When stored in the refrigerator, rennet will not go bad. Even past the 'best by date' it will simply loose potency over time. So, if your rennet is a few weeks past the 'best by date' add slightly more rennet than normal.

How long can you keep rennet?

Use your diluted rennet within 30 minutes or it will lose its effectiveness. Rennet has a limited shelf life – liquid rennet lasts for 7-8 months when refrigerated and tablets last for up to 2 years when stored in the freezer.

Does rennet go bad if not refrigerated?

Rennet does not need to be kept refrigerated at all times. However, it should be stored in the refrigerator to maximize shelf-life.