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For many years I had a typical electric kettle. Whenever I wanted a 75 Celsius / 167 Fahrenheit water to make a green tea, I had to boil it up to 100 Celsius / 212 Fahrenheit (because, that was the temperature, where each my kettle was stopping to heat) and wait.

Now, I've got an electric kettle with a thermometer. Can I boil the water only to 75 Celsius and stop? What is the typical temperature, in which all (most of) germs in water are killed and water becomes drinkable?

I've heard, that this is 70 Celsius / 158 Fahrenheit, so my idea would have a ground. Am I right?

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    Where are you that you need to boil the water to make it drinkable? – ElendilTheTall Sep 27 '14 at 14:57
  • In the prospect issued by my local water-supply company. I'm not from the United States, you know... We don't have a drinkable water comming out of the wall. – trejder Sep 27 '14 at 15:03
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    That's why I asked. – ElendilTheTall Sep 27 '14 at 17:12
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    Your question only implicitly states that you're worried about making your water safe. I originally thought you were asking about water temperatures for brewing tea. It might be clearer if you simply asked how much you have to heat water to make it safe. – Cascabel Sep 27 '14 at 17:57
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    Just on a technical note, if your water is heated to 75 degrees Celsius, it isn't boiling unless you're drastically messing with the pressure. – Preston Sep 27 '14 at 21:22
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Technically, your idea seems sound. But I wouldn't do it.

What you are proposing to do in your electric kettle is very close to the standards for flash pasteurization. According to wikipedia, the standard procedure for flash pasteurization is to heat and circulate the liquid at 71.5 °C (160 °F) to 74 °C (165 °F), for about 15 to 30 seconds, which results in a five log (99.999%) or greater reduction in bacteria. Other journal articles seem to indicate that some protozoa like cryptosporidium are killed by flash pasteurization, but others, like giardia, might survive in small numbers.

I suspect the guideline for boiling water in issues of safety and in recipes is used because steam and bubbles are such convenient guarantors of temperature.

Psychologically, though, this makes me a little nervous. Personally, I would boil the water- if you have time for tea, you have time to boil water. It is possible, though, that you are a more adventurous tea drinker than I am.

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    Perfect: "if you have time for tea, you have time to boil water" – rbp Sep 29 '14 at 18:46
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You cannot boil water at 70 Celsius. Maybe this is a language problem; "boiling" means taking water to the state where there are lively bubbles popping on the surface all the time, and it is steaming profusely. It boils at 100 Celsius at sea level and a little bit below it when you get higher, but the difference isn't that much. Even in the highest towns in the world, at more than 5000 meters, water boils at just below 90 Celsius.

When you get your water to 75 Celsius, you are heating it, not boiling it. Heating water certainly kills bacteria - in fact it is the heat which desinfects water, not the boiling - but we cannot tell you which bacteria are killed and which are not. US/Western Europe guidelines for food safety suggest temperatures up to 70 Celsius for safe food preparation, but these suggestions are based on many things, such as the type of bacteria found in these parts of the world, statistics showing how many people get sick from underheated food, and so on. It is entirely possible that your water is contaminated with something different than whatever is present on US meat.

If you do not have access to tap water, or the water supply in your city is not considered safe, and there is a directive to boil water, then this is what you need to do to be officially safe. And it means real boiling, at 100 Celsius. Nobody is equipped to tell you whether 70 Celsius is sufficient for your case or not.

Update As the commenters suggested (and Wayfaring stranger linked an official source for it): The safety guidelines are not just to bring the water to boiling, but to hold it at a boil for one minute.

This means that the flash-kill temperature for the bacteria must be well over 100 Celsius. A small explanation about food safety: There is no temperature at which all individual cells in a bacterial colony keel over and die. Bacterial death is a function of temperature and time, and some hardy individual cells can withstand a lethal temperature for some seconds. This is why you either have to incinerate them outright with a really high temperature - which seems to not be possible to reach with boiling the water - or wait a bit on a somewhat lower temperature until every bacteria is dead, in this case 1 minute at 100 Celsius at low altitudes, or, because you cannot reach 100 Celsius when you are up in the mountain, 3 minutes boiling at high altitudes.

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    The safest option would probably be to boil it, then let it cool to the desired temperature. – ElendilTheTall Sep 27 '14 at 17:12
  • @ElendilTheTall this is what the OP is currently doing (a bit hidden at the end of the first paragraph). I can understand that he would prefer to stop doing it, but I agree, if his local authority has only tested boiling to be safe and recommends it, we cannot proclaim lower heating to also be safe. – rumtscho Sep 27 '14 at 17:20
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    A Guide to Drinking Water Treatment and Sanitation for Backcountry & Travel Use: cdc.gov/healthywater/drinking/travel/… – Wayfaring Stranger Sep 27 '14 at 17:53
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    When boiling water to sanitize it, I believe all US, state, and local recommendations instruct you to keep the water at a rolling boil for at least a full minute (longer at higher altitudes). Just because your kettle can reach 100°C/212°F doesn't necessarily mean that your water will be considered safe for consumption as soon as the water reaches that temperature. It is always a bad idea to make any assumptions or take any shortcuts when it comes to sanitation issues. Even if your local sanitation instructions say simply to boil the water, I wouldn't tempt fate to save a minute. – Stephen Eure Sep 27 '14 at 18:59
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    You can boil water at 70°C ... it just requires being about 9450 meters (31,000 feet) above sea level, if I'm doing my math right. Of course, Mt. Everest is only 8,848 meters, so you'd probably need a hot air balloon. 5000m should give you a boiling point closer to 83°C. – Joe Sep 27 '14 at 20:07

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