Brownie skin, that thin, shiny, and slightly chewy layer on top of a baked brownie, is believed to form from a foamy layer of air bubbles suspended in starch-starved liquid batter.
Here's a more detailed breakdown:
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The Hypothesis: According to chemical engineer Sörensen, the key idea is that as the brownie bakes, the starch in the batter settles downwards. This leaves the top layer of batter with relatively less starch and more liquid.
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Air Bubbles and Meringue Formation: This starch-poor liquid batter then forms a thin, foamy layer of air bubbles. As it bakes, this layer effectively transforms into a wafer-thin meringue-like structure, creating the characteristic "skin."
In summary, brownie skin is a thin, meringue-like layer created on top of brownies due to the separation of starch in the batter and the formation of air bubbles during baking.
