Rennet Activity

Coagulation Time versus Setting Time

Rennet is generally described in the industry as single, double or triple strength. Single strength is considered to be that concentration where 200 ml is sufficient to set 1,000 kg of milk in 30 - 40 min. at 30 - 32C. Setting time is the point where the curd will break cleanly and exude clear whey. Coagulation time is the point where flecks of curd first appear on a spatula or slide dipped into the milk. Coagulation time is about half of setting time, so typically, coagulation using single strength rennet requires 15-20 minutes followed by setting at 30-40 minutes. The following simple test can be used to check coagulation time which can be measured much more accurately than setting time. The test uses skim milk because the presence of fat globules makes it difficult to see the first sign of coagulation.

Measurement of Coagulation Time

  1. Prepare 200 ml samples of 10% reconstituted low heat skim milk powder in 250 ml beakers. Add 0.02% calcium chloride dihydrate (40 mg per 200 ml).
  2. Temper to 32C in a water bath.
  3. Add 1.0 ml of 5% rennet solution to each sample.
  4. Determine the clotting time by dipping a clean spatula or glass slide into the milk. When coagulation has occurred flecks of curd will appear in the milk film on the slide.

Relative Milk-Clotting Activity Test

A more rigorous test of coagulant activity is the "Relative Milk-Clotting Activity Test" (RMCAT) which measures the activity of rennet and other coagulants in "International Milk-Clotting Units" (IMCU). The method is described in International Dairy Federation standard 157:1992.