Comparing Raw & Pasteurized Milk and Testing Surfaces for Microbial Contamination Hands-on Module Food Safety Scientist Curriculum © 2014 Board of Regents,

© 2014 Board of Regents, South Dakota State University iGrow.org

Comparing Raw & Pasteurized Milk and Testing Surfaces for Microbial ContaminationHands-on ModuleFood Safety Scientist Curriculum

This project was supported by the USDA NIFA grant number 2011-38411-30625

© 2014 Board of Regents, South Dakota State University iGrow.org iGrow.org

3M™ Pertifilm™ Aerobic Count 3M™ Pertifilm™ E.Coli/coliform Count 2 Disposable Pipettes 3M™ Spreader 2 Tablespoons Raw Milk 2 Tablespoons Pasteurized Milk

Materials For Comparing Pasteurized & Raw Milk

Aerobic Petrifilm™Pasteurized MilkRaw Milk

3M™ Spreader

Disposable Pipettes


Procedure

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.

There will be a total of 8 Petrifilm™ for the milk samples

(4-Raw/4-Pasteurized)

Label 4-Aeroboic count Petrifilms™ and 4-E.Coli/coliform Petrifilms™ with:

Sample Source

Your Initials

Date


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.

1 2

3 4


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


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™ Spreader1 cm Swabbing Template

3M™ Quick Swabs


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.


Procedure

The following swabs will be taken from wet and dry surfaces using a different swab for each sample

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

Quick Swabs

Aerobic Petrifilm™

E.Coli/ Coliform Petrilim™

Wet Method 2 2

Dry Method 2 2

Control 1 1


Procedure: Wet Method

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

Dry Surface

Dry Surface: Back of Chair

1 2

3 4


Procedure: Wet Method

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.

8. Repeat 1 through 7 (sampling three other DRY surfaces in a different location using a different swab for each)

Dry Surface (continued)

Light Switch

Computer Mouse

Door Knob

EXAMPLES


Procedure: Dry Method

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

Wet Surface

Wet Surface: Sink


Procedure: Dry Method

5. Break the reservoir now and swish the swab to suspend the bacteria throughout the solution

6. Label the swab, if not already labeled

7. Repeat 1 through 6 (sampling three other WET surface in a different location using different swabs)

Wet Surface

EXAMPLES

Inside mouth

Toilet

Water Fountain


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.


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.


Incubate all samples for 48 hours before analyzing the

results


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

E.Coli Petrifilm™Aerobic Petrifilm™


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


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)

Documents

Comparing Raw & Pasteurized Milk and Testing Surfaces for Microbial Contamination Hands-on Module Food Safety Scientist Curriculum © 2014 Board of Regents,