Question 9 2012-13
a. F value?
At pH=4.0
Spoilage bacteria : D=0.001min----10/0.001=10000D
Bacterial spores: D=0.1 min----10/0.1=100D
Fungal spores: D=0.002 min ----10/0.002=5000D
Actual set F value= D values above
Sterilization: 10 min at 105C
Alternative answer: F=required D.
Bacteria: 10D
Bacterial spores: 8D
Fungal spores: 8D
b. Actual D?
Because actual pH is 4.5
At pH4.5 10 min at 105C
Spoilage bacteria : D=0.01min----10/0.01=1000D
Bacterial spores: D=2 min----10/2=5D (----failed)
Fungal spores: D=0.02 min ----10/0.02=500D
c. Cell densities?
Per ml
Spoilage bacteria : 1000/ml*10^-1000=10^-997
Bacterial spores: 100/ml*10^-5=10^-3
Fungal spores: 100/ml*10^-500=10^-498
d. One more time, 10 min at 105C?
Per ml
Spoilage bacteria : 10^-997/ml*10^-1000=10^-1997
Bacterial spores: 10^-3/ml*10^-5=10^-8
Fungal spores: 10^-498/ml*10^-500=10^-998
OK. Bacterial spores: 10^-8/ml < 10^-6/ml requirement
e. The technician was correct.
f. New process?
1. First calculate time required for 105C sterilization to achieve target reduction of bacterial spores density to 10^-6/ml.
Number of D required for bacterial spores = 8
Bacterial spores D value at 105C = 2 min
Therefore, time required at 105C = 16 min
2. Calculate the cost: (16min/10min)*$20=$32
3. Calculate time required for 120C sterilization to achieve target reduction of all microbes to acceptable densities. Use Z values to get to new D values, then calculate as above.
Time required at 120C = 1.6min
4. Calculate the cost when changed to 120C: (1.6min/1min)*$5=$8
5. Compare the two costs $8<$32
Better to use 120C for 1.6min
Resources for Microbiology. This blog provides resources for the course FNSC3180/4180 Food Microbiology of CUHK. Check this blog often to see what is new. Your comments are welcome. From May, 2011 onwards, news, analyses, and investigation of foodborne microbial diseases are regularly posted in this blog. Analyses of foodborne microbial genomes from Kwan Lab are also posted here.
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