Cheese Making
(12-6-10)
Background
Cheese making is a prehistoric skill that humans developed to preserve surplus milk into a
food that can be stored for weeks to months without refrigeration. It is believed that
the first cheese was made by some ancient people that stored fresh milk in the stomach
of a young sheep, goat or calf. The day-old milk separated into curds (solids) and whey
(liquid). When the curds were removed and dried, they could be preserved for later use.
The addition of salt increased the amount of time the dried curds could be stored. As a
result, an extremely perishable food, milk, could be preserved for long periods of time.
Mammals have rennet (rennin) in their stomachs to slow down the digestion of the
mother’s milk in order to obtain maximum nutrition from the milk. Rennet (rennin) is the
cheese making enzyme found in the stomachs of ruminants. Until 1990, rennet was
produced by grinding up the fourth stomach of pre-weaned ruminants like goats, sheep,
calves. With today’s biotechnology advances, genetically modified fungi produce the
rennet needed for cheese making and the destruction of young ruminants is no longer
necessary to make cheese.
Cheese is basically made in one of four ways:
1. Cheese can be made by allowing the bacteria already in milk to produce lactic acid
which denatures (unwinds) the protein (casein) and makes the milk curdle.
2. Cheese can be made by adding cultures of known bacteria such as Lactobacillus
bacteria. This bacteria produces the lactic acid that makes the protein denature
resulting in curdled milk. Lactobacillus is also one of the good bacteria used to
make yogurt, buttermilk, and sour cream.
3. Cheese can be made by adding purified enzymes such as rennet (rennin) which
comes from the stomach linings of mammals like calves. Rennin cleaves the protein
into smaller parts which settle out of the solution and curdles the milk.
4. Today’s advances in biotechnology allow cheese to be made by using the genetically
engineered rennet (rennin) enzyme called chymosin. Scientists found the gene that
makes the rennet enzyme and cut the gene out using restriction enzymes. They
paste the rennet (rennin) gene into a certain type of fungus which produces the
recombinant protein rennin. Note: Using fungi to make rennin saves the calves!!!
Purpose:
To determine the effect of curdling agents on:
(a) curdling time
(b) volume of cheese produced
Hypothesis:___________________________________________________
____________________________________________________________
Materials:

Test tubes (3 test tubes per student team)

Plastic transfer pipettes (3 per student team)

Whole milk (about 15 mL per student team)

Buttermilk (about 1 mL per student team)

Rennet Solution (1 Junket tablet in ½ cup water)

Water bath, 370C

Test tube rack (1 per student team)

10 mL graduated cylinders (2 per student team)

Filter paper (3 pieces per student team)

Funnel (1 per student team)

Timer (1 per student team)

Corks for test tubes
Procedures:
1. Label test tube #1 “Milk + Milk.” This tube contains the control.
2. Label test tube #2 “Milk + Buttermilk.” This tube contains a variable being tested.
3. Label test tube #3 “Milk + Rennet.” This tube contains a variable being tested.
4. Using a plastic transfer pipette, transfer 7 mL of whole milk into each test tube.
5. Add 0.25 mL of milk to test tube #1. Cap the tube and invert 3 times
6. Add 0.25 mL of buttermilk to test tube #2. Cap the tube and invert 3 times
7. Add 0.25 mL of Rennet solution to test tube #3. Cap the tube and invert 3 times.
8. Place all tubes from all student teams into a 370C incubator OR place tubes in your
armpit for at least 15 minutes. Check each tube every 5 minutes to estimate the
percent of curds formed. Curds are the solidified lumps of milk. Record your
estimates in the first data table. Collect data from the other student groups to
complete the first data table.
9. For the second table, determine and record the average amount of time needed for
the highest percentage curdling.
10. Measure the total volume of curds (solids) and whey (liquid) together in a graduated
cylinder. Record your data in the second table.
11. Using a plastic funnel and filter paper cone, pour the curds and whey mixture
through a filter paper funnel into a second graduated cylinder. Record the amount
of whey (liquid) in mL. Take the total volume of the mixture and subtract the
amount of whey to determine the amount of curds. Record the amount of curds.
Estimated Percent of Curdling Using Different Curdling Agents
Milk + Milk
5 min
10 min
15 min
Milk + Rennet
5 min
10 min
15 min
Milk + Buttermilk
5 min
10 min
15 min
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Group 7
Group 8
Effects of Different Curdling Agents on Curdling Time and Volume of Cheese Produced
Curdling
Agent
Avg. time to
reach highest
percent
curdling (min)
Estimated
highest
percent
curds
Total Volume
of Curds and
Whey (mL)
Volume of Volume of
Whey (mL) Curds (mL)
milk
buttermilk
rennet
Conclusion:
Daugherty, Ellyn. Biotechnology Science for the New Millennium Laboratory Manual. Saint
Paul: Paradigm Publishing, Inc., 2007.
Fankhause, David. “Rennet for Making Cheese.” 03 Nov. 2010
http://biology.clc.uc.edu/fankhauser/cheese/Rennet/Rennet.html.