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Yoghurt investigation Aim The aim of this experiment is to investigate the effect of temperature on yoghurt production by measuring the change in the pH of milk - before and after the starter culture is added.
Introduction Yoghurt is made by the fermentation of lactose (milk sugar) by enzymes from bacteria called Lactobacillus bulgaricus. This process is anaerobic, which means that it occurs in the absence
of oxygen. During the production o f yoghurt, the lactose is broken down by the lactase enzyme (provided by the bacteria) into lactic acid and energy, Lactose
lactic acid + energy
The production of lactic acid causes the yoghurt to have a sour taste. Lactic acid also lowers the pH of the milk which affects the proteins in the milk (casein) causing it to coagulate and precipitate, forming the solid curd that makes up yoghurt. The leftover watery liquid is the whey. Before starting making yoghurt the milk is heated to around 70ºC to kill any other bacteria in the milk, which might also ferment it, so that the only bacteria it contains are the ones we will add. The milk is left to cool, so that the bacteria we are going to add are not also killed, and then the starter culture is added to it.
Variables Dependent variable (what I will measure)
pH range = 2.77 (6.82-4.05=2.77)
Independent variable (what I will change)
Controlled variables (what I will keep the same)
temperature: 4 ,20, 30, 40ºC
volume of milk = 20cmᶾ volume yoghurt= 1cmᶾ type of milk type of yogurt time = 72h
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Safety
Wear a lab coat & goggles to protect you from spills
Do not eat the yoghurt
Method 1. With a marker label 4 test tubes 4x, 20x, 30x & 40x and 4 other test tubes 4y 20y, 30y, 40y. ,
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2. Measure 20cm of milk with a measuring cylinder & pour it into each test tube. 3. Measure 1cm 3 of yoghurt (starting culture) with a pipette & add it to each test tube. 4. Put a bung in each test tube & shake it gently. 5. Measure the pH of mixture in each test tube with a pH sensor & record it on the results table. Have a beaker with water to rinse the probe each time.
6. Cover each test tube with cling film so no bacteria get in it. 7. Place the 2 test tubes labelled 4x & 4y in the fridge, the 2 labelled 20x & 20y at room temperature, the 2 labelled 30x & 30y in a water bath at 30ºC and the other 2 labelled 40x & 40y in a water bath at 40ºC.
8. After 72h observe the mixture in each test tube (if it is liquid or solid) & measure again the pH of mixture in each test tube. Record your observations & measurements on the results table.
Diagram
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Test tube
1cm yoghurt (staritng culture)
3
10cm milk
pH sensor
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Results tables Group1 Madalena, Francisca, Luís
Group2 André, Toninho, Michael
pH before adding yoghurt
Group3 Joana, Catarina, Rita, António
Group4 Benedita, Helena
pH after adding yoghurt
Group1 4x
6.43
4y
6.62
20x
6.61
20y
6.68
30x
6.51
30y
6.61
40x
6.53
40y
6.53
6.43 liquid 6.62 liquid 6.61 liquid 6.68 liquid 6.51 liquid/solid 6.61 liquid/solid 6.53 solid 6.53 solid
Table 1 – Results from group1
pH before adding yoghurt
pH after adding yoghurt
Group1
Group2
Group3
Group4
Group1
Group2
Group3
Group4
4x
6.43
6.55
6.65
6.40
6.33
6.51
6.75
6.45
4y
6.62
6.55
6.75
6.20
6.62
6.52
6.77
6.05
20x
6.61
6.55
6.70
6.47
6.54
6.26
6.55
6.22
20y
6.68
6.49
6.50
6.20
6.46
6.16
6.46
6.22
30x
6.51
6.47
6.51
5.97
4.41
4.35
4.75
6.53
30y
6.61
6.51
6.65
6.35
4.42
4.41
4.73
4.22
40x
6.53
6.57
6.82
6.18
4.14
4.06
4.49
4.17
40y 6.53 6.78 6.69 Table 2 – Results from the 4 groups
6.42
4.12
4.05
4.45
4.14
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Calculations pH change = e.g. Group1 6.33-6.43= -0.10
Total = Add all the pH change (x+y) for each temperature e.g. -0.10 + -0.04 + 0.1 + 0.05 +0.00 +-0.03 + 0.02 + -0.15 = -0.15
Mean =
e.g. -015/8 = -0.019
pH change Group1
Group2
Group3
Group4
4x
-0.10
-0.04
0.10
0.05
4y
0.00
-0.03
0.02
-0.15
20x
-0.07
-0.29
-0.15
-0.25
20y
-0.22
-0.33
-0.04
0.02
30x
-2.10
-2.12
-1.76
0.56
30y
-2.19
-2.10
-1.92
-2.13
40x
-2.39
-2.51
-2.33
-2.01
40y -2.41 -2.73 Table 3 – pH change & means
-2.24
-2.28
Total for each temperature
Mean for each temperature
-0.15
-0.019
-1.33
-0.166
-13.76
-1.720
-18.90
-2.363
Effect of temperature on yoghurt production 0.500 0.000 0
5
10
15
20
25
30
35
40
-0.500 -1.000
pH change -1.500 -2.000 -2.500 -3.000
Temperature (ºC)
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Conclusion From my results I can conclude that the optimum temperature to produce yoghurt is 40ºC. At 40ºC the pH change is the biggest (-2.363), more acidic, the milk coagulates and the mixture gets the texture of yoghurt. For temperatures of 4ºC and 20ºC the pH change is very small and the mixture is still liquid, and for 30ºC although the pH change is -1.720 the milk is not totally coagulated as the mixture is liquid and solid (not the texture of yoghurt). The Lactobacillus bulgaricus present in the starting culture (yoghurt) uses the lactose from the milk as an energy source and the lactase enzyme (provided by the bacteria) produces lactic acid as its fermentation product which lowers the pH of the milk causing it to coagulate. At low temperatures such as 4 and 20ºC the activity of the lactase enzyme is almost none and as lactic acid is not produced the pH of the milk is not lowered and the milk doesn´t coagulate. At 30ºC the activity of the lactase enzyme is increased, however this activity is not fast enough to ferment the milk and produce enough lactic acid to lower the pH of the milk to the optimum value and coagulate it into the proper texture of yoghurt. A temperature of 40ºC encourages the growth of bacteria which allows the lactase enzyme to rapidly break down the lactose in the milk into lactic acid. This causes the pH of the milk to decrease to the optimum value which in turn causes the protein from the milk (casein) to coagulate, forming the solid curd that makes up yoghurt.
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