What new substance is formed when bread turns black burned?

When bread turns black and burned, it doesn't form a single new substance, but rather undergoes a complex series of chemical reactions that create a variety of compounds.

Here are some of the key changes:

* Caramelization: Sugars in the bread break down and react with amino acids, producing a complex mixture of brown compounds responsible for the characteristic flavor and aroma of toast.

* Maillard Reaction: This reaction, similar to caramelization, also involves sugars and amino acids, leading to the formation of hundreds of different flavor molecules, including melanoidins, which contribute to the dark color.

* Pyrolysis: At very high temperatures, the bread undergoes pyrolysis, where the complex organic molecules break down into simpler substances like carbon, water, and volatile gases. This process gives burned bread its black color and bitter taste.

Specific compounds formed include:

* Carbon: The primary black substance you see is mostly carbon.

* Water: The water content in the bread is released as steam.

* Volatile organic compounds: These compounds, including aldehydes, ketones, and furanones, contribute to the strong, often unpleasant smell and taste of burned bread.

* Acids: Acetic acid, formic acid, and other organic acids are formed, adding a sour taste.

In short, burned bread is a complex mixture of compounds, not a single new substance. It's a result of chemical changes that transform the original bread ingredients into a range of compounds, many of which are responsible for the undesirable taste and smell.