Q: 

I am student of your class FNSC4180 and have some questions on past paper during revision, it would be kind of you to help. There are questions on "Hurdle theory” asking whether the conditions and treatments are effective to control the growth and survival of the target organism. I am not sure how to determine whether the method is effective or not. Is it effective once one of the condition out of the tolerance limit of that bacteria?so the explanation just focus on the tolerance limit of the bacteria?
I would like to know if these answers are correct. Thank you very much for your help.

2011-2012. 6
i.                Not effective as Listeria monocytogens is psychrotroph which can grow survive under 4-40oC
ii.              Same as i.
iii.             Not effective, 0.85 is the minimal aw for germ positive bacteria
iv.             Effective, 11% salt is too high for survival of Staphylococcus aureus ,it can only tolerate up to 10%
v.              Not effective as aw0.9 is not low enough to its growth
vi.             Effective, Clostridium botullinum is obligate anaerobe which can be killed by 5% of O2.
vii.           Effective, though there is no O2 but the temperature is too high for a mesopile like Clostridium botullinum and obligate anaerobe survive in low Redox.
viii.          Effective as Pseudomonas spp. is aerobe which require O2 to survive.
ix.             Effective, pH 4.2 is too low for bacteria
x.              Same as ix.

A: Yes, basically one parameter beyond the tolerance limit would control the microbial growth or survival.

























I only have enough time to check your answer to this set of questions:

2011-2012. 6
i.   Not effective as Listeria monocytogens is psychrotroph which can grow survive under 4-40oC
OK
ii.              Same as i.
No. Listeria cannot grow at 40C
iii.             Not effective, 0.85 is the minimal aw for germ positive bacteria
Effective. 0.85 too low for Listeria
iv.             Effective, 11% salt is too high for survival of Staphylococcus aureus ,it can only tolerate up to 10%
OK. Although some reports showed that Staphylococcus can tolerate 11% salt
v.              Not effective as aw0.9 is not low enough to its growth
OK
vi.             Effective, Clostridium botullinum is obligate anaerobe which can be killed by 5% of O2.
OK
vii.           Effective, though there is no O2 but the temperature is too high for a mesopile like Clostridium botullinum and obligate anaerobe survive in low Redox.
OK
viii.          Effective as Pseudomonas spp. is aerobe which require O2 to survive.
No. Pseudomonas can use nitrate for anaerobic respiration
ix.             Effective, pH 4.2 is too low for bacteria
OK. effective for controlling Salmonella
x.              Same as ix.
No. pH 4.2 not low enough to control E. coli O157:H7