FOOD MICROBIOLOGY: MICROBIAL CONTROL
STERILIZATION
Destroys all living microbes, including spores. Autoclave: 121 °C, 15 psi, 15 to 20 minutes. Hot air oven: 160 to 180 °C, 1 to 2 hours.
DISINFECTION
Reduces pathogens on a surface or object to a safe level. Does not necessarily kill spores. Uses chemicals, heat, or UV.
Figure 1: Sterilization kills everything including spores, while disinfection lowers pathogens to a safe level.
Introduction
Sterilization and disinfection differ in how completely they remove microbes. Sterilization destroys all living microbes, including spores, while disinfection reduces pathogens to a safe level without necessarily killing spores.
Quick comparison
Quick comparison: Sterilization and Disinfection
| Aspect | Sterilization | Disinfection |
|---|---|---|
| Goal | Destroy all living microbes, including spores and viruses | Reduce pathogens to a safe level |
| Spores | Killed | Usually survive |
| Result | Sterile: no living microbes remain | Disinfected: safe level, not sterile |
| Main methods | Steam under pressure (autoclave), dry heat, filtration, radiation, ethylene oxide gas | Chemical disinfectants, boiling water, steam, UV radiation |
| Typical settings | Autoclave 121 °C at 15 psi for 15 to 20 minutes; hot air oven 160 to 180 °C for 1 to 2 hours | Chlorine, alcohol or quaternary ammonium solutions at label strength for the label contact time |
| Used on | Media, glassware, instruments, liquids, canned food | Benches, floors, equipment surfaces, hands, water |
| Food example | Canning at 121 °C under pressure targets Clostridium botulinum spores | Sanitizing a washed cutting board or a clean tank surface |
| Check | Indicator tape, spore strips, gauges for temperature, pressure and time | Test strips or titration for strength, plus swabs or ATP checks |
| Related terms | Pasteurization is not sterilization: it kills pathogens but leaves some microbes | Antisepsis is disinfection on living tissue such as skin; sanitization lowers microbes on eating utensils to a safe level |
Scientific explanation
Definition
Sterilization is the destruction of all living microbes, including spores, from an object or material. Disinfection is the reduction of pathogens on a surface or object to a safe level, without necessarily killing spores. Antisepsis is disinfection applied to living tissue such as skin, and aseptic technique is the set of habits that keeps sterile things sterile while they are handled.
Working principle
Moist heat kills by denaturing proteins: steam under pressure reaches temperatures above boiling and carries heat into materials fast, so the autoclave at 121 °C sterilizes media, glassware, liquids and waste in 15 to 20 minutes. Dry heat works by oxidation and is slower, since dry air carries heat poorly, so the hot air oven needs 160 to 180 °C for 1 to 2 hours and suits glassware, metal instruments and powders. Disinfectants kill vegetative cells by damaging membranes, proteins or DNA, but spores resist them, and no disinfectant works well on a dirty surface, so cleaning comes first and disinfection second.
Apparatus
Sterilization uses the autoclave, the hot air oven, membrane filters, and radiation or gas units. Disinfection uses chemical solutions, boiling water, steam and UV lamps. Every autoclave run is checked by temperature and pressure gauges, time, and indicator tape, and the door is never opened until the pressure has fallen to zero.
Measurement procedure
Heat killing follows log-linear death: the D-value is the time for a 90 percent kill at a given temperature, the Z-value is the temperature rise that cuts the D-value tenfold, and the F-value is the total lethal effect at a reference temperature. Pasteurization, for example, holds milk at 72 °C for 15 seconds (high temperature short time) or 63 °C for 30 minutes (low temperature long time), which kills pathogens without sterilizing.
Factors affecting readings
Four factors change how well a treatment works: the number of organisms present, the type of organism (spores resist far more than vegetative cells), the surface being treated, and the cleanliness of that surface. Organic matter shields microbes and reacts with the sanitizer, using it up before it reaches the cells.
Formulas and calculations
Worked example with D-value and Z-value. A D-value is the time at one temperature that kills 90 percent of a population, which is a one-log reduction. If a heat treatment gives a 6-log reduction and the D-value is 1 minute at that temperature, the time needed is 6 x 1 = 6 minutes. The Z-value is the temperature rise that cuts the D-value tenfold: if the D-value is 10 minutes at 110 °C and the Z-value is 10 °C, then at 120 °C the D-value is 1 minute. The F-value is the total lethal effect of a heat process stated at a reference temperature, and canning uses it to prove the process kills Clostridium botulinum spores.
Industrial applications
- Sterilization: culture media and glassware in food labs; canned foods; surgical instruments; sterile pharmaceuticals.
- Disinfection: benches, floors and equipment in food plants; water treatment; hand and surface hygiene in kitchens and dairies.
- Both: food plant hygiene plans that sterilize lab media and disinfect production surfaces on a fixed schedule.
Advantages and limitations
Sterilization: advantages are complete kill including spores and a clear pass or fail check with spore strips; limitations are higher heat, longer time, cost, and damage to heat-sensitive materials.
Disinfection: advantages are speed, low cost, and use on large surfaces and in water; limitations are that spores survive, dirty surfaces defeat it, and strength and contact time must be checked, not guessed.
FAQ
Is pasteurization the same as sterilization?
No. Pasteurization is a mild heat treatment that kills pathogens and most spoilage organisms but does not sterilize: spores are not affected.
Why does the autoclave use steam under pressure?
Steam under pressure reaches temperatures above boiling and carries heat into materials fast, so it kills vegetative cells, spores and viruses in 15 to 20 minutes at 121 °C.
Can a disinfectant sterilize a surface?
No. Disinfection reduces pathogens to a safe level but does not necessarily kill spores, so the surface is disinfected, not sterile.
Why must cleaning come before disinfection?
Organic matter shields microbes and reacts with the disinfectant, using it up before it reaches the cells. A sanitizer cannot reach microbes buried in food residue.
References and further reading
FTN subjects used: Food Microbiology (Units 1, 2, 3, 4, 5 and 8) and General Microbiology (Units 1, 2, 3, 4, 5, 6 and 10), from the notes at ~/workspace/food-tech-notes/notes/.
Books used: Adams, M. R. and Moss, M. O., Food Microbiology, Royal Society of Chemistry, 2007 (book on disk at food-tech-notes/books/Food-Microbiology.pdf; Google Books page: https://books.google.com/books?id=BZNKI088iooC). Ray, B. and Bhunia, A., Fundamental Food Microbiology, 5th edition, as cited in the FTN notes. Essentials of Industrial Microbiology (book on disk at food-tech-notes/books/Essentials-of-Industrial-Microbiology.pdf).
Google Books pages used: Food Microbiology by Adams and Moss, https://books.google.com/books?id=BZNKI088iooC (contents and chapter list read on 2026-11-11).
