Comparing Raw & Pasteurized
Milk and Testing Surfaces for
Microbial Contamination
Hands-on Module
Food Safety Scientist Curriculum
This project was supported by the USDA NIFA grant number 2011-38411-30625
© 2014 Board of Regents, South Dakota State University
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Materials For Comparing
Pasteurized & Raw Milk
 3M™
Pertifilm™ Aerobic Count
 3M™ Pertifilm™ E.Coli/coliform Count
 2 Disposable Pipettes
 3M™ Spreader
 2 Tablespoons Raw Milk
 2 Tablespoons Pasteurized Milk
3M™ Spreader
Aerobic Petrifilm™
Disposable Pipettes
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Raw Milk
Pasteurized Milk
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Procedure
Label 4-Aeroboic
count Petrifilms™
and 4E.Coli/coliform
Petrifilms™ with:
 Sample
 Your
 Date
Source
Initials
There will be a total of 8 Petrifilm™ for the milk samples
(4-Raw/4-Pasteurized)
Sample Source
Aerobic Petrifilm™
E.coli/coliform
Petrifilm
Raw
2
2
Pasteurized
2
2
Control
You do NOT need to do this. The teacher will
incubate 1 Petrifilm™ of each for the entire class.
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Procedure
1.
Use a disposable pipette to collect 1
milliliter (mL) of Pasteurized milk from
bottle
2.
Peel back the top film on the Petrifilm™
plate and squeeze the collected milk in
the middle of the surface
3.
Gently drop the top film onto the sample
to prevent air bubbles from forming.
4.
Place the spreader on top of the
Petrifilm™ with the flat side facing up;
gently press the spreader to evenly
distribute sample.
Repeat the process as needed for the
experimental design – refer to next slide.
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1
2
3
4
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Procedure
5.
Repeat steps 1-4 using the raw
milk
*Each group will have an overall total of 10
Petrifilm™ samples including the two controls
prepared by the teacher
6.
Carefully place all 10 Petrifilms™
inside the incubator at
approximately 90ºF (32ºC) for
48 hours
© 2014 Board of Regents, South Dakota State University iGrow.org
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Materials for Testing Surfaces for
Microbial Contamination
 3M™
Quick Swabs
 1 cm Swabbing Template
 3M™ Pertifilm™ – Aerobic Count
 3M™ Pertifilm™ E.Coli/coliform Count
 Spreader
Aerobic Petrifilm™
3M™ Quick Swabs
© 2014 Board of Regents, South Dakota State University iGrow.org
1 cm Swabbing Template
3M™ Spreader
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Procedure
3M™ Quick Swabs can collect samples from either
wet surfaces or dry surfaces;
a. Dry Swab Method for Wet Surface
Examples: Sinks, facets, water
fountains,
toilets, inside the mouth,
etc.
b. Wet Swab Method for Dry Surface
Examples: tables, cell phones, door
knobs,
book covers, locker doors,
etc.
© 2014 Board of Regents, South Dakota State University iGrow.org
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Procedure

The following swabs will be taken from wet and dry surfaces
using a different swab for each sample
Quick
Swabs
Aerobic
Petrifilm™
E.Coli/ Coliform
Petrilim™
Wet Method
2
2
Dry Method
2
2
Control
1
1

There will a total of 10 Quick Swab samples taken:
4 Wet, 4 Dry, 2 Controls

DO NOT FORGET TO LABEL ALL PETRIFILM & QUICK
SWABS – Location swab was obtained, initials, date
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Procedure: Wet Method
Dry Surface
Dry Surface: Back of Chair
1.
Break the reservoir on the
3M™ Quick Swab
2.
Swish the tube to wet the
cotton tip of the swab
3.
Choose a DRY surface with
the 1 cm swabbing template
to collect bacteria sample.
4.
Swab 1 cm area on the dry
surface
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1
2
3
4
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Procedure: Wet Method
Dry Surface (continued)
5.
Place the swab back in its case
6.
Close the case tightly and swirl it to suspend the
bacteria throughout the solution
7.
Make certain it is labeled correctly: Location swab was
obtained, initials, date.
EXAMPLES
Light Switch
8.
Repeat 1 through 7 (sampling three other DRY surfaces
in a different location using a different swab for each)
© 2014 Board of Regents, South Dakota State University iGrow.org
Computer
Mouse
Door Knob
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Procedure: Dry Method
Wet Surface
Wet Surface: Sink
1.
Choose a WET surface and cover the surface with 1 cm swabbing
template to collect a bacteria sample
2.
DON’T break the reservoir, but remove the dry swab from the case
3.
Swab a 1 cm area on the wet surface
4.
Place the swab into the case and close tightly
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Procedure: Dry Method
Wet Surface
5.
6.
Break the reservoir now and
swish the swab to suspend the
bacteria throughout the
solution
EXAMPLES
Label the swab, if not already
labeled
Inside mouth
5.
Repeat 1 through 6 (sampling
three other WET surface in a
different location using
different swabs)
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Toilet
Water Fountain
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Procedure: Culturing the Quick
Swab Samples

Label 4 aerobic count
petrifilm™ and 4
E.Coli/Coliform
petrifilm™ (if not already
completed) with date,
sample source, and
initials

Label Control Slides for
each petrifilm™.
 Fluid from quick swab
poured onto each type
of petrifilm. Do not do
any swabs.
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Procedure: Culturing the Quick
Swab Samples

Follow the steps from 5 to 12 in milk test to prepare
petrifilm plates. Substitute the liquid in the 3M Quick
Swabs in place of milk.
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Incubate all samples for 48 hours
before analyzing the results
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Interpreting Data

After 48 hours, remove the Petrifilm from the incubator

If the Petrifilm has a number of colonies that can be
easily counted, count all the colonies directly
Leave the top layer on the Petrifilm down
Aerobic Petrifilm™
© 2014 Board of Regents, South Dakota State University iGrow.org
E.Coli Petrifilm™
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Interpreting Data

When the sample has too many colonies to count
directly, estimate the number of colonies by:
 Picking a square in the middle of the sample area and
count the colonies in this square area
 Multiple the number of colonies in this square area by
20, and that is approximately how many bacteria
colonies there are on the petrifilm
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Interpreting Data
 Compare
the number of colonies from different
sample sources and interpreting the results of the
experiment
Source
CFUs (colony-forming
units) on E. Coli Petrifilm
CFUs (colony-forming units)
on Aerobic Petrifilm
(refer to lab report)
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