Food browning

Example of a general reaction of polyphenols by polyphenol oxidase (PPO) that catalyzes enzymatic browning. The production of quinones undergoes more reactions which eventually form brown pigments on the surface of the food.
A desirable enzymatic browning reaction is involved in the process of grapes becoming raisins.
A non-desirable enzymatic browning reaction is involved in the formation of brown spots on the peel of bananas.
Example caramelization of table sugar (sucrose) caramelizing to a brown nutty flavor substance (furan and maltol)

Melanoidins are brown, high molecular weight heterogeneous polymers that are formed when sugars and amino acids combine through the Maillard reaction at high temperatures and low water activity. Melanoidins are commonly present in foods that have undergone some form of non-enzymatic browning, such as barley malts (Vienna and Munich), bread crust, bakery products and coffee. They are also present in the wastewater of sugar refineries, necessitating treatment in order to avoid contamination around the outflow of these refineries.