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I've heard 2 explanations: because of enzymes that get destroyed when cooking (which is chemical) and because plant cells expand when the water inside freezes, and then the cells burst (which is mechanical). Which one is it really? Or both?

On that note, why does cooking (blanching/sautéing) preserve their texture after freezing and thawing? The fact that it does seems to be an argument against the 'burst cells' explanation, since the cells are still full of water after freezing.

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Are you trying to say that cooked frozen spinach will, when thawed, look more like fresh spinach than raw frozen spinach would? Because that's patently absurd. –  Marti Oct 29 '13 at 15:18
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I think OP is saying that the result is better when you cook - freeze - thaw (presumably reheat) rather than freeze - thaw - cook. ie you end up with cooked vegetables either way but the texture is better when you cook first. –  robthewolf Oct 29 '13 at 15:45

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In freezing, it is mostly the bursting of the cell walls as the ice crystals expand.

Much like a water balloon holds its shape when it is intact, but becomes flaccid when punctured, so to the cell walls comprising the vegetable.

See related, for why a similar thing happens when you cook spinach: Why does spinach lose its texture when cooked?

Cooked vegetables are already soft and their metaphorical balloons partially deflated. It is not going to get a lot worse for the freezing. The par cooking is mostly disable enzymes which would change the color of the vegetable to dull olive drab, or negatively affect the flavor.

See: Why blanche vegetables before freezing?

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Your balloon metaphor makes a lot of sense, but in my experience cooking doesn't prevent veggies from discoloring. I noticed that cooked veggies will still turn olive color if they're covered when warm. This happens reliably with mustard greens and Malabar spinach; I haven't tried other kinds. –  Tootsie Rolls Oct 29 '13 at 15:53
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It doesn't prevent, in reduces, and it depends on just how long it was done. Overcook the vegetables, and the chlorophyll will degrade no matter what. –  SAJ14SAJ Oct 29 '13 at 16:01

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