A piece of cut apple left exposed on the dining table, a slice of toasted bread and a piece of meat gradually turning brown when cooked. What do they have in common?
Browning process occurs in food every day, my mum used to teach me to placed cut slices of apple or potatoes in a bowl of water to prevent them from turning brown. Little did I realised that there is a chemical reaction going on when fruits turn brown until I started attending lessons for food chemistry module in my Chemistry course.
Two types of browning occur in food. The browning of cut apples is known as enzymatic browning where polyphenols in fruits or vegetables undergo enzymatic oxidation causing the cut surface to turn brown. This reaction occurs after cutting or other mechanical treatment of food, due to the breaking of cells which is catalysed by the enzyme, polyphenol oxidase in the presence of oxygen in the air. The process converts the polyphenols in fruits and vegetables forming melanins and benzoquinone which give the brown appearance.
chlorogenic acid, a polyphenol found in the flesh of an apple
Non- Enzymatic browning
The non-enzymatic process usually occurs in the cooking of food. There are two types of non-enzymatic browning, caramelization and the Maillard reaction. Caramelization involves decomposition of sugars, often leading to the release of volatile chemicals responsible for the nutty flavour and brown colour of food.
The Maillard reaction refers to a complex set of reactions initiated by reaction between amino acids and reducing sugars, which, at elevated temperature forms insoluble brown products known as melanoidins, a class of compounds which contribute to the brown pigment in food.
The Maillard reaction
In 1912, when a French chemist, Louis-Camille Maillard investigate the reaction between amino acids and sugar, he discovered one of the most important flavour producing reactions in cooking, the Maillard reaction, which was sometimes referred to as “browning reaction”.  The reaction was studied with renewed interest later on by Hodge who published a paper in 1953 that established a mechanism for the Maillard reaction.
Sugar contains glucose, a reducing sugar and meat contains amino acids, which are the building blocks of proteins. During cooking of meat, the outer surface reaches a higher temperature, which triggers the Malliard reaction. Malliard reaction tends to occur at a temperature range of 150 to 260oC.
The Maillard reaction involves three stages. 
A condensation reaction between the carbonyl group of a glucose molecule and the amino group of an amino acid to produce water and an unstable glycosylamine.
The glycosylamine which undergoes rearrangements reactions produces a group of compounds known as ketoseamine.
Ketoseamines further undergo a variety of chemical reactions, such as rearrangements, conversions, additions, and polymerisations to form a variety of molecules, some which give rise to the flavour, aroma, and colour in food.
Some examples of such molecules are ketones such as 2,3-butanedione which contribute to the odour and flavour of cooked food, such as popcorn and grilled steak. Maillard reaction products are also important contributors to the flavour of milk chocolate, caramels, toffees, and fudges, during which reducing sugars also react with milk.
Today, the understanding of factors which affect the rate of Maillard reaction allowed us to improve the texture and flavour of food involving proteins-sugar combinations during cooking. For example, fats were found to play an important role in achieving the flavour of the reaction and controlling the temperature such as to achieve the reaction without eliminating the volatile compounds which contribute to the flavours. 
1. Fennema, O. (1996). Food chemistry. New York: Marcel Dekker.