The Maillard reaction takes its name from the French physician and chemist, Camille Louis Maillard, who first described, in 1921, the reactions that involve amino acids and sugars. It occurs in a wide range of situation and, for example, is observed when roasting a piece of meat on the barbecue at high temperature or at lower temperature in ageing of wine.
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Browning and the Maillard reaction
Browning and the Maillard reaction co-occur in samples, the browning being rather attributed to the effect of pH on the sugars . The Maillard reaction rather takes place in alkaline solutions.
In e-liquids left steeping
The Maillard reaction is observed when aliments become brownish when cooking and render the typical odors of cooking like roasted meat, brown onions, bread, caramel…
The Maillard reaction and browning likely happen also in e-liquids left steeping for a long time. From transparent, a liquid turns golden then brown even in the absence of light and at room temperature. Depending on the flavors added, the browning may or may not occur. The presence of nicotine (that becomes brown for other reasons) tends to accelerate the overall browning of the blend.
From a chemical point of view, a carbonyl group of the sugar reacts with a nucleophilic amino group of the amino acid and forms a mixture of uncharacterized molecules responsible for a range of odors and flavors spanning from smoothly roasted to completely carbonized food.
By-products of the Maillard reaction
By-products of the Maillard reaction include glycosylamines, ketosamines, acetyl-, diacetyl and a wide diversity of aldehydes. Generally occurring around 140°C to 165°C (285°F to 330°F), other products are formed at higher temperature, among which acrylamide, a carcinogen that is also present in cigarette smoke.
 Schroeder, L. J., Iacobellis, M., & Smith, A. H. (1955). The influence of water and pH on the reaction between amino compounds and carbohydrates. Journal of Biological Chemistry, 212(2), 973-984.