Starch drinking the water
Why Does Porridge Go Thick?
Oats and water go into a pan as two things and come out as one, because the starch swells up and traps the water.
-
Step 1 of 5
Starch is stored in tight little grains.
In a raw oat the starch is packed into hard granules that water cannot get into. That is why raw oats sit in milk for a long time before anything happens.
-
Step 2 of 5
Heat lets water in.
Above about sixty degrees the packing loosens and water floods in. Each granule swells to many times its size, taking free water out of the pan with it.
-
Step 3 of 5
Then they burst and tangle.
Swollen granules break open and spill long starch molecules that tangle with each other. The pan becomes a mesh holding the water rather than a liquid containing grains.
-
Step 4 of 5
It thickens again as it cools.
Nothing is added as it cools. The same molecules simply stop sliding. As it cools the loose molecules line up and grab each other more tightly. That is why porridge left in the bowl goes stiff, and why day-old rice goes hard.
-
Step 5 of 5
Stirring and grinding both change the result.
Fine oats release starch quickly and go creamy; coarse ones stay chewy. Stirring knocks more starch loose, which is why some cooks stir hard and others barely at all.
This is the same chemistry as thickening a sauce with flour or cornflour. Porridge is a sauce that happens to be breakfast.
The short version
Porridge thickens because heated starch granules swell, burst and tangle into a mesh that holds the water in place.
Try it yourself
Stir a teaspoon of cornflour into cold water, then warm it. It goes from milky to thick in seconds.