Rennet Viability Test

Please follow the following at home instructions for testing the viability of your milk. Doing this test will prevent you from wasting any milk due to in active rennet.

When using a good quality milk, this is a way to check the performance of your rennet. If the milk has been processed above 172F, the milk will not coagulate as expected.

  • Heat one cup (8 oz) of milk to 90F
  • Dilute 1/4 tsp liquid animal rennet in 1/4 cup cool, non-chlorinated water (1/8 tsp rennet  to 1/8 cup water, if using liquid vegetarian rennet),and stir well
  • From this diluted rennet solution take 2 tablespoons and add it to the heated milk
  • Stir gently from to to bottom for 30 seconds

If rennet is working, the milk surface will begin to firm or form a slight film after about 5 minutes. After 10-15* minutes, it should form a curd that will hold a knife cut.

*Some pasteurized milk may take longer, it is okay to let this set for up to 45 minutes if needed. If no curd has formed after 45 minutes, the rennet is likely not working well.


  • Avoid any milk labeled Ultra Pasteurized, these milks are heated to 280F and will not work for cheese making. Generally, regular pasteurized milk is fine.
  • If you get curds that look more like ricotta or cottage cheese, then the rennet is working, but you may need to try a different milk source.
  • If testing rennet other than liquid, please refer to our Rennet Conversion Chart

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