Testing your rennet and milk
How many times have you started to make cheese only to not have the milk set?
Is it the milk or is it the rennet? Here's what I do:
Heat half a cup of the milk you plan on using in the microwave to 35 Celsius (95 F) and add some rennet. If it sets the rest will set and you are ready to go without the risking of losing all the milk. If the set is weak, add some calcium chloride if you use it or wait longer. I am happy if the milk sets between 30-60 minutes - longer and it becomes a drag!
If your rennet does not want to set raw milk, then you can be pretty sure the issue is with the rennet. Remember to use the rennet according to manufacturer recommendation for specific volume of milk. Rennet strength is measured in IMCU - this is not a bad pick-up line - it stands for International Milk Clotting Units - go figure! IMCU ranges from 134 up to 15000. Obviously with this wide range of strengths we have to make adjustments in the dosage, so beware.
Always and for all cheeses rennet between 32-40 Celsius (90-104 F), this is especially important with animal rennet, as it is more heat sensitive than the microbial ones.
Only things I can think of that would give no or weak set is rennet issues, low protein in milk(nutrition issue or from milk dilution) and low calcium(from dilution mostly) and trying to rennet at too low or too high temps. Oh, and not leaving the milk alone and quiet to set - do not bump the pot or table it is standing on - dogs must not bark and children must be quiet in their rooms ;-) Rennet must be kept in the fridge as it is a protein enzyme that can be denatured(inactivated) over time in the freezer.
Also, remember that rennet is a proteolytic enzyme that likes an acid environment. Calf rennet is extracted from the stomach, so rennet does work much better in an acid environment. You will observe this if the pH of milk is lower due to natural or artificially added(from added citric acid, vinegar, yoghurt, liquid live cultures) acidification. You will also see that cheese milk that has been incubated with the culture for longer, sets faster, as with feta( 90-120 minutes) and mozzarella as opposed to gouda (15-30 minutes).
I hope this helps you to have a good and consistent set in your future cheese making :-)