One of the many rewarding experiences of drinking tea is the savouring of the flavours in each sip. The true appreciation of the depths of flavour in a tea and how it begins, how it develops and then how it lingers afterwards is a never ending learning experience.
In this series of blogs we will look at flavour in three ways. First we will examine how we experience flavour when we drink, then we will look at why different teas have certain flavour. Finally we we investigate the best way to taste tea.
Lets start from the beginning by simply asking : what is flavour?
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Quite simply flavour is a complex mix of sensory data. Messages are sent to our brains by various systems in our bodies and this is then analysed to give us the perception of the flavour of what we have eaten or drunk. These messages include information about taste, smell, any visual stimuli and also sensations such as texture and temperature.
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To understand why we perceive the flavour of tea as we do it is important to understand how we collect this information, the interactions between these and also what can affect each one.
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HOW TASTE IS DETECTED - TASTE BUDS
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Before we look at the specific tastes in more details it is useful to know how taste is detected. This is done by the taste buds which contain taste receptor cells and are located mainly in the tongue but also in the back of the throat, on the roof of the mouth and down the oesophagus. Adults have between 2000 to 4000 taste buds in total. These receptors recognise specific chemicals out of the thousands that are contained in each mouthful of tea and then pass this information on to our brain.
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There are currently thought to be five different types of the receptor cells in each taste bud (classified into four types I to V) and they all collect their information in different ways. Some are able to bind to the specific shapes of chemicals passing over them, while others have ion channels that are activated by different chemicals. When a particular chemical is detected, this information is conveyed along a series of neural pathways to the brain, where taste is perceived.
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Type I cells are mainly concerned with detecting sodium (saltiness). Three different Type II cells pick up bitterness or sweetness or umami respectively. Type III cells are concerned with sourness whilst also transmitting messages for Type II cells to the nervous system. This is a key point as we will see later. Type IV cells make new type I, II, and III cells during the rapid turnover in taste buds every 7-14 days.
At this point it would be prudent to point out a misconception that is prevalent in this area. This misconception is that the tongue has specific areas which pick up each taste especially well. This is not true as sweet, sour, salty, bitter and savoury tastes can actually be detected by all parts of the tongue. However, it is correct to say that the perceived intensity of the certain tastes change in different parts of the tongue, for example bitterness and umami are found to be more intense at the back of the tongue. This is probably to enable us to spit out poisonous or spoiled substances before they are actually swallowed. Hopefully not necessary in our exploration of tea, but possibly useful when we are talking about our tea experiences.
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Taste is usually split into five main elements. These are sweet, sour, bitter, salty and umami. It is taste that gives our brain information about the nutritional qualities of what we are consuming, so that the digestive system can prepare. As I mentioned above these messages can also give warnings in order to avoid ingestion of undesirable products. In tea tastes are usually in the categories of bitterness, sweetness and umami, with sour and salty not generally being present.
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Sweet : This pleasant taste usually indicates sugar and its derivatives. Tea naturally contains monosaccharides (such as glucose, fructose, and galactose), disaccharides (sucrose and lactose) and oligosaccharides (sucrose, lactose and maltose). These are responsible for that immediate sweet taste. However, when they are gone the sweetness subsides quickly. However, many of you will have noticed that in many teas the ‘sweetness’ remains into the aftertaste. This is down to other compounds such as 2,4-dihydrobenzoic acid which is found in teas that were shade-grown (such as the Japanese green tea gyokuro).
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Bitter : Bitter taste is often described as sharp and unpleasant and is recognised by taste receptors in thousands of different chemicals. There are at least 25 different proteins in the sensory cells that respond to bitter substances. This large number of receptors can be explained by a time when recognising that bitterness in plants was important as it often indicated that they were poisonous.
Thankfully, in tea this is not the case, where this bitterness is often a major part of the desirability of the brew. Bitterness in tea comes mainly from methylxanthines, which if you have read our previous series of blogs, is made by a plant to make its taste undesirable to insects and animals. Caffeine is the major methylxanthine alkaloid found in tea along with theobromine.
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Bitterness vs Astringency : At this point it is important to stress the difference between bitterness and astringency, which are often used interchangeably. The key is that (as described above) bitterness is a taste whereas astringency is all about mouthfeel. What exactly mouthfeel is we will discuss later when we look at the trigeminal system.
Savoury or Umami : The quality of ‘umami’ was first used by a Japanese chemist named Kikunae Ikea in 1908. Umami translates as “delicious savoury taste” and describes a rich brothy, sometimes ‘meaty’ taste imparted by glutamates that occur naturally in many foods such as fish, seafood, seaweed, meats, mushrooms and tea! There are eight taste receptors for umami showing how important it is for us to be able to detect it. Unlike bitterness this is not as a warning but more of a necessity, as it indicates the presence of protein, necessary for a healthy body.
The proteins in tea leaves are made up of amino acids. One of the most abundant is L-theanine as it accounts for almost half of the total free amino acids found in tea. It is responsible for the full bodied, umami savouriness and sweetness in teas. Incidentally when the tea plant is growing sunlight converts these amino acids into polyphenols. This decreases the amino acids in the finished tea, thus decreasing the intensity of the umami. This is why some teas, such as Japanese matcha are shaded in the last 2-5 weeks or so of their growth.
Salty : The chemical basis of this taste is sodium and chloride in the form of salt crystal, but the sensation of saltiness can also be gained from mineral salts like the salts of potassium or magnesium.
Sour : Sourness is the taste that detects acidity. It is found mostly in solutions of organic acids such as citric acid (found in lemons), lactic acid or acetic acid. The sour sensation is caused by hydrogen ions (H+), split off by an acid dissolved in a watery solution. These occur in some fruits, but also in fermented foods such as vinegar and wine. As mentioned earlier, sour taste is detected by a small group of cells in the taste buds called Type III taste receptor cells.
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Next in the systems used to inform us of the flavour of our tea(or anything else we consume) is the nose. Similar to taste, smell is the detection of chemicals, but this time they are volatile (or vaporised) molecules that are present in the air. There are two paths in which these molecules enter the nose. The first is through our nostrils when we breath in. The molecules enter and then attach to receptor cells in the roof of the nose, which then transmit information to the brain as a smell. This is called orthonasal olfaction and is usually our initial experience of each tea we drink. The second path occurs when we take a sip of tea and more volatile molecules are released into the mouth from the tea liquor. These also enter the nose, but this time via the back of the mouth. This is called retronasal olfaction. Both paths impact on the brains analysis of what we about to consume, or have consumed.
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In tea there are hundreds and even thousands of volatile molecules making up its aroma. These vary from the fresh tea leaves to the processed leaves and of course this varies according to which processes have been used. The specific combinations of these volatiles also has a play what you smell.
The final part of how our brain decides what the flavour of our tea is the trigeminal nerves and the systems they are parts of. These nerves travel from the eyes, nose and palate (nasal receptor cells), and the lower mouth and tongue (taste buds). They are concerned with sensations that originate, for example, in the nose, on the lips, and throughout the entire mouth and throat. This can be texture (mouthfeel) and temperature, but these sensations are often associated with compounds found in what is being consumed, such as menthol in mint giving a coolness, or capsaicin in chillis providing heat.
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In tea the best example is that of astringency. This is picked up by the trigeminal system in combination with taste bud receptors, but it is mainly a sensation rather than a taste. The dry, puckering feeling often felt on the roof of the mouth and the tongue is caused by the shrinking effect on the tissues caused by certain chemicals. In brewed teas these are polyphenols. Polyphenols can be split into many categories, with the most important being flavanoids. The most common type of flavanoid in tea are Flavan-3-ols (commonly called flavanols), which contain catechins. These are often incorrectly referred to as 'tannins' a word which most people associate with bitterness. In fact it is astringency not bitterness that they are causing!
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It is thought that the trigeminal system can act in muting or amplifying flavours depending on how the nerves are activated. When combined with smell and taste components, this can enrich the perception of flavour. The best description of this is as a 'volume dial' for flavour. This was in the book 'Tea: A Nerd's Eye View' by Virginia Utermohlen Lovelace MD which was the main source of information for this blog. If this short exploration has ignited your interest then we recommend buying a copy!
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We hope you have enjoyed this first blog looking at 'flavour' and specifically how we experience flavour when we drink tea. You can also check out our Instagram Live to see us talking about this topic! We hope you will join us next time when, in two weeks, we will focus on why different teas have certain flavour.
References for this blog :
Tales of the Tea Trade : Comins Tea
'Tea: A Nerd's Eye View' : Virginia Utermohlen Lovelace MD
Article : Nicole Schöbel, Debbie Radtke, Jessica Kyereme, Nadine Wollmann, Annika Cichy, Katja Obst, Kerstin Kallweit, Olaf Kletke, Amir Minovi, Stefan Dazert, Christian H. Wetzel, Angela Vogt-Eisele, Günter Gisselmann, Jakob P. Ley, Linda M. Bartoshuk, Jennifer Spehr, Thomas Hofmann, Hanns Hatt; Astringency Is a Trigeminal Sensation That Involves the Activation of G Protein–Coupled Signaling by Phenolic Compounds, Chemical Senses, Volume 39, Issue 6, 1 July 2014, Pages 471–487
