Tea serving temperature
Fun question: at what temperature to serve tea?
The first systematic study I found to answer this question was provided by Ragita Pramudya and Han-Seok Seo, which studied liking and emotional response to coffee and green tea.*
They used bagged pan-fired Korean green tea which can be expected to have a roasted flavor. They prepared the tea by brewing the bags with boiling water for 5 minutes. Under these conditions they had a greater chance of leaching out indole with its animalic quality, and catechins, with their bitterness. They then served the tea at 5ºC/41ºF, 25ºC/77ºF, and 65ºC/149ºF. I’ve graphed their results for the proportion of participants attributing each quality to the teas here:
As you can see, more people assigned potentially unpleasant qualities to the tea served at 5ºC than to the tea served at 65ºC, and more people assigned positive qualities to the tea served at 65ºG than at 5ºC. The tea served at (more or less) room temperature had intermediate qualities.
Pungency is the term used to describe the sensations induced by activation of the cold trigeminal receptor TRPA1. Catechins also activate cool/cold receptors, so serving green tea at a cold temperature may bring out their bitterness. That's because the trigeminal system serves as a volume dial, so that when there is congruence between the actual and perceived temperature of a compound, the flavor of that compound is amplified.
Type II taste bud cells responsive to sweet use TRPM5, a warm receptor, in their response pathway. In fact you can simply apply mild heat to the tongue, and many if not most people will interpret the sensation as “sweet.” That is why the warmer tea is sweeter, even when no sweetener has been added to it. It is also milder because the bitterness and pungency are dialed down.
Remember how I mentioned that color and flavor can be related? In this same study they asked participants about the color of the green tea, which in fact was a yellow. More participants saw a brown color in the tea when it was hot than when it was cold, and virtually none saw a green color in the hotter brew.
In the emotional realm, the warmer tea was associated with positive emotional feelings and the colder teas with more negative feelings.
One more important conclusion of the study to include here: “Since females could better detect sample temperature-induced changes in sensory attributes than males, sensory attributes might contribute to likings of coffee and green tea samples among female participants, but not [as strongly] among male participants.”
In the US, and especially in the South, there is a strong tradition of consuming sweetened iced tea. The sweetness added to the tea counteracts the bitterness and astringency of the cold tea. An open question is whether sweetness can also add to the emotional positivity of the person drinking it!
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* Ragita C. Pramudya & Han-Seok Seo. 2018. Influences of product temperature on Emotional Responses to, and Sensory Attributes of, Coffee and Green Tea Beverages." Frontiers in Psychology, Gale Academic Onefile, https://link.gale.com/apps/doc/A522502274/AONE?u=nysl_sc_cornl&sid=AONE&xid=9c9e7154. Accessed 7 Sept. 2019.