1. PHT382
Lab. No. 11

2. Food Microbiology
Water
Milk

3. Bacteriological Examination
of milk

4. Bacteriological Examination of Milk
Normal flora of milk:
Enterococcus faecalis
Streptoccus lactus
Lactobacillus sp.
Candida albicans (yogurt)

5. Bacteriological Examination of Milk
Pathogenic bacteria may be present in milk:
E.coli
Streptoccus pyogenes
Mycobacterium tuberculosis
Salmonella sp.
Brucella sp.

6. Bacteriological Examination of Milk
Microscopical Examination of milk:
Note the cells of the normal flora
Enterococcus faecalis
Streptoccus lactus
Lactobacillus sp.
Candida albicans (yogurt)

7. Bacteriological Examination of Milk
Determination of viable bacterial count :
Using the pour plate method
after preparation of 10 fold serial dilution from the milk sample with ringer solution.

8. Viable Bacterial Count
Using 10 fold serial dilution method
1 ml milk
1 ml
1 ml 1 2 3
9 ml Saline
Milk sample
1/10 x 1/10
1/100
1/100 x 1/10
1/1000
1/10
Melted NA
1 ml
1 ml
1 ml 1 2 3

9. Viable Bacterial Count
Results:
Viable Bacterial Count Y
Dilution factor 1 2 3 X
X . y 10 x1
X1.y1
102 x2
X2.y2
103 x3
X3.y3
No. of colonies per plate
No. of cells per 1 ml =
X1.y1 + X2.y2 + X3.y3 3

10. Viable Bacterial Count
Results:
Permissible number of bacterial flora in pasteurized milk is 5 x 104 cfu/ml
Permissible number of bacterial flora in long life milk is 10 cfu/ml

11. Bacteriological Examination of Milk
Methylene Blue Reduction Test:
To determine quality of the milk
Increasing the number of bacterial flora will reduce the colour of methylene blue more rapidly due to increasing consumption of oxygen.
i.e: The speed of reduction of methylene blue colour is directly proportional to the number of bacteria present in milk sample.

12. Bacteriological Examination of Milk
Methylene Blue Reduction Test:
Results:
The shorter the decolorization time, the higher the number of bacterial flora present in milk, and the poor quality of milk
Decolorization time
Result
30 min – 2 hrs
Poor quality
2 – 6 hrs
fair quality
6 – 8 hrs
good quality
Over 8 hrs
excellent quality

13. Bacteriological Examination of Milk
Test for coliforms:
Done by inoculation of MacConkey’s broth with 0.1 ml of milk sample.
Examine for the production of acid detected by changing the color of the medium from purple to yellow.

14. Thank you
كل عام وأنتم بخير
With my Best Wishes,,,
Manal Abu El-Khair