1. Why does a drink taste too sweet when it gets warm?
A canned juice taken straight from the fridge might taste just right, but if you leave it out and it becomes lukewarm, it can suddenly feel overly sweet. This change in the impression of sweetness due to temperature is quite common. In a Q&A column of a British science magazine, it is explained that coldness makes it harder to perceive sweetness. Therefore, commercial drinks are designed to be drunk cold, and they taste too sweet when they warm up [5]. However, this is a general explanation for drinks overall. It does not describe the specific formulation or design of a particular canned coffee. This article will not discuss how the sweetness of canned coffee is created.
Instead, the point of interest is why coldness weakens sweetness. The clues lie in small components inside the tongue.
2. How does the TRPM5 channel in taste cells react to temperature?
The tongue has small organs called taste buds, which are made up of taste-sensing cells. A channel called TRPM5 (a part like a door for cells) within these cells changes its function based on temperature. This was reported in the scientific journal *Nature* in 2005 [1]. In the experiment, a sweet liquid was applied to a mouse's tongue, and the response of the nerves transmitting taste was measured while changing the liquid's temperature from 15°C to 35°C. The response changed with temperature, but in mice that did not have TRPM5, the response did not change even when the temperature was altered [1].
A university report introducing this study explains that TRPM5 barely opens at 15°C, and its sensitivity increases by more than 100 times at 37°C [2]. It is important to note that the "100 times" refers to the sensitivity of the channel, not that the sweetness becomes 100 times stronger. Also, this was an experiment on mice. It did not measure the taste of canned coffee when humans drank it.
3. Can changing only the tongue's temperature make people feel sweetness?
There are experiments where temperature changes are applied only to parts of a human tongue, and participants report what they feel. It is reported that many answers were "sweet" when warming and "bitter" or "sour" when cooling [3]. The temperature range where sweetness began to be felt was 22–38°C, and bitterness was reported more often when the temperature dropped from 32°C to 18°C [3]. In this study, some people began to feel sweetness at temperatures lower than body temperature.
However, this study targeted people who could taste with temperature alone, and it is written that there are large individual differences in how people feel things [3]. This does not mean everyone feels it the same way.
4. At what temperature did students at one junior high school find drinks sweetest?
Students at Tennoji Junior High School attached to Osaka Kyoiku University conducted a free research project (2013) where they prepared 7 types of drinks and compared them at temperatures of about 3–45°C. However, Mitsuya Cider was excluded from the results discussion because its carbonation had gone flat [4]. The results showed that for 3 types, including apple juice, it was sweetest around 35°C. When it was too cold, hardly any taste was felt, and when it was too hot, sourness appeared. For citrus juice and some others, sweetness seemed to increase with temperature [4].
The point that sweetness stands out strongly around 35°C, close to body temperature, aligns with the direction of the tongue experiments mentioned earlier. However, the peak of sweetness differs depending on the drink, and this is a small-scale result from a middle school free research project. It cannot be stated as a law. This material also introduces differences based on the type of sugar. It is said that fructose becomes less sweet as temperature rises, while sucrose and glucose do not change much with temperature [4]. However, the source of the original data is not written in this material, so it remains at the level of "it is said."
5. How far can these experiments be applied to canned coffee?
The experiments so far used sweet liquid on mice, temperature stimulation on human tongues, and juice comparisons. None of them used canned coffee itself. Canned coffee also has bitterness, and how bitterness changes with temperature is not clear from the materials used in this article. What can be said is only that "the perception of sweetness can change depending on temperature."
Hot coffee is a separate matter. In a small study of 87 people in Germany, the average temperature at which hot coffee felt "too hot" was 66°C (range 58–71°C), and the average preferred temperature was 63°C (range 55–70°C) [6]. This "too hot" refers to a sensation close to pain, not a discussion of the taste's sweetness. Even within "temperature and drinks," we need to separate how we perceive taste from the boundary where we feel heat.
6. How can you compare the same juice when cold and when warm?
You can try this with fruit juice you have at home. Divide one bottle of juice into two cups. Put one in the fridge to cool it, and leave the other in the room for a while to make it lukewarm. Take the cold one out of the fridge just before comparing. Do not heat the juice; hot drinks can be dangerous.
Close your eyes, take a sip of the cold one first, rinse your mouth with water, then take a sip of the lukewarm one. Compare with your family to see which tastes sweeter. Since how people feel things differs, it is fine if the answers are split. If you want to read the original sources, there is the 2005 *Nature* paper [1], the paper by Skinner et al. on human tongue experiments [3], and the middle school free research [4]. The free research material is the one readable in Japanese.