Question

a strain of E.coli has a D-value of 1.3 minutes at 60 degrees celcius. which of the following statements is correct:

A. each increase or decrease in temperature of 1.3 degrees celcius will result in a 1 log change in D-value

B. at 60 degrees celcius, the number of viable E.coli will increase by 1 log in 1.3 minutes

C. at 60 degrees celcius, the number of viable E.coli will be non-existent in 1.3 minutes

D. at 60 degrees celcius, the number of viable E.coli will be reduced by 1 log in 1.3 minutes

E. none of the above

Answer 1

The correct answer is option D) at 60 degree celcius, the number of viable E.coli will be reduced by 1 log in 1.3 minutes.

A strain of E.coli has a D value of 1.3 minutes at 60 degree celcius. It's meaning is at 60 degree celcius, the number of viable E.coli will be reduced by 1 log in 1.3 minutes because D value means decimal reduction time. When a homogeneous suspension of microorganisms like E.coli is heated at a constant temperature, microorganisms are destructed in logarithmic order (exponential decay) So, 90% of target microorganisms (1log cycle) are reduced in particular heating time at that temperature and this heating time in minutes is expressed as D-value. D-value basically denoted that 1 log cycle means 90% of total viable microorganisms will be reduced by the exposure of constant temperature for a particular time(in minutes). Only 10% of total microorganisms are viable after exposure. Therefore, here the given statement indicates that due to the exposure of 60 degree celsius heat for 1.3 minutes on a suspension of E.coli strain, 90%(also called 1log cycle) of total viable E.coli will be reduced. Microorganisms are reduced by a factor of ten, here assume 1000 to 100. Means, if total number of viable E.coli is 1000 initially after exposure of heat only 100 E.coli cells are viable, 900 are reduced by heat. Therefore, percentage of reduction = [(1000-100)/1000] x 100 = [900/1000] x 100 = 90% = 1 log cycle. D-value is same for each log cycle like 10,000 to 1000 or 100 to 10. Hence, we can understood that remaining options are not correct.