**5. The Maillard reaction**

Different types of food darkening can occur in both home and industrial kitchens. Enzymatic darkening and discoloration originates from reactions catalyzed by an enzyme known as polyphenol oxidase (PPO). This enzyme acts mainly on fruits and vegetables, generating negative consequences such as economic losses, decreased nutritional value, and undesirable changes in flavor and appearance of foods such as such as tea, coffee, cocoa, and prunes [10].

There is also a nonenzymatic browning that can occur. It is slower than enzymatic browning because it does not have the reaction catalyzing enzyme. This nonenzymatic browning is characterized by caramelization, the Maillard reaction,

### *Dulce de Leche—Chemistry and Processing Technology DOI: http://dx.doi.org/10.5772/intechopen.82677*

and ascorbic acid (vitamin C) oxidation. Each food has its own specific darkening profile since reaction speed is dependent on the nature of the reactive components of the food [11].

 Among the nonenzymatic darkening reactions, the Maillard reaction will be highlighted here. The reaction was discovered in 1912 by Louis-Camille Maillard while attempting peptide synthesis under physiological conditions. It is of great interest at present time as it has been shown to be related to chemical, organoleptic, nutritional, toxicological, and in vivo manifestations [12].

The Maillard reaction is in fact a complex cascade of reactions (**Figure 10**), which take place primarily during heating and prolonged storage of food products. These reactions can result in positive or negative changes in food quality because it favors the formation of compounds responsible for aroma, flavor, and color of heattreated foods. The Maillard reaction is divided into three stages: the initial stage, intermediate stage, and final stage [10].

The initial stage involves the condensation of the carbonyl group of the reducing sugar with the free amino group of amino acids, peptides, or proteins. It occurs through the nucleophilic attack of the nitrogen's electron pair of the amino group, leading to the beginning of the reaction. As a result of the condensation, an unstable Schiff base is formed, which in turn releases water and then forms a glycosylamine. The Schiff base undergoes these sequential rearrangements to produce a reasonably stable aminoketose known as the Amadori product (aldose sugar) or Heyns product (ketose sugar). These initial stage products are stable and do not have color, fluorescence, or ultraviolet visible absorption. As a result, there is a huge variety of products in different proportions [12].

The second phase of the Maillard reaction takes place upon prolonged heating prolongation or storage. The Amadori products or Heyns products become fragmented and give rise to a series of reactions including dehydration, enolization, and

retro-aldolization. In this intermediate stage, dicarbonilic compounds, redutones, furfural derivatives, and Strecker degradation products appear and induce the appearance of a furan derivative that becomes the origin of a hexose commonly known as 5-hydroxymethylfurfural.

The compounds that originate during the intermediate phase are fluorescent and absorb ultraviolet radiation. They are cyclic and highly reactive, polymerizing with lysine or arginine residues in proteins to create stable compounds that culminate in the formation of dark pigments known as melanoidins. These pigments lead to the desirable or undesirable coloring of foods that make up part of the final stage of the Maillard reaction [12].

The Maillard reaction can be affected by temperature and pH, among other factors. The Maillard reaction rate is slower at lower temperatures and practically doubles at every 10°C increase between 40 and 70°C. pH also exerts an effect on the intensity of the reaction, with maximum discoloration occurring in an alkaline range between pH 9 and 10 [12].

The amine type directly influences onset of the Maillard reaction. Highly reactive amino acids such as lysine, glycine, tryptophan, and tyrosine facilitate the reaction, whereas proline, leucine, isoleucine, hydroxyproline, and methionine show medium reactivity and histidine, threonine, aspartic acid, glutamic acid, and cysteine demonstrate low reactivity. Lysine, because it has the free epsilon amino group, demonstrates high reactivity because it is more susceptible to the reaction (carbonylamino). It may therefore reduce the nutritional value of the food [10].

 A reducing sugar is essential for the Maillard reaction to occur; pentoses are more reactive than hexoses, which in turn are more reactive than disaccharides. This type of browning occurs more frequently with intermediate values of water activity (0.5 and 0.8) and a relative humidity between 30 and 70%. At low water activity (0.2– 0.25), the velocity tends to zero due to a decrease in solvent. At high water values (0.9), the reactants are extremely diluted, which decreases the darkening rate [10].

In addition to these factors, metal ions (iron and copper), sulfite, storage conditions, light, type, time, and temperature of the heat treatment and cooking methods may interfere with the reaction [12].

The extent to which the Maillard reaction occurs can be monitored by the appearance of certain compounds, including furosine, hydroxymethylfurfural and carboxymethyllysine. The appearance of these compounds offers an indication of the intensity of the thermal processing and nutritional changes related to the reaction as well [16].

Thus, the color of the dulce de leche is basically due to the Maillard reaction. The use of different ingredients, with modification of, for example, the type of sugar and the amount of acidity reducer during processing may influence the development of the Maillard reaction. Among the ingredients used to make DL, glucose and acidity reducers exert the most influence over the development of this nonenzymatic darkening reaction.

Glucose, an optional ingredient in dulce de leche production, is a monosaccharide made up of monomeric units that form sugars of greater size when united. It is less sweet than sucrose, which enables it to reduce the development and growth of lactose crystals while increasing the viscosity and brightness of the final product. The maximum glucose/sucrose substitution is 40% according to the Technical Regulation of Identity and Quality of DL. The authors suggest adding 2 g.100 g<sup>−</sup><sup>1</sup> of glucose by weight of milk in order to improve the final product's texture and brightness. The color of dulce de leche made with glucose tends to be darker because glucose is a reducing sugar, which promotes an increase of the nonenzymatic darkening reaction.

Baking soda is a fundamental element in DL technology because it plays the role of acidity reducer. Adding bicarbonate at the beginning of the manufacturing *Dulce de Leche—Chemistry and Processing Technology DOI: http://dx.doi.org/10.5772/intechopen.82677* 

process helps reduce initial acidity of the syrup (milk + sugar) and maintains the pH of the milk during the concentration stage. Thus, sodium bicarbonate acts as an extra source of alkalinity. It prevents the destabilization of casein micelles caused by the decrease of the pH that occurs during evaporation. This decrease could be due to the concentration of calcium phosphate and the formation of organic acids that occur during lactose degradation and phosphoric casein ester hydrolysis [1].
