Maillard reaction
What is the Maillard reaction?
Who first described the Maillard reaction?
How can Maillard reaction products improve foods?
Maillard reaction, is a non-enzymatic reaction that occurs between amino acids and reducing sugars under heat. It is best known for its role in cooking, specifically the browning of cooked foods, where it contributes to flavor and aroma.
Discovery and naming
The Maillard reaction is named for French chemist Louis-Camille Maillard, who first described it in 1912. Maillard discovered the reaction during attempts to synthesize proteins with applications in medicine; specifically, he noticed that a solution containing glucose (a reducing sugar) and the amino acid glycine became brown and released a toasted smell similar to cooked food when heated. He hypothesized that the reaction was responsible for modifying nutrients in heated foods and that it was relevant in diabetes, potentially being driven by hyperglycemia (high blood sugar).
Maillard’s research was revisited following World War II, coincident with a growing interest in food processing and preservation methods. In 1953 chemist John E. Hodge described the chemical processes behind the Maillard reaction. In particular Hodge proposed that the reaction occurs in three stages: early, intermediate, and advanced.
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Stages
The Maillard reaction typically is initiated at a temperature of about 250 °F (120 °C), with an optimal range of 280–330 °F (140–165 °C). At higher temperatures the rate of reaction accelerates quickly, such that in the case of cooking, temperatures above 350 °F (176 °C) result in charring or burning. The reaction can also occur at ambient temperatures (68–77 °F [20–25 °C]), albeit at a greatly reduced reaction rate. Reduced moisture levels, such as in hot grills, accelerate the reaction as well.
The early stage of the Maillard reaction is characterized by the formation of an imine (or Schiff base) and its isomerization into a more stable product. In imine formation the carbonyl portion of a reducing sugar reacts with the amino group on an amino acid, resulting in a hemiaminal product. This unstable compound rapidly undergoes dehydration to form an imine. Also unstable, the imine then undergoes isomerization via either Amadori rearrangement or Heyns rearrangement, depending on the type of sugar involved. Specifically, sugars such as glucose are transformed through Amadori rearrangement, resulting in a ketosamine; sugars such as fructose are altered through Heyns rearrangement, resulting in an aldosamine.
In the intermediate stage sugar molecules are dehydrated and degraded, along with amino acids. In the case of foods and certain beverages, breakdown of the amino acids results in volatile Strecker aldehydes, which are responsible for aroma, and in aminoketones, which impart flavor and coloration, particularly browning. These effects are evident, for example, in the flavor and aroma of chocolate, beer, freshly baked bread, and roasted coffee. As the Maillard reaction progresses to the advanced stage, thermal breakdown of Strecker aldehydes and aminoketones results in the generation of free radicals and reactive carbonyl (carbon-oxygen-containing) species. These highly reactive products form rapidly, leading to significant oxidative damage, substantial browning, and off-flavors and aromas.
In the human body the Maillard reaction is referred to as non-enzymatic glycation, which is a normal process that happens throughout life and generally is considered a spontaneous reaction. Under certain conditions it may contribute to complications related to aging, chronic inflammation, and diabetes. In such cases, carbonyl compounds generated by glycation attach to amino groups on lipids, proteins, and nucleic acids, causing the formation of advanced glycation end products (AGEs). The accumulation of AGEs fuels oxidative stress.
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Impact on health and nutritional qualities of foods
Maillard reaction products (MRPs), which are generated during the reaction, can be beneficial, adding to the nutritional and culinary qualities of food, or harmful, causing nutritional losses of foods and introducing health risks. For example, the reaction, through its production of end products such as melanoidins and reductones, can increase antioxidant activity in various foods and beverages; antioxidants also help prolong the shelf life of food products. Several MRPs are also known to have bactericidal or anti-allergenic properties.
Health risks linked to the Maillard reaction are associated primarily with the chemical acrylamide, which is also found in cigarette smoke. Acrylamide is a probable carcinogen in humans and is generally considered a chemical contaminant of food. Fried potato products and breakfast cereals are major contributors to dietary acrylamide exposure.