A. Formaldehyde is toxic and cannot be used to soak food, so A is wrong;

B. Adding iodized salt can not only adjust the taste of the soup, but also provide iodine to prevent big neck disease. occurs, so B is correct;

C. The toxicity of plasticizer is much higher than that of melamine, so it cannot be put into juice to affect health, so C is wrong;

D. Asia Sodium nitrate is a food additive, but it must be added in an appropriate amount. Adding too much sodium nitrite will affect human health, so D is wrong.

So choose B.