About salt tolerance

Q: On the web, we can find that salt tolerance limit of Clostridium perfringens is 6%, different from the 10% we used in the notes. How to judge?

A: You can always find different limits for  the parameters of a microbe. There are many factors affecting the reporting of the limits. The better way to deal with this is to use the least stringent one to assume the growth or survival of the microbe. For example, if you find 6-10% as the salt content limit for C. perfringens, take it that it can grow at 10% salt. You will have a good margin to be safe.

In the real world, you will need to do the experiment to determine the limits for your own target food and microbe.

The main effect of adding salt is to lower water activities. Low water activities will never kill all the microbes.

Past paper 201617

Q. B3e.  How to calculate?

A: Target set: 900/ml to 1/ml. Reduction is ~1000-fold. You have the D value of the temperature 98C and the time of 21 sec. Is the combination giving 1000-fold reduction or more? If yes, then the industrial standard is sufficient.

Past paper 201415

Q: B4e. How to do it?   "The canning temperature can be raised 15°C without any adverse effect on the quality of the canned hams. Hence, the technician would like to use l20°C in the canning process to treat this lot of contaminated ham. Calculate the treatment time required at 120°C to achieve (i) the acceptable cell density for this contaminated lot and (ii) the acceptable cell densities of the microorganisms of the regular lot of ham after canning with this new process. Justify your answer and explain. (10 marks)"

A: Use the Z value to calculate the new D value at 120C. Use this new D value to calculate your treatment timed for the targeted reductions for bacteria and for spores. You need to calculate twice, one for the contaminated lot and one for the regular lot to complete your answer.

Past paper 201617

Q: A3, How to judge whether the bacteria would be killed or could not grow under the condition?

A: You have to know the characteristics of the bacteria. The bacteria in this question are those with some distinct characteristics. You have to match their characteristics to the treatment or process to judge whether they can be killed or still be able to grow.

Past examination paper 201617

Q: A3, which parameter would be the most important? what about the parameters not mentioned in the treatment or process? 

A: Importance of parameter depends on the parameter itself and the target microbe, no strict rules. The parameters not mentioned may remain the same, but changing one thing may have multiple effects and change other parameter. for example, adding salt changes also the water activities, or removing oxygen would change also the Redox.

Salt content limits

Q: salt content higher than the maximum limit, will the bacteria die?

A: salt content primarily affects water activities, bacteria would survive at low water activities.

Past paper 201617

Q: A2--NaCl prevented heat damaged bacteria to recover?

A: Yes

why gram positive bacteria survive better than gram negative bacteria?

A: Depends on the type of challenges, if it is high temperature, spore formers of Gram positive bacteria can survive a lot better.

Thermal death time curve and F value?

A:  F value is your target thermal death value, can be 2D, 3D, ................12D, any value depend on your target thermal reduction.  You can draw the curve in many ways to get the temperature-time combination to reach your target. One example is given in the notes.

Past paper 201617 C

Q: A2. Recovery not back to log colonies number of  9.4/ml ? but only 8.5?

A: There was actual killing of the bacteria at 52C. Recovery can never go back to 9.4.

Q;  a2 vi. c. botulinum killed by low level of  oxygen?

A: yes.

Past paper 201617 B

Q: B3 a., d ?

A: B3 a. No need. High acid food--low pH

B3 d. 3D reduction--900/1000 /ml = 0.9/ml. Temperature does not matter.

Past paper 201617 A--Please note the change in answers

Q: A3 iii, iv, ix

A: iii, G, S           iv. NG.S.  G, S. was a mistake  ix. G. S.

Q: A4 Soy sauce chicken--bacteria? Bagel?

A. C. perfringens. should be none  Bagel: Aspergillus

Review question 1 201617 paper B4

Q: B4 f. 

A: Find the odd one.  It was No. 24 patient. Incubation period much longer than the other patients in the two outbreaks.

Question from a student

From CCCC,

Dear Professor Kwan,

I am xxxxxxxxxx from class FNSC3180. For the open book examination, may I ask what types of materials can we bring for the final?

Thank you for your attention.

Q: Any book, notes, or charts can be used in the OPEN BOOK examination. Of course, only paper materials.

Notes of Open Book Examination

Please note the following about Open Book Examination

1. You need to familiarize with the materials, you may not have enough time to search for everything. If you can answer without reading through your notes and books, answer right away, confirm later on. The time will always be tight.

2. Know the basic principles of outbreak analysis, thermo-treatment calculations, microbial growth, etc. You should know how to apply them.

3. Questions may not be always similar to previous years examinations.

4. Even if the questions looks similar to previous years, they may differ greatly in details. Don't answer yester-years' questions.

5. Pay attention to details.

I wish you acquire and use these skills and capabilities:

1. logical
2. attention to details
3. organized
4. thorough
5. careful in calculation
6. familiarize--internalize knowledge
7. read questions carefully
8. good time management
9. writing clearly and concisely

Open Book Examination

Dear Students,

The final examination of FNSC3180 Food Microbiology will be Open Book examination. Please check the final examination time table to make sure you know the date and time: 26 April 2018, 9:30am to 11:30am at Thomas H C Cheung Gymnasium, UC.  Let me know if you have any questions.

Best,

HS Kwan

USDA Food Safety and Inspection Service Daily Digest Bulletin

FSIS Notice 21-18 04/16/2018 FSIS Notice 21-18 Scientific Integrity Training at FSIS. FSIS Directive 2100.3 04/16/2018 FSIS Directive 2100.3 Information Technology Procurement Process Improvement.   Stay Connected:   Subscriber Services: Preferences | Unsubscribe | Help | Questions? Contact Us This email was sent to hskwan288.course4180@blogger.com using GovDelivery Communications Cloud on behalf of the USDA Food Safety and Inspection Service · United States Department of Agriculture · 1400 Independence Ave., S.W. · Washington D.C. 20250 · 800-439-1420

Question from a student 20170503e

Q:

Sorry for bothering. May I know what is the minimum pH limit of

i) E.coli

ii) Salmonella?

Is adjusting pH to 4.2 effective in controlling their growth respectively?

Since there are many different data that confuse me.

A:
pH 4.2 should be effective in controlling the growth of most E. coli and Salmonella strains. However, some E. coli serotypes, such as O157:H7, can survive at pH 4.2. You have to note that minimum, optimum, and maximum of whatever microorganisms are affected by many factors, including their own biology, other environmental factors, and growth stages, there is no absolute limit. They are at best approximations. That is a reality. We can only note the limits as general features not absolutely applicable to all strains of a species.  

Question from a student 20170503d

Q:

I have some questions concerning 2014-2015 Past Paper B2.

I think the implicated microorganism should be Staphylococcus aureus but the incubation period of 11-40 hours is not so consistent with that of the Staphylococcus aureus. Does it mean that as long as the calculated incubation period is longer than the incubation period of the microbe itself, it can be considered as consistent?

And the difference in attack rate of roast turkey is also quite high and there are also Clostridium perfringens detected in the roast turkey. The incubation period also matches. Can we also deduce it is the agent that cause the outbreak?

One more question is that if the microbes mentioned in the questions do not fit in the analysis, we can suggest some possible microbes by ourselves?

And may I also ask if Listeria monocytogens can grow at 40C? If yes, that means heat treatment at 40C is not effective in suppressing its growth?

A: 

The incubation period is a little too long. You have to discuss this point when you answer the question. But the pizza has the highest attack rate difference, which is an important factor for the implicated food. The roast turkey can also be a suspected food but it is less likely than the pizza. 

If the microbes resulted from the investigation do not fit totally with the deduction, you can of course suggest other possibilities.

Listeria can grow at 40C, so 40C is not effective in suppressing its growth

Question from a student 20170503c

Q:

From the outbreak investigation questions in the past papers, part (a) asks about the suspected food and part (b) asks about the implicated food.

I would like to ask:

     1) for answering the suspected food, do we ONLY use the attack rate difference to deduce the answer?

     2) is it necessary that the suspected food MUST BE THE SAME as the implicated food?

For 2012-2013 Q8, are these answers correct?

       (a) Pizza is the suspected food as it has the highest attack rate difference among the others. One point needs to be aware that there is no bacteria found in pizza.

       (b) B. cereus is the implicated microorganism as it is consistent to the shortest incubation period (5 hours) while C. perfringens is not. Therefore, the implicated food is bread

A: 

I do not have the 2012-13 question at hand, so I just answer according to my memory of the questions.

1. You can use whatever evidence you think appropriate to deduce the suspected food. The attack rate difference is an important factor in the deduction or elimination. Especially when the difference is very small or negative.
2. Suspected food and implicated food can be different. 

a. No bacteria does not mean the food is clean. There can be toxins.

b. The bread, if having a very small attack rate difference, or negative attack rate difference, would probably not the implicated food. It is possible that the pizza had Staphylococcus aureus grown on and toxin produced.