The
Maillard reaction is a form of nonenzymatic browning similar to
caramelizationCaramelization is the browning of sugar, a process used extensively in cooking for the resulting nutty flavor and brown color. As the process occurs, volatile chemicals are released, producing the characteristic caramel flavor....
. It results from a
chemical reactionA chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
between an
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
and a
reducing sugarA reducing sugar is any sugar that either has an aldehyde group or is capable of forming one in solution through isomerisation. This functional group allows the sugar to act as a reducing agent, for example in the Tollens' test or Benedict's test.-Chemistry:...
, usually requiring
heatIn physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
.
Vitally important in the preparation or presentation of many types of food, it is named after chemist
Louis-Camille MaillardLouis Camille Maillard was a French physician and chemist.-Early days:He was admitted to the Faculty of Science in the University of Nancy at the age of 16...
, who first described it in the 1910s while attempting to reproduce biological protein synthesis.
The reactive carbonyl group of the sugar reacts with the nucleophilic
amino groupAmines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
of the amino acid, and forms a complex mixture of poorly-characterized molecules responsible for a range of odors and flavors. This process is accelerated in an alkaline environment, as the
aminoAmines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
groups are deprotonated and, hence, have an increased nucleophilicity. The type of the amino acid determines the resulting flavor. This reaction is the basis of the flavoring industry. At high temperatures,
acrylamideAcrylamide is a chemical compound with the chemical formula C3H5NO. Its IUPAC name is prop-2-enamide. It is a white odourless crystalline solid, soluble in water, ethanol, ether, and chloroform. Acrylamide is incompatible with acids, bases, oxidizing agents, iron, and iron salts...
can be formed.
In the process, hundreds of different flavor compounds are created. These compounds, in turn, break down to form yet more new flavor compounds, and so on. Each type of food has a very distinctive set of flavor compounds that are formed during the Maillard reaction. It is these same compounds
flavor scientistsA flavorist, also known as flavor chemist, is someone who uses chemistry to engineer artificial and natural flavors. The tools and materials used by flavorists are almost the same as that used by perfumers with the exception that flavorists seek to mimic or modify both the olfactory and gustation...
have used over the years to make reaction flavors.
Foods and products with Maillard reactions
The Maillard reaction is responsible for many colors and flavors in foods:
- The browning of various meats like steak
A steak is a cut of meat . Most are cut perpendicular to the muscle fibers, improving the perceived tenderness of the meat. In North America, steaks are typically served grilled, pan-fried, or broiled. The more tender cuts from the loin and rib are cooked quickly, using dry heat, and served whole...
- Toasted bread
Toast is bread that has been browned by exposure to radiant heat. This browning reaction is known as the Maillard reaction. Toasting warms the bread and makes it firmer, so it holds toppings more securely...
- Biscuits
- French Fries
French fries , chips, fries, or French-fried potatoes are strips of deep-fried potato. North Americans tend to refer to any pieces of deep-fried potatoes as fries or French fries, while in the United Kingdom, Australia, Ireland and New Zealand, long, thinly cut slices of deep-fried potatoes are...
- Malted barley as in malt
Malt is germinated cereal grains that have been dried in a process known as "malting". The grains are made to germinate by soaking in water, and are then halted from germinating further by drying with hot air...
whiskey or beerBeer is the world's most widely consumed andprobably oldest alcoholic beverage; it is the third most popular drink overall, after water and tea. It is produced by the brewing and fermentation of sugars, mainly derived from malted cereal grains, most commonly malted barley and malted wheat...
- Fried onion
The onion , also known as the bulb onion, common onion and garden onion, is the most widely cultivated species of the genus Allium. The genus Allium also contains a number of other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion The onion...
s
- Dried or condensed milk
Condensed milk, also known as sweetened condensed milk, is cow's milk from which water has been removed and to which sugar has been added, yielding a very thick, sweet product which when canned can last for years without refrigeration if unopened. The two terms, condensed milk and sweetened...
- Roasted coffee
Coffee is a brewed beverage with a dark,init brooo acidic flavor prepared from the roasted seeds of the coffee plant, colloquially called coffee beans. The beans are found in coffee cherries, which grow on trees cultivated in over 70 countries, primarily in equatorial Latin America, Southeast Asia,...
- Dulce de leche
Dulce de leche is a thick,creamy, caramel-like milk-based sauce or spread.Literally translated, dulce de leche means "sweet from milk". It is prepared by slowly heating sweetened milk to create a product that derives its taste from caramelised sugar. It is a popular sweet in Latin America, where...
- The burnished surface (crust) of brioche
Brioche is a highly enriched French pastry, whose high egg and butter content give it a rich and tender crumb. It is "light and slightly puffy, more or less fine, according to the proportion of butter and eggs" It has a dark, golden, and flaky crust, frequently accentuated by an egg wash applied...
, cakes, yeast, and quick breadBread is a staple food prepared by cooking a dough of flour and water and often additional ingredients. Doughs are usually baked, but in some cuisines breads are steamed , fried , or baked on an unoiled frying pan . It may be leavened or unleavened...
s
- Roasted meat
Meat is animal flesh that is used as food. Most often, this means the skeletal muscle and associated fat and other tissues, but it may also describe other edible tissues such as organs and offal...
- Maple syrup
6-Acetyl-2,3,4,5-tetrahydropyridine6-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....
is responsible for the biscuit or cracker-like flavor present in baked goods like
breadBread is a staple food prepared by cooking a dough of flour and water and often additional ingredients. Doughs are usually baked, but in some cuisines breads are steamed , fried , or baked on an unoiled frying pan . It may be leavened or unleavened...
,
popcornPopcorn, or popping corn, is corn which expands from the kernel and puffs up when heated. Corn is able to pop because, like sorghum, quinoa and millet, its kernels have a hard moisture-sealed hull and a dense starchy interior. This allows pressure to build inside the kernel until an explosive...
, and
tortillaIn Mexico and Central America, a tortilla is a type of thin, unleavened flat bread, made from finely ground maize...
products. The structurally-related compound
2-acetyl-1-pyrroline2-Acetyl-1-pyrroline, abbreviated 2AP, with the IUPAC name 1-ethanone, is an aroma compound and flavor that gives white bread, jasmine rice and basmati rice, the spice pandan , and bread flowers their typical smell...
has a similar smell, and occurs also naturally without heating and gives varieties of cooked
riceRice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...
and the
spiceA spice is a dried seed, fruit, root, bark, or vegetative substance used in nutritionally insignificant quantities as a food additive for flavor, color, or as a preservative that kills harmful bacteria or prevents their growth. It may be used to flavour a dish or to hide other flavours...
pandan (
Pandanus amaryllifoliusPandanus amaryllifolius is a tropical plant in the screwpine genus which is known commonly as pandan and is used widely in Southeast Asian cooking as a flavoring. The plant is rare in the wild but is widely cultivated. It is an upright green plant with fan-shaped sprays of long, narrow, bladelike...
) their typical smells. Both compounds have odor thresholds below 0.06 ng/
l
.
The browning reactions that occur when meat is roasted or seared are complicated, and occur mostly by Maillard browning with contributions from other chemical reactions, including the breakdown of the tetrapyrrole rings of the muscle protein
myoglobinMyoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. The only time myoglobin is found in the...
.
CaramelizationCaramelization is the browning of sugar, a process used extensively in cooking for the resulting nutty flavor and brown color. As the process occurs, volatile chemicals are released, producing the characteristic caramel flavor....
is an entirely different process from Maillard browning, though the results of the two processes are sometimes similar to the naked eye (and tastebuds). Caramelization may sometimes cause browning in the same foods in which the Maillard reaction occurs, but the two processes are distinct. They both are promoted by heating, but the Maillard reaction involves amino acids, as discussed above, whereas caramelization is simply the
pyrolysisPyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
of certain sugars. The following things are a result of the Maillard browning reaction:
- Caramel
Caramel is a beige to dark-brown confection made by heating any of a variety of sugars. It is used as a flavoring in puddings and desserts, as a filling in bonbons, and as a topping for ice cream, custard and coffee....
made from milkMilk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...
and sugarSugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
, especially in candies: Milk is high in protein (amino acids), and browning of food involving this complex ingredient would most likely include Maillard reactions. See references below.
- Chocolate
Chocolate is a raw or processed food produced from the seed of the tropical Theobroma cacao tree. Cacao has been cultivated for at least three millennia in Mexico, Central and South America. Its earliest documented use is around 1100 BC...
and maple syrupMaple syrup is a syrup usually made from the xylem sap of sugar maple, red maple, or black maple trees, although it can also be made from other maple species such as the bigleaf maple. In cold climates, these trees store starch in their trunks and roots before the winter; the starch is then...
- Lightly roasted peanut
The peanut, or groundnut , is a species in the legume or "bean" family , so it is not a nut. The peanut was probably first cultivated in the valleys of Peru. It is an annual herbaceous plant growing tall...
s
In making
silageSilage is fermented, high-moisture fodder that can be fed to ruminants or used as a biofuel feedstock for anaerobic digesters. It is fermented and stored in a process called ensiling or silaging, and is usually made from grass crops, including corn , sorghum or other cereals, using the entire...
, excess heat causes the Maillard reaction to occur, which reduces the amount of energy and protein available to the animals who feed on it.
The process
- The carbonyl group of the sugar reacts with the amino group of the amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
, producing N-substituted glycosylamineGlycosylamine is a class of biochemical compounds consisting of an amine with a β-N-glycosidic bond to a carbohydrate, thus forming a cyclic hemiaminal ether bond .Examples include nucleosides such as adenosine....
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...
, forming ketosamineA ketosamine is a compound word used to describe the combination of two organic chemistry functional groups, derived from ketose and amine....
s
- There are several ways for the ketosamines to react further:
- Produce 2 water and reductones
- Diacetyl
Diacetyl is a natural byproduct of fermentation. It is a vicinal diketone with the molecular formula C4H6O2...
, aspirinAspirin , also known as acetylsalicylic acid , is a salicylate drug, often used as an analgesic to relieve minor aches and pains, as an antipyretic to reduce fever, and as an anti-inflammatory medication. It was discovered by Arthur Eichengrun, a chemist with the German company Bayer...
, pyruvaldehyde and other short-chain hydrolytic 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 at high temperatures and low water activity. Melanoidins are commonly present in foods that have undergone some form of non-enzymatic browning, such as barley malts , bread...
s
Factors
High temperature, intermediate moisture levels, and alkaline conditions all promote the Maillard reaction. In cooking, low moisture levels are necessary mainly because water boils into steam at 212 °F (100 °C), whereas the Maillard reaction happens noticeably around 310 °F (154.4 °C): significant browning of food does not occur until all surface water is vaporized.
The rate of Maillard reactions increases as the
water activityWater activity or aw was developed to account for the intensity with which water associates with various non-aqueous constituents and solids. Simply stated, it is a measure of the energy status of the water in a system...
increases from 0.0 or as it decreases from 1.0, reaching a maximum at water activities in the range of 0.6 to 0.7. Since the Maillard reaction produces water, further increases in water activity may inhibit Maillard reactions.
PentoseA pentose is a monosaccharide with five carbon atoms. Pentoses are organized into two groups. Aldopentoses have an aldehyde functional group at position 1...
reducing sugars react more than
hexoseIn organic chemistry, a hexose is a monosaccharide with six carbon atoms, having the chemical formula C6H12O6. Hexoses are classified by functional group, with aldohexoses having an aldehyde at position 1, and ketohexoses having a ketone at position 2....
s, which react more than
disaccharideA disaccharide or biose is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides form an aqueous solution when dissolved in water...
s. Different amino acids produce different amounts of browning.
Physiology
The Maillard reaction also occurs in the human body. It is a step in the formation of
advanced glycation endproductAn advanced glycation end-product is the result of a chain of chemical reactions after an initial glycation reaction. The intermediate products are known, variously, as Amadori, Schiff base and Maillard products, named after the researchers who first described them. An advanced glycation...
s (AGEs). It is tracked by measuring
pentosidinePentosidine is a biomarker for advanced glycation endproducts, or AGEs. It is a well characterized and easily detected member of this large class of compounds.-Background:...
.
Although the Maillard reaction has been studied most extensively in foods, it has also shown a correlation in numerous different diseases in the human body, in particular degenerative eye diseases. In general, these diseases are due to the accumulation of AGEs on nucleic acids, proteins, and lipids. Though AGEs have numerous origins, they can form from the oxidation and dehydration of Amadori adducts, which themselves are products of nonenzymatic Maillard reactions. Apart from ocular diseases, whose correlation with Maillard chemistry has been more recently studied, the formation of AGEs has also proven to contribute to a wide range of human diseases that include
diabetic complicationsDiabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...
,
pulmonary fibrosisPulmonary fibrosis is the formation or development of excess fibrous connective tissue in the lungs. It is also described as "scarring of the lung".-Symptoms:Symptoms of pulmonary fibrosis are mainly:...
, and
neurodegenerationNeurodegeneration is the umbrella term for the progressive loss of structure or function of neurons, including death of neurons. Many neurodegenerative diseases including Parkinson’s, Alzheimer’s, and Huntington’s occur as a result of neurodegenerative processes. As research progresses, many...
.
Receptor systems in the body have been suggested to have evolved to remove glycation-modified molecules, such as AGEs, to eliminate their effects. The adverse effects of AGE accumulation appear to be mediated by numerous different AGE receptors. Examples include AGE-R1, galectin-3, CD36, and, most noted, RAGE, the receptor for AGEs.
Advanced glycation in numerous different locations within the eye can prove detrimental. In the
corneaThe cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...
, whose endothelial cells have been known to express RAGE and galectin-3, the accumulation of AGEs is associated with thickened corneal stroma, corneal edema, and morphological changes within patients with diabetes. Within the lens, Maillard chemistry has been studied extensively in the context of cataract formation. Advanced glycation is known to alter fiber membrane integrity in the lens, and dicarbonyl compounds are known to cause increased aggregate formation within the lens. This effect is exacerbated by both diabetes and aging. Furthermore, it is thought that AGE-inhibiting compounds are effective in preventing cataract formation in diabetics.
Glycation in Maillard reactions may lead to destabilization of the vitreous gel structure within the eye via unnecessary cross-linking between collagen fibrils. Again, this process is more strongly observed within diabetic patients.
Within the
retinaThe vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
, the accumulation of AGEs in the Drusen and Bruch’s membrane has been associated with age, and has also been observed at a higher level among patients with age-related macular degeneration (AMD). This is manifested by the thickening of the Bruch’s membrane. Furthermore, it has been observed that AGE levels increase with age within the lamina cribosa, and the products of the Maillard reaction have been observed there, as well.
A wide range of ocular diseases, particularly diabetic retinopathy, may be prevented by the inhibition of the Maillard reaction. This may be achieved in numerous ways: preventing the formation of AGEs, reducing the effectiveness of the AGE signaling pathway and the receptor-ligand interactions, or breaking the AGE crosslinks. This latter method has already been achieved to some extent by the breaker ALT-711, though its effectiveness against retinopathy is unknown. Another method is by the use of amadorins, which are able to prevent the reaction of Amadori intermediates, which form into AGEs by scavenging the reactive carbonyls.
See also
- Advanced glycation end-product
- Baking
Baking is the technique of prolonged cooking of food by dry heat acting by convection, and not by radiation, normally in an oven, but also in hot ashes, or on hot stones. It is primarily used for the preparation of bread, cakes, pastries and pies, tarts, quiches, cookies and crackers. Such items...
- Beer
- Wok hei
External links