ADD Citric Acid at 30 degrees?

I assume you are trying or wanting to make mozzarella cheese? In that case, the citric acid is diluted and added to the milk while it is cold or cool, before the warming process starts. It alters the pH of the milk, so you can make cheese quicker than the old fashioned method using a lactic acid bacteria starter. So no, it doesn't destroy the milk- at least not for cheesemaking purposes. If you add too much acid, then yes, it will cause curds to form before you add rennet. That is how cheeses like ricotta, queso fresco or paneer are made, utilizing only an acid to coagulate the milk solids. I suggest you research cheese making recipes so you can get an idea of when and how to add things, and understand what it happening with each step. What you add and when has a great impact on your final product, and to get what you want requires following a recipe designed to produce a specific cheese.