micro milk

Dr/ Walaa Mahmoud Elsherif

Food MicrobiologyFood Microbiology

Water Milk

Normal flora of milk:Normal flora of milk:

Bacteriological Bacteriological Examination of MilkExamination of Milk

Enterococcus faecalisStreptoccus lactusLactobacillus sp. Candida albicans (yogurt)

Pathogenic bacteria may Pathogenic bacteria may be present in milk:be present in milk:


E.coli Streptoccus pyogenes Mycobacterium tuberculosis Salmonella sp. Brucella sp.


1-Microscopical Examination 1-Microscopical Examination of milk:of milk:

Note the cells of the normal floraEnterococcus faecalisStreptoccus lactusLactobacillus sp. Candida albicans (yogurt)

2-Determination of viable 2-Determination of viable bacterial count :bacterial count :

Using Using the pour plate methodthe pour plate method after preparation of 10 fold serial dilution after preparation of 10 fold serial dilution

from the milk sample with ringer solution. from the milk sample with ringer solution.


Using 10 fold serial dilution methodViable Bacterial Count

9 ml Saline1 2 3Milk sample

1 ml milk 1 ml 1 ml

1/101/10 x 1/10

1/1001/100 x 1/101/1000

1 ml 1 ml 1 mlMelted NA

1 2 3

Results:Results:Viable Bacterial Count

DilutioDilution n

factorfactor 11 22 33 XXX . yX . y

1010 xx11 X1.y1

101022 xx22 X2.y2 101033

xx33 X3.y3

No. of colonies per plateY

No. of cells per 1 ml = X1.y1 + X2.y2 + X3.y3

3

Permissible number of bacterial flora Permissible number of bacterial flora in pasteurized milk is 5 x 10in pasteurized milk is 5 x 1044 cfu/ml cfu/ml

Permissible number of bacterial flora Permissible number of bacterial flora in long life milk is 10 cfu/mlin long life milk is 10 cfu/ml

Results:Results:

Viable Bacterial Count

3-Methylene Blue Reduction 3-Methylene Blue Reduction Test:Test:


Increasing the number of bacterial flora Increasing the number of bacterial flora will reduce the colour of methylene will reduce the colour of methylene blue more rapidly due to increasing blue more rapidly due to increasing consumption of oxygen.consumption of oxygen.

i.e: The speed of reduction of methylene i.e: The speed of reduction of methylene blue colour is directly proportional to blue colour is directly proportional to the number of bacteria present in milk the number of bacteria present in milk sample.sample.

To determine quality of the milk


Methylene Blue Reduction Methylene Blue Reduction Test:Test:ResultResult

s:s: The shorter the decolorization time, The shorter the decolorization time, the higher the number of bacterial flora the higher the number of bacterial flora present in milk, and present in milk, and the poor quality of the poor quality of milkmilkDecolorization time Result

30 min – 2 hrs Poor quality 2 – 6 hrs fair quality 6 – 8 hrs good quality Over 8 hrs excellent quality


4-Test for coliforms:4-Test for coliforms:

Done by inoculation of MacConkey’s Done by inoculation of MacConkey’s broth with 0.1 ml of milk sample.broth with 0.1 ml of milk sample.

Examine for the production of acid Examine for the production of acid detected by changing the color of detected by changing the color of the medium from purple to yellow.the medium from purple to yellow.

Figure 3.2

Loop Dilation MethodLoop Dilation Method Loop dilation, or pour plate, method- sample Loop dilation, or pour plate, method- sample

inoculated serially in to a series of liquid inoculated serially in to a series of liquid agar tues to dilute the number of cells in agar tues to dilute the number of cells in each successive tubeseach successive tubes Tubes are then poured in to sterile Petri dishes Tubes are then poured in to sterile Petri dishes

and allowed to solidifyand allowed to solidify

Figure 3.3 c,d

Spread Plate MethodSpread Plate Method Spread plate method- small volume of liquid, Spread plate method- small volume of liquid,

diluted sample pipette on to surface of the diluted sample pipette on to surface of the medium and spread around evenly by a sterile medium and spread around evenly by a sterile spreading toolspreading tool

Figure 3.3 e,f

Streak Plate MethodStreak Plate MethodStreak plate method- small droplet of Streak plate method- small droplet of

culture or sample spread over surface culture or sample spread over surface of the medium with an inoculating loopof the medium with an inoculating loopUses a pattern that thins out the sample Uses a pattern that thins out the sample

and separates the cellsand separates the cells

Figure 3.3 a,b

Blood agar

Mycobacterium tuberculosis on Lowenstein-Jensen (LJ) medium

Figure 3.25

Figure 3.10

Figure 3.7

Figure 3.8

Figure 3.9

Figure 3.1