Maillard reaction: Phases and Degradation of Strecker

The Maillard reaction is the name given to chemical reactions between amino acids and reducing sugars that obscures food during roasting, baking, roasting and frying. Brown compounds are formed responsible for the color and aroma of products such as bread crust, roast beef, French fries and baked cookies.

The reaction is favored by the heat (temperatures between 140 to 165 ˚C), although it also occurs at a lower speed, at room temperature. It was the French physician and chemist Louis-Camille Maillard who described it in 1912; it is from there that his name derives.

Darkening occurs without the action of enzymes, as well as caramelization; hence both are called nonenzymatic browning reactions. However, they differ in that in caramelization only carbohydrates are heated, whereas for the Maillard reaction to occur proteins or amino acids must also be present.

Phases of the reaction

Although it seems easy to achieve the golden color in food by culinary cooking techniques, the chemistry involved in the Maillard reaction is very complex. In 1953 John Hodge published the scheme of the reaction that is still admitted in a general way.

In a first step a reducing sugar such as glucose is condensed with a compound containing a free amino group, such as an amino acid, to give an addition product that is transformed into an N-substituted glycosylamine.

After a molecular arrangement called Amadori rearrangement, a molecule of the type 1-amino-deoxy-2-ketose (also called Amadori compound) is obtained.

Once this compound is formed, two reaction routes are possible:

- There may be a split or break of molecules in carbonyl compounds lacking nitrogen, such as acetol, pyruvaldehyde, diacetyl.

- It is possible that an intense dehydration occurs that gives rise to substances such as furfural and dehydrofurfural. These substances are produced by heating and decomposition of carbohydrates. Some have a slight bitter taste and aroma of burnt sugar.

Stecker degradation

There is a third way of reaction: the degradation of Strecker. This consists of a moderate dehydration that generates reducing substances.

When these substances react with the unaltered amino acids, they are transformed into aldehydes typical of the amino acids involved. By this reaction, products such as pyrazine are formed, which gives the characteristic aroma to potato chips.

When an amino acid intervenes in these processes the molecule is lost from a nutritional point of view. This is particularly important in the case of essential amino acids, such as lysine.

Factors that influence the reaction

Nature of the amino acids and carbohydrates of the raw material

In free state almost all amino acids have a uniform behavior. However, it has been shown that among the amino acids included in the polypeptide chain, the basic ones -especially lysine- have a high reactivity.

The type of amino acid involved in the reaction determines the resulting taste. The sugars must be reductive (that is, they must have a free carbonyl group and react as electron donors).

In carbohydrates it has been found that pentoses are more reactive than hexoses. That is, glucose is less reactive than fructose and, in turn, than mannose. These three hexoses are among the least reactive; followed by pentose, arabinose, xylose and ribose, in increasing order of reactivity.

Disaccharides, such as lactose or maltose, are even less reactive than hexoses. Sucrose, not having a free reducing function, does not intervene in the reaction; it only does so if it is present in an acidic food and is then hydrolyzed into glucose and fructose.

Temperature

The reaction can develop during storage at room temperature. For this reason it is considered that heat is not an indispensable condition for it to occur; however, high temperatures accelerate it. For this reason, the reaction occurs mainly in cooking, pasteurization, sterilization and dehydration operations.

When increasing the pH, the intensity increases

If the pH rises, so does the intensity of the reaction. However, the pH between 6 and 8 is considered to be the most favorable. A decrease in pH makes it possible to attenuate the browning during dehydration, but adversely modifies the organoleptic characteristics.

Humidity

The speed of the Maillard reaction has a maximum of between 0.55 and 0.75 in terms of water activity. Therefore, dehydrated foods are the most stable, as long as they are kept away from humidity and at a moderate temperature.

Presence of metals

Some metallic cations catalyze it, such as Cu ⁺² and Faith ⁺³ . Others like the Mn ⁺² and the Sn ⁺² inhibit the reaction.

Negative effects

Although the reaction is generally considered desirable during cooking, it presents a disadvantage from a nutritional point of view. If foods with a low water content and the presence of reducing sugars and proteins (such as cereals or milk powder) are heated, the Maillard reaction will lead to the loss of amino acids.

The most reactive in decreasing order are lysine, arginine, tryptophan and histidine. In these cases it is important to delay the onset of the reaction. With the exception of arginine, the other three are essential amino acids; that is, they must be contributed by the feeding.

If a large number of amino acids of a protein are found bound to sugar residues as a result of the Maillard reaction, amino acids can not be used by the body. The proteolytic enzymes of the intestine can not hydrolyze them.

Another drawback noted is that, at high temperatures, a potentially carcinogenic substance such as acrylamide can be formed.

Food with organoleptic characteristics product of the Maillard reaction

Depending on the concentration of melanoidins, the color may change from yellow to brown or even black in the following foods:

- Roast.

- Fried onions.

- Roasted coffee and cocoa.

- Baked goods such as bread, cookies and cakes.

- Chips.

- Malt whiskey or beer.

- Powdered or condensed milk.

- Caramel.

- Roasted peanuts.

References

1. Alais, C., Linden, G., Mariné Font, A. and Vidal Carou, M. (1990). Biochemistry of food.
2. Ames, J. (1998). Applications of the Maillard reaction in the food industry. Food Chemistry
3. Cheftel, J., Cheftel, H., Besançon, P. and Desnuelle, P. (1992). Introduction à la biochimie et à la technologie des aliments.
4. Helmenstine A.M. "The Maillard reaction: Chemestry of food browning"(June 2017) in: ThoughtCo: Science. Retrieved on March 22, 2018 from Thought.Co: thoughtco.com.
5. Larrañaga Coll, I. (2010). Control and hygiene of food.
6. Maillard reaction. (2018) Retrieved on March 22, 2018, from Wikipedia
7. Tamanna, N. and Mahmood, N. (2015). Food Processing and Maillard Reaction Products: Effect on Human Health and Nutrition. International Journal of Food Science.