MadSci Network: Microbiology

Re: Would baking water remove all bacteria, and how to test?

Date: Tue Mar 19 20:41:40 2002
Posted By: Dean Cliver, Faculty, Food Safety Unit, Uiversity of California, Davis
Area of science: Microbiology
ID: 1014589830.Mi

	The way that heat is applied to water has very little to do with 
whether bacteria in the water live or die.  What matters is the 
temperature.  Most of the things in water that might make us sick 
(bacteria, viruses, parasites) are killed by boiling, but some bacteria 
form spores that are not killed by boiling.  Fortunately, they are not 
common in water and are not likely to cause disease if water containing 
them is drunk.
	The key to your question is that water (pure, at sea level) boils 
at 212 degrees Fahrenheit (100 degrees Celsius), no matter how the heat is 
applied.  If you put a pan of water to boil on a stove top burner and 
measure the temperature with the right kind of thermometer, it will be 
somewhere near 212  depending on your altitude and the atmospheric 
pressure that day.  If you put another pan of water in the oven at, say, 
350 degrees Fahrenheit, the water won't get any hotter than it did on the 
stove top because it keeps itself "cool" by boiling.  A thermometer will 
prove this.  When all of the water has boiled away, the pan will get to 
350.  If we want to kill the bacteria that form spores, we do what the 
canning companies do  use a pressure cooker to raise the boiling point 
(temperature) of the water.  
	So, you can show that the temperature of boiling water is the same 
as the temperature of "baking" water with a thermometer that will register 
such temperatures (say, a candy thermometer).  If you dissolve a lot of 
salt or sugar in the water, you can raise its boiling temperature 
	If you want to know whether the bacteria have all died, you can buy 
some sterile "petri plates" of medium (say, nutrient agar, for a common 
one) and spread a few drops on the agar surface.  You can transfer the 
water, a drop at a time,  from the pan to the agar with a metal or glass 
rod that has been wiped off with rubbing alcohol and let dry.  You'll need 
to put a few drops of unheated water on one plate, a few drops of boiled 
water on another, and a few drops of "baked" water on a third.  "Disinfect" 
your rod again with rubbing alcohol after you finish each plate.  Technique 
is tricky, though; it's very easy to get bacteria from fingers, your 
breath, or the air onto the agar surface and ruin the experiment.  You 
can touch the agar in another plate with just the "disinfected" rod (no 
water) to see whether anything dropped in that was not from the water. Put 
the lid back on each plate as soon as you have put maybe 4 drops of water 
at various places on the agar surface.  After a half hour, carefully turn 
the plates over and put them somewhere where the temperature is comfortable 
(for you) and preferably away from light.  Look at them the next morning, 
and daily till the agar dries out.  You should see little clumps of 
bacteria ("colonies") growing on the agar surface where the water drops 
were.  If there were a lot of bacteria, the whole area wet by the water 
drops will be covered by bacterial growth.  If boiling or baking the water 
killed all the bacteria, you won't see anything where those drops landed.  
You may see a few after a couple of days, if there were spores in the 
	You can try to look for water bacteria with a microscope, but 
that's not a really good way to do this kind of experiment because dead 
bacteria (killed by the heat) may stay around for some time and look just 
like the ones that were alive.  Preparing bacteria to be seen with a 
microscope requires "fixing" them (mild heating kills them and sticks them 
to a glass slide).  It is usually necessary to stain the bacteria and to 
look at them with a microscope lens that needs oil under it to magnify the 
bacteria enough to be seen.  If you really want to see the bacteria, rather 
than just learn whether they are alive or dead, you will need to find a 
nearby science teacher to give you a hands-on lesson.
	Good luck!

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