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The Maillard reaction ( ; ) is a
chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
between
amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
and reducing sugars that gives browned food its distinctive flavor.
Seared Searing (or pan searing) is a technique used in grilling, baking, braising, roasting, sautéing, etc., in which the surface of the food (usually meat such as beef, poultry, pork, seafood) is cooked at high temperature until a browned crust forms ...
steaks, fried dumplings, cookies and other kinds of biscuits, breads, toasted marshmallows, and many other foods undergo this reaction. It is named after French chemist
Louis Camille Maillard Louis Camille Maillard ( ; ; 4 February 1878 – 12 May 1936) was a French physician and chemist. He made important contributions to the study of kidney disorders. He also became known for the "Maillard reaction", the chemical reaction which he ...
, who first described it in 1912 while attempting to reproduce biological
protein synthesis Protein biosynthesis (or protein synthesis) is a core biological process, occurring inside Cell (biology), cells, homeostasis, balancing the loss of cellular proteins (via Proteolysis, degradation or Protein targeting, export) through the product ...
. The reaction is a form of
non-enzymatic browning Browning is the process of food turning brown due to the chemical reactions that take place within. The process of browning is one of the chemical reactions that take place in food chemistry and represents an interesting research topic regarding ...
which typically proceeds rapidly from around . Many
recipe A recipe is a set of instructions that describes how to prepare or make something, especially a dish of prepared food. A sub-recipe or subrecipe is a recipe for an ingredient that will be called for in the instructions for the main recipe. His ...
s call for an oven temperature high enough to ensure that a Maillard reaction occurs. At higher temperatures,
caramelization Caramelization is a process of browning of sugar used extensively in cooking for the resulting sweet nutty flavor and brown color. The brown colors are produced by three groups of polymers: caramelans (C24H36O18), caramelens (C36H50O25), and c ...
(the browning of sugars, a distinct process) and subsequently
pyrolysis The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
(final breakdown leading to burning and the development of acrid flavors) become more pronounced. The reactive
carbonyl group In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...
of the sugar reacts with the
nucleophilic In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
amino group In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent such ...
of the amino acid and forms a complex mixture of poorly characterized molecules responsible for a range of aromas and flavors. This process is accelerated in an alkaline environment (e.g.,
lye A lye is a metal hydroxide traditionally obtained by leaching wood ashes, or a strong alkali which is highly soluble in water producing caustic basic solutions. "Lye" most commonly refers to sodium hydroxide (NaOH), but historically has been u ...
applied to darken pretzels; see lye roll), as the amino groups () are
deprotonated Deprotonation (or dehydronation) is the removal (transfer) of a proton (or hydron, or hydrogen cation), (H+) from a Brønsted–Lowry acid in an acid–base reaction.Henry Jakubowski, Biochemistry Online Chapter 2A3, https://employees.csbsju.edu ...
, and hence have an increased
nucleophilicity In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
. This reaction is the basis for many of the
flavoring A flavoring (or flavouring), also known as flavor (or flavour) or flavorant, is a food additive used to improve the taste or smell of food. It changes the perceptual impression of food as determined primarily by the chemoreceptors of the gusta ...
industry's recipes. At high temperatures, a probable
carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substan ...
called
acrylamide Acrylamide (or acrylic amide) is an organic compound with the chemical formula CH2=CHC(O)NH2. It is a white odorless solid, soluble in water and several organic solvents. From the chemistry perspective, acrylamide is a vinyl-substituted primary ...
can form. This can be discouraged by heating at a lower temperature, adding
asparaginase Asparaginase is an enzyme that is used as a medication and in food manufacturing. As a medication, L-asparaginase is used to treat acute lymphoblastic leukemia (ALL) and lymphoblastic lymphoma (LBL). It is given by injection into a vein, muscl ...
, or injecting carbon dioxide. In the cooking process, Maillard reactions can produce hundreds of different flavor compounds depending on the chemical constituents in the food, the temperature, the cooking time, and the presence of air. These compounds, in turn, often break down to form yet more flavor compounds. Flavor scientists have used the Maillard reaction over the years to make artificial flavors.


History

In 1912, Louis Camille Maillard published a paper describing the reaction between amino acids and sugars at elevated temperatures. In 1953, chemist John E. Hodge with the
U.S. Department of Agriculture The United States Department of Agriculture (USDA) is the federal executive department responsible for developing and executing federal laws related to farming, forestry, rural economic development, and food. It aims to meet the needs of comme ...
established a mechanism for the Maillard reaction.


Foods and products

The Maillard reaction is responsible for many colors and flavors in foods, such as the browning of various meats when seared or grilled, the browning and
umami Umami ( from ja, 旨味 ), or savoriness, is one of the five basic tastes. It has been described as savory and is characteristic of broths and cooked meats. People taste umami through taste receptors that typically respond to glutamates and ...
taste in fried onions, and
coffee roasting Roasting coffee transforms the chemical and physical properties of green coffee beans into roasted coffee products. The roasting process is what produces the characteristic flavor of coffee by causing the green coffee beans to change in taste. ...
. It contributes to the darkened crust of
baked goods Baking is a method of preparing food that uses dry heat, typically in an oven, but can also be done in hot ashes, or on hot stones. The most common baked item is bread but many other types of foods can be baked. Heat is gradually transferred ...
, the golden-brown color of
French fries French fries (North American English), chips (British English), finger chips ( Indian English), french-fried potatoes, or simply fries, are '' batonnet'' or ''allumette''-cut deep-fried potatoes of disputed origin from Belgium and France. Th ...
and other crisps, browning of malted barley as found in
malt Malt is germinated cereal grain that has been dried in a process known as " malting". The grain is made to germinate by soaking in water and is then halted from germinating further by drying with hot air. Malted grain is used to make beer, wh ...
whiskey and beer, and the color and taste of
dried Drying is a mass transfer process consisting of the removal of water or another solvent by evaporation from a solid, semi-solid or liquid. This process is often used as a final production step before selling or packaging products. To be consider ...
and
condensed milk Condensed milk is cow's milk from which water has been removed (roughly 60% of it). It is most often found with sugar added, in the form of ''sweetened condensed milk'' (SCM), to the extent that the terms "condensed milk" and "sweetened condens ...
, ''
dulce de leche ''Dulce de leche'' (; pt, doce de leite), also known as caramelized milk, milk candy or milk jam in English, is a confection from Latin America prepared by slowly heating sugar and milk over a period of several hours. The resulting substance, w ...
'',
toffee Toffee is a confection made by caramelizing sugar or molasses (creating inverted sugar) along with butter, and occasionally flour. The mixture is heated until its temperature reaches the hard crack stage of . While being prepared, toffee is ...
,
black garlic Black garlic is a type of aged garlic that is colored deep brownish-black. The process is of East Asian origin. It is made by placing garlic (''Allium sativum'') in a warm, moist, controlled environment over the course of several weeks, a pro ...
,
chocolate Chocolate is a food made from roasted and ground cacao seed kernels that is available as a liquid, solid, or paste, either on its own or as a flavoring agent in other foods. Cacao has been consumed in some form since at least the Olmec civ ...
, toasted marshmallows, and
roasted peanuts The peanut (''Arachis hypogaea''), also known as the groundnut, goober (US), pindar (US) or monkey nut (UK), is a legume crop grown mainly for its edible Seed, seeds. It is widely grown in the tropics and subtropics, important to both small ...
.
6-Acetyl-2,3,4,5-tetrahydropyridine 6-Acetyl-2,3,4,5-tetrahydropyridine is an aroma compound and flavor that gives baked goods such as white bread, popcorn, and tortillas their typical smell, together with its structural homolog 2-acetyl-1-pyrroline. 6-Acetyl-2,3,4,5-tetrahydrop ...
is responsible for the biscuit or cracker-like flavor present in baked goods such as bread, popcorn, and tortilla products. The structurally related compound 2-acetyl-1-pyrroline has a similar smell and also occurs naturally without heating. The compound gives varieties of cooked rice and the herb ''pandan'' (''
Pandanus amaryllifolius ''Pandanus amaryllifolius'' is a tropical plant in the ''Pandanus'' (screwpine) genus, which is commonly known as pandan (; ). It has fragrant leaves which are used widely for flavouring in the cuisines of Southeast Asia and South Asia. Occurre ...
'') their typical smells. Both compounds have odor thresholds below 0.06 nanograms per liter. The browning reactions that occur when meat is roasted or seared are complex and occur mostly by Maillard browning with contributions from other chemical reactions, including the breakdown of the tetrapyrrole rings of the muscle protein
myoglobin Myoglobin (symbol Mb or MB) is an iron- and oxygen-binding protein found in the cardiac and skeletal muscle tissue of vertebrates in general and in almost all mammals. Myoglobin is distantly related to hemoglobin. Compared to hemoglobin, myoglobi ...
. Maillard reactions also occur in
dried fruit Dried fruit is fruit from which the majority of the original water content has been removed either naturally, through sun drying, or through the use of specialized dryers or dehydrators. Dried fruit has a long tradition of use dating back to th ...
.
Caramelization Caramelization is a process of browning of sugar used extensively in cooking for the resulting sweet nutty flavor and brown color. The brown colors are produced by three groups of polymers: caramelans (C24H36O18), caramelens (C36H50O25), and c ...
is an entirely different process from Maillard browning, though the results of the two processes are sometimes similar to the naked eye (and taste buds). Caramelization may sometimes cause browning in the same foods in which the Maillard reaction occurs, but the two processes are distinct. They are both promoted by heating, but the Maillard reaction involves amino acids, whereas caramelization is the
pyrolysis The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
of certain sugars. In making
silage Silage () is a type of fodder made from green foliage crops which have been preserved by fermentation to the point of acidification. It can be fed to cattle, sheep and other such ruminants (cud-chewing animals). The fermentation and storage p ...
, excess heat causes the Maillard reaction to occur, which reduces the amount of energy and protein available to the animals that feed on it.


Archaeology

In archaeology the Maillard process occurs when bodies are preserved in
peat bog A bog or bogland is a wetland that accumulates peat as a deposit of dead plant materials often mosses, typically sphagnum moss. It is one of the four main types of wetlands. Other names for bogs include mire, mosses, quagmire, and muskeg; a ...
s. The acidic peat environment causes a
tanning Tanning may refer to: *Tanning (leather), treating animal skins to produce leather *Sun tanning, using the sun to darken pale skin **Indoor tanning, the use of artificial light in place of the sun **Sunless tanning, application of a stain or dye t ...
or browning of skin tones and can turn hair to a red or ginger tone. The chemical mechanism is the same as in the browning of food, but it develops slowly over time due to the acidic action on the bog body. It is typically seen on Iron Age bodies and was described by Painter in 1991 as the interaction of anaerobic, acidic, and cold (typically ) sphagnum acid on the
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...
s. The Maillard reaction also contributes to the preservation of
paleofeces Paleofeces (or palaeofaeces in British English) are ancient human feces, often found as part of archaeological excavations or surveys. The term coprolite is often used interchangeably, although coprolite can also refer to fossilized animal ...
.


Chemical mechanism

# The
carbonyl group In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...
of the sugar reacts with the amino group of the amino acid, producing N-substituted
glycosylamine Glycosylamines are a class of biochemical compounds consisting of a glycosyl group attached to an amino group, -NR2. They are also known as N-glycosides,. as they are a type of glycoside. Glycosyl groups can be derived from carbohydrates. The glyc ...
and water # The unstable glycosylamine undergoes
Amadori rearrangement The Amadori rearrangement is an organic reaction describing the acid or base catalyzed isomerization or rearrangement reaction of the ''N''-glycoside of an aldose or the glycosylamine to the corresponding 1-amino-1-deoxy-ketose. The reaction is i ...
, forming ketosamines # Several ways are known for the ketosamines to react further: #* Produce two water molecules and
reductone A reductone is a special class of organic compounds. They are enediols with a carbonyl group adjacent to the enediol group, i.e. RC(OH)=C(OH)-C(O)R. The enediol structure is stabilized by the resonance resulting from the tautomerism with the adjace ...
s #*
Diacetyl Diacetyl (IUPAC systematic name: butanedione or butane-2,3-dione) is an organic compound with the chemical formula (CH3CO)2. It is a yellow liquid with an intensely buttery flavor. It is a vicinal diketone (two C=O groups, side-by-side). Diacet ...
, pyruvaldehyde, and other short-chain
hydrolytic Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysi ...
fission products can be formed. #* Produce brown nitrogenous polymers and
melanoidin Melanoidins are brown, high molecular weight heterogeneous polymers that are formed when sugars and amino acids combine (through the Maillard Reaction) at high temperatures and low water activity. They were discovered by Schmiedeberg in 1897. Melan ...
s The open-chain Amadori products undergo further dehydration and deamination to produce dicarbonyls. This is a crucial intermediate. Dicarbonyls react with amines to produce Strecker aldehydes through Strecker degradation.
Acrylamide Acrylamide (or acrylic amide) is an organic compound with the chemical formula CH2=CHC(O)NH2. It is a white odorless solid, soluble in water and several organic solvents. From the chemistry perspective, acrylamide is a vinyl-substituted primary ...
, a possible human
carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substan ...
, can be generated as a byproduct of Maillard reaction between reducing sugars and amino acids, especially
asparagine Asparagine (symbol Asn or N) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...
, both of which are present in most food products.


See also

* Akabori amino-acid reaction *
Advanced glycation end-product Advanced glycation end products (AGEs) are proteins or lipids that become glycated as a result of exposure to sugars. They are a bio-marker implicated in aging and the development, or worsening, of many degenerative diseases, such as diabetes, at ...
*
Baking Baking is a method of preparing food that uses dry heat, typically in an oven, but can also be done in hot ashes, or on hot stones. The most common baked item is bread but many other types of foods can be baked. Heat is gradually transferred " ...
*
Caramelization Caramelization is a process of browning of sugar used extensively in cooking for the resulting sweet nutty flavor and brown color. The brown colors are produced by three groups of polymers: caramelans (C24H36O18), caramelens (C36H50O25), and c ...
*


References


Further reading

* Van Soest, Peter J. (1982)
''Nutritional Ecology of the Ruminant''
(2nd ed.). Ithaca, NY: Cornell University Press. . . {{Authority control Food chemistry Name reactions