Is there a reason to not add salt when making a soup?

Question

On several occasions I have had friends make soup without salt and then tell me that salt should be added only when the soup is served.

I've read Why is it important to add salt during cooking? But what is the reason, if any, for withholding salt during cooking?

I have only heard this advice in context of Chinese soup/broth, namely a Chinese-style chicken soup and a brothy oxtail soup. I've asked a friend about the reason, but didn't quite understand the response (something about damaging the proteins).

Answer 1

Because as you are cooking your soup, water in your soup is evaporating away as steam. You might salt a soup perfectly halfway through, but after evaporation, your now thicker soup is too salty.

When adding salt, wait until the end of the cooking process, as soups will reduce and concentrate the flavors as the liquid evaporates.

[ Source: http://allrecipes.com/howto/cooking-questions-soup/ ]

Answer 2

Normally one would salt a soup or sauce to taste before serving, not early in the cooking process. When a soup boils, it's flavors concentrate as water is evaporated off. It is easy for it to become too salty if you add salt to taste before concentrating the broth, and it's much easier to add a little salt than try to remove it....

Also, the amount of salt that tastes good to individuals is highly variable. Your friends might be sensitive to salt or know that someone else in the group doesn't like food salty so they opt to just have everyone salt their own.