Your rennet should state the best before date on the packet.
But if you have disposed of the packet or the date is not visible this is how we test our rennet:
- Heat 1 cup (250 ml) of milk to 32°C (90°F) (do not add citric acid).
- Dissolve 1/4 rennet tablet (or 1/4 tsp liquid rennet) in 2 Tbsp of cool, non-chlorinated water and stir well.
- Add the rennet to the milk at 32°C (90°F).
- Stir gently from the bottom to the top for 30 seconds.
If the rennet is working, the milk surface will begin to firm or form a slight film after two minutes. After 6 minutes, it will have formed a curd that will hold a knife cut.