Dry Heat Sterilisation

Question 1

How should each heat-sterilisation cycle be recorded? Provide an example.

Answer 1

Should be recorded by means of appropriate equipment of suitable accuracy and precision

> e.g. on a time/temperature chart with a suitably large scale.

> Chart or photocopy of it should form part of the batch record

Question 2

What must be allowed for the whole of the load to reach the required temperature before measurement of the sterilising time is started?

Answer 2

SUFFICIENT TIME

Question 3

What should be done after a high-temperature phase of a heat sterilization cycle?

Answer 3

Precautions should be taken against contamination of a sterilized load during cooling

• Any cooling fluid or gas in contact with the products should be sterilized

Question 4

Dry Heat Sterilisation refers to a variety of methods, what is the common factor between these methods? Describe some of these methods.

Answer 4

Common factor is the absence of water from the heating environment

> heating in a direct flame [“flaming”]

• loop (red hot)
• forceps (alcohol dipped and ignited)

Question 5

Where is dry heat sterilisation usually carried out? How is heat transferred?

Answer 5

Usually carried out in a hot-air oven

• Heat is transferred by radiation, convection and to a lesser extent conduction

Question 6

What are the conditions required for dry heat sterilisation/terminal sterilisation (BP)?

Answer 6

For this method of terminal sterilisation the reference conditions are a minimum of 160 °C for at least 2 hours

Question 7

For hot air oven (BP specification);

A) What are the equipment requirements?

B) How to load the products in the oven?

C) How is temperature monitored?

D) How to check the efficacy of sterilisation for a biological assessment?

Answer 7

A)

• carried out in an oven equipped with forced air circulation

> perforated tray

B)

• steriliser is loaded in such a way that uniform temperature is achieved throughout the load –> spaced out the products evenly

C)

• knowledge of temperature within the steriliser during the sterilisation procedure is usually obtained by means of temperature-sensing elements inserted into containers at coolest part of loaded steriliser.
• temperature throughout each cycle is suitably recorded

D)

• Using a suitable biological indicator

Question 8

How to load the hot air oven when its;

A) cold load operation (recommended procedure)

B) hot load operation

Answer 8

A)

• Recommended procedure
• Oven at room temperature
• Load at room temperature

B)

• Oven at sterilising temperature BUT the LOAD, initially at room temperature
• Requires a “lag time” to be known for the particular process (product & the loading)
• Handle HOT loads
• No cooling between loads

Question 9

What are the THREE steps for the sterilization procedure?

Answer 9

1. Heating the chamber to the selected sterilising temperature
2. Sterilising the load (heating penetration time + holding time)
3. Cooling the load

Question 10

Why is a biological indicator placed in the hot air oven together with products undergoing sterilisation?

Answer 10

To see if there is growth after sterilisation

• Most heat resistant bacterial spores
• If spores are killed, then less resistant microbes in the product are killed more readily

Question 11

The heat sensitivity of microorganisms is known as the D value, what does the D value mean?

Answer 11

D value (decimal reduction time) –> unit is TIME

• Time under sterilisation exposure conditions (at a specific temperature) required to reduce a surviving microbial population by a factor of 90%. Or 90% reduction in count.

> viable population falls to 10% of its initial value after a certain time period

> eg 100 –> 10 (ten-fold reduction)

Question 12

Why is it a characteristic of first-order kinetics that the same percentage change in concentration occurs in successive time intervals?

Answer 12

• Provides a means of establishing dependable sterilisation cycles
• Initial microbial load on a product to be sterilised becomes an important consideration

Question 13

What are some examples of the D-values of the tetanus spores at the following temperaatures;

A) 180 degrees

B) 170 degrees

C) 160 degrees

D) 150 degrees

Answer 13

A) D value = 1 minute

B) D value = 5 minutes

C) D value = 12 minutes

D) D value = 30 minutes

Question 14

After heating a batch of products known to contain tetanus spores at 180°C for 5 minutes, the amount of tetanus spores was found to be 1. The biorburden of the products was 10⁶. Estimate the D value of tetanus spores at 180°C

Answer 14

Originally there was 10⁰ = 1

10⁶ –> 10⁰ = 6 D value reduction

6 D = 300 seconds

1 D = 50 seconds

Question 15

Why is the minimum of 160 °C for at least 2 hours required for terminal sterilisation?

Answer 15

Employed to give a SAL (sterility assurance level ) of 10^-6 or better

Question 16

What is the sterility assurance level (SAL)?

Answer 16

Calculated probability that a microorganism may survive a sterilisation process and that a corresponding proportion of treated articles may be unsterile.

(The SAL for a given process is expressed as the probability of a non-sterile item in that population)

• PROBABILITY OF FINDING 1 NON-STERILE PRODUCT OUT OF OF 1 MILLION

> An SAL of 10^-6, for example, denotes a probability of one item per one million processed may be unsterile –> 10^-7 would be better SAL value as its smaller.

> SAL can be used to describe the microbial population that was destroyed by the sterilization process

Question 17

What is the bioburden approach (sterilisation approach)? Provide an example.

Answer 17

Approach refers to when there is:

• Known microbial load (number and sensitivity), known bioburden in other words
• Use ‘x’ D values exposure to produce 1 survivor
• Add a further 6D values to a produce a SAL of 10^-6

> If 10⁵ at the start –> 11 D values to get to 10^-6

Question 18

What is the overkill approach (sterilisation approach)

Answer 18

Approach refers to when:

• Don’t know the bioburden
• Assume bioburden is log 6 (10⁶ ) and resistant
• 12D rule to give SAL of 10^-6
• Assume product is dirty

Question 19

Provide TWO advantages and ONE disadvantage of dry heat sterilisation.

Answer 19

Advantages

• Ability to penetrate solids, non-aqueous liquids and closed cavities
• Lack of corrosion in the sterilisation of non-stainless metals, instruments with fine cutting edges

Disadvantages

• High temperatures and the long time required for sterilisation

Question 20

What some applications of dry heat sterilisation for non-aqueous products?

Answer 20

• Oils and oily products
• Powders (medical, surgical)
• Apparatus and containers (glassware, metal)
• Depyrogenation (dry heat at temperatures 230 degrees celsius is frequently used for sterilisation and depyrogenation of glassware)

Question 21

What does the term terminal sterilisation mean?

Answer 21

A process in which the product is sterilised in its final container