The same browning chemistry flavours your toast and reddens ancient bog bodies
Browned crusts, seared steak, roasted coffee and golden fries all owe their colour and much of their flavour to a single reaction between amino acids and certain sugars. French chemist Louis Camille Maillard first described it in 1912, and it also helps explain reddish hair on Iron Age bog bodies.
The chemistry starts when the carbonyl group of a reducing sugar meets the amino group of an amino acid, giving off water and forming a fragile glycosylamine. That intermediate rearranges into ketosamines, which can then split down several paths, yielding reductones, dicarbonyls, Strecker aldehydes and, eventually, brown pigments called melanoidins. Depending on the ingredients, the heat, the cooking time and the air present, hundreds of distinct flavour molecules can emerge. Maillard stumbled on the reaction while trying to mimic how living things build proteins, and John E. Hodge of the U.S. Department of Agriculture worked out its mechanism in 1953.
It runs fast at roughly 140 to 165 degrees Celsius, which is why so many recipes demand a hot oven. Alkaline conditions speed it up, which is the trick behind brushing pretzels with lye. Push the heat higher and caramelisation, a separate process in which sugars break down without any amino acids involved, becomes more prominent, followed by outright burning and acrid tastes. High temperatures can also generate acrylamide, a probable carcinogen that forms especially from the amino acid asparagine; cooking cooler, adding the enzyme asparaginase or injecting carbon dioxide all reduce it.
Particular molecules carry particular smells. One called 6-acetyl-2,3,4,5-tetrahydropyridine gives bread, popcorn and tortillas their cracker-like note, and a close chemical cousin, 2-acetyl-1-pyrroline, perfumes cooked rice and the herb pandan even without heating. Both can be smelled at concentrations below 0.06 nanograms per litre. Flavour chemists have long exploited the reaction, with most related patents aimed at meaty tastes, and the Nobel laureate Jean-Marie Lehn judged it the most widely practised chemical reaction on Earth.
Its reach goes well beyond the stove. It darkens dried fruit and contributes as champagne ages in the bottle. In animal feed and overheated silage it is a nuisance, locking up protein and energy the livestock would otherwise digest. In cold, acidic peat bogs at about 4 degrees Celsius the same process works slowly on buried bodies, tanning skin and turning hair ginger, and it even helps preserve ancient faeces.
Source: Maillard reaction
