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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:

  1. For maximal sweetness, how long ought they freeze them?

  2. 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?

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    Sounds like a case of sour grapes to me. – Strawberry Oct 10 at 17:13
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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.

  • 4
    Having eaten thawed banana (in chocolate, I was unaware they had thawed), I can add that it does taste sweeter, but it also turns to mush so it wasn't particularly nice to eat. Might be okay if used as an ingredient though since the flavor wasn't bad. – Flater Oct 10 at 17:49
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    Freezing bananas ruins their structure and turns them to much, but they make great banana bread! – GdD Oct 10 at 19:55
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    The "turning to mush" part is the point of the exercise, it's what makes the bananas sweeter, see the article the OP linked. – rumtscho Oct 11 at 6:53
  • Frozen bananas are perfect for smoothies or fake icecream (e.g.: try to mix frozen bananas and cocoa in a food processor) – dolma33 Oct 15 at 21:19
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You may be thinking of sweet ice wine. But the grapes used for making that are not only frozen at some point, but juiced while frozen. So the juice is a more concentrated solution of, well, everything that was in the grape before, because ice (from the water inside the grape) likes to form without any foreign molecules in the crystal lattice.

After thawing, though, everything is dissolved again in the same amount of water as before, so the taste should not change just because of that.

Edit: There are fruits that do change permanently in taste after freezing, even if thawed again (e.g. in sloe, the tartness is reduced). But at least to my knowledge, grapes do not belong to that group.

  • Sometimes, more of the juice will still be released and tasted first, which can result in a stronger flavor when eaten, but the biggest effect is as you point to for making things like ice wine or ice cider, when the fruit is juices while still frozen. In that case you actually concentrate the "contaminants". If the fruit is overly acidic, unless that acid stays in the flesh at a higher rate than the sugar or has a more preferred freezing tendency than sugar water, it will also concentrate even in juicing defeating even that attempt to concentrate the sugar. – dlb Oct 10 at 12:45
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    Actually, there are other effects going on when harvesting fruit or vegetables during a frost. The living plant itself increases its sugar content as a reaction to the frost, to lower the freezing point of its cell plasma. This doesn't happen if the freezing is done after harvest. I don't even know if the part of "more concentrated" holds, you are juicing everything together, including the water. – rumtscho Oct 11 at 6:58
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    Other parts of the plant than fruits will contain more sugar (and other compounds) as "antifreeze". But the plant has no "incentive" to increase the sugar content of the fruits during a frost, as the fruits are going to be discarded (by the plant) anyway. And yes, you are juicing everything together, but while the water is still frozen, so it stays with the solids. – Matthias Brandl Oct 11 at 8:02
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When you freeze grapes you drive water out of the fruit, all that is going to do is intensify the flavor that is already there. If it's sour to begin with it will be more sour, if it was sweet to begin with they will be sweeter.

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As Matthias mentioned: you might be thinking about Sweet Ice Winem which, traditionally, is left on the plant til the first frosts hit, and freeze the grape. Micro-ruptures then allow some water to escape, concentrating the 'stuff' that is inside.

But these very late harvest grapes are high in sugar, because they have been left to ripen as long as possible. So it is not the freezing as such that makes them sweeter - the freezing just concentrates the sugar that is already there. If the grape was sour, the freezing should simply concentrate the sourness eve further.

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