Saturday, November 30, 2013

Another question from student via email

Dear Prof. Kwan,

Excuse me, I have 2 questions about the answers you have just posted: 

1. Could Listeria monocytogenes really grow under pH=4.0 condition ?

A: Probably not, so note changes in the last reply

2. As Listeria monocytogenes is gram +ve, should I believe that it could grow under as low as 0.85 water activity condition?(I took this information from your lecture notes...)

A: 0.85 Aw is too low for L. monocytogenes to grow.

Thank you in advance =]

Questions from a student by email--Revision

Dear Prof Kwan,

I would like to ask below 5 questions. Please kindly advice. Thanks for your
reading & helping.

1.For fresh apple juice, the condition is acidic & high sugar & low salt & keep
at 25C. How could I determine whether it is yeast/ecoli 0157/lactic acid
bacteria (as all seems favour to this condition)?

A: Lactic acid bacteria: more competitive than E. coli and grow faster than yeast

2.For ground beef, the condition is high protein & salt& not acidic, if stored
at 25C, is it resulted as S. aureus? But if stored at 5C, it turns to be L.
monocytogenes?
But if it is high protein & store at 25C,but not salty & acidic,then it became
Ecoli 0157?

A1: Yes, S. aureus, if 5C, L. monocytogenes

A2: Yes, E. coli O157 should grow better.
3. From the pastpaper:
a)pls see attached picture, see if the organism is correctly stated (based on
highlighted reason)
b)the most toxic natural bacterial toxin -> C. botulinum?
c)Culturing does not detected the pathogen in oysters in winter but it appears
in summer to cause liver infection -> hepatitis A rather than V.cholera?

A (a) Go to the end of this post

A (b) Yes

A (c) Yes, hepatitis A.


4. S. aureus make the toxin become stonger if reheat the meat again,
so thats mean we cannot reheat something like sui mei that already cooked & put
in ambient temp (which contain S. aureus)?

A: Yes, the siu mei would still contains toxin

5. One of the controlled method of spore forming bacteria(C. perfringen & B.
cereus) is fully reheated the food before consumption. But it is known that
heating activate endospore to germinate, so why/ how could the food is still
considered to be safe once reheated?

A: The food reheated and then consumed right after reheating would activate endospores to germinate but they would not have enough time to grow and cause problem


Q3 A (a)


From left to right:

Listeria monocytogenes, Salmonella spp./E. coli O157:H7 is ok too, Aspergillus oryzae,
None (pH 4.0 and low temp, high salt), C. perfringens




Sunday, November 24, 2013

One example of the FNSC5110 Examination Papers of Professor Kwan's part

Section A.       Answer question 1, 2, and 3, and any ONE question from questions 4 and 5.

Answer Question 1,2 and 3.                            (15 marks)

1.
Draw a simple labeled diagram to illustrate the pathogenic mechanisms of the "hybrid" Escherichia coli  in the infection of intestinal cells. The "hybrid" E.coli refers to the one that caused serious EHEC outbreak in Germany last year, 2011.           (4 marks)

2a.
Describe the approach that we can use to evaluate and predict the spoilage of foods.
                                                                                                                            (3 marks)
  b.
Write short notes on Vibrio chlorae and Vibrio parahaemolyticus
                                                                                                           (5 marks)


3.
An epidemiological analysis hypothesized that over the years there is a correlation between the control of Salmonellosis in Chicken by Salmonella serotypes and the increased occurrence of human Salmonellosis caused by Salmonella serotypes.  Based on that hypothesis, answer the following questions.
a.
Indicate which of the answers correctly describes the trend curves and descriptions of chicken and human Salmonella cases.                                                               (1 mark)




Figure 1.  Changes in the number of cases of Salmonellosis from 1935  to 1989  
               i.             
National Poutry Improvement Plan (USDA) started in 1935 and S. Pullorum cases in chicken dropped over the years but S. Typhimurium cases remained at similar levels since 1962

             ii.             
National Poutry Improvement Plan (USDA) started in 1935 and S. Pullorum cases in chicken dropped over the years but S. Enteritidis cases increased since 1962

           iii.             
National Salmonella Surveillance Program started in 1935 and S. Pullorum cases in chicken remained at similar levels over the years but S. Enteritidis cases increased since 1962

           iv.             
National Salmonella Surveillance Program started in 1935 and S. Typhimurium cases in chicken decreased over the years but S. Pullorum cases increased since 1962

             v.             
National Poutry Improvement Plan (USDA) started in 1935 and S. Enteritidis cases in chicken decreased over the years but S. Pullorum cases increased since 1962


  1.  
 i and iii
  1.  
 ii and iii
  1.  
 iii and iv
  1.  
 i and ii
  1.  
 i and v


b.
According to the hypothesis, the control of Salmonellosis in chicken has an impact on Salmonellosis in human. Which of the following factors would be important for the trends indicated in Fig.1?  Explain your answer briefly.                                (2 mark)

  1.  
Humans become more susceptible because they have been eating more undercooked  chicken over the years
  1.  
Humans become more susceptible because their immunity are weakened over the years due to greater prevalence of AIDS and other immuno-compromising diseases
  1.  
The antibodies produced in chicken in Salmonellosis against S. Pollorum prevented the growth of S. Typhimurium in chicken after chicken Salmonellosis was under control.
  1.  
The antibodies produced in chicken in Salmonellosis against S. Enteritidis prevented the growth of S. Pullorum in chicken before chicken Salmonellosis was under control.
  1.  
The antibodies produced in chicken in Salmonellosis against S. Pollorum prevented the growth of S. Enteritidis in chicken before chicken Salmonellosis was under control

Answer JUST ONE of the following Questions.                  (10 marks)

4.
Indicate ONE of the organisms that may most preferentially grow (the one most suitable to grow), among all organisms listed, in the food with the particular characteristics and storage condition.  If none of the organism can grow, put in None.

(10 marks)

                      i.           
Polio virus


                     ii.          
Clostridium botulinum


                    iii.         
Listeria monocytogenes


                    iv.         
Salmonella species


                     v.          
Aspergillus oryzae  (fungus)


                    vi.         

Saccharomyces cerevisiae (yeast)



                   vii.        
Norovirus


                  viii.       
Escherichia coli O157: H7


                    ix.         
Clostridium perfringens




Soy Sauce
Chicken
Raw Cleaned Chicken
Sweet Bread
Pickled Pork Knuckle
Stuffing of roasted Chicken

Water Activity
0.90
0.95
0.85
0.97
0.95

pH
5.2
6.5
5.0
4.0
6.2

NaCl content w/v
9.0%
0.5%
0.5%
9.0%
2.0%

Redox (mV)
-50
200
200
150
-200

Sugar content
Moderate
Moderate
Moderate
Low
Moderate

Protein content
High
High
Low
High
Moderate

Keeping temperature
25C
25°C
25°C
4°C
40°C

Handled by hand
Yes
Yes
No
No
Yes

Organism





9.
You are the senior technical manager of an apple cider bottling factory.  You are responsible for food safety management system ISO22000 based on HACCP.  The following happened last week:

i.          On Tuesday, your technician rushed into your office and informed you that the quality control laboratory have made a mistake in measuring the pH of the apple cider  delivered yesterday for bottling.  The actual pH of the food was 4.5 and the laboratory used a malfunctioning pH meter and reported a pH value of 4.0
ii.         Some of the apple cider was bottled and pasteurized already, using the pasteurization process designed for apple cider of pH 4.0.
iii.        The technician thought that the pasteurization process was not sufficient and suggested a corrective action that the bottled apple cider should be processed one more time for 10 minutes at 105°C.
iv.        He provided you with the information from the quality control laboratory as described below. 
v.         You had to judge whether the technician’s decision was correct or not and answer the questions below.



Approximate microbial flora in apple cider (Number/ml)
Acceptable cell density in bottled apple cider (Number/ml)

Spoilage bacteria
1000
less than
0.0000001

Bacterial spores
100
less than
0.000001

Fungal spores
100
less than
0.000001



The problematic pasteurization process:     10 minutes at 105°C.

For apple cider:                                                                                                     

D values and Z values of the microbes at pH4.0:


D value at 105°C (minute)

Z value (°C)

Spoilage bacteria
0.001
2
Bacterial spores
0.1
10
Fungal spores
0.002
5

D values and Z values of the microbes at pH4.5:


D value at 105°C (minute)

Z value (°C)

Spoilage bacteria
0.01
5
Bacterial spores
2
15
Fungal spores
0.02
10


Answer the following questions.  Show your calculations :

a.           
Calculate the F values set in the pasteurization for these target microorganisms?                                                                                                                                                                                                                (1 marks)  

b.           
Calculate the number of D’s that actually occurred in the problematic pasteurization process for these target microorganisms?                                                            (2 marks)  

c.            
Calculate the cell densities of the microorganisms in the processed apple cider with actual pH of 4.5 after they were processed for 10 minutes at 105°C?   
                                                                                                                            (2 marks)

d.           
Was the proposed additional pasteurization process needed and if needed, was it sufficient for the processing of the one-time processed apple cider with wrong pH measurement?  Explain briefly.                                                                                                  (1 mark)

e.            
Was the technician correct in his suggestion of the additional remedial processing?  If he was wrong, fire him and suggest alternative pasteurization time to treat the problematic bottled apple cider at 105°C.  Explain briefly.                                (1 marks)

f.             
Suggest a new process with a combination of pasteurization temperature and time for the remaining batches of apple cider (pH4.5)These data may be useful for your consideration: heating for 10 minutes at 105°C for ten bottles of apple cider costs HK$1 and heating for 1 minute at 120°C for ten bottles of apple cider costs HK$5. Explain briefly your process with justification.  
(3 marks)

- END --