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In college, I recall an experiment where we boiled water by reducing the pressure in a flask that contained water until the water boiled. No heat was added.

Would this be considered a melacha of "cooking" on Shabbat? Or does "cooking" specifically mean by heat, only?

  • You can do that experiment with a plastic water bottle. :) – ezra Nov 13 '17 at 19:43
  • I'm not convinced that's cooking regardless even if you first reduced the pressure to get it almost to boiling, then did the final boiling by adding heat. It never gets to yad soledet bo. – Heshy Nov 13 '17 at 22:31
  • @Heshy "It never gets to yad soledet bo". Yes, I see this term a lot. Is this criteria the foundation of the melacha of bishul? – DanF Nov 13 '17 at 23:01
  • My understanding is yes for liquids. But don't quote me on that. – Heshy Nov 13 '17 at 23:08
  • Yay for physics. For the curious, increasing pressure increases the heat required to boil; by significantly decreasing the pressure, you can theoretically make water boil at room temperature. – DonielF Nov 15 '17 at 0:35
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R Daniel Braude (Learn Shabbos, pp. 120ff) writes that it is only cooking if using a heat source and if it is the normal way of cooking. Since "pressure-heating" water doesn't have a heat source and is not the normal way to cook, it wouldn't be prohibited.

Specifically, he writes

  • The Torah prohibition of cooking is transgressed by using fire and those things that derive their heat from fire because that is the normal way to cook. However any heat source that gets red-hot is also considered fire
  • Cooking under the sun is permitted, as this kind of cooking is not normal and therefore not called cooking in a Halachic sense (but Chazal prohibited cooking using anything that derives its heat from the sun as it is very similar to something that derives its heat from the fire)
  • A microwave could theoretically be permitted (ignoring electricity for the moment) as it does not actually have a heat source. However it is prohibited since it is (now) the normal way to cook
  • The prohibition of cooking only applies when the heat is capable of causing the food to become yad soledes bo

I checked this with R Binyamin Tabady but of course, consult your rabbi before implementing anything you learn here.

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