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Food Science

The Maillard Reaction Turns 100

Scientists celebrate the centennial of a reaction that makes cooked food tasty, but also produces worrisome molecules in our meals and bodies

by Sarah Everts
October 1, 2012 | APPEARED IN VOLUME 90, ISSUE 40

Credit: Wikimedia Commons
Credit: Wikimedia Commons

Baked bread, roasted coffee, and grilled steak owe their enticing smell to a sequence of chemical reactions that was first reported 100 years ago by the French chemist Louis-Camille Maillard.

Credit: Shutterstock


Credit: Shutterstock


Credit: Shutterstock


Credit: Shutterstock


Credit: Shutterstock


Credit: Shutterstock


Credit: Shutterstock


The Maillard reaction creates many of the delicious colors and odors of cooked food.

His 1912 paper took a first stab at explaining what happens when amino acids react with sugars at elevated temperatures, and in doing so, Maillard set the foundations of serious food science (Compt. Rend. 1912, 154, 66).

Although it is responsible for some of cooked foods’ more delightful flavors as well as the brown color of bread crusts, soy sauce, and barbecued meat, the Maillard reaction also has a dark side: It can produce cancer-causing acrylamide and furans in food, particularly highly processed or burnt meals. Medical researchers have also discovered that the Maillard reaction takes place spontaneously in human tissue, and its products have been linked to a variety of diseases, including diabetes and cataracts.

Credit: Sarah Everts/C&EN
The conference in celebration of Maillard’s reaction ended with a sweet homage.
Credit: Sarah Everts/C&EN
The conference in celebration of Maillard’s reaction ended with a sweet homage.

“The Maillard is, by far, the most widely practiced chemical reaction in the world,” said chemistry Nobel Prize winner Jean-Marie Lehn late last month in Nancy, France, some 20 miles from the village of Pont-à-Mousson, where Maillard was born. That’s because the reaction takes place daily in households around the globe whenever food is cooked, Lehn told the group of 270 international scientists who had gathered on Maillard’s home turf to honor the reaction’s centennial and attend this year’s International Maillard Reaction Society conference.

Prior to Maillard’s paper in 1912, which described the reaction between reducing sugars and amino acids, “there wasn’t much of what you could call flavor chemistry,” said Alan Rocke, a historian at Case Western Reserve University. “There were lots of ideas and anecdotes, but no proper science.”

Yet even with the simplest of reactants, Maillard chemistry was so complicated and produced so many products—hundreds of them—that the research world would largely ignore it until around the time of World War II, Rocke said. That’s when the military became interested in producing on an industrial scale food that both was palatable and had a long shelf life. Because the Maillard reaction is responsible for the appealing aromas of freshly cooked food as well as some of the unwelcome ingredients in processed or long-stored food, scientists began to seriously study the reaction, Rocke explained.

Then in 1953, an African American chemist named John E. Hodge, who worked at the U.S. Department of Agriculture in Peoria, Ill., published a paper that established a mechanism for the Maillard reaction (J. Agric. Food Chem.1953, 1, 928).

“Maillard discovered the reaction, but Hodge understood it,” said Vincenzo Fogliano, a food chemist at the University of Naples, Federico II. In fact, because citations of Hodge’s paper far outnumber those of Maillard’s, there has been some discussion of renaming it the Maillard-Hodge reaction, Fogliano said. But that idea hasn’t yet caught on.

According to Hodge’s model, the Maillard reaction has three stages. First, the carbonyl group of a sugar reacts with an amino group on a protein or amino acid to produce water and an unstable glycosylamine. Then, the glycosylamine undergoes Amadori rearrangements to produce a series of aminoketose compounds. Last, a multitude of molecules, including some with flavor, aroma, and color, are created when the aminoketose compounds undergo a host of further rearrangements, conversions, additions, and polymerizations.

The reaction forms thousands of compounds in food, said Thomas Hofmann, the chair of food chemistry and molecular sensory science at Technical University of Munich. And of those, only a small subset repeatedly contribute to the odor and flavor of cooked food, such as 2,3-butanedione in popcorn and grilled steak.

Over the past several decades, there’s been a huge effort by food scientists to figure out how to influence the end products, Fogliano said. They’ve looked at various starting sugars and proteins as well as how different temperatures, pH levels, moisture levels, and other ingredients affect the creation of desired and undesired odor and flavor products. The idea, he added, is to figure out how to control the unruly Maillard process as it happens in food.

For example, Hofmann said, “it’s primarily the amino acid that drives the odor quality, not the sugar.” Glycine reactions produce beerlike odors, valine reactions produce characteristic rye-bread smells, and cysteine is the amino acid responsible for many meat and cracker scents, he said.

Maillard reactions can also change the texture and consistency of food, said Thomas Henle, a food chemist at Dresden University of Technology. For example, the Maillard reaction is used to append sugars to the protein lactalbumin, which is then used to make yogurt more gelatinous. Meanwhile, adding sugar to a protein called β-lactoglobulin in processed cheese makes the product softer and creamier, he said.

Sometimes a Maillard product that is appealing in some processed foods is undesirable in others. Case in point: 2-acetyl-1-pyrroline. This molecule gives crusty bread, popcorn, and basmati rice a desirable odor and flavor, but its presence in ultra-high-temperature pasteurized milk, because of the processing, results in an off-putting aftertaste that many consumers dislike, Hofmann says.

More notorious outcomes of the Maillard reaction in food are 5-hydroxymethylfurfural (HMF) and acrylamide, both potential carcinogens. Ten years ago, Stockholm University food chemists Margareta Törnqvist and Eden Tareke published a paper that sent shock waves through the food regulatory and science community: They showed that heavily processed food such as french fries, chips, and biscuits contained milligram levels of acrylamide (J. Agric. Food Chem., DOI: 10.1021/jf020302f).

The discovery came after a sequence of unusual, coincidental events. A Swedish construction team using polyacrylamide to plug leaks in a tunnel became ill, and Törnqvist was asked to investigate the workers’ exposure to acrylamide. But when she looked for healthy members of the public to use as controls, they too had unexpectedly high levels of acrylamide. At the same time, Tareke was studying acrylamide levels in wild animals and domesticated pets, and she found unexpectedly high levels of the compound in pets, she told C&EN. Because a main difference between wild and domesticated animals is their consumption of highly processed pet food, the researchers began to wonder whether the levels of acrylamide in humans are also attributable to consumption of highly processed food. Then they showed that processed food did in fact have substantial levels of the chemical.

Since 2002, food scientists and the food industry have devised a multitude of ways to reduce levels of acrylamide and other health-concerning molecules such as HMF in food. And according to Monica Anese, a food scientist at the University of Udine, in Italy, they have implemented some of the methods.

One of the most promising techniques for acrylamide removal, she said, is the preprocessing use of an enzyme called asparaginase, which can break down the amino acid asparagine. Acrylamide is produced when asparagine reacts with sugar, so removing the amino acid at the outset of processing helps reduce acrylamide levels in the final foodstuff. Another strategy is to lower cooking temperature, although this makes cooked food such as cookies and bread less brown—which consumers typically don’t like, she said.

Although the past decade has witnessed an increase in concern about the possible long-term consequences of consuming unwanted Maillard products in processed food, medical researchers have been worried about the outcome of endogenous Maillard reactions in our own bodies since the 1960s. After all, we are chock-full of proteins and sugars, and the Maillard reaction can take place at lower temperature as well, albeit at much slower rates of reaction, said Vincent M. Monnier, a medical researcher at Case Western Reserve.

One reaction hot spot is the lens of the human eye, where Maillard-based chemistry is partly responsible for nuclear cataracts. In this prevalent form of the disease, the cataracts darken and need to be extracted, he said. Because lens cells don’t regenerate over a lifetime and they have high levels of ascorbic acid, which can enhance Maillard reactions, “the lens is a trash can for human Maillard reactions,” he added.

Diabetes is another major area of medical Maillard research. The increased levels of sugar in the bloodstream result in Maillard reactions that activate the body’s inflammation response and contribute to many of the liver and cardiovascular complications of the disease, Monnier said.

In fact, the human body has several endogenous systems in place to remove these Maillard reaction products, said Paul Thornalley, a researcher at England’s Warwick Medical School. Thornalley studies enzymes that our body produces to eliminate methylglyoxal, a common Maillard reaction product circulating in our bloodstream. Left unchecked, methylglyoxal wreaks all sorts of damage, including interfering with cell surface proteins needed to keep blood vessel cells attached to each other. Although the enzymes responsible for breaking down methyl­glyoxal work 99.7% of the time, some methyl­glyoxal still “slips under the fence and does damage, particularly in diabetics,” he said.

After 100 years of studying the reaction, we’ve come to realize “there’s really a Maillard paradox,” Monnier said. “Cooking kills bacteria, increases shelf life, and creates attractive aromas.” But these same processes can create harmful chemicals in food. And in our body, the reaction is linked with inflammation, diabetes, and cardiovascular disease, he adds.

Maillard himself probably would have been more fascinated with the modern medical applications of the reaction rather than the initial food ones, Rocke said. When Maillard discovered the reaction, the scientist was looking for ways to synthesize proteins in vitro, Rocke said. The odors and colors emerging from his lab bench probably directed him more toward food chemistry applications, “but he was really a biochemist at heart,” Rocke said.



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Sidney Holt (October 8, 2012 9:16 AM)
Extremely interesting. I'm 86 and I thinkI'll stop eating!
Bob Buntrock (October 9, 2012 1:44 PM)
Sidney: I'm 71 (almost 72) and I'll keep on eating. If we made it this far, acrylamide or not, I think we can make it sometwhat further. Besides, I treasure eating (although I'mnot really overweight).
Jessy Van Wyk (October 8, 2012 11:01 AM)
The "birthday" cake (the "pile" of profiteroles) was sweet indeed - we sampled it thoroughly! The Symposium was also great - a very good mix of the Cinical/Medical/Cosmetic as well as food aspects of the Maillard reaction. It was very fitting that it was held so close to Louise Camille Maillard's birthplace(Pont-à-Mousson with the dinner at Abbaye Pont-à-Mousson an occassion to remember.
Robert Buntrock (October 9, 2012 1:52 PM)
Fascinating reaction and article. I had no idea that diacetyl was formed by a Maillard Rxn. or that it contributed to the odor and flavor of grilled steak.

Thanks for making this available to non-subscribers. I'm sending a link to my niece who teaches a HS food science class (with science credit) and a local internal HS STEM academy.
Vincent Monnier (October 15, 2012 3:01 PM)
Wonderful article on this topic of growing importance to food technology and medicine!
Darby Brooke (January 9, 2013 9:20 PM)
Very interesting article. However - call me old-fashioned, but should not some kind of (simplified, as necessary) diagram/scheme of a typical Maillard reaction have been included, too, especially in a chemistry-themed journal?
The paragraph describing Hodge's model is good, but not enough, really: a picture would've spoken a thousand words, and all that...
Dr. Aliza Stark (January 12, 2013 9:15 AM)
I will definately be giving this article to my Nutrition students to read in their Food Science class. It helps illustrate not only the importance of the Maillard Reaction in foods, but also how the same reactions can cause devastating results in the human body.

Dr. Aliza Stark, The Hebrew University of Jerusalem, Rehovot Campus
Dean Busquaert (March 30, 2013 4:20 PM)
There is one aspect of Maillard Reaction I don't understand. In order for the browning to occur it requires a reducing sugar and amino acid. Where is the sugar in a steak or chicken breast? Nutritional facts tell us these meats have 0 carbs no sugars. So when does the reducing sugar come from?
James R Etchison, Ph.D. (August 10, 2013 2:42 PM)
I agree. I've been pooh-poohing food "chemists" who attribute meat browning to "The Maillard Reaction" for years. I'm a retired glycoprotein biochemist and spent my career studying protein glycosylation and carbohydrate chemistry/biochemistry.

As anyone who cooks meat knows, dark meat browns more readily and more extensively than white meat. Red meat (e.g. beef steak) contains more myoglobin and mitochrondria than white meat (e.g. chicken breast). Both lean meats contain very little to no carbohydrate or sugars. Red meat is geared up for aerobic metabolism (high myoglobin and mitochondria) whereas white meat produces energy primarily by anaerobic metabolism (glycolysis of glucose). If anything, white meat should contain more carbohydrate (as glycogen) than red meat. In actuality, the glycolysis continues after the source animal is dead an butchered and virtually all glycogen is metabolized post slaughter.

While many reactions producing browning in meat may produce end products similar to those of The Maillard Reaction, calling the browning of meat a "Maillard Reaction" is almost certainly a misnomer.

In the absence of carbohydrate, the browning reactions in meat probably are related to reactions arising from the pyrrolysis of the tetrapyroles of myoglobin and mitochondrial cytochromes or other structures present in the mitochondria.

Of course, when sugar is added to the meat (e.g. barbecue sauce, etc.), then the Maillard reaction is certainly an appropriate label for the flavor reactions of barbecued meat.
Maheswor Gautam, MSc food science (student) (September 16, 2013 8:56 AM)
In a not so recent review Motram (1998) (Food Chemistry, Vol. 62, No. 4, pp. 415-424) cited Morton et al. (1960) (Flavouring substances and their preparation. GB Patent 1960; 836-694) for the explanation of meat-flavour during cooking. Ribose (source of reducing sugar) and cysteine (source of amino acid) was lost during cooking, accompanied by the formation of meat flavour. Similar flavour was also shown to develop in a model system including ribose and cysteine, clearly indicating Maillard reaction. Many other researchers have also confirmed the finding. Lipids and heterocyclic compounds are also involved in the quality of fried or roasted meat.

However, the question raised by Dr. James is really an eye-opener. But, the role of glycogen in meat browning (if any) needs to be elucidated.
James Etchison, Ph.D. (August 11, 2013 4:13 PM)
I'm a carbohydrate chemist/biochemist and I've been pooh-poohing this for years. There is no reducing sugar in meat. All of the glycogen is metabolized to lactic acid after slaughter. Dark meat browns more readily/extensively than white meat. The browning reactions in meat are due to reactions arising from the breakdown of myoglobin, mitochondrial cytochromes, and other mitochondrial components. Meat browning is NOT due to a Maillard reaction. The browning reactions may be similar to those arising from a Maillard reaction but do not originate from sugar/protein reactions.
Jorge Macdonaldinho (April 9, 2013 4:51 AM)
I'm a little concerned with the emphasis on "(highly) processed foods". Is a food which I have cooked at home, e.g. french fries made from raw potatoes, any less "processed" than one cooked in a factory and then packaged? Doesn't my home frying procedure produce the same undesirable acrylamide? I shudder to think of the result of sauteing some asparagus. I'm no fan of factory processed food but I'd hate for people to think that their home fry-up is somehow a more healthy option if it's not.
James Etchison, Ph.D. (August 11, 2013 2:22 PM)
Good article (in spite of propagating the myth of meat browning being due to the Maillard Reaction).

Dark meat browns more than white meat and, generally, dark meat has more flavor than white meat (e.g. pork shoulder vs. center cut pork loin)- probably due to both browning and fat.

Dark (red) meat comes from muscles that are well-exercised and are geared up biochemically for aerobic metabolism. They have much more myoglobin (red) and mitochondria (brown). White meat comes from poorly exercised muscles, like chicken breast, and those rely on anaerobic metabolism - i.e. glycolysis of glucose from stored glycogen. After slaughter, all of the glycogen in the muscles is broken down and metabolised to lactic acid within several hours. The pH of slaughtered meat actually drops from about 7.2-7.4 to about 6 or less within 24 to 48 hours. This helps to keep the meat from spoiling.

Just prior to slaughter, white meat probably has as much or more glycogen as red meat. After butchering, both red and white meat are depleted of glycogen stores by continued anaerobic metabolism. The only carbohydrate (sugars) remaining in the meat is in the cell membranes of the muscle cells and those are all "non-reducing sugars" and do not react with protein amino groups. So, those membrane carbohydrates (less than 0.5% by weight) cannot give rise to the Maillard Reaction browning.

Going back to the fact that red meat browns more readily and extensively than white meat, it must be those properties of red muscle that makes it brown more than white meat. Most likely candidates are the myoglobin and mitochondrial cytochromes. Pyrrolysis of the tetrapyrrole rings of those proteins could give rise to a series of chemical reactions that are similar to those of the Maillard reactions and produce a lot of the same or similar flavor chemicals.

But, classically, The Maillard Reaction refers specfically to reaction of a reducing sugar (e.g. glucose) with a primary amino group (e.g. amino acid) which subsequently undergoes an Amadori rearrangement that leads to a series of reactions which initiate a complicated cascade of browning reactions.

No reducing sugar; No Maillard Reaction.
Sarah E (September 16, 2013 1:43 PM)
James: Can you pleaseprovide links to the peer-reviewed references that support your claim?
Martha (August 10, 2014 2:29 PM)
I'm thinking the raw foodists and minimal cooking and no processed sugar or processed food may have a point. And I think the enzymes are mostly in raw food.

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