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I had a nice bowl of clear yellow chicken broth which was not salted. It was in a serving bowl when this happened. I poured the cold unsalted soup into the bowl and then microwaved it and it came out clear still. To make it more palatable, I added a small spoonful of salt and the below happened. It only became frothy after I added the salt. It was much 'foamier' when I first added the salt, then it slowly subsided.

What is causing this?

Foamy yellow broth in a bowl

Catija
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Trogdor
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  • Hmm, interesting. Was it on the heat or in a serving bowl when it happened? – rumtscho Mar 08 '15 at 21:53
  • It was in a serving bowl when this happened. I poured the cold unsalted soup into the bowl and then microwaved it, but the it came out clear still. It only became frothy after I added the salt. – Trogdor Mar 09 '15 at 00:24
  • Hm. How hot was the broth when you took it out? Just quite warm or (near) boiling? – Stephie May 07 '15 at 09:21
  • It was fresh from the microwave so probably 50-60 degrees C – Trogdor May 07 '15 at 09:27
  • Theory: Moisture is rapidly drawn out of some particle matter by the salt... – rackandboneman Mar 31 '17 at 08:00
  • @user110084 It's really confusing to have to look for a comment to see where you got the "microwaving" tag from. Please, if there's content in the comments that is important to the question, copy it in rather than just adding tags that end up looking random. – Catija May 16 '17 at 22:18
  • @catija, I added the word "microwaving" to the title of the question. "if there's content in the comments that is important to the question, copy it in rather than just adding tags that end up looking random" - what should I have done instead? The question body mentioned heating by microwave. – user110084 May 16 '17 at 22:25
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    See my edit @user110084 :) – Catija May 16 '17 at 22:26
  • @Catija, understood, hoping not to repeat that! Still finding my way around. Always thankful for any guidance. – user110084 May 16 '17 at 22:28
  • Edits are always appreciated, @user110084 I hope you don't think they aren't! But we do like to make sure they make sense as well as we can. We appreciate your work. – Catija May 16 '17 at 22:29

1 Answers1

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There are likely two or three things happening.

When clear liquids come out of a microwave, it is quite common for it to froth as soon as you put something into it. A spoon, or crystals of salt or sugar forms nucleation sites for over-energised water molecules to make vapour bubbles. Water forms vapour at any temperature, not just at its boiling point.

In conventional heating, convection starts from the layer closest to the heated bottom of the pan and the hottest liquid always rises to the surface and there is plenty of movements in the liquid body. In a microwave energy goes into the water molecules directly and the container is heated by the energiesed liquid, the opposite of conventional heating. There is very little time for convection currents to develop. Molecules within the body of the liquid that are excited may not find sites to form vapour and become locally superheated. Vapour pressure above the liquid may be actually less than what it should be at that temperature until the body is disturbed. Lots of tiny vapour bubbles are released when disturbed. They look foamy until they escape from the liquid. This is the most likely cause.

Salting out of proteins could be another cause, though probably less likely. You would probably see a layer of scum on the surface that would not disappear.

Emulsion breaking. Heating and addition of salt will encourage any emulsion to split. From your picture, there is plenty of oil around. There could have been some oil droplets suspended in the water phase of the soup. These could clump together at the start of emulsion breaking giving a cloudy appearance before floating to the top as a continuous layer. Not likely either.

user110084
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