Answer to questions from students 20141029

Dear LLLLLLLLLL,

The Answers are put under each question below. These Q&A will be posted on the blog also.

Best,

HS Kwan

-----Original Message-----
From: LLLLLLLLLLLL
Sent: Wednesday, October 29, 2014 10:51 PM
To: hoishankwan@cuhk.edu.hk
Subject: FNSC 5110 questions

Dear Professor Kwan,

I have several questions about the sample midterm that I have been struggling with, and it’d be great if you could clarify the following for me:

1. Calculation for attack rates

      Is the attack rate just basically the # of ill / total #

A: We have not discussed this topic in the class. Not included in the exam.

2. Question on the exam is: What are the F values set in the canning process for these target microorganisms?) I know that for low acid canned foods (pH>4.5), the calculation for F values is F = 12D. However, the exam asks for F values at pH 4.0 and 4.5. What approach should I take?

A: The F values now is set as the number of D attained in the process. If the process was designed for pH4.0, use the D for pH4.0 to calculate F.

3. Question on the exam is: What are the number of D's that actually occurred in the problematic canning process for these target microorganisms?

      I am not too sure about what the question is asking for. Dose it mean how many fold-decrease for each of the microbial (spoilage bacteria, bacterial spores, and fungal spores) occurred for 20 minutes at 105 degree C of processing (10 min per one time of processing, and the canned pig knuckles with incorrectly reported pH were processed twice) ?

A: Yes.

4. Question on the exam is: What would be the cell densities of the microorganisms in the processed pickled pigknuckles with actual pH of 4.0 after they were processed for 10 minutes at 105 degree C?

      Do I basically just calculate the number of fold-decrease using the known D value for microbes at pH 4.0, and then divide the approximate microbial flora in picked pig knuckles by the fold-decrease?

e.g. Spoilage bacteria (D value at 105 degree C) = 0.001 minutes

      That means there’s a 10-fold decrease in 0.001 minutes. So in 10 minutes, there is a 100,000 fold-decrease.

        Approximate spoilage bacteria in pickled pig knuckles = 1000/mL

      So 100,000 fold-decrease results in 0.01/mL after 10 minutes treatment at 105 degree C I felt like the calculations are wrong, because it happened that none of the microbial flora decreased to an acceptable cell density after treatment. Please let me know if I have misinterpreted anything.

A: Your calculation approach is basically correct. But 10/0.001=10,000, not 100,000. The decrease is log decrease, not fold decrease. A 10,000 log decrease of 1000/ml-->1000x10<-10>=10<-997> definitely blow the acceptable cell density. You need to know how to calculate.

Thank you very much for you attention, and I hope to hear back from you soon!

Best Regards,