Lecture 4: Microbial Destruction Kinetics

Question 1

in the thermal destruction of microbes, enzymes and nutrients follow what reaction rate?

what equation represents this?

Answer 1

first order rxn rate

-[dC/dt] = kC

Question 2

in the plot of log C vs t, what represents the intercept?

Answer 2

intercept = logCo

Question 3

in the plot of log C vs t, what represents the slope?

Answer 3

slope = -k/2.303

Question 4

what is FS?

Answer 4

flat sour

a type of microorganism that causes spoilage

Question 5

what is the difference between FS and other spoilage microorganisms?

Answer 5

when FS is active, it doesn’t produce gas (it only produces acidity). Thus, the presence of FS does not cause bulging of a can

however, when other microorganisms are active, this causes gas production, which results in bulging of the can

Question 6

what is D?

Answer 6

• decimal reduction time
• a heating time at any given temperature that results in:
  1. 90% destruction of the microbial population
  or
  2. one decimal reduction in the surviving population

Question 7

what is Co?

Answer 7

microbial concentration at time t0

Question 8

what is k?

Answer 8

reaction rate

Question 9

what is TDT?

Answer 9

thermal death time

the time in minutes required to destroy a microbial population at any given temperature

used exclusively in in process calculations

Question 10

what is the destruction time set between?

Answer 10

initial time and no-survivor time

Question 11

death time depends on _____

Answer 11

initial population (higher population means longer TDT)

Question 12

how are TDT and D related?

Answer 12

• TDT represents a multiple of D value (shown by: TDT = nD and their parallel slopes)
• TDT and D also vary semi-logarithmically with temperature (they

Question 13

complete destruction is ____

Answer 13

theoretically impossible

Question 14

what indicates the temperature sensitivity of TDT (or D) vs T graph?

Answer 14

the negative slope

therefore, when temperature increases, D or TDT value decreases.

A steeper slope means higher temperature sensitivity

A less steep slope means less temperature sensitive

Question 15

what is the z value?

Answer 15

• the temperature range between which TDT or D value curve passes through one log cycle
• the slope indicator of TDT curve or D value curve
• negative reciprocal slope of the regression line

Question 16

how can you calculate the z value from the slope of logTDT vs temp?

Answer 16

slope = -1/z

Question 17

what is the reference D value?

Answer 17

Do

Question 18

what is the reference k value

Answer 18

ko

Question 19

what is the sterilization reference temp?

Answer 19

250degF or 121degC

Question 20

what is the pasteurization reference temp?

Answer 20

150degF or 180degC

Question 21

what is N and No?

Answer 21

N = final count of microorganisms
N0 = initial count

Question 22

what is MPN

Answer 22

most probable number per given unit

MPN = (2.303/a) log (n/q)

a = amount of sample
n = total # of replicates
q = number of negative samples

Question 23

what is the MPN equation used in?

Answer 23

stumbo’s thermoresistor approach for D

to determine initial and final counts