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just some cocoa getting stirred

You're getting ready to make hot chocolate, or maybe a roux. You put the cocoa or flour in the milk. You stir. The milk is still milky. There are lumps, clumped together, of the powders. If you "pop" a lump with your spoon, you will see that inside the lump there still exists dry cocoa/flour. What is the reason for these poppable lumps? Why does liquid not seep into the inside, even over a long, milk-spoiling time? My guess is that the part of the lump that does get the liquid forms a protective shield around the inside of the lump, preventing liquid from seeping in.

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If you put the starchy cocoa or flour into warm milk, then you can get the outer lump surface to gelatinize, forming a protective layer around the dry contents.

There is also a low-temperature version of this, where I think surface tension is doing the job: the starchy powder is mildly hydrophobic, and once there is a bubble of water around it it is stabilized by surface tension pushing inwards, resisted by the powder.

This is why sifting powders is so useful in cooking, why you often want to add the starch before things become too hot, or add them to a fatty liquid like molten butteror cream when making sauce thickening. Or use industrial mixing.

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  • $\begingroup$ Warm? Wow, I had cold in mind when asking this question, should have said that out loud. $\endgroup$ Commented Jun 17, 2021 at 17:29
  • $\begingroup$ @JohnnyApplesauce - It happens in either case, and I think the gelation issue is cool physics. But surface tension is likely the dominant effect. $\endgroup$ Commented Jun 17, 2021 at 18:59
  • $\begingroup$ Also cocoa is processed with alkali to improve its mixability with water. I didn't find much about it. Here is a paper - Impact of alkalization on the antioxidant and flavanol content of commercial cocoa powders. You could try googling. $\endgroup$
    – mmesser314
    Commented Jun 17, 2021 at 22:42

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