Why are some kind of packets-soups boiled up in lukewarm water, while normally just stirred into already boiling water?

Question

Inscription on packets-soups from supermarket states:

Stir the soup-meal into lukewarm water, boil it up, and wait 5-10 minutes. Ready to eat.

Normally the inscription says that you should boil up water and then stir the soup-meal into it. At least for all noodle and broccoli soups I regulary buy.

Whats the reason, that, in my case, a potato-soup and very few noodle soups needs to/should be boiled up in lukewarm water. Can somebody explain the cooking-physics/purpose behind this procedure?

I cannot imagine is has to do with cooking time like other boiling-tagged questions at first made me think (because the inscriptions are different on packets of same company, there seems to be a different reason), I suppose more that those soups would get lumpy/agglutinate? Cannot think of other reasons.

Answer

Some ingredients do not dissolve well in hot water - the starch swells and thickens, forming lumps that may have raw powder in them and are nasty. They need to be added to cold or lukewarm water and heated after they are dissolved. Other ingredients, most notably pasta, will partially dissolve in cold water making a thick gloppy soup. But if you add them to hot water they will "seal" (in a way) and stay together as they cook.

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