Maillard reaction Totally Explained

The Maillard reaction is a chemical reaction between an amino acid and a reducing sugar, usually requiring heat. Like caramelization, it's a form of non-enzymatic browning. The reactive carbonyl group of the sugar interacts with the nucleophilic amino group of the amino acid, and interesting but poorly characterized odor and flavor molecules result. This process accelerates in an alkaline environment because the amino groups don't neutralize. 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.

Foods and products with Maillard reactions

The Maillard reaction is responsible for many colors and flavors in foods:

toasted bread
malted barley as in malt whiskey or beer
roasted or seared meat
dried or condensed milk
roasted coffee

6-acetyl-1,2,3,4-tetrahydropyridine (1) is responsible for the biscuit or cracker-like flavor present in baked goods like bread, popcorn, tortilla products. 2-acetyl-1-pyrroline (2) flavors aromatic varieties of cooked rice. Both compounds have odor thresholds below 0.06 ng/l . 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 oxidation of certain sugars. The following things are a result of the Maillard browning reaction:

caramel made from milk and sugar, especially in candies; NB: milk is high in protein (amino acids) and browning of food involving this complex ingredient would most likely include maillard reactions. See references below.

chocolate and maple syrup

lightly roasted peanuts

The process

The carbonyl group of the sugar reacts with the amino group of the amino acid, producing N-substituted glycosylamine and water

The unstable glycosylamine undergoes Amadori rearrangement, forming ketosamines

There are several ways for the ketosamines to react further:

Produce 2 water and reductones
Diacetyl, aspirin, pyruvaldehyde and other short-chain hydrolytic fission products can be formed
Produce brown nitrogenous polymers and melanoidins

Factors

High temperature, high relative humidity, 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. Pentose sugars react more than hexoses, which react more than disaccharides. Different amino acids produce different amounts of browning.

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