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Since recently i have to cook for myself and a lot of products mention i should boil some water with salt and when it boils add the product.

Why on earth do you have to add salt to the water?

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up vote 11 down vote accepted

This is generally to season the thing being boiled whilst it is cooking. The salt will infiltrate the innards of the thing being cooked infusing it with some seasoning.

Try with something basic like pasta or rice to see the difference between cooking in plain water and cooking in salted water.

This quesion and this one might also provide some insight.

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Thanks, all i needed to know :) – Aerus Mar 4 '11 at 15:42
    
@Aerus: Definitely worth trying the difference to convince you of the necessity of salt. Starch foods like potato, pasta, rice without salt in the cooking liquid are barely palatable, though of course it is personal preference. – Orbling Mar 5 '11 at 1:02
    
@orbling "barely palatable" is subjective of course. Many of us have trained our palates to demand salt, but it's possible to untrain your palate, and if you have kids, to avoid training them that way in the first place. My grandparents demanded huge amounts of salt. I'm down to a scant half-teaspoon in two portions of rice. – slim Mar 9 '11 at 14:58
    
@slim: Well I guess it's subjective, not everything requires salt, but some of the foods we eat, particularly in that category are exceptionally bland without enhancers like salt. Not sure what your portion size is, but I would guess I add about 4 tsps to 250ml of dried rice. – Orbling Mar 9 '11 at 15:26
    
@orbling Yes, for me that's loads. I put half a teaspoon for about 170ml of dried rice. That's with the absorption cooking method, so all the salt ends up in the rice. I suggest that salt masks the actual flavour of the food -- but you get used to the salt, so you taste through it. – slim Mar 9 '11 at 15:42

There are also several reasons beyond seasoning that apply when boiling vegetables:

  1. Salted water will cause vegetables to become tender faster than plain water because it speeds the breakdown of vegetables' cell walls.

  2. Salted water will preserve the natural flavor of vegetables. Using plain water actually draws the natural salt out of the vegetables, into the water.

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Thanks for elaborating a bit more on vegetables :) – Aerus Mar 5 '11 at 10:19
    
I think both of these are old wives' tales. Heston Blumenthal's cookbook details some experiments he did on the matter. – slim Mar 9 '11 at 14:56
    
Harold McGee provides scientific justifications for both in On Food and Cooking. And certainly for #2, vegetables have some salt content higher than water, and it follows that boiling them will pull the salt out of them. It's pickling in reverse. – yacomink Mar 11 '11 at 16:21

Something I've learned recently is that if you are cooking dried beans (not from a can) then you do NOT use any salt until the beans are fully cooked! If you salt the boiling water for the beans, it actually toughens their skins to a level that won't go away and isn't pleasant. It doesn't affect the flavor, but the texture is too chewy.

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This goes for all legumes. Beans, lentils. etc. – daniel Mar 9 '11 at 4:38
    
This is true, though not "fully" cooked, near the end of cooking, so the salt has some penetration, just not early when they will toughen. – Orbling Mar 9 '11 at 15:47

Adding salt just slows down or stops osmosis, so the cells of the vegetables do not blow up with water or burst. Salts cannot pass the cell walls, they are SEMI permeable, so there is no leakage of salts into the boiling water... otherwise you would not have osmosis at all. Likewise, the salt cannot get into the cells. Other nutrients may leak into the water, though I do not know that...well obviously some do, otherwise the water wouldnt get any colour or taste, but how salt infuences this I do not know. Oversalting would result in water leaking out of the cells, and so more nutrients would be lost, I think.

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Sodium can pass through cell membranes. I'm less clear on how that works in (dead) vegetables than in (living) human cells, but it's definitely able to get through. – Erica Feb 13 at 11:23
    
You are right, but not quite. Sodium transport through the membrane is an active process, that cost energy. So they cannot pass, but have to be pumped. The question is indeed, does this still happen in dead cells, killed by boiling water? I guess not. I guess that is the reason why starting in boiling water (killing them quickly) is good. But that is another question. – Marc Luxen Feb 13 at 12:09
    
Can you include some sources for your answer, then? Guesswork is OK, but backing it up with research is better. – Erica Feb 13 at 12:11
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I meant in the answer, since you are speculating about nutrient loss. – Erica Feb 13 at 14:57
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It's up to you. I'm just saying there were things in your answer that I found confusing, as well as some outright speculation. And that's fine, I'm just one user and not the OP, but it is a possible path to making your answer more thorough. I honestly don't know enough about the science (as we've established :) ) to say what the appropriate sources or explanation might be. – Erica Feb 13 at 16:15

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