When boiling water without any other ingredients in it, why start from cold?

Erik P.
  • When boiling water without any other ingredients in it, why start from cold? Erik P.

    A question about probably the most mundane subject in cooking: boiling water.

    For cooking techniques where you drop ingredients in simmering or boiling water - such as for vegetables, pasta, many rice recipes - I have often seen the recommendation that you start by putting cold water into a pot, then bring it to a boil. Why would you not start with hot water from the tap? It's going to be quicker than heating cold water, and your water heater is going to be way more energy efficient than your stove top at heating the stuff.

    In particular, is there any physical or chemical process that starting from cold water encourages or prevents from happening?

    (To reiterate: in the case where you add stuff to cold water and then start heating it, there clearly is a difference with starting with hot water; this question is about the case where you drop your ingredients in pure water that's already boiling.)

  • There has been a lot of discussion over the years in cooking circles about whether cold water or hot water comes to a boil faster, and the people that wrote those recipes are passing on their determination on to you. There is zero functional difference between one pot of boiling water and another. Once a roiling boil has been achieved, the water will always be the exact same temperature. That is one of the reasons that we use water as a cooking medium.

  • Some people say cold water boils faster than hot water, this is false, found here and here.

    One reason might be (from the first link): "Some water heaters may introduce additional sediment into the water, giving you another reason to consider starting with cold—at least, if time is not of the essence."

  • There have been plumbing systems in which the hot water was likely to have dissolved more [toxic|unsightly|unpleasant-tasting] material from the pipe walls or joints than the cold. In particular any system that uses lead-based solder, can leach minute{*}, but detectable amounts of lead into the drinking water, and the hot water is more efficient at this.

    In this case that advice amounts to "use the clean water".

    {*} Really minute. Like "Use this water all you life and not suffer any ill-effects" minute. But it can be detected, and who wants to chug down a glass of lead solution...

  • My high-school chemistry teacher claimed that hot water is inferior at storing dissolved gasses, and that hot water has probably been sitting in the plumbing system for some time, so a great deal of the dissolved oxygen gas has been expelled. On the other hand, cold water from the taps is more "fresh" and enriched with oxygen gas. Even after boiling water, the trapped gases will take some time before they escape. So if you start with cold water, whatever you're cooking with the water will become more enriched with oxygen gas than if you started with warm water.

    Dissolved oxygen gas = tasty?

  • My hot water tap is supplied by a combi boiler which heats the water on-demand, supplied by the same cold water source as my cold water tap. As a result, I'm confident that the water is reasonably fresh and clean. I don't use it for brewing tea or coffee, but I'm happy to boil vegetables and rice in it. It saves a couple of minutes bringing the water to the boil.

    My parents' hot tap is supplied from an insulated immersion heater tank. The same water can sit in there for days, and it may heat and cool several times in that period. It is supplied by a header cistern in the attic. Last time I looked at the header cistern, there was a crop of dead flies floating on the surface, and some unidentifiable gunge settled at the bottom. This hot water is suitable for bathing and cleaning; it's not suitable for cooking.

    If you don't know the details of your plumbing, and you're not sure it's safe, don't cook with water from your hot tap.

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