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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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.
There are also several reasons beyond seasoning that apply when boiling vegetables:
Salted water will cause vegetables to become tender faster than plain water because it speeds the breakdown of vegetables' cell walls.
Salted water will preserve the natural flavor of vegetables. Using plain water actually draws the natural salt out of the vegetables, into the water.
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.
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.