A simple, effective, sterilizing method used today is to heat the substance being sterilized to 121°C for 15 minutes in a pressured system. If sterilisation under pressure is not possible because of lack of equipment, or the need to sterilise something that will not withstand the higher temperature, unpressurized heating at a temperature of up to 100°C, the boiling point of water, may be used. The heat will kill the bacterial cells, but bacterial spores capable of later germinating into bacterial cells may survive. Tyndallization can be used to destroy the spores.
Tyndallization essentially consists of heating the substance to boiling point (or just a little below boiling point) and holding it there for 15 minutes, three days in succession. After each heating, the resting period will allow spores that have survived to germinate into bacterial cells; these cells will be killed by the next day's heating. During the resting periods the substance being sterilized is kept in a moist environment at a warm room temperature, conducive to germination of the spores. When the environment is favourable for bacteria, it is conducive to the germination of cells from spores, and spores do not form from cells in this environment (see bacterial spores).
The Tyndallization process is usually effective in practice. But it is not considered totally reliable—some spores may survive and later germinate and multiply. It is not often used today, but is used for sterilizing some things that cannot withstand pressurized heating, such as plant seeds.
- Tyndallization briefly explained in a short tutorial about techniques for Dealing with Microbes in a Science Lab.
- Gould GW (September 2006). "History of science--spores". J. Appl. Microbiol. 101 (3): 507–13. doi:10.1111/j.1365-2672.2006.02888.x. PMID 16907801.
- Mesquita, J. A. M.; Teixeira, M.A.; Brandao, S. C. C. (1998). "Tindalization of goats' milk in glass bottles." (PDF). J. Anim. Sci. /J. Dairy Sci. 76, Suppl. 1 / Vol. 81, Suppl. 1/: 21. Archived from the original (PDF) on 22 March 2012.