How much does freezing grapes longer sweeten them more? My grandparents in Toronto bought 10 kg of Ontario coronation grapes from Loblaws on Sep 1 2019, as they have been doing the past 10 years. They don't know why, but the grapes taste too sour and acidic this year, and they couldn't eat any more after tasting a few. So they placed them in a freezer to sweeten them.

Today they thawed 20 grapes but these still taste inordinately sour and acidic! So:


For maximal sweetness, how long ought they freeze them? 
How does the duration of freezing affect the sweetening? To wit, what are the 1st and 2nd derivatives of freezing w.r.t. sweetening? Did I formulate my question correctly with calculus?
The time doesn't matter. What you need is a single freeze-thaw cycle. Once your grapes have been frozen solid, holding them frozen won't do anything more. 

I get it that you are not happy with the results you got so far, but that is just because your freezing idea is not really suited to your situation. I have never tried eating thawed bananas, so don't know how large the effect is there, but the article states the two reasons for more sweetness: juices leaking out of cells, and amylase converting starch to sugars. First, grapes are a juicy fruit, unlike bananas, and you easily have access to their juice once you bite into them. Second, grapes have practically no starch. 

The problem with your grapes are not that they are a mealy banana, it's that they are sour. The acid is not going anywhere when you freeze them, and you can't create more sugar out of nothing. They will stay the way they are, freezing or no.