Micro milk

Food MicrobiologyFood Microbiology
 Water
 Milk

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

Pathogenic bacteria may bePathogenic bacteria may be
present in milk:present in milk:
of Milkof Milk
 E.coli
 Streptoccus pyogenes
 Mycobacterium tuberculosis
 Salmonella sp.
 Brucella sp.

of Milkof Milk
1-Microscopical Examination of1-Microscopical Examination of
milk:milk:
Note the cells of the normal flora
Enterococcus faecalis
Streptoccus lactus
Lactobacillus sp.
 Candida albicans (yogurt)

2-Determination of viable2-Determination of viable
bacterial count :bacterial count :
UsingUsing the pour plate methodthe pour plate method
after preparation of 10 fold serial dilutionafter preparation of 10 fold serial dilution
from the milk sample with ringer solution.from the milk sample with ringer solution.
of Milkof Milk

Using 10 fold serial dilution method
Viable Bacterial Count
9 ml Saline1 2 3
Milk sample
1 ml milk 1 ml 1 ml
1/10
1/10 x 1/10
1/100
1/100 x 1/10
1/1000
1 ml 1 ml 1 mlMelted NA
1 2 3

Results:Results:Viable Bacterial Count
DilutionDilution
factorfactor 11 22 33 XX X . yX . y
1010 xx11 X1.y1
101022 xx22 X2.y2
101033
xx33 X3.y3
No. of colonies per plate
Y
No. of cells per 1 ml =
X1.y1 + X2.y2 + X3.y3
3

Permissible number of bacterial flora inPermissible number of bacterial flora in
pasteurized milk is 5 x 10pasteurized milk is 5 x 1044
cfu/mlcfu/ml
Permissible number of bacterial flora inPermissible number of bacterial flora in
long life milk is 10 cfu/mllong life milk is 10 cfu/ml
Results:Results:
Viable Bacterial Count

3-Methylene Blue Reduction3-Methylene Blue Reduction
Test:Test:
of Milkof Milk
Increasing the number of bacterial flora willIncreasing the number of bacterial flora will
reduce the colour of methylene blue morereduce the colour of methylene blue more
rapidly due to increasing consumption ofrapidly due to increasing consumption of
oxygen.oxygen.
i.e: The speed of reduction of methylene bluei.e: The speed of reduction of methylene blue
colour is directly proportional to the numbercolour is directly proportional to the number
of bacteria present in milk sample.of bacteria present in milk sample.
To determine quality of the milk

of Milkof Milk
Methylene Blue ReductionMethylene Blue Reduction
Test:Test:ResultsResults
:: The shorter the decolorization time, theThe shorter the decolorization time, the
higher the number of bacterial flora presenthigher the number of bacterial flora present
in milk, andin milk, and the poor quality of milkthe 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

of Milkof Milk
4-Test for coliforms:4-Test for coliforms:
 Done by inoculation of MacConkey’s brothDone by inoculation of MacConkey’s broth
with 0.1 ml of milk sample.with 0.1 ml of milk sample.
 Examine for the production of acidExamine for the production of acid
detected by changing the color ofdetected by changing the color of
the medium from purple to yellow.the medium from purple to yellow.

Loop Dilation MethodLoop Dilation Method
 Loop dilation, or pour plate, method- sampleLoop dilation, or pour plate, method- sample
inoculated serially in to a series of liquid agar tuesinoculated serially in to a series of liquid agar tues
to dilute the number of cells in each successiveto dilute the number of cells in each successive
tubestubes
 Tubes are then poured in to sterile Petri dishes andTubes are then poured in to sterile Petri dishes and
allowed to solidifyallowed to solidify
Figure 3.3 c,d

Spread Plate MethodSpread Plate Method
 Spread plate method- small volume of liquid, dilutedSpread plate method- small volume of liquid, diluted
sample pipette on to surface of the medium and spreadsample pipette on to surface of the medium and spread
around evenly by a sterile spreading toolaround evenly by a sterile spreading tool
Figure 3.3 e,f

Streak Plate MethodStreak Plate Method
 Streak plate method- small droplet of cultureStreak plate method- small droplet of culture
or sample spread over surface of the mediumor sample spread over surface of the medium
with an inoculating loopwith an inoculating loop
Uses a pattern that thins out the sample andUses a pattern that thins out the sample and
separates the cellsseparates the cells
Figure 3.3 a,b

Mycobacterium tuberculosis on
Lowenstein-Jensen (LJ) medium

Micro milk

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