Bacterial fermentation causes milk to spoil over time. Lactic acid bacteria and coliforms are the main types of bacteria present in milk. When milk is left at room temperature for 1 day, its pH decreases steadily as the lactic acid bacteria ferment the milk's lactose into lactic acid, causing acidification. Monitoring the milk's pH drop over 24 hours showed the rate of change in pH remaining constant as fermentation occurred.

1. Bacterial Fermentation
AZFATINI BT NAWI
D20081032320
ZETY LIYANA BT ZAINALABIDIN
D20081032351
SYAFIQAH SHAARI
D20091034832
YIP SOOK HAN
D20091034841

2.  Milk is contaminated by bacteria even before
it leaves the udder, although at this point
they are harmless and few in numbers.
 The two main groups of bacteria in milk are
Lactic acids and coliforms. Lactic acid is the
normal bacteria present in milk and dairy
products. Coliforms are the main reason for
milk spoilage.

3. Before After 1 day
What happen to the milk? Why?

4. Planning and Doing Experiment – observation of
the change in pH when the milk is left for 1 day.

5. Result
Time (hour) pH Time (hour) pH
1 5.442 13 5.186
2 5.442 14 5.174
3 5.442 15 5.158
4 5.380 16 5.149
5 5.360 17 5.142
6 5.340 18 5.133
7 5.299 19 5.121
8 5.278 20 5.117
9 5.258 21 5.112
10 5.237 22 5.096
11 5.230 23 5.074
12 5.197 24 5.063

6. Graph
4.8
4.9
5
5.1
5.2
5.3
5.4
5.5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
pH
Time (hour)
pH versus Time (hour)
pH

7. Discussion
1. Did you see a reduction of pH shortly after
starting the experiment?
2. What causes the reduction in the pH of
milk?
3. Does the speed of change in pH stay
constant over the whole experiment
period?
4. Calculate the extent of change in pH
obtained in the process.
5. What would be the results of incubating
UHT milk over the same time period?

8. School Milk Program has to be postponed for a time being
because the food poisoning case among students due to milk
spoilage. What are the factors causing the milk spoilage and
how to overcome the problems?

9. By using data
logger, we do not
need a constant
observation.
The graph can
be plotted
easily.
Low cost
because we
only use
milk.
The result can be analyzed
and interpreted easily.
Easy to perform.

10. Measuring the pH of Milk. Retrieved on 31st
August 2010 from
http://www.picotech.com/experiments/ph_
of_milk/ph_of-milk.html