Eli5 Why do powdered donuts feel cold?

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Why do powdered foods (powdered donuts, cornstarch, etc) feel cold when you touch them or eat them despite not being cold?

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It has to do with heat displacement. When you touch a regular solid surface you’re touching one object which holds temperature well (in heat it holds heat, in cold it hold cold). When it comes to powdered donuts, they are covered in millions of tiny surfaces each of which can hold heat for a very little time due to its small size. The powdered sugar is basically a heatsink. It feels cold because it is generally going to be colder than your body temp as a normal donut would stay about ambient room temp but these millions of tiny surfaces are colder.

Tldr: it feels colder because it IS colder.

What? They feel cold to you? I can’t say I’ve ever experienced that.

It feels cool to your tongue but not your fingers. The powdered sugar takes some heat from your tongue as it dissolves into your saliva thereby making your tongue cooler. Your fingers are dry so no dissolving happens and heat transfer is minimized.

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Feeling temperature for humans is a bit weird. We don’t actually feel temperature like thermometers but more like heat flow from and into our body. For example, if you place something made out of metal and a book in a freezer and leave them overnight when you take them out the metal will feel colder than the book even though they are both at the same temperature. But the metal feels colder because it is a better conductor of heat and can absorb heat faster from your body in comparison to the book.

Now the second thing you need to know about heat transfer is that it is a function of both material properties and surface area available for transferring heat. For example, if you have seen a radiator or a heat sink for a processor you will notice that they are ribbed not just a solid block. Even though a solid block of metal can transfer more heat, but to give that heat to air you need a lot of surface area, i.e. more parts where the metal and the air are in contact with each other. This is why they are designed to look like this.

If you combine the 2 previous concepts, powdered sugar or powdered stuff in general have more surface area, if you want to understand that consider a grain of sugar and now crush it into tiny pieces, you basically created more surface even though the volume is the same. More surface area equals more heat transfer and that’s why it feels colder.

This is such an amazing question. I’ve had this sensation before myself but I never thought to ask anyone why.