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I read this at this webpage: http://www.whfoods.com/genpage.php?dbid=90&tname=nutrient#impactcookingstorageprocessing

How do cooking, storage, or processing affect potassium?

Potassium losses from cooking of high-potassium foods can be significant. In the case of spinach for example, potassium levels have been shown to drop from 6.9 to 3.0 grams in 3 and 1/2 ounces of spinach after blanching for several minutes (a loss of about 56%).

I don't understand this. How can potassium disappear after cooking ?

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As Eric has mentioned in his answer, the potassium present in foods will leach out into water when you cook them.

Potassium is present in food in the form of potassium ions $(\ce{K+})$. These $\ce{K+}$ ions happen to be extremely soluble in water.

When food is boiled, the cells break down and release their contents more easily which makes the $\ce{K+}$ ions free to move into the water. The $\ce{K+}$ ions remain in the water since they are so highly soluble in it.

This happens to all water-soluble nutrients like the B vitamins, vitamin C, sodium, etc. Some ways to avoid the loss of all these nutrients would be to steam such foods instead of boiling them since the food wouldn’t come into contact with water.

Side note: The same problem occurs when you deep fry foods containing fat-soluble nutrients like vitamins A and E. These leach out easily into oil and so all those nutrients would also be lost in the process.

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    $\begingroup$ Water does contact food when steamed, otherwise it wouldn't get heated. Also, the food drips water. You do loose less nutrients since less water comes in contact with the food. $\endgroup$
    – LDC3
    Apr 1 '14 at 4:35
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    $\begingroup$ Side note: The constant addition of heat will destroy most of the vitamins, esp. when deep frying. Minerals however will be watered out. $\endgroup$ Apr 1 '14 at 6:18
  • $\begingroup$ @Martin What is the process by which heat "destroys" vitamins? $\endgroup$
    – ProfRob
    Jan 15 '15 at 8:58
  • $\begingroup$ @RobJeffries It might be worthwhile asking this as a separate question on the site, it is probably too complex for a comment and I am not an expert on this. I believe they undergo denaturation and often will be deactivated due to oxidation with readily available oxygen from air. But there are certainly other factors. $\endgroup$ Jan 15 '15 at 9:54
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The potassium is leeched out into the water used to cook it, and unfortunately poured down the drain -- vitamins and all.

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