The action of rennin on milk
8)
Rennet is an extract prepared from tissue in the stomach of a calf. It contains
the enzyme rennin. Microbial rennin is also available nowadays.
9)
(B)
Rennin catalyses coagulation of the milk protein called casein. As a result the
milk clots to form curds. Rennet is used in the production of most cheeses.
Warning! Rennin is an enzyme and care should be taken in case of allergic
reactions. Do not put the liquids near your face and wash off immediately any
spilt on your skin.
1)
2)
3)
(C)
How long does it take for the milk to coagulate? (Time it in
minutes.)
Does the milk in both test tubes coagulate?
Put 10 mL of milk at 35 oC and 0.5 mL of rennet essence into a
test tube.
Shake the test-tube.
Time how long it takes for the milk to coagulate.
Repeat (B) at 40 oC, 50 oC, 60 oC and 70 oC.
Tabulate your results like this.
Experiment 1 Effect of Temperature
This experiment attempts to find answers to the following questions.
(a) How does rennin act on milk?
(b) At which temperature does rennin work best?
(c) At which temperature is rennin inactivated?
You will need:
 3 mL rennet essence
 70 mL milk
 2 mL plastic pipette and a measuring cylinder
 7 large test-tubes + rack
 6 water baths set at 30oC, 35 oC, 40 oC, 50 oC, 6 oC , 70 oC
 thermometer
 stop watch
Test tube
1
2
3
4
5
6
7
Temperature (oC)
30
30
35
40
50
60
70
Time taken to coagulate (minutes)
Show your results in a graph like this.
Temperature (oC)
Method
(A)
1)
2)
3)
4)
5)
6)
7)
Place 10 mL of milk in each of two test tubes.
Stand the tubes in a water bath at 30 oC until the milk reaches
30 oC.
Do not add rennet essence to test-tube 1. (This is the control.)
Use pipette to add 0.5 mL of rennet essence to test-tube 2.
Shake test tubes 1 and 2 immediately.
Start the stopwatch.
Observe the contents of both tubes.
Time taken to coagulate
(minutes)
Use your results to answer the questions at the start of the experiment.
Experiment 2 Effect of pH
pH has an effect on enzyme -controlled reactions. This experiment attempts to
find answers to the following questions.
(a) At which pH does rennin work best?
(b) At which pH is rennin inactivated?
You will need:
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
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

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50 mL milk
rennet essence
0.1 M lactic acid solution
0.1 M sodium hydroxide solution
3 2 mL pipettes - one for acid, one for alkali, one for rennet
5 large test-tubes + rack
pH papers or pH meter
water bath at 35 oC
thermometer
stop watch
Method
1)
Label the five test tubes 1, 2, 3, 4 and 5.
Use appropriate pipettes to prepare the following:
• test tube 1 10 mL milk + 1 mL lactic acid
• test tube 2 10 mL milk + 0.5 mL lactic acid
• test tube 3 10 mL milk only
• test tube 4 10 mL milk + 2 mL sodium hydroxide
• test tube 5 10mL milk + 2.5 mL sodium hydroxide
Tabulate your results.
Show your results in a graph.
Use your results to answer questions (a) and (b) at the start of the experiment.
Experiment 3 Effect of Milk Heat Treatment
Milk may be heat treated in different ways.
This experiment attempts to find if the type of heat treatment effects the action
of rennin.
You will need:
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10 mL samples of milk of three types:
pasteurised milk
pasteurised milk which has been boiled and cooled
UHT milk
rennet essence
a 2 mL plastic pipette and a measuring cylinder
3 large test-tubes + rack
water bath at 35 oC
thermometer
stop watch
Method
1)
2)
Place 10 mL of each type of milk into each of the three test tubes.
Place the test tubes in a water bath at 35 oC and leave until each sample
has reached 35 oC.
Add 0.5 mL of rennet essence to each test-tube and mix.
Start the stopwatch.
Note the time taken for the milk to coagulate in each test-tube.
2)
Measure and note down the pH of each solution.
3)
Place the five test tubes in a water bath at 35oC and leave until the
solution in each test-tube has reached 35oC.
4)
Add 0.5 mL of rennet essence to each test-tube and mix.
3)
4)
5)
5)
Start the stopwatch.
Tabulate and explain your results.
6)
Note the time taken for the milk to coagulate in each test-tube.
Explain the significance of this activity to the cheese-making industry.