The Maillard reaction , also known as non-enzymatic glycosylation, is a chemical phenomenon in which reducing sugars react with the amino groups of proteins, without the participation of any enzyme. This reaction proceeds at low temperatures, but reaches its maximum speed when the reagents are heated in a range between 140-165ºC.
When we cook, the use of the oven or oil ensures that the temperature is optimal for the development of the Maillard reaction, since food contains sugars and proteins that are sensitive to react with each other. Therefore, this reaction takes place in all areas of gastronomy where heat or fire browns or toasts food.
The Maillard reaction is very complex and has not been fully described to date. It is subject to several parameters, such as the composition of the food, the temperature, the pH, the heating time, etc., each one having weight in the final flavors and smells that we are normally used to.
Although many of its applications are reserved for cooking, the truth is that it also takes place within the body, being of interest for medical studies aimed at cardiovascular problems and diabetes.
The Maillard reaction is a conglomerate of many reactions, which end up generating hundreds of products, most of which have not even been characterized.
However, there are three stages in which this reaction can be divided and they are: formation of the Schiff’s base, formation of the Amadori product, and the reactions of this product.
Schiff base formation
First, the carbonyl group of the reducing sugar must be nucleophilically attacked by the nitrogen atom of an amino group (see first row of the upper image). This amino group belongs to any of the amino acids located on the surface of the protein, accessible to react with the closest reducing sugar.
Note that the double arrows indicate that it is a reversible reaction. When the reducing sugar binds to the amino group, what is known as a glucosamine is formed. Glucosamine rearranges its electrical charges to form the Schiff’s base (second row, fourth structure).
Amadori Product Training
The Schiff’s base, with its positively charged nitrogen atom surrounded by an OH – ion , is rearranged to reduce its instability. In the process, it is transformed into the 1,2-enaminol compound (center structure), which in turn rearranges its structure to give rise to a ring, corresponding to the Amadori product molecule, which can also exist as an open chain (last row and last three structures).
From the Amadori product, cyclic or linear, different reactions can be triggered. This is where Maillard’s reaction gets tricky.
On the one hand, this product can be dehydrated or fragmented to form dicarbonyl compounds, which also react with the amino groups of other proteins and give rise to other compounds that are not fully characterized. Among some of these products we have hydroxymethylfurfural (HMF), pyruvaldehyde, or reductones.
The relative predominance of these products depends on the pH. For example, HMF is formed under acidic conditions, which is when we add lemon juice or acidic ingredients to food as it cooks. Instead, reductones predominate under basic conditions, which is when baking powder is used.
The above products, derived from the Amadori product, can undergo aldol condensation to assemble polymers known as melanoidins. Melanoidins are responsible for the golden, brown or brown colorations (according to their average molecular weights) that we see in cooked foods.
Also, during this stage small and aromatic molecules are released, such as furanones (smell of caramel), thiophenes (smell of scorched meat), pyrroles (smell of nuts and cereals), among many others.
Effects of the Maillard reaction
In the food
The Maillard reaction, in all its complexity, has relatively simple sensory effects: the browning or browning of food, and the release of aromas. We talk about tastes and smells.
That is why this reaction, although it cannot be described, can be manipulated in such a way that the desired flavors and smells are obtained for a specific dish.
In the body
The Maillard reaction can proceed at room temperature, or within the body itself. Only the speed of the process changes. Proteins and reducing sugars react reversibly, and finally, after the Amadori product, the reactions continue irreversibly.
The impact that this reaction has on the body depends on how proteins work once they have sugars bound to them, and on the beneficial or toxic effects of reversible or irreversible products. Some of these products, such as carboxymethyl-lysine, are associated with diabetes and cardiovascular problems.
Other products, such as acrylamide, are even considered carcinogens, and the food industry therefore seeks to minimize the formation of acrylamide during the Maillard reaction.
However, products with bactericidal, antioxidant and antiallergic properties have also been found, which also increase the activities of enzymes.
Applications of the Maillard reaction
The products of the Maillard reaction are enjoyed in the following foods or products:
- Coffee, by roasting the beans,
- Chicken, on the golden and brown surfaces of its grilled or fried pieces,
- Beef meats, in the smells and colors of their fillets
- Breads, in their crusts or brown edges
- Cookies and cakes, in their brown tones
- French fries
The Maillard reaction can be confused with the caramelization reaction. The latter occurs at a higher temperature, and its mechanism is due to the pyrolysis of sugars, and not to a reaction between reducing sugars and proteins.