Question from a student on examination question

From: ZZZZZZZ
Sent: Saturday, 26 November 2016 6:51 PM
To: hoishankwan@cuhk.edu.hk
Subject: FNSC5110 Exam Question

Dear Professor,

I re-do the past exam paper as to prepare for the final exam, but not sure on one question (See attachment). Please let me know if I made any mistake or suggest a better a solution. 


Thank you,

●     What are the numbers of D's targeted in the standard canning process for each of these microorganisms? (2 marks)

                        Spoilage bacteria: 25/0.25=100D                                                                                           Bacteria Spores: 25/2.5= 10D                                                                                                          

●     Calculate the cell densities of the microorganisms remaining in the contaminated ham after a standard canning process treatment. (2 marks)

                        Spoilage bacteria: 10⁵ / 10¹⁰⁰ = 10^-95 (per ml)                                        

                        Bacteria Spores: 10⁴ / 10¹⁰ =10^-6 (per ml)                                                                                                          

●     Was the standard canning process sufficient for the canning ofthe contaminated ham? Explain briefly. (2 marks)

                        Insufficient. Bacteria Spores -> 10^-6 /ml, larger than 10^-8                             

                        To make it sufficient : additional 25 mins at 105°C

                      Bacteria spores at 10^-6 for additional 10D reduction ->10^-6 x10^-10 =10^-16       

Thus, sufficient after 10D reduction.

(Actually may only need additional 5 minutes at 105°C,

                         because 10⁴ /10^30/2.5=10^-8 (per ml))                                              

Better if only 5mins? Additional 25mins is too long?     

●     According to Chart I, what are the Z values                                                        ofthe two types of microbes? (4 marks)           

                        Spoilage Bacteria: -(95-85)/ log 5- log 50=10°C       

Bacteria Spores: -(105-90)/ log 5 - log 50 =15°C

                                                                                                           

●     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 at120°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)

            At 120°C:                                                     

            Spoilage bacteria:

            D120°C=D105°C *10^[(105-120)/10]=0.25*10^(-1.5)=7.9*10^(-3)min

           Bacteria Spores:

            D120°C=D105°C *10^[(105-120)/15]=0.25*10^(-1)=0.25min

          

          Treatment time for regular:

          Spoilage bacteria: 10D120°C=7.9*10^(-2)min

           Bacteria Spores: 9D120°C=2.25min

           Treatment time for contaminated:

           Spoilage bacteria: 13D120°C=0.1027min

           Bacteria Spores: 12D120°C=3min

Your answers to the questions are correct except one: Your answer to the last question is incomplete: you can only use one treatment time to treat each lot. You need to use the time for the most heat-resistant microorganism, in this case, bacterial spores. So the times should be: 

Regular--2.25 min

Contaminated--3 min