Sterilisation Kinetics W8

Question 1

The death of microorganisms using sterilisation follows….

Answer 1

First order kinetics

Question 2

First-order kinetics

Answer 2

refers to a process where the rate of reaction is directly proportional to the concentration of the reactants

Question 3

What is the rate of death at any time dependant on

Answer 3

No.micro organisms present

Question 4

Over time what happens in micro-organum killing kinetics

Answer 4

The time increases in minutes and the no.viable m-organisms present in cfu/ml reduces rapidly

Question 5

What graph does time vs cfu/ml give

Answer 5

Curved but we want straight

Question 6

No?
Nt?

Answer 6

• N0 = Number of micro-organisms present at time 0
• Nt = Number of micro-organisms present at time t

Question 7

equation for killing kinetics

Answer 7

dN/dt=-constant x No

-> ln (Nt/No) = -constant x t

-> ln(any number) = 2.303 x log10(that number)

So :

ln(Nt/No) = -constant x t

-> 2.303xlog10(Nt/No) = -constant x t

-> log10(Nt/No) = (-constant x t / 2.303)

-> log 10(Nt/No) = -k x t

Put into y=mx+c

-> log10(Nt) = log10(No) -k x t
Y = Mx +c

Gives a straight line graph of log(Nt) vs t

Question 8

D value

Answer 8

• relate to a particular micro-organism and temperature, which must both be specified
• D values tend to be around 0.5 to 2 minutes at 121°C
• Can use D values to predict how long it takes to reduce the number
of micro-organisms to a specific level or by a specific amount

Question 9

The D value of a particular micro-organism at 121°C is 1.5 minutes.
How long will it take to reduce the bioburden from 10^8 to 10^0 cfu/mL?

Answer 9

D = 1x decimal reduction

10^8 to 10^7 = 1x decimal reduction = 1.5 mins
10^8 to 10^0 = 8x decimal reductions = 12 mins

Question 10

The D value of a particular micro-organism at 115°C is 2 minutes.
How many micro-organisms will remain after 10 minutes if the starting bioburden was 10^7 cfu/mL?

Answer 10

10/2=5 decimal reductions

10^7 to 10^2 = 5

Question 11

The effect of temperature on D values

Answer 11

Higher temperatures lead to lower D values for each micro-
organism

Question 12

The effect of temperature on D values

K vs D graphs

Answer 12

K vs T curves up

D vs T curves down

Log10K vs T straight line up

Log10D vs T straight line down

Question 13

Z value

Answer 13

defined as the “the change in temperature required to cause a 10- fold change in the D value

Z = (T1-T2) / log10(Dt2) - log10(Dt1)

• around 6 to 13°C in the temperature range
100 to 130°C
• Can use Z values to predict the effect of changing temperature on
the rate of kill of micro-organisms

Question 14

The D value of a particular micro-organism at 121°C is 3 minutes. Its Z value is 4°C. What is its D value at 113°C?

Answer 14

How many Z cycles = temp difference / Z

8/4=2 x Zcycle

Each cycle increases D by 10

D113 = 100xD121
D113 = 100x3=300min

Question 15

Traditionally, moist heat sterilisation has been conducted using

Answer 15

121 degrees for 15mins

Question 16

If a sterilisation process has an F0 value of 10, this means that:

Answer 16

The process s equivalent to 10 mins at 121 degrees

Question 17

F0

Answer 17

F0 = tT1 x 10^((T1-121/z))