What Causes Browning?

"Bake till golden brown" is something commonly found in cookbook instructions. But what does that actually mean?

Crust lover or hater?

I remember while growing up, we used to fight over the cake crusts - especially when mom was making black forest cakes for order. She would apportion the unwanted crusts to appease us and sometimes cut a little extra cake with the crusts.
So what causes this noticeable difference between the 'skin' of a cake and it's inner portion? After all, the same batter was used to make the entire cake! And all of the cake was baked together at the same time in the same oven. Is it by some super power magical trick the cake is brown on the outside and yellow on the inside? On top of that the brown crusts tastes so good! (Disclaimer: some may favour the cake over its crust) The same goes for other foods such as breads, grilled steak, pan-seared fish etc.

Is the crust actually burnt cake where the surface of the batter has been directly exposed to the heat of the oven? But the crust doesn't taste burnt (unless it actually is burnt).

This browning effect is actually the Maillard reaction in action. This occurs at around 140 to 165°C where amino acids (RNH₃⁺) and reducing sugars (RC=O) react to form distinct aromas and flavours. Different combinations of amino acids will give rise to different flavours. Interestingly it is used to make artificial maple syrup and gives beer its colour among others.

Now isn't this just superb? Char siew Maillard perfection