1. D and Z values determination
D value
The time requires to inactivate 1 log scale (90%) of bacterial population at a particular temperature.
Z value
The difference of temperature between 1 log scale of D-values.

2. Materials and Methods Bacteria: Escherichia coli Medium: TSA
Dilution buffer: phosphate buffer saline (PBS, pH 7.4)
Water bath at 60, 70, 75°C
Pipette
Petri-dish
incubator

3. Procedures Incubate two flasks of E. coli culture.
Turn on water bathes before class begins. Make sure water bathes have enough water.
Each group has one tube containing phosphate buffer saline (PBS) into water baths.
Place the tube into the water bath.
Aliquot the E. coli culture into tubes. Each table has one tube. (teaching assistant)
Take the PBS tube from water bath.
Transfer 1 mL E. coli culture into a tube containing 9 mL PBS. Place the culture into one tube immediately.
Bacterial density in each tube should be at 108 CFU/mL.

4. Start timing. Gently shake the tubes during heating.
When heating time is achieved, place the tubes under running tap water. Gently rotating the tube during cooling for one minute.
Decimally serial dilution
For 0 dilution using pour plate :transfer one mL of dilutant into a petri-dish. Gently mixed agar and bacterial suspension.
For other dilutions using spread plate: transfer 0.1 mL onto the agar surface.
Spread the plates, DO NOT pack the plates vertically.
Place the plates into 37C incubator. Incubate at 37C for 24 hour. Count colonies.

5. group
temperature
time
dilutions 1
60°C
12 min -1 -2 -3 2
8 min -4 3
6 min -5 4
4 min -6 5
70°C 6 7 8
2 min 9
75°C 10 11
1 min
值日組(1)
No heating
control -7 -8

6. 1 mL
9 mL
9 mL
Bacterial culture
Heating
After cooling
1 mL
0.1 mL
0.1 mL
0, pour plate -1 -2 -3 -4 -5 -6 -7
Duplicate plates/dilution

7. Control
1 mL
9 mL
Bacterial culture -1 -2 -3 -4 -5 -6 -7
0.1 mL -6 -7 -8
Duplicate plates/dilution

8. Record colony number: 25-250
Calculate average number of the duplicate plates
Average number x dilution = bacterial population
Example: 42 x 103= 4.2 x 104=4.62 log10 CFU/mL
Calculate bacterial population of control
Example: 42 x 108= 4.2 x 109=9.62 log10 CFU/mL
Therefore, bacterial population in the first dilution tube (the same dilution used for heating process) should be = 4.2 x 108=8.62 log10 CFU/mL
The control number is time 0 number (the number on Y axie)

9. Record bacterial population.
Draw a figure. Use regression function.
Y axle: bacterial population (log CFU/mL)
X axle: heating time.
Obtain an equation.
Obtain D value.

11. Small D values mean more sensitive to heat
y = x
when y= 6, x=2.339
When y=5, x= 3.234
When y=4, x=4.308
D value = 0.98 minute
Small D values mean more sensitive to heat
Time to reduce 90% (log scale) population
Different temperature different D values
Higher temperature, smaller D values
12D
Reduce 12 log scale

12. min
Population (log CFU/mL)
60℃
70℃
75℃

13. Z values Combine data from all groups.
Draw a figure. Use regression function.
Y axle: D values(log scale)
X axle: temperatuare
Obtain an equation.
Obtain Z value.

14. Z values (℃ or ℉) log D values vs. temperature
The time between 1 log scale D values
log D values
Z value
Z value
temperatures