Anyone who has ever eaten an apple has probably noticed that by the time you’re nearly finished and come back around to where you took your first bite, that part of the apple has started to turn brown. Why? Toasting bread turns it brown; frying potatoes turns them brown; a few days after Hallowe’en your pumpkin will start to turn brown. Why?

It turns out, there’s a number of reasons why foods turn brown:

Caramelization
Caramelization occurs when a sugar is heated to a high temperature, over 248°F (120°C). At these temperatures the sugar undergoes a process called thermal degradation and breaks down into two compounds: one, which provides the caramel aroma, and a residue that creates the familiar brown colouring.

The process of camelizing sugar is complex and there are a number of different flavours and colours that can be produced depending on the acidity of the sugar compound, the heat used and the duration the sugar is heated.

Maillard Reactions
This reaction is named after the French scientist, Louis Camille Maillard, who discovered it in 1912. It occurs when a carbohydrate (such as sugar) reacts with a protein at high temperatures. More specifically, the aldehydes in the sugar molecule react with the nitrogen (amino acids) in the protein, to produce melanoids, which in turn produce the flavours associated with the process.

Enzymatic browning
And now we come back to the apple. Enzymatic browning, or oxidation, doesn’t require heat and generally isn’t used to enhance the flavour of food, although the flavour of some foods like tea and coffee are enhanced by controlled enzymatic browning.

This type of browning occurs in fruits like apples and bananas, vegetables like potatoes, and seafood like shrimp. A chemical reaction takes place between the enzymes (polyphenol oxidase or tyrosinase) in the food and the oxygen in the air, producing melanins. The process damages the cells of the food and changes the colour and flavour.

In fruit and vegetables, the skin of the food prevents oxidation but it will brown quickly once cut. When cut, the damaged cells release the enzymes and react with the air. Bruised fruit will also turn brown since the damaged cells in the bruise will react with the air in the food.

Removing the enzymes before they have a chance to react with the air can control enzymatic browning. For example, blanching (boiling in water for a short time) will prevent fresh shrimp from browning.

The reaction can also be prevented by not allowing the food to come in contact with oxygen. Ever notice that when your Mom is cutting potatoes, she puts them directly into a pot of water rather than letting them sit in an open bowl? There’s less oxygen in the water. Apples can be stored for months at a time by flooding the storage container with carbon dioxide and displacing the oxygen.

Carmelization and Maillard Reactions are also referred to as non-enzymatic browning.


Sources
http://www.scienceyear.com/outthere/index.html?page=/outthere/diner/why/browning.html
http://www.geocities.com/perfectapple/brown.html
http://www.exploratorium.edu/cooking/candy/caramels-story.html
http://www.agsci.ubc.ca/courses/fnh/410/colour/3_81.htm

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