Sterilization by heat

• Sterilization means the freeing of an article from all living forms that includes viruses, bacteria and their spores, fungi and their spores, both pathogenic and non-pathogenic forms.
• There are four different methods of sterilization. They are

a) Heat

b) Filtration

c) Irradiation

d) Chemical disinfection.

The most widely applicable and reliable method is sterilization by heat which is performed under carefully controlled conditions at different temperatures.

• Two types of heat are used in which moist heat is considered more effective than the dry heat.
• Moist heat kills the microorganisms by coagulating and denaturing the enzymes and structural proteins.
• Dry heat is believed to kill the microorganisms by destroying the essential cell constituents by oxidation.
• Slight charring of paper, cotton and other organic materials are some consequences of high temperature.

A) Sterilization by dry heat

• Various methods are used in this method of sterilization. They are:

i) Red heat

• The materials like inoculating wires, points of forceps and searing spatulas are sterilized by this method.
• They are heated in the flame of Bunsen burner until they are seen to be red hot.

ii) Flaming

• Scalpels and needles are exposed for a few seconds in a gas or spirit flame which causes the destruction of microorganisms but it is uncertain.
• The materials are treated in methylated spirit and the spirit is burnt off to produce heat.
• This method may not produce sufficiently high temperatures required for complete sterilization.

iii) Hot air oven

• This is the main means of sterilization by dry heat.
• The temperature of oven is maintained by the use of thermostat.
• The oven is heated with electricity till the chosen temperature is achieved.
• A fan is used to assist circulation of air in the oven.
• A temperature of 160⁰C is maintained for 1 hour.
• Materials like dry glass ware, forceps, scalpels, scissors, throat swabs and syringes are sterilized by this method.
• Dry materials in sealed containers, powders, fats, oils, greases that are impermeable to moisture are also sterilized.
• The oven must not be overloaded and spaces must be left for circulation of the air through the load.
• Initially, the oven may be cold or warm when loaded but after loading it is heated till the temperature of 160⁰C is attained that is indicated by thermostat.

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iv) Infra- red radiation

• Infra-red radiation is also employed in this method of dry heat.
• The infra-red rays are directed from an electrically heated element on to the objects that are to be sterilized.
• All glass syringes, surgical instruments are sterilized by this method where temperature of 180⁰C or 200⁰C or above 200⁰C is attained.
• Cooling is hastened and oxidation is prevented by admitting filtered nitrogen to the chamber during the cooling period.

B) Sterilization by moist heat

• Moist heat kills the microorganisms only when they are in contact with hot water or steam.
• The microorganisms are subjected only to the dry heat if they are protected from wetting, as by grease or in a sealed impervious container at the same temperature.
• Moist heat can be employed at different temperatures.

i) At temperatures below 100⁰C

ii) At a temperature of 100⁰C i.e. in boiling water or free steam

iii) At a temperature above 100⁰C i.e. in saturated steam under increased pressure in an autoclave

• The first two temperatures are generally used for disinfection purposes.
• The third temperature ensures sterilization and killing of highly resistant spores.

a) Moist heat at temperature below 100⁰c

• It is used for pasteurization of milk where temperature employed is either 63⁰C to 66⁰C (145⁰-150⁰F) for 30 minutes commonly known as holder method.
• The temperature of 72⁰C (162⁰F) for 20 seconds might also be used in the name of flash method.
• All the non-spore forming pathogens such as Mycobacterium tuberculosis, M. bovis, Brucella abortus and various Salmonella are destroyed by this method.
• Coxiella burnetti is heat-resistant and may survive pasteurization by holder method.
• The treatment by this method usually reduces those microorganisms to less than an infective dose unless large numbers of the organisms are present.
• Vaccines prepared from pure cultures of non-sporing bacteria may be inactivated at a low temperature in a water bath.
• Heating in vaccine bath for 1 hour at 60⁰C is sufficient for this.
• High temperatures may diminish the immunizing power of the vaccine.
• Disinfection of eating utensils, clothing, bed clothes and some items of nursing equipment is done by washing in water for several minutes at 70⁰C to 80⁰C.

b) Moist heat at a temperature of 100⁰C

• Boiling at 100⁰C for 5 to 10 minutes is sufficient to kill all non-sporing and many, though not all, sporing organism.
• Sterility might not be ensured by this method.
• To some extent they are satisfactory for certain purposes in bacteriology and medicine where sterility is not essential.
• The instruments should be allowed to dry after they are removed from boiling water before they are used.
• This prevents the contamination of the working end with skin bacteria from the fingers.
• The culture media is sterilized by using either of the two ways as follows:

i) By a single exposure at 100⁰C for 90 minutes

• This may not be effective to spores of some thermophilic and rarely mesophilic bacteria.

ii) intermittent exposure at 100⁰C e.g. for 20-45 minutes on each of the three successive days

• Culture media rich in sugar contents and gelatin use this method of moist heat.
• The successive heating may give time to spores to get converted to vegetative form in presence of nutrients which will be killed again on next heating.
• For less volume i.e. about 100ml, heating can be done for 20 minutes but for larger volume the time should be increased.
• Thermophilic, anaerobic and other bacteria which spores will not germinate in the particular medium or under the conditions of storage may survive this heating method.

c) heat at temperatures above 100⁰C

• This is the more efficient method of sterilization where saturated steam is used.
• It provides the greater lethal action of moist heat and partly because it is quicker in heating up the exposed particles.

• It can also easily penetrate porous materials such as cotton plug, paper and cloth wrappers, bundles of surgical linens and hollow apparatus.
• The cooler surface of articles helps in condensing the steam on contact, into a small volume of water.
• This liberates its considerable latent heat to that surface.
• This contraction in volume brings more steam to the same site and the process continues rapidly till the article’s temperature will be same to that of the steam.
• The condensation process ensures moist condition for killing of the exposed microbes.
• Pure steam must be used in complete absence of air which may hinder the penetration if present in small amount.
• The saturated steam under pressure higher than atmospheric pressure attains the higher temperature than 100⁰C.
• This additional advantage of steam as a sterilizing agent is exploited in the process known as autoclaving.

References: 

i) https://tuttnauer.com/blog/sterilization-heat

ii) https://study.com/academy/lesson/what-is-the-moist-heat-sterilization-method.html

iii) https://microbeonline.com/dry-heat-sterilization-principle-advantages-disadvantages/

Sterilization by heat