The Maillard reaction is a chemical reaction between an amino acid and a reducing sugar, usually requiring heat. Like caramelization, it is a form of non-enzymatic browning. The reactive carbonyl group of the sugar reacts with the nucleophilic amino group of the amino acid, and forms a variety of interesting but poorly characterized molecules responsible for a range of odors and flavors. This process is accelerated in an alkaline environment as the amino groups are deprotonated and hence have an increased nucleophilicity. This reaction is the basis of the flavoring industry, since the type of amino acid determines the resulting flavor.
In the process, hundreds of different flavor compounds are created. These compounds in turn break down to form yet more new flavor compounds, and so on. Each type of food has a very distinctive set of flavor compounds that are formed during the Maillard reaction. It is these same compounds that flavor scientists have used over the years to create artificial flavors.
Although used since ancient times, the reaction is named after the chemist Louis-Camille Maillard who investigated it in the 1910s.
The Maillard reaction is responsible for many colors and flavors in foods:
6-Acetyl-2,3,4,5-tetrahydropyridine is responsible for the biscuit or cracker-like flavor present in baked goods like bread, popcorn, tortilla products. The structurally related compound 2-acetyl-1-pyrroline has a similar smell and occurs also naturally without heating and gives varieties of cooked rice and the spice pandan (Pandanus amaryllifolius) their typical smell. Both compounds have odor thresholds below 0.06 ng/l.
The browning reactions which occur when meat is roasted or seared have often been referred to as Maillard reaction browning. However, lean meat contains very few, if any, reducing sugars. Furthermore, red meat undergoes more extensive browning than does white meat. The browning reactions in lean meat are most likely due to the breakdown of the tetrapyrrole rings of the muscle protein, myoglobin. Thus, the browning of meat is technically not a Maillard browning since it does not involve the reaction with a reducing sugar.
Caramelization is an entirely different process from Maillard browning, though the results of the two processes are sometimes similar to the naked eye (and tastebuds). Caramelization may sometimes cause browning in the same foods in which the Maillard reaction occurs, but the two processes are distinct. They both are promoted by heating, but the Maillard reaction involves amino acids, as discussed above, while caramelization is simply the pyrolysis of certain sugars. The following things are a result of the Maillard browning reaction:
In making silage, excess heat causes the Maillard reaction to occur, which reduces the amount of energy and protein available to the animals that feed on it.
High temperature, low moisture levels (low moisture levels are mainly necessary because water boils into steam at 212 Fahrenheit (100 Celsius), whereas the Maillard reaction happens noticeably around 310 Fahrenheit (155 Celsius): by the time something is in fact browning, all the water is vaporized), and alkaline conditions all promote the Maillard reaction.
The rate of Maillard reactions increases as the water activity increases, reaching a maximum at water activities in the range of 0.6 to 0.7. However, as the Maillard reaction produces water, further increases in water activity may inhibit Maillard reactions.