The walcoren states 914 IMCU...which I am assuming is per gram.
I think that's probably correct.
So I figure 1 gram of dry is at a ratio of 4.15 to the liquid rennet.
If a recipe calls for say 2.5ml of rennet is it safe to calculate:
2.5 ml ÷ 4.15 = 0.6
This would mean 0.6 grams of dry
Since 1/16 tsp = 0.3 g then 1/8 tsp = 0.6g
I would do the math like this:
The recipe calls for 2.5 ml of single strength rennet. The rennet is 220 IMCU/ml.
2.5 * 220 = 550
914 / 550 = 0.62 g
So you are right on. However, I'd urge you to move away from the written recipe and instead rely on your own calculations. Most people who right recipes are essentially putting in arbitrary amounts of rennet. Instead, I would look at 4 things:
- The ripening time for the milk
- The ripening and coagulation temperature (almost always the same)
- The flocculation time for the rennet
- The flocculation multiplier for the type of cheese you are doing
This is what *really* determines the amount of rennet to use. The speed of coagulation is 100% dependent on the amount of rennet, the pH of the milk and the temperature. This means that you can ballpark the amount of rennet that you need my guessing the pH and temperature of the milk, and the flocculation time.
Unfortunately, most recipes don't give flocculation time. They just give the time to a "clean break" (because most home cheese makers don't understand the benefit of having firmer or sloppier breaks). But you can kind figure it out. Imagine that the "time to clean break" is stated as 50 minutes. Let's also say that the normal multiplier for that kind of cheese is 3x. 50 minutes / 3 = ~17 minutes until flocculation. That's kind of slow. Normal flocculation time is usually between 12-15 minutes. So either the temperature is low, the acidity is low, or the amount of rennet is low. You can see the temperature in the recipe. You can guess the acidity of the milk based on the ripening time. Therefore you can kind of guage the rennet amount.
It's hard to do this at first, but if you keep good notes, you should be able to see a pattern and be able to guess the flocculation time for a recipe. I'm usually within 1 minute of my estimates these days and can even estimate the IMCU of my rennet (which is *long* past it's best before date). Just keep in mind that virtually every recipe uses the "normal" amount of rennet. Often when they don't it's a mistake -- I've seen some recipes with up to double the amount of rennet you would expect and you will often see people complaining that the cheese is bitter, or that they are getting too much moisture in the curds, etc, etc.
