cakes

[phl666]

Why do cakes rise more in the middle than around the edges?

[Skids]

Because the middle isn't being held back by the cake tin.

[gen2]

Skids' answer is only part of the reason.  The heat from the oven causes the cake mixture to first 'rise' and then 'set'.  The outside cooks first and the heat reaches the centre last so that part has much longer to rise and so rises higher. The top surface has probably also set and the pressure from beneath often cracks it as it continues to rise.

[NetSquirrel]

Skids is onto the right track.  The cake mixture, when heated, is forced to expand roughly equally in all directions (as if it were made out of many tiny, delicious balloons, and each balloon is being inflated).  The cake tin prevents the cake mixture from expanding past its walls, so the mixture is forced upwards - the only place it can go.

 

As for why the middle always rises more than the edge, consider the directions that certain areas of the cake mixture can expand:

 

� The mixture next to the bottom of the tin can only expand upwards.

� The mixture round the edges can expand upwards and towards the middle.

� The mixture "floating" in the middle is pushed, as it expands, by the mixture from the bottom and edges.  The overall direction of push is upwards and towards the middle.  This is what causes the middle of a cake to bulge upwards.

 

As gen2 has pointed out, part of the reason that a cake retains this mountain-like shape - rather than "flowing" back to flat - is because the edges receive more heat, and therefore solidify faster, than the middle.  And, of course, cake mixture is sticky and doesn't flow as easily as water anyway, but that's not very scientific =)

 

Good point about the crust cracking - it hadn't occurred to me what the reason behind that was.  Incidentally, you can prevent this happening to a cake by placing a sheet of aluminium foil over the top of the tin while it cooks.  This, I assume, delays hardening of the crust until the mixture has fully expanded.