The sweetness of sucrose, which we usually call sugar, does not change with temperature, while the sweetness of fructose, which is found in fruits, syrups, and soft drinks, varies. , which varies greatly with temperature.
To conclude first, fructose becomes sweeter when cooled.
As shown in the table above, there are α-type and β-type glucose and fructose, respectively, and the ratio of α and β changes depending on the temperature zone. Simply put, the lower the temperature range, the more α-type, and the lower the temperature, the more β-type. The β-type of fructose is three times sweeter than the α-type, so when it is cooled, the β-type increases and becomes sweeter.
In other words, fructose is about 1.5 times sweeter than sucrose at 5°C, but only 0.8 times sweeter at 60°C.
*Sweetness is determined by a sensory test (judgment by people actually eating).
From here on it's subjective.
By the way, invert sugar contains fructose and glucose in a ratio of about 1:1. In other words, since honey is mostly made of invert sugar, it is thought that it becomes sweeter when cooled.
However, sweetness is best felt at around body temperature, and is weaker at temperatures higher or lower. (*At the moment, there is a hypothesis that the temperature of the food itself is irrelevant, and the temperature of the tongue affects how it feels.)