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I recently heard that you should steam vegetables instead of boiling them, because boiling washes away the vitamins.

Is this really true? Is there any scientific backing to this claim? Can you really wash away vitamins or nutrients?

Source of Notability: http://wyanjen.hubpages.com/hub/high-vitamin-foods-vitamin-rich-foods

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  • It's obvious, to any casual observer, that boiling vegetables causes something to be leeched from the veggies--because the water you pour off is no longer pure. So I think the answer may depend on how you define "important." – Flimzy May 10 '12 at 16:55
  • Another related question (tangentially): skeptics.stackexchange.com/questions/528/… – JasonR May 10 '12 at 17:42
  • During steaming, there is also water, and something leeches into the water. The water at the bottom of the steaming pot is no longer pure. What you want to do is steaming with a small quantity of water, so that it is practical to consume the left over water. Pressure steaming is good for this because it is fast, which allows a small quantity of water to be used: as little as a cup. – Kaz May 10 '12 at 21:34
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TL;DR It depends on the vegetable, and what other preparation method you're comparing it to.

A study in the Nov 2003 issue of Journal of the Science of Food and Agriculture (Paywall--some key points explained here). From the abstract:

High-pressure boiling, low-pressure boiling (conventional), steaming and microwaving were the four domestic cooking processes used in this work. ... The results showed large differences among the four treatments in their influence on flavonoid and hydroxycinnamoyl derivative contents in broccoli. Clear disadvantages were detected when broccoli was microwaved, namely high losses of flavonoids (97%), sinapic acid derivatives (74%) and caffeoyl-quinic acid derivatives (87%). Conventional boiling led to a significant loss of flavonoids (66%) from fresh raw broccoli, while high-pressure boiling caused considerable leaching (47%) of caffeoyl-quinic acid derivatives into the cooking water. On the other hand, steaming had minimal effects, in terms of loss, on both flavonoid and hydroxycinnamoyl derivative contents. Therefore we can conclude that a greater quantity of phenolic compounds will be provided by consumption of steamed broccoli as compared with broccoli prepared by other cooking processes.

Now whether the nutrients studied here constitute "important vitamins" is an exercise left to the reader.

According to another study from the Journal of Food Science (paywall), summarized here:

Results showed that, depending on the vegetable, cooking on a flat metal surface with no oil (griddling) and microwave cooking maintained the highest antioxidant levels.

Note that these studies are not limited to the "washing away" or leeching effect on nutrients, but also consider the destruction of nutrients during cooking. The latter obviously does not directly answer your claim, but is probably equally relevant to anyone interested in food nutrition.

In summary, the preferred cooking method likely depends on your vegetable, but boiling is known to leech nutrients (sometimes this is even beneficial, as in the case of Swiss chard, where boiling allegedly reduces the acidic content [source]). I've also seen (unreferenced) claims that Spinach ought to be boiled briefly, or blanched, to help release vitamins.

  • Does the first study refer to pressurized steaming, or steaming just atmospheric pressure? – Kaz May 10 '12 at 21:40
  • @Kaz: The abstract does not say, so I presume atmospheric pressure, since high-pressure boiling is specifically mentioned as such. – Flimzy May 10 '12 at 21:44
  • +1 for "not limited to the washing away or leeching effect on nutrients, but also consider the destruction of nutrients during cooking" – Henry May 10 '12 at 22:05
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Boiling vegetables does reduce vitamins, sometimes substantially. The method of boiling makes a difference: boiling thawed vegetables reduces Vitamin C more than boiling direct from frozen. Even the pan makes a difference: the 2000 study Vitamin C losses in some frozen vegetables due to various cooking methods includes the result:

While boiling spinach, peas, green beans, and okra without thawing resulted 46.5, 25.2, 18.2, and 21.6% vitamin C loss in double based stainless steel pan, boiling them in pyrex pan resulted 58.5, 36.0, 42.1, and 28.2% vitamin C loss, respectively

For related comments on fresh, frozen and canned vegetables see the earlier question Are frozen vegetables typically healthier to buy and eat than fresh ones?

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