Dry heat sterilizers are important equipment used in various industries, especially in the healthcare and pharmaceutical industries. Unlike steam sterilization, dry heat sterilizers rely primarily on hot air to achieve sterilization. dry heat sterilization is used for items that are heat-resistant but moisture-sensitive, such as glass, metal equipment, oils, and powders. In this article, we will explore the definitions, processes, and validation methods associated with dry heat sterilization.

Design principle of Dry heat sterilizer

Dry heat sterilizer refers to the thermodynamic sterilization under non-saturated humidity (RH≠100%). It uses resistance wire or halogen heater to heat the air, and the dry heat air circulates in the sterilizer through the fan to kill the microorganisms in the sterilized articles by high temperature.
The sterilization principle of dry heat sterilization is to make protein denaturation, microbial oxidation or electrolyte concentration through dry heat. Dry heat sterilizer at 160 ℃ under the conditions of 10min and 40min respectively can be thermophilic fat bacillus (ATCC 7593 strain) and Bacillus subtilis black variant (ATCC 9372 strain) of the spores are completely killed. 160 ℃ 120min, 170 ℃ 60min, 180 ℃ 30min can be killed general bacterial spores. For bacterial propagules, fungi and general viruses set sterilization time and temperature of 120 ℃, 30min, dry heat sterilizer can achieve sterilization effect.

Steps for testing the sterilizing effect of dry heat sterilizers:

  1. For each test, take 2 slices as a group and place them flat in a sterile flat dish without overlapping. Cover, respectively, in the dry heat sterilizer, diagonal inside, middle and outside of the 3 points, adjacent layers of diagonal cross placement.
  2. Design program time to run 1 cycle for sterilization. After sterilization, wait until the temperature of the sterilizer naturally drops to below 50 ℃ and then open the cabinet door.
  3. Remove the petri dishes and inoculate the slices into test tubes containing 5.0 mL of TSB, and place them in a constant temperature incubator at 36 ℃±1 ℃ for 7 days. Observe the results daily from the 3rd day, the turbid surface with wrinkled bacterial film indicates that there is bacterial growth, judged as positive; clarification indicates that there is no bacterial growth, judged as negative.

If there is contamination by non-test bacteria, the cause should be found and the test should be repeated.

Dry heat sterilizer types

Dry heat sterilizers are classified into continuous and batch types according to the way they are used:
Batch type dry heat sterilizer can be divided into electric oven and dry heat air sterilizer.
Electrically heated oven using the bottom heating and upper and lower ventilation channels, hot air convection is slow. Hot air sterilizers use the bottom and both sides of the heating method, the overall air duct, rapid warming.
Continuous dry heat sterilizer, such as tunnel oven, can be used for small volume injection production. According to the heating method can be divided into: radiation heating-based thermal radiation dry heat sterilizer and convection heating-based thermal laminar flow heating dry heat sterilizer and so on.

Scope of application of Dry heat sterilizer

Dry heat sterilizers can be used to sterilize items that are not damaged, deteriorated or evaporated at high temperatures.
Sterilization and disinfection of glass products, metal products, ceramic products, ointments, etc. can be done with dry heat sterilizer.
Surgical instruments in the scalpel, scissors, forceps, petroleum jelly gauze, talcum powder.
Orthopedic hammers, saws, steel pins, bone splints and so on.
Pharmaceutical supplies and medicines such as storage containers, metal appliances for transfer and conveyance, mixing hoppers, proprietary medicines, powders, granules, pills and adhesive plugs for preparations.
Sterilization of utensils, pipettes, etc. used in microbiology laboratories can be accomplished using dry heat sterilizers.

Precautions for Dry Heat Sterilizer

  1. In order to prevent sterilization failure or carbonization of dirt to be sterilized items, the sterilized items should be washed sufficiently before dry heat sterilization, and the items should not be overlapped, avoiding contact with the inner wall of the heating box.
  2. The objects to be sterilized should not be overlapped and avoid contact with the inner wall of the heating box. 3.
  3. After the sterilization is completed, the temperature inside the box must be reduced to 40 ℃ before opening the door to prevent danger.
  4. The package of items to be sterilized should not be too large, the total height of the items should not exceed 2/3 of the height of the inner chamber of the sterilizer. in order to facilitate the convection of hot air in the items between the items to be sterilized should be left between the items.
  5. Sterilization time should be recorded from the time the sterilization chamber reaches the set temperature to start counting.
  6. In order to ensure that the goods are not subject to secondary microbial contamination, open the door and other operations should be completed in accordance with the aseptic operation method.