Thursday, June 13, 2013

Maillard Reaction Mechanism and Its Applications to Your Cooking

 



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.

Chemistry of Maillard Reaction

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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