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I'm a beginner brewer, but I've noticed that on every batch I've done so far, when the wort is boiling, and I stir the wort, it ends up behaving this way. Can anyone explain why this happens?

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up vote 2 down vote accepted

There is a scientific paper on this, but it is locked behind a paywall, and I doubt I could understand it anyway. I am not a scientist, but I think this is what is happening in layperson's terms:

  • In a water-based liquid, boiling occurs at around 212°F at Standard Temperature and Pressure (STP) when liquid water undergoes phase conversion (from liquid to gas and vice versa) at that temperature. Little bubbles of water vapor nuclease in the liquid, and then rise to the surface to break because they are lighter than the liquid -- they increase in size as they rise due to both merging with other bubbles and through expansion as the water pressure decreases as they get closer to the surface. This is what boiling is. (Incidentally, bubbles are also formed because the liquid starts with gas dissolved in it, and the solubility of gas in that liquid decreaes as temperature rises -- these are the tiny bubbles that form on the side of the boil kettle before it starts boiling)
  • By stirring the wort in a circle in one direction, you are creating a vortex.
  • In the vortex, the wort in the center of the vortex is moving much more slowly than the wort at the outer edge of the vortex.
  • As the wort heats, it rises in a convection current, and it is much easier for it to move in a mostly upward vector in the center of the vortex because the wort is not moving as fast. Likewise, on the edge of the vortex, the wort is moving on a more diagonal vector.
  • Heating the kettle keeps adding energy to the vortex, and sustains its movement.
  • The water vapor formed in the wort finds it easier to nuclease into bubbles in the slower-moving wort in the center of the vortex. Perhaps the water vapor is phase-shifting back to liquid in the faster-moving wort, the temperature differential between the center and rest of the vortex is affecting the bubble formation, the bubbles merge in the center of the vortex, or a combination of those factors. I suspect that a major factor is that bubbles are forming in the higher pressure areas on the edges of the vortex, and then when they hit the lower-pressure area in the center (think of the eye of a hurricane), they are expanding rapidly and quickly rising and bursting.

Remind me to tell my theory on why the moon is made of cheese. :)

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nice. can you say anything about how the heat energy is turned into kinetic energy in the vortex to keep it moving? –  mdma Nov 9 '13 at 0:03
    
I'd give it a shot, but I tried to replicate that tonight, and realized that I could produce the same effect when whirlpooling soon after my kettle was put in an ice bath. I am sure there is a kernel of truth in that answer, but the added energy from the heat source is not a critical part of the equation. I think the pressure difference in the vortex definitely plays a part in the violent eruption of the bubbles, but I'm not sure how the vortex is sustained. –  Chino Brews Nov 9 '13 at 5:50
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