A. The alkaline substances added during the production process of preserved eggs, while vinegar is acidic and can remove the astringency of preserved eggs, so A is correct;

B. Nitrogen The chemical properties of fruit juice are inactive and non-toxic, so food bags are filled with nitrogen to extend the shelf life of food, so B is correct;

C. Although adding "plasticizer" to fruit juice can make it taste better , but it will harm your health, so C is wrong;

D. Putting an appropriate amount of iodized salt into the cooked chicken soup can supplement iodine for the human body, so D is correct.

So choose: C.