that tea will extract stronger with alkaline water and therefore we would expect more astringency.
Clay pots have a similar effect of softening astringency so perhaps it is more with specific mineral interaction rather than alkalinity of water.
CONCLUSION: The eggshell trick works to lower astringency but we are not sure if this is related to pH or other chemical activities similar to clay pots softening tea.
10. EXTRA INGREDIENTS
We all know that milk and lemon reduce the astringency of tea. The mechanisms behind this are probably as follows: the fat in milk turns off TRPV1 receptors and the acid in lemon displaces the polyphenols from TRPV1. The addition of cooling essential oils like Bergamot activate the cooler receptors like TRPA1 which switches off TRPV1.
But we do not have to add ingredients to reduce astringency as I believe good tea does this naturally. In fact, one of my key markers for high quality tea is the transformation of astringency to juiciness and I am convinced that there are compounds in good quality tea which naturally displace astringency after a short while. What these compounds are is the subject of even more study!
CONCLUSION: The addition of fats, acidity or cooling flavours like mint can reduce astringency.
So there you go, the geeky lowdown on astringency. Go put on the kettle and start playing to craft your perfect astringency levels and create even better quality tea.
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Three Basic Teas and How to Brew Them - Virginia Utermohlen Lovelace MD
Special thanks to Virginia Utermohlen Lovelace MD who has revealed so much about taste and tea.