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.

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.

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 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. 

Question from a student 20170503b

Q:
I found that the incubation periods (IP) of some of the bacteria are inconsistent in concept maps and the food microbiology notes. For instance, the IP of Clostridium botulinum is 8hours to 8 days as written on the concept map and said during class but that in the note of p.A-1 is 1.5-3days. For Staphylococcus aureus, the IP written on concept map is 2-6hrs and that on p.A-1 of the note is 1-6hrs. For Campylobacter jejuni, the IP is 2-9days on ppt notes but that on p.A-2 of note is 2-7 days. Should I refer the incubation period of bacteria and virus on the notes or on the concept maps? Thanks for your kind attention!

A:
As I have mentioned many times in my lectures that the incubation period for each pathogen is only an approximation. You will find that the incubation periods from different sources are not exactly the same, even from my concept map and notes. Due to two reasons, at least,

1. Many environmental factors would affect the incubation period.
2. Each species may have many strains and incubation periods for them may be different.

So use the approximate incubation period from either notes or concept maps would be acceptable.

Question from a student 20170503a

Q and A

• 2014-2015 B2. If I know that the suspected food is pizza from (a), do I also need to calculate the incubation period assuming dinner is the problem? 

Yes, you should check every piece of information to ensure your conclusion is close to the truth.

• I dont understand what you mean by elimination. Do I need to refer to every other food served at the picnic and its attack rate? (eg. Although Salmonella entericaserotype Typhimurium is found in both the sushi and faeces of patients, it seems that sushi is not likely to be the food of problem as it has the lowest positive difference between the attack rate of those who ate and those who didn’t. ) 

Yes, eliminate other possibilities with information at hand. For example, S Typhimurium in this case would probably not the agent because of the attack rate and incubation time are not consistent with it being the agent. You have to do this because disease may not be caused by a single pathogen but may be caused by more than one. You also need to explain any inconsistence observed.

• 2013-2014 QB1 (b) x : Occurred frequently in child-care centers : Human rotavirus. Is this correct? 

No, should be Norovirus

• 2014-2015 QB1 (b)viii : Occurrence and growth in fresh cream even stored in refrigerator: Listeria monocytogenes

Yes, it can grow at refrigeration temperature and may exist in fresh cream

• 2014-2015 QB1 (b) Occurred frequently in child-care centers and caused vomiting: Is it nora virus or human rotavirus? 

Norovirus is the better choice.

Question from a student 20170502a2

Q & A:

Thank you Professor Kwan. I am sorry that I would like to ask 2 more questions. 

1) Can Salmonella and E. coli tolerate high salt content? And in specific, can E. coli survive/grow in food with salt content 7%?

E. coli cannot grow at 7% salt, but it may survive.

2) Clostridium botulinum cannot grow in food with oxygen 5% since it is an obligate anaerobe?

It cannot grow with 5% oxygen.

Question from a student 20170502a1

Q: 

I would like to ask how to determine 'growth' and 'survival' of microorganisms from some given data. 

For 'growth', if the parameters are out of the minimum or maximum limit of microorganisms, it means that they cannot grow?

A: They cannot grow

Q:

For 'survival', I remembered that you have mentioned microorganisms can actually survive at low temperature and low water activity, even if the conditions are out of their limit. How about pH? Will they just die if the pH is below minimal limit/above maximum limit? And for temperature that is higher than the maximal temperature for growth, does it mean that the microorganisms will die?

A: 

pH below minimum or above maximum: Die.

temperature higher than maximum: Die.

But of course, the length of time for the microorganisms expose to these parameters needs to be taken into consideration. If brief exposure, there is D value or similar factors affecting the survival.

Question from a student 20170502d

Question about  pastpaper 2013-2014

I would like to know  how to calculate new D value like question in B5 (d).
And do you mind checking if the following ans in B5 is correct?
a. no. of D's 
Spoilage bacteria = 100D
Bacterial spores = 10D
Fungal spores = 50D
b.

Spoilage bacteria = 10^-95

Bacterial spores = 10^ -6

Fungal spore = 10^-49

c. the standard canning process for  Spoilage bacteria and  Fungal spore are sufficient, but not for bacterial spores

A: You are correct, but you should show your calculation too, also need to explain.

Please see https://fnsc3180hsk.blogspot.hk/2017/05/question-from-student-201617-20170501a.html?showComment=1493708599366#c8295958764595042514

Question from a student 20170502c

Q:
     I have a question popped up in my mind when I looked at the notes of fermentation.  "Lactic acid production can lower the pH to make the fermented food resistant to spoilage." If this is true, then why do fermented foods like cheese, yoghurt still subject to spoilage? Thank you for your attention.

A: 
1." Resistant to spoilage" does not mean never spoiled, only less likely to be spoiled, not high risk for spoilage.
2. In fact, cheese and yogurt do not spoil easily.

Question from a student 20170502b

Q&A:    

 I would like to ask that whether notes content in pages below will be covered in final examination? Coz I am not sure about these contents are related to lectures topics or not. Thanks.

1. Soybean paste 

NO

2. Clostridium difficile 

NO

3. Notes P.34-40 Protozoa and parasitic worms

NO

4. Notes P.41-45 fungal toxins in foods 

NO

5. Notes P.101-106 controversial preservation: nitrite,nitrate,antibiotics and food irradiation 

NO

6. Notes P.107-111 legal considerations and food safety programs

Yes, we have discussed HACCP. HACCP is included.

Question from a student 20170502a

Q&A:
 I would like to ask for the answer of B2 and B3a in 14-15 pastpaper. For B2, is the most suspected food Roast Turkey?

No. According to differences in attack rates and incubation period, and other evidences, it is probably Pizza and Staphylococcus aureus.


 For B3a, drying means heat drying that lowers the aw? For the comparison made, that means in this case, we have to point out lowering pH is more effective in preserving food? 

Yes, lowers Aw and water content.

No, lowering pH is not more effective in preserving food.



Lastly, can I answer all the questions in exam? Thanks a lot!

Yes, but if you answer all the questions, I will only count the number of questions required which have higher marks.

Some more Q&A from another email:


Also, as I see from the approach of doing 14-15 B4(e), the last question about justifying the answer and explain, that means we have to first find out the D value at 120°C, then calculate the treatment time required for both bacteria in regular blot ( spoilage bacteria: 10D x 0.0079= 0.079min, spores: 9D x 0.25= 2.25min), both bacteria in contaminated blot (spoilage: 0.1027min, spores: 3mins), then we use 3 mins at 120 degree celsius as the new time, then we follow part a to c to calculate the corr. D values, cell densities for both bacteria, then we come with conclusion to see if new method is ok. Am I correct?

You have to carry out all the calculations and answer both (i) and (ii). Your calculation to reach the two treatment times already shows whether the times you concluded to use are long enough to achieve the goals. "Justify" means you have to indicate why the times you use are good ones, "explain" means you need to give some details on your calculation and justification.