Chemistry in Food

Cooking is a chemical laboratory in everyday life, only as a rule is based on its experience and "approximately". This is especially true when carrying out a complex of reactions, called the reaction of formation of brown products. It just seems like we are frying meat or making caramel. In fact, we launch and partly control a whole chain of chemical transformations.
There are three large groups of brown formation reactions. One of them is enzymatic, where nature does without our participation, except for the initial launch. This reaction of the enzymatic interaction of phenolic components with oxygen, catalyzed by internal plant enzymes - polyphenol oxidase. It appears in the form of darkening on a cut of apples, bananas, avocados.
The other two reactions are not enzymatic, in order to pass them, in addition to substrates, it is necessary to create reaction conditions, most often an increased temperature, but sometimes the pH of the medium, time or pressure. This is the reaction of caramelization and the reaction of Maillard.
Bringing a detailed description would be too boring. The degree of control over each reaction is small. At each stage, reaction products interact with each other and with the products of the previous stages, as a result, the list will be large.

At the most general level of description, caramelization is a chain of transformations of monosaccharides and more complex sugars, and the Mayar reaction is a chain of reactions between sugars and amine compounds (amino acids, peptides, proteins). An example of the reaction of caramelization is burnt sugar. An example of a Maillard reaction is a steak crust or a crust of baked bread, as well as roasted coffee. The description is complicated by the fact that often all these reactions occur simultaneously. For example, when caramelized onions, not only caramelization occurs, but also the reaction of Maillard. And in the reactions involved not only sugars and proteins, but also other compounds, for example, in onions are sulfur-containing components that impart onion its smell in raw form. And often these reactions are combined with the reactions of lipid decomposition, which is observed, for example, when grilling.
Formation of yellow-brown colored products. The most famous example is substances in caramel, which are used as pigments for alcoholic and nonalcoholic beverages (whiskey, coca cola). When caramelized from the products of the decomposition of sugars, polymers with a brown color (sugar color) are formed. With the reaction of Maillard, a whole group of colored substances is formed-melanoidin pigments.
The special value of these reactions for cooking is the formation of unique aromatic substances. With caramelization - it is mainly the products of decomposition and dehydration of sugars - furans, furfurales, cyclopentenes. For example, maltol has the smell of freshly baked bread.
The variety of aromas of the Mayar reaction depends on the flow conditions and on the starting products. Some amino acids have a sulfur atom in their composition, which further increases the number of possible reaction products. These groups of compounds are formed in the course of Maillard reactions - pyrroles, pyridines, pyrazines, imidazoles, oxazoles, furanthols, Shtrekker aldehydes, alkylpyrazines, etc. From concrete examples:
2-furanylmethanethiol with the aroma of freshly roasted coffee, 2-acetyl-2-thiazoline in the aroma of baked meat.