The Maillard Reaction: A Taste of Food Chemistry

“How come we never learned anything practical in high school?”

He placed his steak knife parallel to a grill line and began to cut along the blackened indentation atop his expertly-seared steak. In between bites, he chugged from his beer obnoxiously.

“For instance, I NEVER use chemistry,” he continued. His train of thought was interrupted by the promise of dessert as the delicious sweetness of a freshly-baked apple pie and a large pot of coffee were brought to the middle of the table.

A food scientist would immediately identify the irony at this cookout: Grilled steak, baked delicacies, beer, and coffee are all made possible through a chemical reaction called the “Maillard reaction.”

Pronounced “mai-YAR,” the reaction was first described by the French chemist Louis Camille Maillard in the early 1900s. At temperatures between 110-150 degrees Celsius, a sugar molecular can bond with an amino acid, resulting in a “Maillard reaction product.” (Although one initially thinks of the word “sugar” as referring to table sugar, or sucrose, the chemical definition of a sugar encompasses a wide variety of molecules. However, only simple reducing sugars will work in this reaction!) Because of the inherent variety of sugar compounds and amino acids that exist on foods, several thousand different Maillard reaction products can be formed when these two classes of molecules react with one another. Some of these resultant compounds, like the melanoidins, contribute to the golden-brown color of cooked foods. Other compounds can add mouth-watering aromas with a range of flavor descriptors: pyrazines give us roasty and toasty notes, acylpyridines remind us of cereal, and furanones give us sweet or caramel tastes.

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Golden-Brown Color of Cooked Meat

Are you salivating yet? You can thank our evolutionary ancestors for giving us this craving for cooked flavors. One hypothesis put forward by Richard Wrangham in Catching Fire suggests that our early ancestors who enjoyed the taste of cooked foods were more successful than their neighbors who had no preference between raw and cooked foods. According to his theory, eating cooked foods allowed early hominids to evolve into the big-brained Homo sapiens we are today. Cooking changes the chemistry of the food we eat, making more calories, minerals, and vitamins available during digestion. Raw food-preferring hominids, on the other hand, would be forced to spend the whole day foraging for fruits and berries just to get the same nutritional content that comes from cooked food. (As an added pro-survival bonus, cooking food has the benefit of killing many of the harmful, disease-causing bacteria and parasites that thrive on raw meats.)

By random evolutionary good luck, some of these early hominids might derive pleasure from smelling and tasting Maillard reaction products. Their discerning palate would net these individuals more nutritional content and less risk of contracting foodborne illnesses from their meals, thus increasing their likelihood of viability. As selection theory would argue, they would be more likely to pass on their Maillard-loving genes to the next generation.

Although some of the flavor compounds produced in the wake of the Maillard reaction may act as an evolutionarily-preserved signal of desirability and nutrition, not all of them are appealing to the palate. Some reaction products that fall in the class of alkylpyridines can enhance the “meat-ness” of a food by acting at the umami taste receptor. The taste of some alkylpyridines, however,   can be described as bitter, astringent, or burnt – words that are often used as critiques of someone’s skill on the grill.

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Worse yet, some Maillard reaction products may carry health risks. For example: acrylamide, found in cooked carbohydrate rich foods like potatoes, is suspected to be carcinogenic. In animal studies, exposure to acrylamide results in tumor growth in the lungs, reproductive organs, and thyroid and adrenal glands. In humans, on the other hand, evidence is still inconclusive. Acrylamide seems to be formed only after direct heat cooking, so your Grandma’s recipe for boiled potatoes might be the way to go.

Researchers and clinicians are also starting to become more concerned with Advanced Glycation End products (AGEs), another class of compounds that can be formed by a Maillard reaction. AGEs such as glucosepane can alter the structure of proteins irreversibly, resulting in abnormal and sometimes unpredictable biological function. are implicated in a variety of disorders, including diabetes, macular degeneration, cellular inflammation, and neurodegenerative diseases.

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Glucosepane, an AGE molecule.

The phrase “Maillard reaction” gets brought up in the kitchen and the laboratory, but you might not suspect where else this chemical process is mentioned: the beauty salon. As it turns out, tanning lotions and spray tans contain a sugar called dihydroxyacetone (DHA) which can react with amino acids in the surface layers of your skin. The resulting compounds are melanoidins, which are, chemically related to the melanin that darkens your skin naturally, and gives you that sought-after, golden-brown tan. The skin darkening properties of DHA were discovered by accident – Dr. Eva Wittgenstein was testing the chemical as a treatment for a glycogen storage disease when she observed that DHA darkened the bare skin that it came into contact with.

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Melanoidin

Now that you know a little something about the science of the Maillard reaction, you might be eager to impress the guests at your next grill session with a “practical chemistry demonstration.” Here are a couple quick tricks to nail the perfect Maillard reaction.

    1. Remove as much as water from the surface while heating. High water content at the surface of the meat can prevent the temperature from getting up to the sweet spot of 110-150 ºC. Instead of roasting nicely, the steam produced by the boiling water keeps the meat around 100 ºC, resulting in a tough gray steak. To prevent the meat from steam-cooking, dry off the surfaces with a paper towel before dropping it onto your preheated grill. Additionally, sprinkling some salt onto the steak will help pull water away from the surface.
    2. Flip the meat repeatedly. This prevents one side of the meat from being exposed to the heat source for too long. Extended exposure to high heat in excess of 180 ºC results in a different chemical reaction called pyrolysis – the breakdown of molecules at high temperatures. The resulting products are various simple carbon compounds, and these pyrolyzed products are often bitter to the taste and highly undesirable. Also, flipping the meat frequently allows the surface water to boil off more rapidly, resulting in less steam-cooking.
    3. Add a dash of baking soda. The Maillard reaction proceeds faster in high pH conditions, and giving your steak a quick dusting of sodium bicarbonate will bump up the pH. But use sparingly – too much baking soda will result in off-flavors in the final product. And act quickly! The baking soda will start to act as a chemical tenderizer and denature proteins in the meat if you wait too long.

And in the meantime, if anyone questions what you learned in chemistry while you work the grill, you’ll be armed with the most essential facts about this mouth-watering chemical reaction.

Austin Lim is a dancer, artist, lover of all things brain-related, and a Postdoctoral Fellow with the Surmeier lab at Northwestern University. He holds a Ph.D. in Neurobiology from The University of Chicago. You can find him on Twitter and on his own website.