The following demonstration provides an example of the use of a protein
produced by biotechnology for making ice cream, custard, and cheese. Chymosin
is a protein that occurs in rennet, which has been used for decades by
cheesemakers. Until 1990, the chymosin (rennet) was obtained from the
stomach of animals, preferably milk-fed calves. In 1990, the U.S. Food
and Drug Administration approved the use of chymosin produced by genetically-engineered
bacteria. The chymosin from the bacteria is identical to that obtained
The demonstration can be done by the teachers or the students. Only kitchen
equipment and utensils are needed.
- Chymosin :Iowa teachers and extension specialists can
obtain the enzyme from the Office of Biotechnology at Iowa State University.
It should be stored in a refrigerator, not a freezer, when not in use.
The chymosin is provided to Iowa State University by Pfizer.
- 1 medicine dropper :Used for adding a drop of chymosin
- 1 glass measuring cup : The one-cup size works well,
but any available size is fine.
- an oven or hot water bath : The oven/bath must maintain
temperatures between 95° F (35° C) and 110° F (43°C)
for 30 minutes.
- an oven or candy thermometer : Used to check the temperature
of the oven or hot water respectively.
- whole milk : Milk must be at room temperature. Milk with less fat
content will work, but the results won't be as dramatic.
- Pour about 50 ml of warm milk in the measuring cup. Use
the ml marks on the measuring cup and pour the milk directly from the
container into the cup.
- Add only one drop of chymosin to the milk. It is unnecessary
to use more than one drop in up to 200 ml of milk.
- Swirl the milk in the cup gently for 10 seconds,
then place the cup in the oven or hot water. Within 30 minutes, the
milk protein and fat will have coagulated and a solid curd will be present.
The demonstration is complete.
One drop of chymosin can be used to coagulate 200 ml of milk. The curd
is the most firm when 50 ml of milk is used. Use of about 200 ml of milk
does not provide as solid a curd as with only 50 ml, but does make the
whey (water) more obvious, if this is important for the student to see.