Louis-Camille Maillard never
worked on food. He was a physician who concerned himself with the biochemistry
of living cells. His work led him to investigate how the amino acids would react
with sugars both of which could be found inside cells. In 1912 Maillard
published a paper trying to explain what happens when amino acids react with
sugars at elevated temperatures. However, it was American chemist John E. Hodge, working at the U.S. Department
of Agriculture in Peoria, Illinois, who published a paper in 1953 that established
a mechanism for the Maillard reaction. Today, Maillard's name is
inextricably linked with food
science. You cannot pick up any textbook to do with food and fail to find
his name in the index. The reason is simple. Long after Maillard died, it was realized
that all the meaty flavors that develop during cooking are caused by reactions
of amino acids with sugars. Maillard's pioneering work in the area led
to the whole group of complex reactions being given his name.
Maillard Reaction Defined
The Maillard reaction
(/maɪˈjɑr/ my-YAR;
French pronunciation: [majaʁ]) is a form of nonenzymatic browning. It
results from a chemical reaction between an amino acid and a reducing sugar,
usually requiring heat.
The reactions are very complex and the details are by no means
understood today, despite many chemists devoting their lives to study the
reactions. The complexity comes from the fact that there are many different
sugars and amino acids that can react together and from the fact that the
actual reaction products from anyone sugar amino acid pair depend on the
temperature at which the reaction takes place, the acidity of the environment,
the other chemicals that are nearby as well as random chance.
The reactive carbonyl
group of the sugar reacts with the nucleophilic amino group 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 (e.g., lye applied to darken
pretzels), as the amino 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, acrylamide 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 scientists have used over
the years to make reaction flavors.
However, this very
complexity offers the chef a range of interesting possibilities. Nearly all the
molecules generated in the Maillard reactions (so far well over a thousand have been identified) are volatile
enough to count as "flavor molecules”.
So from the same
starting ingredients the control of the temperature and environment can lead to
a range of differing flavors.
Chemistry of Maillard Reaction
In the Maillard reactions
the amino acids can come from any proteins and the sugars from any
carbohydrates. In the first stage of the reactions the proteins and
carbohydrates are degraded into smaller sugars and amino acids. Next the sugar
rings open and the resulting aldehydes and acids react with the amino acids to
produce a wide range of chemicals. These new molecules then react amongst
themselves to produce the main flavor compounds. The list of identified compounds
includes several important classes of molecules. Pyrazines are the molecules that give fresh green notes to fruit
and vegetables. Furanones and Furanthiols have fruity odors. Other compounds
such as the di-sulphides have pungent and even unpleasant smells.
One particularly
important molecule that is generated has been associated with meaty odors - if
it is missing the meaty smell is absent. When present even in very small
quantities there is a strong meaty smell. This molecule is called bis-2-methyl-3-furyl-disulphide and is
now widely used in the flavor industry to prepare artificial meat flavors.
Maillard Reaction Process
1. The carbonyl group of the sugar
reacts with the amino group of the amino acid, producing N-substituted glycosylamine
and water
2. The unstable glycosylamine
undergoes Amadori rearrangement, forming ketosamines
3. There are several ways for the
ketosamines to react further:
A. Produce 2 water and reductones
B. Diacetyl, aspirin, pyruvaldehyde
and other short-chain hydrolytic fission products can be formed
C. Produce brown nitrogenous polymers
and melanoidins
What
Temperature Does the Maillard Reaction Occur? How to Get Maillard Reaction on
Steak or Other Meat
Controlling the
Maillard reactions is a tricky business - really it is a part of the art of a
good chef to know how much heat to apply to a piece of meat to get the flavor
he wants. There are however, a few simple guidelines that can be helpful. The
Maillard reactions only take place at all quickly
at high temperatures (above about 140°C) so you need to
cook meats at high temperatures to develop "meaty"
flavors. Since these high temperatures will only occur at the surface of
the meat (inside there will be water which
cannot be heated above 100°C without turning to steam) you will develop the
flavor more quickly if you increase the surface area of the meat. You can
increase the surface area by cutting the meat into small pieces, or thin
slices before you cook it.
A second important
point to bear in mind is that as the temperature rises above about 200°C, so
new molecules start to appear at the end of the Maillard reactions. Some of
these molecules that are formed at high temperatures can be carcinogenic and
don't taste very pleasant. So it is wise to avoid over-heating your meats. The
potential for dangerously over-heated meats is greatest on barbecues
where some cooks may completely carbonize the outside of the food.
Figure 1. Diagram of Millard Reaction
Chemistry. (click to enlarge)
The main point to
remember is that Maillard reaction can occur at a wide range of temperatures,
but the lower limit is not well-defined. It can even occur at room temperature,
providing some flavoring components (for example) to ripening cheeses and Seranno
ham. At high
temperatures (over 300F), it will noticeably occur on many foods in a matter of
minutes, so you can actually watch things "brown." At lower
temperatures, it may take hours, days, or even years for the effects to be
noticeable. Water inhibits the faster reactions, but at lower temperatures it
actually can help the reaction by allowing proteins and sugars more freedom to
circulate.
According to Harold
McGee (On Food And Cooking: The Science And
Lore Of The Kitchen),there
are exceptions to the rule that browning reactions require temperatures above
the boil. Alkaline conditions, concentrated solutions of carbohydrates and
amino acids, and prolonged cooking times can all generate Maillard colors and
aromas in moist foods. For example, alkaline egg whites, rich in protein, with
a trace of glucose, but 90% water, will become tan-colored when simmered for 12
hours. The base liquid for brewing beer, a water extract of barley malt that
contains reactive sugars and amino acids from the germinated grains, deepens in
color and flavor with several hours of boiling. Watery meat or chicken stock
will do the same as it's boiled down to make a concentrated demiglace.
Persimmon pudding turns nearly black thanks to its combination of reactive
glucose, alkaline baking soda, and hours of cooking; balsamic vinegar turns
nearly black over the course of years.
Note that while
alkaline conditions help, they are clearly not necessary (e.g., balsamic
vinegar). Another standard example for non-alkaline conditions is traditional
pumpernickel bread, which is steam baked for 12-24 hours usually at oven
temperatures ranging from 225 to 250F. The interior of the bread does not get
much above normal boiling temperature, but a significant color change can
clearly be seen in such a humid, relatively low-temperature environment.
Interestingly,
despite the information in many cooking sources, many of the earliest studies
of Maillard reactions were in systems varying from room temperature to slightly
above body temperature, from the browning reactions that create
the color of soil
to internal reactions in the human body that are now thought to contribute
significantly to the aging process and some diseases. Maillard reactions also play a role
in the natural changes in moist food observed to happen at room temperature
when stored over years, like when you discover a jar or can of food in the back
of the pantry
and find that the food has turned brownish.
At very high or very
low temperatures, Maillard reactions are often secondary to other processes
such as caramelization and enzymatic browning. Below is the explanation of
Figure 1 above.
1) Above
400F - mostly caramelization, with the possibility of burning with
prolonged heating
2) ~330-400F
- increasing caramelization with higher temps, which uses up sugars and thus
inhibits Maillard at the high end of this range
3) ~300-330F
- Maillard progresses at a fast pace, causing browning noticeably within
minutes
4) ~212-300F
- Maillard gets slower as temperature goes lower, generally requiring many
hours near the boiling point of water
5) ~130-212F
- Maillard requires water, high protein, sugar, and alkaline conditions to
advance noticeably in a matter of hours; generally can take days
6) Below
130F - Enzymatic browning is often more significant in many foods than
Maillard, but Maillard will still occur over periods from days or months to
years, with progressively longer times at lower temperatures
(In some cases, certain reactions can be activated by a short time at a
high temperature, which then can lead to faster browning below boiling or even
near room temperature.)
One final, but very
important, note: the Maillard reaction is a very general process that occurs
between all sorts of amino acids and sugars. It thus also can produce a lot of
different flavor components and products, in addition to the browning.
Different reactions between particular amino acids and sugars will also occur
at different rates depending on temperature.
This, I think, may be part of the reason for the confusion
among various professional cooking sources about the "minimum"
temperatures. Many of the reactions that produce the classic "Maillard
taste" and "Maillard smell" components don't really begin to
happen appreciably until about 250F, and they won't happen fast until 300F or
so. Maillard reactions at lower temperatures produce different taste and smell
components, which often could be characterized as more "earthy."
While browning still happens at a slower pace, the results will actually taste
different. But because reaction products will always depend on the exact amino
acids and sugars involved, as well as other conditions (moisture, pH), it's
difficult to divide temperature ranges into clear flavor zones.
Foods
and Products with Maillard Reactions
The Maillard reaction is responsible for
many colors and flavors in foods:
1.
The browning of various meats like steak
2.
Toasted bread
3.
Biscuits
4.
French fries
5.
Malted barley as in malt whiskey or beer
6.
Fried onions
7.
Dried or condensed milk
8.
Roasted coffee
9. Dulce de leche
11.
The darkened crusts of baked goods
12.
Maple syrup
6-Acetyl-2,3,4,5-tetrahydropyridine is responsible for
the biscuit or cracker-like flavor present in baked goods like bread, popcorn,
and tortilla products. The structurally related compound 2-acetyl-1-pyrroline
has a similar smell, and occurs also naturally without heating and gives
varieties of cooked rice and the spice pandan (Pandanus
amaryllifolius) 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 myoglobin.
Maillard
Reaction vs Caramelization
Caramelization 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 both are
promoted by heating, but the Maillard reaction involves amino acids, as
discussed above, whereas caramelization is simply the pyrolysis of
certain sugars.
The following things are a result of the
Maillard browning reaction:
1. Caramel made from milk and sugar,
especially in candies: Milk is high in protein (amino acids), and browning of
food involving this complex ingredient would most likely include Maillard
reactions.
2. Chocolate and maple syrup
3. Lightly roasted peanuts
In making silage, excess heat causes the
Maillard reaction to occur, which reduces the amount of energy and protein
available to the animals who feed on it.
When cooking, the Maillard reaction can be
achieved at lower temperatures (for
example, when using the sous-vide method or when searing meats) by increasing the pH of the item being
cooked. The most common method for accomplishing this is by using baking
soda as a catalyst to facilitate the reaction. Additionally, a pressure cooker
is well-suited for achieving the higher temperatures often required for the
Maillard reaction to occur (depending upon what is being prepared).
If you have time you can watch this video
on sous vide cuisine solutions or you can
bookmark this page and watch it later.
Maillard Reaction on Physiology
The Maillard
reaction also occurs in the human body. It is a step in the formation of advanced glycation endproducts (AGEs).
It is tracked by measuring pentosidine. 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
complications, pulmonary fibrosis, and neurodegeneration.
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 cornea, 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 retina, 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.
References:
Chichester, C. O. (1986). Advances in Food Research (Advances in Food and Nutrition Research).
Boston: Academic Press. ISBN 0-12-016430-2.
Everts, S. (2012). "The Maillard Reaction Turns
100". Chemical & Engineering News 90: 58–60.
Grandhee, SK, VM Monnier (June 25, 1991). "Mechanism
of formation of the Maillard protein cross-link pentosidine. Glucose, fructose,
and ascorbate as pentosidine precursors". J. Biol. Chem. 266 (18): 11649–53. PMID 1904866.
Harrison, T. J., G. R. Dake (2005). "An
expeditious, high-yielding construction of the food aroma compounds
6-acetyl-1,2,3,4-tetrahydropyridine and 2-acetyl-1-pyrroline". J.
Org. Chem. 70 (26): 10872–4.
doi:10.1021/jo051940a. PMID 16356012.
Hodge, J.E. (1953). "Dehydrated Foods, Chemistry
of Browning Reactions in Model Systems". Journal of
Agricultural and Food Chemistry 1
(15): 928–43. doi:10.1021/jf60015a004.
Maillard, L. C. (1912). "Action of Amino Acids on
Sugars. Formation of Melanoidins in a Methodical Way.". Compt.
Rend. 154: 66.
McGee, Harold (2004). On Food and Cooking: The Science and
Lore of the Kitchen. Scribner, New York. ISBN 978-0-684-80001-1.
Soest, Peter J. Van. 1982. Nutritional
Ecology of the Ruminant published by Cornell University Press
Stitt, Alan W. (2005). "The
Maillard Reaction in Eye Diseases". Annals of the New York
Academy of Science 1043:
582–97. doi:10.1196/annals.1338.066. PMID 16037281.
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