# Bacteria and Microwave

## Main Question or Discussion Point

If I place a quart jar of water that has been tested positive for bacteria into a 1000w microwave, how long will it take to kill 100% of the bacteria.
Thank you, Dell

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cristo
Staff Emeritus
This sounds like a homework question. Please post your thoughts, as we do not do homework for students!

I am 60 years old and I have bacteria in my water. Why are you so confrontational?

Hootenanny
Staff Emeritus
Gold Member
This sounds like homework, as does the other two identical questions you have posted in physics and biology. Tell me, if this isn't a homework question, how have you tested for bacteria?

cristo
Staff Emeritus
Sorry if my post sounded confrontational. I (incorrectly) thought that this was a homework problem, for which we cannot help unless work is provided. So, apologies again for that misunderstanding.

With regard to your question; generally I think the guidelines are that most of the bacteria will be killed by boiling the water for 1 minute.

In a 1000w microwave, to bring one quart(~1 litre) of water to boiling point (i.e. from 25 Celcius to 100 Celcius) a quick calculation shows that this will take about 5 mins. So, including the minute of boiling this will be approx 6 minutes in the microwave.

Note that, when heating liquids in the microwave, it is a good idea to include a wooden(or glass) rod, or spoon, to avoid the water becoming "super-heated." (see here for more details)

For this reason, I would advise against boiling water in a microwave- use a saucepan, bring the water to the boil, and boil for a minute.

Hope this was (something like) what you were looking for!

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iansmith
Staff Emeritus
Gold Member
Percentage killing will depend on the amount of bacteria (CFU/ml), the type of bacteria and, of course, the time.

Without knowing the amount of bacteria it's impossible to tell you how long you need. However, 15 minutes of boiling is usually the standard to ensure that you get virtually everything killed. If there any heat resistant bacteria then a longer time might be required.

As cristo pointed out, boilling in a pot is probably better than boiling water in a microwave.

The water was tested by the County Health Dept as it was a replacement well. I posted the other thread because I thought noone would answer here.
I was under the impression microwaves killed bacteria without needing to boil.

iansmith
Staff Emeritus
Gold Member
Microawave would be able to kill the bacteria but the water would need to reach a certain temperature and maintained the temperature longed enough to kill everything. It's all about the thermic effect.

It's hard to determine the time needed if you don't know how many and the type of bacteria that is present in the water.

Moonbear
Staff Emeritus
Gold Member
Boiling should only be a very short-term measure while working on eliminating the source of contamination.

Here is some information on decontaminating a well with shock chlorination, and preventing recontamination if bacteria continue to be detected.
http://www.extension.umn.edu/distribution/naturalresources/DD5941.html [Broken]
http://ohioline.osu.edu/aex-fact/0318.html [Broken]
http://www.hc-sc.gc.ca/ewh-semt/water-eau/drink-potab/well_water-eau_de_puits_e.html [Broken]
http://cahe.nmsu.edu/pubs/_m/m-115.html

Each of these has essentially the same information, but also differ in the amount of details they offer, so reading the full set seems to give more complete information than reading any one of them. If you need more assistance in dealing with the decontamination, contacting your own state or county agricultural extension office is a good source to start with, especially if you need help remediating a recurring source of contamination (they'll know the reputable contractors to help with that sort of job, and if there are any specific sources of contaminants in your area that should be suspected first).

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I was under the impression that bacteria (a majority) would be killed by microwaves as well. I often see kitchen tips that include throwing your washcloth in the microwave after washing dishes for about 45 seconds to kill bacteria on the cloth. It does help with the smell.

iansmith
Staff Emeritus
Gold Member
Microwave will killed bacteria but it's hard to tell how efficient it is when there is several variable to take into account.

Sanitation, disinfection and sterilization is dependent on several variable. For example, the temperature on a surface will probably reached a killing effect much faster than in a 1L of water or any other liquid. Once the temperature is reached, it has to be maintained long enough to kill bacteria. Also, 45 sec in a microwavemight be reach 99.99% but that's only effective if you less than 10 000 bacteria per ml. If you have 100 000, 1 million, 10 million, 1 billion, then you're left with 10, 100, 1 000 and 100 000. In some cases, 10 bacteria is sufficient to cause disease. Also, bacteria can form spores that are resistant to heat and some bacteria are capable of resisting heat killing for a longer period. An 99.99% kill for one bacteria with one set of temperature and time might be a 90% kill for a different bacteria. For example, for pasteurized milk, 65c for 30 sec is sufficient to kill 99.999% of the Mycobacterium tuberculosis and Coxiella burnetii that might be present. However, there's still bacteria left in the milk and it's not consider sterile.

Killing is dependent on temperature, time and concentration of living/sporulated bacteria.

DaveC426913
Gold Member
I was under the impression that bacteria (a majority) would be killed by microwaves as well. I often see kitchen tips that include throwing your washcloth in the microwave after washing dishes for about 45 seconds to kill bacteria on the cloth. It does help with the smell.
There is a small enough amount of water in the cloth that the mw will boil it off quite quickly. Not the same as a containerful.

Moonbear
Staff Emeritus
Gold Member
There is a small enough amount of water in the cloth that the mw will boil it off quite quickly. Not the same as a containerful.
You're also not going to eat the cloth, so if a few bacteria survive, you don't have to worry so much about it as long as you rinse the dishes you wash with that cloth thoroughly. Ian's explanation should make it clear why this is more of a concern for something that will be consumed, or if someone desires a completely sterile (killing 100% of the bacteria) solution.

Ouabache
Homework Helper
I am 60 years old and I have bacteria in my water....
Welcome to PhysicsForums!! So you're just a baby boomer . Many of us on here on PF are of the same generation.
By the way, I have a good friend who went and earned his Bachelors degree at 75.. So you're certainly young enough to be a student.

dellbertt said:
I was under the impression microwaves killed bacteria without needing to boil.
Perhaps you are thinking microwaves damage the bacteria directly.
Microwaves are in the non-ionizing end of the spectrum. They are considered radio waves and have longer wavelengths than visible light. Non-ionizing radiation has enough energy to excite molecules, causing them to vibrate. A microwave oven generates a frequency that excites water molecules by vibration. The induced kinetic energy is sufficient to heat to boiling.

Ionizing radiation is much shorter in wavelength (upper end of the ultraviolet, X-rays, Gamma rays). They cause direct damage to living tissue by breaking chemical bonds and ripping electons off of atoms.

It is possible to use high energy microwave pulses to sterilise without heating and this is done commercially for packaged foods.
An industrial microwave steriliser does not cook the food it just blats most of the pathogens so that it will last longer before spoiling.
How much energy at what frequency is determined by medium, flow rate, legal requirements for food/animal feed.
If you are serious about sterilising using microwaves then look for bulk food equipment manufacturers.
IMHO the energy requirements would mean that it is not cost effective for water which generally has a low unit cost. Even if you are in the middle of nowhere a few years running an industrial steriliser would cost more than piping in from the nearest point of civilisation. It might work on a small town scale though.

Further to my previous, I new I had a brochure somewhere.

A company called ROmiLL in the Checz Republic does microwave hygienization lines. Whether these could be used for flowed water is not my field but they build custom solutions based around your requirements.

Ouabache
Homework Helper
It is possible to use high energy microwave pulses to sterilise without heating and this is done commercially for packaged foods
Please reread the references (underlined text) listed in my last post. Microwaves don't blast anything. They induce heating by vibration of the molecules. Your http://www.romill.cz/index.php?lang=eng&show_dir=36 [Broken] also acknowledges the thermal effect.

"Microwave energy is supplied via applicators to the chamber scattered in order to reach optimal heating – usually maximum thermal homogeneity of the product."

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You need to take into account the effects of peak radiation power as well as average power.
Using low peak power then the water molecules are slowly heated and bacteria are killed off.
Using High Peak pulsed power causes little heating of the medium but still effectively kills off bacteria.
If you look at the safety guide lines for RF radiation the average power of pulsed waveforms at acceptable exposure limits are much lower than CW because other factors come into play.
Biological effects of RF Radiation are highly complex and not simply the average energy absorption.

Ouabache
Homework Helper
Panda said:
It is possible to use high energy microwave pulses to sterilise without heating and this is done commercially for packaged foods.

Using High Peak pulsed (*microwave) power causes little heating of the medium but still effectively kills off bacteria.

*included for clarity

iansmith
Staff Emeritus