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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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4 Answers

up vote 15 down vote

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 boiling vegetables doesn't destroy the vitamin C. It's still there; it's just in the water rather than the vegetable. This means that if you're boiling vegetables for a soup or stew, you probably haven't lost any nutritional value, assuming you're cooking the vegetables in the soup's broth.

Additionally, heat doesn't reduce the vitamin C content of the vegetables; steaming and microwaving are recommended cooking methods if you want to preserve nutrient content.

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up vote 5 down vote

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
up vote 4 down vote

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
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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
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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
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My point is that the temperature at which you boil vegetables doesn't destroy vitamin C. – kiamlaluno Sep 2 '11 at 14:58
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up vote 0 down vote

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

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