Microbiological examination of milk

Question 1

What are the variations of levels of microorganisms?

Answer 1

5000/ml to more than 1 000 000 /ml

Question 2

What techniques can be used to analyze dairy products for microorganisms?

Answer 2

Microscopic and plate

Question 3

Describe sample preparation by dilution technique:

Answer 3

1 ml sample placed into a sterile tube with 9 ml saline = 10^-1 solution
1 ml 10^-1 into sterile tube with 9 ml saline = 10^-2 solution
AND SO ON

Question 4

Describe sample preparation by inoculation on agar medium:

Answer 4

Spread plate technique: 0.1 ml of diluted sample is placed on solid agar medium and spread around, after incubation bacterial colonies appear on surface

Pour plate technique: 1 ml of diluted sample is placed onto an empty agar plate, then nutrient medium is poured onto. Once solidified, another layer is poured over.

Question 5

Total bacteria count indicates:

Answer 5

How many microorganisms are present in a sample that can grow and form countable colonies on standard agar after incubation at 30*C for 72 hours

Question 6

What is the formula used for counting?

Answer 6

N (number of colonies per ml) = Sum of all colonies on all plates counted / (Volume x (1xn1) + (0.1xn2)xdilution from the first counts)
n1 = number of plates in first dilution counted
n2 = number of plates in second dilution counted

Question 7

Plate values:

Answer 7

Normal: 10-300, all CFU are counted. Record dilutions and total number of colonies.

Estimated aerobic plate counts: less than 10 CFU

Too numerous: over 300 CFU

Question 8

Legislation:

Answer 8

Raw cow milk = plate count < 100 000 CFU at 30*C

Raw goat, sheep and buffalo: Not heat treated < 500 000 CFU, heat treated < 1 500 000 CFU

Question 9

Microorganisms:

Answer 9

Enterobacteriaceae: Violet Red Bile Glucose Agar, 37*C for 24 hours. Glucose + and oxidase -

E. coli and coliform: Violet red bile agar, endo agar for detection and isolation

Staphylococcus aureus: baird-parker agar - black, shiny, convex colonies

Bacillus cereus: mannitol-egg yolk polymyxin agar - rough, dry with pink background, egg yolk precipitate

Salmonella: destruction accelerated by UV and heat

Listeria monocytogenes: most virulent foodborne, meningitis in newborns, grow as low as 0*C

Enterobacter Sakazakii: infant formula, isolation agar at 44*C 24 hours, tryptone soya agar