MAILLARD REACTION: PHASES AND STRECKER DEGRADATION - CHEMISTRY - 2025

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

The reaction is favored by heat (temperatures between 140 to 165 ˚C), although it also occurs at a slower rate, at room temperature. It was the French physician and chemist Louis-Camille Maillard who described it in 1912.

Darkening occurs without the action of enzymes, as well as caramelization; for this reason both are called non-enzymatic browning reactions.

However, they differ in that only carbohydrates are heated during caramelization, while for the Maillard reaction to occur, proteins or amino acids must also be present.

Phases of the reaction

Although it may seem easy to achieve the golden color of food through culinary cooking techniques, the chemistry involved in the Maillard reaction is very complex. In 1953 John Hodge published the scheme of the reaction which is still generally accepted.

In a first stage 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 1-amino-deoxy-2-ketose type (also called Amadori's compound) is obtained.

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

- There may be a cleavage or breakdown 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 burnt sugar aroma.

Stecker degradation

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

When these substances react with unchanged amino acids, they transform into typical aldehydes 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 influencing the reaction

Nature of the amino acids and carbohydrates of the raw material

In the 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- show great reactivity.

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

In carbohydrates, pentoses have been found to be 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; It is 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, because it does not have 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 above all in cooking, pasteurization, sterilization and dehydration operations.

By 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 the most favorable.

A decrease in pH makes it possible to attenuate browning during dehydration, but unfavorably modifies the organoleptic characteristics.

Humidity

The rate of the Maillard reaction has a maximum of between 0.55 and 0.75 in terms of water activity. For this reason, dehydrated foods are the most stable, as long as they are stored away from moisture and at a moderate temperature.

Presence of metals

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

Negative effects

Although the reaction is generally considered desirable during cooking, it has a disadvantage from a nutritional point of view. If foods with low water content and presence of reducing sugars and proteins (such as cereals or powdered milk) 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 appearance of the reaction. Except for arginine, the other three are essential amino acids; that is, they must be provided by food.

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

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

Foods with organoleptic characteristics product of the Maillard reaction

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

- Roast.

- Fried onions.

- Coffee and roasted 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 alimentants.
4. Helmenstine AM "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). Food control and hygiene.
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.