Watermelon is rich in fructose, and below 40 ° C, the sweetness of fructose will increase with the decrease of temperature, up to 1.73 times that of sucrose. There are two types of fructose, furan fructose and pyran fructose, the latter being about 3 times sweeter than the former. Fructose "transforms" at different temperatures, and as the temperature decreases, the furan fructose changes to the pyran fructose. The same goes for apples, lychees, cantaloupes, and grapes.
Source: Zhongshan Release