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It's generally known that boiling vegetables removes a large fraction of vitamin C, but in what way?

Does the high temperature destroy it?

Is it merely absorbed by the boiling water?

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It isn't really "absorbed" by the boiling water; more precisely, it is leached into the water. As kiamlaluno said, Vitamin C is water soluble. An important thing to note is that the leaching of vitamin C into water, by itself, doesn't destroy the vitamin C. It's still there; it's just in the water rather than the vegetable. If you consume the liquid you cooked in, you'll reclaim some of the "lost" vitamins.

High heat can reduce the vitamin C content of the vegetables, and when heat and water are combined, as they are in boiling, you can see significant reduction of vitamin C. (One study found that boiling reduced the vitamin C content in broccoli by 45 to 64 percent.) This is because the vitamin is first leached out of the food into the water, and then degraded by the heat. Heat alone will cause some reduction in vitamin C, but not as much as when combined with loss of nutrients through leaching. Steaming and microwaving are recommended cooking methods for preserving as much of the nutrient content as possible because they involve minimal exposure of food to both water and high temperatures.

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"heat doesn't reduce the vitamin C content of the vegetables;" => this is plain inaccurate; see David N. Andrews' answer for explanations. – Tootsie Rolls Jun 12 '13 at 1:55

Actually, vitamin C degrades with heat. The following, by dietician Jill Irvin, says it all:

Vitamin C is one of the least stable of all vitamins in solution and is oxidized readily in light, air and when heated. It is also water soluble. This means that heating in water, (like cooking broccoli in boiling water) causes the vitamin to leach out of the food into the water and also to be oxidized, first to dehydroascorbic acid and then to diketogulonic acid. This last compound has no Vit[amin] C activity at all and is irreversible.

She goes on to say that normal cooking doesn't affect levels of the vitamin too much, but the main issue being queried here is how boiling removes vitamin C from food, and this quotation tells how that happens.

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M. Ed. = Master of Education C. P. S. E: = Certificate of Professional Specialisation in Education I'm not a cook or a dietician, but my background is in sciences. – David N. Andrews MEd CPSE Nov 4 '12 at 8:59
This is the only answer that "answers" the OP's question. Vitamin C gets degraded with temperature. That's the reason you shouldn't heat babys milk above 40C/100F (or why they gave orange juice to babies in the past, when they gave boiled milk's cow diluted in water). Or the reason you don't find traces of it in baked bread, despite they add E300 to enhance the dough. – J.A.I.L. Nov 5 '12 at 23:12
From it appears that enzymes within the vegetables themselves also play a significant role in Vit C degradation, without necessarily leaching out into the cooking water. – Didgeridrew Jun 29 '13 at 18:01

Vitamin C, as most of the vitamins, is soluble in water; one of the few vitamins that is not soluble in water is the vitamin D, which is fat soluble.

The melting point of vitamin C is 190 °C (374 °F), which means the temperature at which you boil the vegetables cannot destroy the vitamin C.

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Unless you boil away all the water, surely the vitamin C is never solid - it's in solution. – Jefromi Sep 1 '11 at 13:35
It's what I said: When the water is evaporated (100 °C), the vitamin is still solid. – kiamlaluno Sep 1 '11 at 14:25
But when people boil vegetables, they don't boil them dry. They boil, then drain. The soluble vitamins are poured out. (And the question was about boiling vegetables.) – Jefromi Sep 2 '11 at 1:28
When people boil vegetables, the vitamin C is dissolved in water; that is what soluble in water means. – kiamlaluno Sep 2 '11 at 9:40
My point is that the temperature at which you boil vegetables doesn't destroy vitamin C. – kiamlaluno Sep 2 '11 at 14:58

Since Vitamin C is a water soluble vitamin, boiling the vegetables cause the vitamins to get dissolved in the water.

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