EDITORIALS
Effect of Heat on the Sterilization of Artificially
Contaminated Water
Howard D. Backer
Groh et a1 have published a simple, yet informative
experiment in this issue of thelournal ofTravel Medicine
to challenge a commonly held belief that water must be
boiled for up to 10 minutes to render it safe to drink.
They conclude that sustained boiling is not necessary;
furthermore, achieving the boiling temperature of 1OOOC
is not necessary. While I agree with their conclusions,
they are making an assumption that cannot be supported by their data, which are based only on testing
enteric bacteria. However, other data exist to help support their conclusion.
While enteric bacteria, especially enterotoxigenic
Escherichiu coli account for the majority of travelers’ diarrhea, other classes of microorganisms, including viruses
and protozoa, regularly account for a small percentage
of cases. At certain tinies and locations, these other
pathogens may even be the predominant etiologic agents.
Adequate preventive measures are important because
these pathogens will not respond to the presumptive
antibiotic therapy recornmended for traveler’s diarrhea.’
Water treatment is an essential component in the prevention strategy of enteric infections.The issue of heating or boiling water is important because this method
is used worldwide by local residents,travelers and campers
to provide safe drinking water. It is also recommended
in countries with nornidly safe drinking water as backup
in emergencies or when water systems have beconie
contaminated by floods or a lapse in water treatment plant
efficacy. In addition to heat, methods available for iinproving microbiologic and aesthetic qualities of water for travelers include halogens (iodine and chlorine), filtration,
and clarification techniques.’ The main advantages of
using heat are that it is widely available and it imparts no
additional taste to the water. Efficacy of heat treatnient
is not compromised by contaminants or particulates in
the water, as is treatment by halogenation and filtration.
The major disadvantage of heat in many areas is the use
of scarce resources. O n e kilogram of wood is required
to boil 1 L of water.’ Liquid fuels are expensive in developing countries and heavy to carry for the wiIderness
traveler. Another disadvantage is that heat does not
improve the taste, smell, or appearance of poor quality
water.
To understand discrepancies in recommendations,
it is useful to define some terms. Disinfection, the desired
result of field water treatment, means the removal or
destruction of harmful microorganisms. Technically, it
refers only to the use of chemical means such as halogens, but the term can be applied to heat and filtration.
Pasteurization is sindar to dsinfection, but specifically refers
to the use of heat, usually at temperatures below 100°C,
to kill most pathogenic organisms. Disinfection and pasteurization should not be confused with sterilization,
which is the destruction or renioval of all life f o r n i ~ . ~ T h e
goal of disinfection is to achieve potable water; potability indicates only that a water source, on average over a
period of time, contains a “minimal niicrobial hazard”
with an acceptable statistical likelihood of illness.’ P ~ Y $ catioti is the renioval of organic or inorganic chemicals
and particulate matter to remove offensive color, taste,
and odor. It is frequently used interchangeably with disinfection; however, purification may not remove or kill
enough microorganisms to assure microbiologic safety.
Heat inactivation of microorganisms is exponential
and follows first-order kinetics.” Thus, thermal death
point is reached in shorter time at higher temperatures,
while lower temperatures are effective with a longer
contact time. Pasteurization uses this principle to kill
enteric food pathogens and spoiling organisnis at temperatures between 60-7O0C, well below boiling (Table
I).’Therefore, the minimum critical temperature is well
below the boiling point. Heat resistance varies with different microorganisms, but common enteric pathogens
are readily inactivated by heat (Table 2).’Although results
are inconsistent, they cannot be directly compared because
of different experimental methods.
Howard D. Backer, M5:The Permanente Medical Group,
Kaiser Hospital, Hayward, California, Past President,
Wilderness Medical Society, Indianapolis, Indiana.
Reprint requests: Howard D. Backer, MD, 27400 Hesperian
Boulevard, Hayward, CA 94545
J Travel Med 1996; 3:l-4.
1
J o u r n a l of T r a v e l M e d i c i n e , V o l u m e 3, N u m b e r 1
2
Table 1 Grade A Milk Pasteurization Temperature
and Timeg
Temperature ("C)
Time Required to Achieve
Pasteurization (seconds)
72
89
90
94
96
100
15.0
1.o
0.5
0.1
0.05
0.01
Bacterial spores (e.g. CIortridiurn spp.) are the most
resistant; some can survive 100°C for long periods.
Clostridicrm spores are ubiquitous in soil, lake sediment,
tropical water sources, and the stool of animals and
humans, but they are not likely to be waterborne enteric
pathogens. C.pe$ingens type C causes enteritis necroticans and has the potential for waterborne transmission in
the tropics, but its epidemiology relates infection to foodborne sources affecting small children and malnourished
adults.*'Thus, water sterilization is not necessary, since the
most resistant organisms are not enteric human pathogens.
Protozoal cysts, including Giardia, Entarnoeba histolyticu, and Cryptorporidiurn are the most susceptible to
heat, killed rapidly at 55 to 60°C. Parasitic eggs, larvae,
and cercariae are equally susceptible to heat. For most
helminth eggs and larvae (which are more resistant than
are cercariae and Cyclops), the critical lethal temperature
is 50 to 55°C.29
Other work has confirmed the results of Groh et a1
for heat inactivation of vegetative bacterial enteric pathogens.The work by Bandres et a1 was most comparable
and reached similar conclusions.*' After adding lo7organisms to water, they heated 100-mL aliquots to the desired
temperatures, then allowed them to cool (see Table 2 ) .
Viruses are more closely related to vegetative bacteria than
to spore-bearing organisms.6
Given its environmental stability and clinical virulence, there is special concern for hepatitis A virus (HAV).
HAV should respond to heat similarly to other enteric
v i r ~ s e s ,but
~ " data indicate that it has increased thermal
resistance compared with some other enteric viruses.
Widely varying data are likely due to variation in models for infectivity and destruction and in testing media.
Table 2 Data on Heat Inactivation of Microorganisms*
Organism
Lethal Temperature and Time
Giardia
55°C (131°F) X 5 minutes
100"C/immediateIy
50°C X 10 minutes (95% inactivation)
60°C X 10 minutes (98% inactivation)
70°C X 10 minutes (100%inactivation)
55°C
Similar to above
50-55°C
10
45°C X 5-20 minutes
55°C warmed over 20 minutes
64.2"C X 2 minutes
72°C heated up over 1 minute
55OC X 30 minutes
65°C X < 1 minute
16
E. histolytica
Nematode cysts
Helminth eggs
Larvae, cercariae
Cryptosporidium
E. coli
Salmonella
Shigella
Vibrio cholerae
E. coli
E. coli
Salmonella
Shigella
Campy10bacter
Viruses
Hepatitis A
Hepatitis E
Bacterial spores
'Particularly enteric pathogens
"Cultured immediately after heating from 23'C
6Ck62"C X 10 minutes
100°C X 30 seconds
6OoC**/3-logreduction
Reference
11
12
13
14
15
17
18.19
20
21
65°C" (all but 10 Campylobacter)
75°C" (100% kill)
55-60°C X 20-40 minutes
70°C X < 1 minute (even in milk)
98°C X 1 minute
60°C X 19 minutes (in shellfish)
61°C X 10 minutes (50% disintegrated)
85OC X 1 minute
60°C X 30 minutes
z 100°C
8.22
23
24
25
26
26.21
26
8
Editorials
3
Table 3 Effect of Elevation on Boiling Temperature
of Water
Elevation
10,000
14,000
19,000
fl)
Boiling Point (“C)
90
86
81
only one was 65”C, and several were 52°C. They concluded that hot water from hot water taps is not safe to
drink. However, if water has been sitting in the hot
water tank at temperatures near 60°C for a prolonged
period, the enteric pathogens dbe sigdcantly reduced,
likely to potable levels. As a result, Neumann’s suggestion is reasonable if no other means of water treatment
is available.
Heat remains an important and widely used method
to assure microbiologic safety of drinking water. Given
the resistance of Cryptosporidiwn oocysts to halogens3’and
the inadequacy of filters for viruses, heat is the only single step process that inactivates all microorganisms.A combination of filtration and halogen (as in the filters that
combine iodine resins and microfiltration) is the other
alternative.*
It would be very helpful to have more definitive work
done with hepatitis A in water, not in food.The most
applicable experimental format would include time
required to heat the water to a specific temperature.
In recognition of the difference between pasteurizing water for drinking purposes and sterilizing for surgical purposes, most sources now agree that boiling for
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Transportation of sea weed to be dried at Jambiani Beach, Zanzibar, Tanzania. Submitted
by Danielle Gyurech, M D and Julian Schilling, MD.