Unit 6 assignment 2 Project planning:
Testing the spoilage of milk in different fat contents:
How does the rate of bacterial growth become affected by different fat
contents in milk?
Equipment:
Whole milk
Semi-skimmed
Skimmed
Graduated pipette.
Cello tape
Sharpie
pH reader
1cmᵌ syringe
9cmᵌ syringe
Molten agar cooled to 50 ͦC.
Disinfectant
25 ͦC incubator
Distilled water.
18x screw cap bottles
18x 9cm petri dishes
Measuring cylinder
Funnel
Three small beakers
Test tube with distilled water with pH probe
Test tube rack for the test tube containing pH probe.
Method:
1. Clean the work surface before starting (to remove any bacteria already
there).
2. Pour 20ml of each different milk types into three small beakers by using
the measuring cylinder.
3. Test the pH of the milk using the pH reader and record this in results.
(Before switching to each milk rinse then reset the pH reader)
4. Note down the pH results.
5. Using the graduated pipette transfer 1cmᵌ of one of the milks into a
screw cap bottle. As well as 9.9cmᵌ of distilled water. Label using the
sharpie 10-2
-10
6. Repeat until a 10 solution is produced:
, 7. swirl each bottle to gently mix it.
8. Repeat steps 3 to 7 for the other two milk types.
Culturing bacteria:
1. Take 5 petri dishes and label them 10-2, 10-4, 10-6, 10-8 and 10-10
2. Take the molten agar out of the hot water bath and take the funnel and
put it in the measuring cylinder.
3. using a graduated pipette add 12 cmᵌ of molten MRS agar into a
measuring cylinder and add this to the petri dish. Swirl to mix and to
evenly cover the bottom of the dish. Add 1cmᵌ of the 10 -2 milk dilution
into the centre of the corresponding Petri dish.
4. Add 2 to 4 pieces of sticky tape to tape the base of the Petri dish to the
lid.
5. Repeat steps 2 to 4 for the four other milk dilutions.
6. Leave the Petri dishes at room temperature until the agar has solidified.
7. Repeat these steps for the other two milk types.
8. Clean the work surface after the experiment.
9. Incubate at 25°C for four days. A temperature of 25°C discourages the
growth of bacteria pathogenic to humans. Store upside down to prevent
condensation disrupting the culture.
Bacterial count:
1. Remove the Petri dishes from the incubator. Observe the plates and
select the dilution that produces the most distinct colonies.
2. Count the number of bacterial colonies present on the selected plate.
Use a marker to highlight each colony counted on the Petri dish to
prevent re-counting.
3. Estimate the bacterial count of the initial fermented milk sample. Each
bacterial colony arises from a single cell, enabling the estimation of the
number of cells in the initial culture.
Testing the spoilage of milk in different fat contents:
How does the rate of bacterial growth become affected by different fat
contents in milk?
Equipment:
Whole milk
Semi-skimmed
Skimmed
Graduated pipette.
Cello tape
Sharpie
pH reader
1cmᵌ syringe
9cmᵌ syringe
Molten agar cooled to 50 ͦC.
Disinfectant
25 ͦC incubator
Distilled water.
18x screw cap bottles
18x 9cm petri dishes
Measuring cylinder
Funnel
Three small beakers
Test tube with distilled water with pH probe
Test tube rack for the test tube containing pH probe.
Method:
1. Clean the work surface before starting (to remove any bacteria already
there).
2. Pour 20ml of each different milk types into three small beakers by using
the measuring cylinder.
3. Test the pH of the milk using the pH reader and record this in results.
(Before switching to each milk rinse then reset the pH reader)
4. Note down the pH results.
5. Using the graduated pipette transfer 1cmᵌ of one of the milks into a
screw cap bottle. As well as 9.9cmᵌ of distilled water. Label using the
sharpie 10-2
-10
6. Repeat until a 10 solution is produced:
, 7. swirl each bottle to gently mix it.
8. Repeat steps 3 to 7 for the other two milk types.
Culturing bacteria:
1. Take 5 petri dishes and label them 10-2, 10-4, 10-6, 10-8 and 10-10
2. Take the molten agar out of the hot water bath and take the funnel and
put it in the measuring cylinder.
3. using a graduated pipette add 12 cmᵌ of molten MRS agar into a
measuring cylinder and add this to the petri dish. Swirl to mix and to
evenly cover the bottom of the dish. Add 1cmᵌ of the 10 -2 milk dilution
into the centre of the corresponding Petri dish.
4. Add 2 to 4 pieces of sticky tape to tape the base of the Petri dish to the
lid.
5. Repeat steps 2 to 4 for the four other milk dilutions.
6. Leave the Petri dishes at room temperature until the agar has solidified.
7. Repeat these steps for the other two milk types.
8. Clean the work surface after the experiment.
9. Incubate at 25°C for four days. A temperature of 25°C discourages the
growth of bacteria pathogenic to humans. Store upside down to prevent
condensation disrupting the culture.
Bacterial count:
1. Remove the Petri dishes from the incubator. Observe the plates and
select the dilution that produces the most distinct colonies.
2. Count the number of bacterial colonies present on the selected plate.
Use a marker to highlight each colony counted on the Petri dish to
prevent re-counting.
3. Estimate the bacterial count of the initial fermented milk sample. Each
bacterial colony arises from a single cell, enabling the estimation of the
number of cells in the initial culture.
