My son is one week old. He had to be in NICU for first couple days and they gave him similac formula. So now we are home he only likes a bottle. So i pump breast milk and other times give give similac formula. However, with this formula once you open it, you can put rest in fridge and keep using it. However it doesn't say anything about warming it up. Formula is already to use which I don't need to do anything. Is it okay to use cold formula. It doesn't say anything about warming it up.