The application of heat is a simple, cheap and effective method of killing
pathogens. Methods of heat application vary according to the specific appli-
& Pasteurization. This is the antimicrobial treatment used for foods in li-
quid form (milk):
— Low-temperature pasteurization: 61.5 8C, 30 minutes; 71 8C, 15 seconds.
— High-temperature pasteurization: brief (seconds) of exposure to 80–85 8C
in continuous operation.
— Uperization: heating to 150 8C for 2.5 seconds in a pressurized container
using steam injection.
& Disinfection. Application of temperatures below what would be required
for sterilization. Important: boiling medical instruments, needles, syringes,
etc. does not constitute sterilization! Many bacterial spores are not killed
by this method.
& Dry heat sterilization. The guideline values for hot-air sterilizers are as
follows: 180 8C for 30minutes,160 8C for 120minutes,whereby the objects to
be sterilized must themselves reach these temperatures for the entire pre-
& Moist heat sterilization. Autoclaves charged with saturated, pressurized
steam are used for this purpose:
— 121 8C, 15 minutes, one atmosphere of pressure (total: 202 kPa).
— 134 8C, three minutes, two atmospheres of pressure (total: 303 kPa).
In practical operation, the heating and equalibriating heatup and equalizing
times must be added to these, i.e., the time required for the temperature in
the most inaccessible part of the item(s) to be sterilized to reach sterilization
level.When sterilizing liquids, a cooling time is also required to avoid boiling
The significant heat energy content of steam, which is transferred to the
cooler sterilization itemswhen the steamcondenses on them, explainswhy it
is such an effective pathogen killer. In addition, the proteins of microorgan-
isms are much more readily denatured in a moist environment than under
& Nonionizing radiation. Ultra-violet (UV) rays (280–200 nm) are a type of
nonionizing radiation that is rapidly absorbed by a variety of materials. UV
rays are therefore used only to reduce airborne pathogen counts (surgical
theaters, filling equipment) and for disinfection of smooth surfaces.
& Ionizing radiation. Two types are used:
— Gamma radiation consists of electromagnetic waves produced by nuclear
disintegration (e.g., of radioisotope 60Co).
— Corpuscular radiation consists of electrons produced in generators and
accelerated to raise their energy level.
Radiosterilization equipment is expensive. On a large scale, such systems are
used only to sterilize bandages, suture material, plastic medical items, and
heat-sensitive pharmaceuticals. The required dose depends on the level of
product contamination (bioburden) and on how sensitive the contaminating
microbes are to the radiation. As a rule, a dose of 2.5 !104 Gy (Gray) is con-
One Gy is defined as absorption of the energy quantum one joule (J)
Liquids and gases can also be sterilized by filtration. Most of the available
filters catch only bacteria and fungi, but with ultrafine filters viruses and
even large molecules can be filtered out as well. With membrane filters, re-
tention takes place through small pores. The best-known type is the mem-
brane filtermade of organic colloids (e.g., cellulose ester). Thesematerials can
be processed to produce thin filter layers with gauged and calibrated pore
sizes. In conventional depth filters, liquids are put through a layer of fibrous
material (e.g., asbestos). The effectiveness of this type of filter is due largely
to the principle of adsorption. Because of possible toxic side effects, they are
now practically obsolete.