I never understood this. The kettle hisses loudly, but the water appears perfectly still. Then the hissing stops when the water boils and bubbles furiously, emitting comparatively little sound.
I never understood this. The kettle hisses loudly, but the water appears perfectly still. Then the hissing stops when the water boils and bubbles furiously, emitting comparatively little sound.
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Good question. I was wondering about that today.
As the water approaches the boiling point, you get local boiling right near the heating element, but those little water vapor bubbles almost immediately collapse in the relatively cooler water above. This is similar to what happens when propellers cavitate, it’s noisy and can cause damage.
I would guess it’s because of cavitation, as in it’s just hot enough to form some steam bubbles that violently collapse immediately but once it gets hot enough the bubbles start to reach the surface and leave as vapour.
Water close to the heat source is turning to steam but then immediately collapsing back to liquid water because the water above it is cooler. They’re basically micro bubbles collapsing making that sound.
In the initial phase of boiling water in a pot, the water just touching the pot gets hot enough to boil to steam for a fraction of a second, forming little bubbles. But as the bubbles grow up and away from the base of the pot, the bubble brushes up against water that’s a lot colder. This condenses the steam almost immediately and forces the bubble to rapidly collapse, which is called cavitation. The thing you’re actually hearing during the noisy phase of boiling is the water’s surface slapping back against the bottom of the pot every time one of those tiny little bubbles collapses.
When the water gets warm enough to the point where the steam bubbles grow very quickly and the surrounding water isn’t so cold that it shocks them back down to nothing, the bubbles will gently float up to the surface and break. That still makes a non-zero amount of noise, but way less noise than cavitation.
Steaming milk for your latte does the same thing. We used to call it screaming. You could reduce it by “bumping” it, getting the steam wand just below the surface and making a quick cloud of bubbles.
These aren’t official terms or anything, just what we used to call it at the Starbucks I worked during degrees. One of which, I was surprised to learn, was that Rochester one that was in the news.