Why/when would grapes (and other fruits) have better flavor/texture when served chilled? [closed]

It's because different taste buds are activated differently depending on temperature.

A new study reveals why our taste perception is enhanced as the temperature of food and beverage products increases, explaining why beer is more bitter and ice cream is sweeter when consumed warm.

......The study... identified microscopic channels in our tste buds - termed TRPM5 - as being responsible for different taste perception at different temperatures.

According to our researchers, the reaction of TRPM5 in our taste buds is much more intense when the temperature of food or fluid in increased, sending a stronger electrical signal tot he brain and resulting in enhanced taste.

Note, this particular article is over 10 years old. There has been a lot of research that has both confirmed and extrapolated greater details on this.

Food temperature affects taste, reveal scientists - Food Navigator

Other, more recent articles -

The Guardian (2013): Hot or not? How serving temperature affects the way food tastes

Flavors Fluctuate with Temperature - Scientific American (2012)

In the case of your grapes, there is probably some kind of flavor profile or combination that is greatly enhanced by temperature. If the skins have a pungent bitterness that is enhanced at a greater temperature, it might overpower the flavors you like, while at a lower temperature, it gives a hint of those flavors that is more pleasing, while the grape also has enough sweetness that still carries through while cold.

That is probably a much more likely scenario than the particular food rapidly ripening or decomposing to that degree.

It tastes different because chemical and biological reactions are dependent on temperature.

Being alive means that the organism is expending energy to keep its metabolism running and its biochemistry at an equilibrium which couldn't exist without the whole system geared to support it. Once a part is separated from the organism, in all but the lowest organisms, its metabolism changes. The flesh of animals starts decomposing, but the metabolism of plants also changes (although they can continue to be alive and even procreate if you supply them with good conditions). Some reactions stop, barriers break down, enzymes which have been kept in check start acting on the cytoplasm, etc.

And what happens when the equilibrium is disturbed depends on the external conditions. So a bunch of grapes which has spent some hours in the fridge will taste differently than a bunch of grapes which has spent some hours out in the sun, or some hours in a closed warm tub. So some kind of difference is to be expected, and that's what you are tasting. It is not unique to grapes, or to fruit, or even to plant matter, it is actually much more pronounced in meat.

As for why you prefer the taste from the fridge, this is not something we can guess. Personal tastes are exactly that, personal. There is no known mechanism to find out for sure how an individual started liking a given taste, although in some cases guesses can be made (sugar content, or memorable events connected to a smell, or simple familiarity are common). So you just have to take it as an information you discovered about yourself - it is no different than "I like spinach" or "I like honey". You simply like cooled grapes.