Corinne Trang, author of Noodles Every Day, explains that contrary to popular belief, fish sauce doesn't last forever. As soon as you open the bottle, the air gets to it and it starts fermenting again.
- Buy it in small quantities, and refrigerate it so that it stays fresh.
- You'll know when it's too old by the color. What you're looking for is a fresh amber or golden colored fish sauce.
- If it gets too dark, almost like soy sauce, or if it forms salt crystals at the bottom, buy a new bottle of sauce.