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I made chicken katsu tonight following this recipe. I added peanut oil to the pan, let it get hot, then threw in my semi-frozen breaded chicken thighs. As you can see, I cooked them longer than called for because they hadn't totally dethawed yet. Suprisingly they came out very moist and delicious!

Anyway, I noticed a foam forming in the pan about midway through cooking. I took the picture below at the very end when I was taking out the chicken, and the foam was still there. What's the cause of this?

enter image description here

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2 Answers 2

In 2 words: Water vapour.

Remember that the water boils at 100C while oil boils at much higher temperature. Therefore a (close-to) boiling oil cannot contain a liquid water and it's evaporated instantly as it leaks out of the stuff you fry.

This should start shortly after the oil gets to 100C, and it ends as soon as there's no more water to leak out of the stuff.

(Side note: Low quality meat can quite often be injected by water, the sellers increase the weight hence the price in this very nasty way. My experience is that if you buy a fresh meat from butchers', this effect is much lower and you can make more portions of the meal of the same weight of meat.)

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The water causes the bubbles, but if it's actually foaming, I suspect it's proteins that have leached into the water from the chicken. (similar to the scum that forms when making stock) –  Joe May 16 '13 at 12:20
    
@Joe That's quite possible, But as well, in a "dirty" oil the water bubbles needn't be so large as in a boiling water... –  tohecz May 16 '13 at 12:27
    
ahh interesting. I read that if peanut oil is heated past the smoke point (about 320F), it'll foam. Not sure if that is true or not. My coworker also mentioned that the oil could be bad- but I think that's unlikely since I just bought it a week ago. –  mdegges May 17 '13 at 13:17

Most probably, the peanut oil experienced over-heat* too high temperature, making the volatile compounds of oil breakdown. In that fact of the foaming is due to the formation of polymerised oil. According to Dr. Udo Erasmus, all good oils are sensitive to heat, light and oxygen.

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