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FHSN2100 Lab 7 1 LABORATORY SESSION 7 TITLE: BACTERIAL COUNT OF A FOOD PRODUCT PREREQUISITES: Required readings: • Text: “Prescott, Harley, & Klein’s Microbiology” (7th or 8th Ed.). • Laboratory notes OBJECTIVES: At the conclusion of this unit the student will be expected to be able to: 1. Explain why the standard plate count is used in food quality control. 2. Determine the number of bacteria in a food sample by performing a standard plate count. 3. Statistically compare the number of bacteria between food samples. LABORATORY SEQUENCE: 1. Pre-lab discussion on the program for the session. 2. Assess Durham tubes for the presence of acid and/or gas (Lab 6). 3. Assess sugar disks from Lab 6. 4. Make dilutions of your mince slurry from 102 to 106. 5. Set up pour plates of your dilutions in plate count agar (PCA). 6. Incubate the plates in an inverted position for 24 to 48 hours at 35ºC. EVALUATION/FOLLOW UP: Prepare a laboratory report. THIS LAB REPORT IS WORTH 15% FHSN2100 Lab 7 2 Bacterial Count of a Food Product Safety Considerations No mouth pipetting. Always handle raw meat with caution. Materials per Group of Students 20g of hamburger mince (fresh) 20g of hamburger mince (left at RT for 1 hour) 180 ml of sterile distilled water weighing paper scale or balance Bunsen burner Colony counter Plate count agar (Standards Methods Agar) 2 x 99ml sterile saline dilution blanks sterile 1ml pipettes with pipettor 5 sterile petri plates 35 ºC incubator wax pencil boiling water bath 48º to 50º C water bath for cooling tubes. Suggested Reading in Textbook Food Spoilage Processes. Chapter 43 FHSN2100 Lab 7 3 Principles The sanitary control of food quality is primarily concerned with testing food products for the presence of specific microorganisms. Food products are the primary vehicle responsible for the transmission of microbial diseases of the gastrointestinal system. For this reason, food products are routinely examined for the presence of bacteria. The heterotrophic plate count can be used to determine the number of viable bacteria in a food sample. The larger the count, the greater the likelihood that specific pathogens capable of causing disease will be present and also that the food will spoil. Normally, raw hamburger mince should not contain over 106 bacteria per gram. One of the limitations of the heterotrophic plate count is that only bacteria capable of growing in the culture medium under the environmental conditions provided will be counted. As a result, a medium that supports the growth of most heterotrophic (requiring organic carbon) bacteria is commonly used. Heterotrophic plate count: A test used to estimate the total number of all types of bacteria in an environmental sample, usually water (WHO 2003). FHSN2100 Lab 7 4 Procedure: First Period 1. Weigh out 20g of raw hamburger mince. 2. Blend the 20g of the meat in a blender with 180ml of sterile distilled water for 5 minutes. This gives a 1/10 dilution. * STEPS 1 AND 2 PREPARED BY YOUR DEMONSTRATOR IN ADVANCE * 3. Make dilutions from 102, 103, 104, 105, 106 . 4. Using the marker pen, label the petri plates with your name, date and dilution. 5. Using aseptic technique, pipette aliquots from the dilution blanks to the petri plates. 6. Melt the plate count agar (PCA) pours in a water bath and cool to about 48º to 50ºC. Add cooled PCA agar to the plates. Gently swirl on a flat surface in a figure-eight motion and allow to harden. 7. Incubate the plates in an inverted position for 24 to 48 hours at 35ºC. Second Period 1. Arrange the plates in order of lowest to highest dilution. 2. Count the number of colonies on the plates that have between 25 to 250 colonies. Designate plates with fewer than 25 colonies as too few to count (TFTC) and plates with more than 250 colonies as too numerous to count (TNTC). 3. Calculate the average number of bacteria per gram of hamburger as follows: Number bacteria/gram = number of colonies x _____1_____ dilution factor x _____1______ weight of sample FHSN2100 Lab 7 5 For example, if 200 colonies were counted from a 10-3 dilution of a 20 gram sample: Number bacteria/gram = 200 x 1/10-3 x 1/20g = 10,000 per gram 4. Complete a statistical analysis of the number of bacteria within the food samples: i) Compare the number of bacteria in fresh hamburger mince versus mince left at RT. Record your results in your Laboratory Manual. Questions: What do these results mean? What do they tell you about the importance of safe food handling, etc.?

FHSN2100 Lab 7
1
LABORATORY
SESSION 7
TITLE: BACTERIAL COUNT OF A FOOD PRODUCT
PREREQUISITES:
Required readings:
• Text: “Prescott, Harley, & Klein’s Microbiology” (7th or 8th Ed.).
• Laboratory notes
OBJECTIVES: At the conclusion of this unit the student will be expected to be able to:
1. Explain why the standard plate count is used in food quality control.
2. Determine the number of bacteria in a food sample by performing a standard plate
count.
3. Statistically compare the number of bacteria between food samples.
LABORATORY SEQUENCE:
1. Pre-lab discussion on the program for the session.
2. Assess Durham tubes for the presence of acid and/or gas (Lab 6).
3. Assess sugar disks from Lab 6.
4. Make dilutions of your mince slurry from 102 to 106.
5. Set up pour plates of your dilutions in plate count agar (PCA).
6. Incubate the plates in an inverted position for 24 to 48 hours at 35ºC.
EVALUATION/FOLLOW UP:
Prepare a laboratory report. THIS LAB REPORT IS WORTH 15%
FHSN2100 Lab 7
2
Bacterial Count of a Food Product
Safety Considerations
No mouth pipetting. Always handle raw meat with caution.
Materials per Group of Students
20g of hamburger mince (fresh)
20g of hamburger mince (left at RT for 1 hour)
180 ml of sterile distilled water
weighing paper
scale or balance
Bunsen burner
Colony counter
Plate count agar (Standards Methods Agar)
2 x 99ml sterile saline dilution blanks
sterile 1ml pipettes with pipettor
5 sterile petri plates
35 ºC incubator
wax pencil
boiling water bath
48º to 50º C water bath for cooling tubes.
Suggested Reading in Textbook
Food Spoilage Processes. Chapter 43
FHSN2100 Lab 7
3
Principles
The sanitary control of food quality is primarily concerned with testing food products
for the presence of specific microorganisms. Food products are the primary vehicle
responsible for the transmission of microbial diseases of the gastrointestinal system.
For this reason, food products are routinely examined for the presence of bacteria.
The heterotrophic plate count can be used to determine the number of viable bacteria
in a food sample. The larger the count, the greater the likelihood that specific
pathogens capable of causing disease will be present and also that the food will spoil.
Normally, raw hamburger mince should not contain over 106 bacteria per gram.
One of the limitations of the heterotrophic plate count is that only bacteria capable of
growing in the culture medium under the environmental conditions provided will be
counted. As a result, a medium that supports the growth of most heterotrophic
(requiring organic carbon) bacteria is commonly used.
Heterotrophic plate count: A test used to estimate the total number of all types of
bacteria in an environmental sample, usually water (WHO 2003).
FHSN2100 Lab 7
4
Procedure:
First Period
1. Weigh out 20g of raw hamburger mince.
2. Blend the 20g of the meat in a blender with 180ml of sterile distilled water
for 5 minutes. This gives a 1/10 dilution.
* STEPS 1 AND 2 PREPARED BY YOUR DEMONSTRATOR IN
ADVANCE *
3. Make dilutions from 102, 103, 104, 105, 106 .
4. Using the marker pen, label the petri plates with your name, date and
dilution.
5. Using aseptic technique, pipette aliquots from the dilution blanks to the
petri plates.
6. Melt the plate count agar (PCA) pours in a water bath and cool to about
48º to 50ºC. Add cooled PCA agar to the plates. Gently swirl on a flat
surface in a figure-eight motion and allow to harden.
7. Incubate the plates in an inverted position for 24 to 48 hours at 35ºC.
Second Period
1. Arrange the plates in order of lowest to highest dilution.
2. Count the number of colonies on the plates that have between 25 to 250
colonies. Designate plates with fewer than 25 colonies as too few to count
(TFTC) and plates with more than 250 colonies as too numerous to count
(TNTC).
3. Calculate the average number of bacteria per gram of hamburger as
follows:
Number bacteria/gram
= number of colonies x _____1_____
dilution factor
x _____1______
weight of sample
FHSN2100 Lab 7
5
For example, if 200 colonies were counted from a 10-3 dilution of a 20 gram sample:
Number bacteria/gram = 200 x 1/10-3 x 1/20g
= 10,000 per gram
4. Complete a statistical analysis of the number of bacteria within the food
samples:
i) Compare the number of bacteria in fresh hamburger mince versus mince
left at RT.
Record your results in your Laboratory Manual.
Questions:
What do these results mean?
What do they tell you about the importance of safe food handling, etc.?

Interested in a PLAGIARISM-FREE paper based on these particular instructions?...with 100% confidentiality?

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