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Why does simmering sauce makes bubbles at different places. Is this like boiling water? Why or why not?

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You can get a somewhat similar effect in a pot of water if you bring the water to a boil and then turn the heat down a bit. Do it just right and you'll see bubbles of steam form at the bottom of the pot, separate from the bottom, start to rise, only to vanish partway up. There's a vertical temperature gradient in that pot of simmering water. The water bottom of the pot is at the boiling point while the water at the top is well below the boiling point. This temperature gradient also exists in a simmering skillet or pot of food.

That said, there are marked differences between a simmering skillet or pot of food and a simmering pot of water. One key difference is the food. You need to stir the bottom lest the food stick and burn. Food doesn't burn at 100°C; the bottom of the pan (at least underneath the food) is well over 100°C. Another key difference is viscosity. The sauce isn't just water. It's water plus a bunch of other stuff. That "other stuff" changes the nature of the liquid. The combination of huge temperature gradients and viscosity makes the bubbles of steam much bigger and longer-lasting in the simmering pan of food and sauce than in the simmering pot of water.

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Water transitions from liquid to vapor when it hits the boiling point. With a pot of water approaching boiling, you'll see (and hear) boiling begin as small bubbles across the bottom of the pan, where the water is locally at the boiling point. The bubbles collapse, depositing their heat energy, when they reach cooler (higher) areas of the water. As the temperature of the bulk of the water gets closer and closer to boiling, the bubbles will reach higher and higher into the water, until they'll finally break the surface and start a rolling boil. This happens fairly evenly across the pot since the warmer water convects, distributing the heat throughout the pan.

With sauce, the same thing happens, except that since it's thick there is far less convection going on, and thus the temperature can vary more. You'll see a spot on the surface of the sauce bump up and down as bubbles form and collapse on the bottom of the pan. These bubbles will heat the sauce immediately above them when they collapse and deposit their heat energy, so the next bubbles will reach further. Finally, you'll get a column of bubbles making it all the way to the surface and blurping bits of the hot sauce across your stove.

Since there's almost no convection, the bulk of the sauce can still be below boiling when it starts blurping: for proof, stir the sauce well and the boiling will go away for a while as the cooler parts bring the warmer parts below boiling. After a few of these cycles, stirring will no longer stop the boiling, as all the sauce will be at the boiling temperature.

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