Effect of Heat on the Sterilization of Artificially Contaminated Water

ABSTRACT

Background: The objective of the study was to evaluate indirect methods commonly used in the field to decontaminate water as follows: boiling for 5–10 minutes and heating until "too hot to touch." Water perceived to be "too hot to touch" is defined as water in which a subject is unable to bear having the right index finger fully immersed for 5 seconds.

Methods: Five flasks of water at 25°C, 50°C, 60°C, 70°C, and 100°C were inoculated with 1.82 × 10⁶Escherichia coli bacteria. At the time of inoculation, and at 1 minute, 5 minutes, and 10 minutes, samples were withdrawn from each flask. The samples were plated and incubated for 18 to 24 hours. The numbers of colonies were then counted. Finally, subjects attempted to immerse, fully, their right index fingers into water at 50°C, 55°C, 60°C, and 65°C for 5 seconds. Subjects were normal, healthy individuals who worked at a microbiology laboratory. The main outcome measures were the number of colonies and the subjects' intolerance to heat.

Results: Results showed that water at 50°C has no effect on the number of bacteria, whereas water maintained at 60°C for 5 minutes and at 70°C and 100°C for any time period effectively kills E. coli bacteria. The majority of subjects found 60°C or cooler to be "too hot to touch."

Conclusions: For travelers or campers, water is safe to drink if heated to boiling, but heating water until it is "too hot to touch" is inadequate for safe drinking purposes.