Why Food Smells So Good When It’s Browning

A complex chemical reaction called the Maillard Reaction is responsible

The Maillard Reaction's best-known application is in tasty, tasty food. iStock

This weekend, millions of Americans will fire up the grill and cook up steaks, hot dogs, burgers and some tofu dogs (why not). Why will folks walking by the patio stop and sniff (besides the hickory wood chips tossed into the barbecue)? The Maillard Reaction.

First described by French scientist Louis Maillard in a 1912 paper, the reaction has been intuitively known to cooks since time immemorial. It’s what happens when you apply heat to amino acids and sugars (i.e. food), and it's why browned food smells and tastes so good. But scientific knowledge of the process that Maillard described and later scientists studied can help cooks make better food.

“Understanding the reaction, even on a surface level… is a gateway to understanding the chemical and physical processes of cooking,” writes Eric Schulze for Serious Eats.

Humans are the only animals that cook food rather than eating it raw, Schulze writes, so we’ve evolved to seek out cooked food. The reason the Maillard Reaction is so important to making food tasty is because it signals two things that make human mouths water: the food is likely harmless (because it’s been cooked) and nutritious (because it contains proteins and sugars that we need).

So far, so simple.  

Maillard’s research, published in the journal of the French Academy of Science, was the first to describe “the reaction between reducing sugars and amino acids,” writes Sarah Everts for Chemical and Engineering News. The chemist hit on something new, but he didn't have all the answers.

What happens during any kind of Maillard Reaction is so complicated that scientists largely ignored his research, Everts writes. When World War II rolled around, though, the military was looking for ways to produce huge amounts of pre-packaged food that had a long shelf life and tasted okay. They turned to the Maillard Reaction for answers.

It took until 1953 for an American chemist named John E. Hodge to publish a paper actually establishing how the reaction worked, she writes. Hodge’s work was so fundamental to understanding the complex, three-stage reaction that some people called for renaming it the Maillard-Hodge Reaction, although that didn't happen.

What Hodge found was that the Maillard Reaction isn't actually one reaction at all, but a complex series of small reactions that fit into seven approximate steps. An eighth step was added by a later scientist. At the end of the reaction, sugars and amino acids have transformed to create melanoidins, which are polymers that show up as "browning."

The Maillard Reaction continued to be studied mostly in the context of food and food flavor until the 1970s, according to a paper from biologist Thomas O. Metz and colleagues. But the reaction happens in more than just cooking, they write, so it continues to be studied. The reaction also is also part of processes in paper and textile making, as well as certain branches of pharmaceutical medicine and in soil. Inside our bodies, the Maillard Reaction takes place and is linked with chronic conditions, inflammation and diabetes. A scientist in the 1980s also figured out the eighth step of the reaction, which produces potentially cancer-causing particles—not so tasty.

That’s actually why Maillard began studying the reaction in the first place, writes Joe Palca for NPR. “He thought it would be important for medicine and diabetes,” chemist and doctor Vincent Monnier told Palca. He didn’t immediately recognize it would be important for food.”

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