**1. Introduction**

280 Food Industrial Processes – Methods and Equipment

Zahrim, A.Y.; Tizaoui,C. & Hilal, N. (2011). Coagulation With Polymers for Nanofiltration

Zhao, C., Zhou, X., & Yue, Y. (2000). Determination of Pore Size Distribution On the Surface

Zhao, Y., Xing, W., Xu, N., & Wong, F. (2005). Effects of Inorganic Electrolytes On Zeta

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The Maillard reaction was first reported in 1912 by Louis-Camille Maillard, who described that upon gently heating sugars and amino acids in water, a yellow-brown color developed. The reaction that leads to these colorful compounds, firstly described from a simple observation, is actually the result of a complicated pathway of chemical reactions. The Maillard reaction is often described in food systems but it also occurs in living organisms, and in this case, it is called glycation. In biological systems, the ramifications of the Maillard reaction have been observed and analyzed, as this reaction has become important in the field of food science and medicine (Finot, 2005; Gerrard, 2002a).

The consumption of Maillard Reaction Products (MRPs) has increased in recent decades and there are evidences that these substances are absorbed and may participate in pathological processes such as, cataract, diabetes, degenerative diseases, atherosclerosis and chronic renal failure. On the other hand, these compounds are responsible for essential sensory attributes of thermally processed food products, contributing to their appearance, flavor, aroma and texture.

This chapter will cover the chemistry of Maillard reaction products generation, the role of these products in food acceptability, the analysis of these compounds both in food products and in the human body and the biological activities attributed to these compounds, since this is a contemporary and controversy subject in food science and nutrition field.

### **2. The chemistry of Maillard reaction products generation**

Since the first description of a browning reaction of glycine with glucose by Louis Maillard, the knowledge on chemical structures derived from the reaction of carbonylic and amino compounds has considerably increase (Nass *et al*., 2007).

Amino-carbonyl and related interactions of food constituents comprise those changes commonly termed as "non-enzymatic browning reactions". Specifically, reactions of amines, amino acids, peptides, and proteins with reducing sugars and vitamin C (Maillard reaction, caramelization, ascorbic acid degradation) and quinones (enzymatic browning) cause deterioration of food during storage and processing (Friedman, 1996).

living organisms have other pathways that are linked to glucose metabolism and lipid peroxidation, whose products are termed Advanced Lipoxidation End Products (ALEs)

277

Fig. 1. Main stages in Maillard reaction proposed by Hodge (adapted from Nursten, 2005) MRPs/AGEs generated in food and biological systems are shown in Figures 2 and 3. Nguyen (2006) describes the MRP/AGE content of selected popular foods, such as roasted almonds (66.5 kU/g), oil (120.0 kU/g), butter (94.0 kU/g), mayonnaise (265.0 kU/g), broiled chicken for 15 minutes (58.0 kU/g), fried chicken for 15 minutes (61.0 kU/g), homemade pancakes (10.0 kU/g), bread (0.5 kU/g). It is noteworthy that temperature and cooking process are more relevant for the formation of MRPs than time of cooking or other

(Goldberg *et al.*, 2004; Nass *et al.*, 2007).

parameters (Nguyen, 2006).


Non-enzymatic browning reactions depend on many parameters (Table 1), such as, temperature, water activity (aw), pH, moisture content and chemical composition. In general, maximum browning occurs at aw between 0.60 and 0.85 and the browning rate increases with increasing pH, up to a pH of around 10 (Gerrard, 2002a; Morales & Van Boekel, 1997).

Table 1. Main differences between non-enzymatic browning reactions (based on Finot, 2005)

From 1940, amino-carbonyl reactions and the resulting browning pigments have been investigated by many chemists. In 1953 Hodge proposed a three stages (initial, advanced and final) scheme for the Maillard reaction (Figure 1) (Nursten, 2005). The initial stage starts from the sugar amine systems leading to browning pigments generation. Amadori colorless compounds are formed in this stage, and an increased content of unsaturated carbonyl compounds is observed. During the intermediate stage, both fluorescence and radiation absorbing properties of the system increase due to the formation of small molecules with chromophores. Aldehydes formed by the Strecker degradation of amino acids can condense either with themselves, sugar fragments, furfurals, or with other dehydration product forming brown pigments. Although Strecker's pathway is not the major color-producing reaction, it is responsible for the origin of off-flavours usually associated with Maillard browning. The final stage is characterized by the formation of unsaturated, brown nitrogenous polymers (melanoidins) which may also be generated from the condensation reaction of furfurals or dehydro reductones (Finot, 2005; Hodge, 1953; Morales & Van Boekel, 1997).

Browning development occurs after an induction period, characterized by the production of fluorescent uncolored intermediates. Fluorophores are considered precursors of brown pigments and allow detecting the progress of the reaction before any visual change occurs. Fluorescence from the Maillard reaction is attributed to molecular structures with complex bonds between carbon and nitrogen, and the contribution of sugar caramelization to global fluorescence is insignificant in amino-acid containing systems (Matiacevich & Buera, 2006; Rozycki *et al.,* 2010).

Maillard reaction ratio is proportional to the heat-treatment severity during food processing, when temperatures range from 100 to 250 ºC (baking, grilling, frying, extruding and roasting) and/or during storage for long periods at room temperature. This reaction is of most importance for roasted products such as coffee, chocolate and peanuts. In the medical arena, several authors describe the role of the Maillard reaction during ageing and chronic diseases, as diabetes and renal failure (Gerrard, 2002a; Nguyen, 2006; Nursten, 2005).

Maillard reaction occurs in biological systems and the final products are refered as Advanced Glycation End Products (AGEs). AGEs are a heterogeneous group of compounds that arise non-enzymatically by the reaction of reducing sugar and other α-carbonylic compounds with amino groups on proteins, lipids and nucleic acids. Actually, glycation in

Non-enzymatic browning reactions depend on many parameters (Table 1), such as, temperature, water activity (aw), pH, moisture content and chemical composition. In general, maximum browning occurs at aw between 0.60 and 0.85 and the browning rate increases with

> pH optimum

> > acid

No Yes Basic/acid Medium Medium/high

Temperature aw

Medium Medium/high

increasing pH, up to a pH of around 10 (Gerrard, 2002a; Morales & Van Boekel, 1997).

Caramelization No No Basic/acid High Low

Table 1. Main differences between non-enzymatic browning reactions (based on Finot, 2005) From 1940, amino-carbonyl reactions and the resulting browning pigments have been investigated by many chemists. In 1953 Hodge proposed a three stages (initial, advanced and final) scheme for the Maillard reaction (Figure 1) (Nursten, 2005). The initial stage starts from the sugar amine systems leading to browning pigments generation. Amadori colorless compounds are formed in this stage, and an increased content of unsaturated carbonyl compounds is observed. During the intermediate stage, both fluorescence and radiation absorbing properties of the system increase due to the formation of small molecules with chromophores. Aldehydes formed by the Strecker degradation of amino acids can condense either with themselves, sugar fragments, furfurals, or with other dehydration product forming brown pigments. Although Strecker's pathway is not the major color-producing reaction, it is responsible for the origin of off-flavours usually associated with Maillard browning. The final stage is characterized by the formation of unsaturated, brown nitrogenous polymers (melanoidins) which may also be generated from the condensation reaction of furfurals or dehydro reductones (Finot, 2005; Hodge, 1953; Morales & Van

Browning development occurs after an induction period, characterized by the production of fluorescent uncolored intermediates. Fluorophores are considered precursors of brown pigments and allow detecting the progress of the reaction before any visual change occurs. Fluorescence from the Maillard reaction is attributed to molecular structures with complex bonds between carbon and nitrogen, and the contribution of sugar caramelization to global fluorescence is insignificant in amino-acid containing systems (Matiacevich & Buera, 2006;

Maillard reaction ratio is proportional to the heat-treatment severity during food processing, when temperatures range from 100 to 250 ºC (baking, grilling, frying, extruding and roasting) and/or during storage for long periods at room temperature. This reaction is of most importance for roasted products such as coffee, chocolate and peanuts. In the medical arena, several authors describe the role of the Maillard reaction during ageing and chronic

Maillard reaction occurs in biological systems and the final products are refered as Advanced Glycation End Products (AGEs). AGEs are a heterogeneous group of compounds that arise non-enzymatically by the reaction of reducing sugar and other α-carbonylic compounds with amino groups on proteins, lipids and nucleic acids. Actually, glycation in

diseases, as diabetes and renal failure (Gerrard, 2002a; Nguyen, 2006; Nursten, 2005).

NH2 requirement

Yes/No No Slightly

Mechanism Oxygen

Maillard reaction

Ascorbic acid degradation

Boekel, 1997).

Rozycki *et al.,* 2010).

requirement

living organisms have other pathways that are linked to glucose metabolism and lipid peroxidation, whose products are termed Advanced Lipoxidation End Products (ALEs) (Goldberg *et al.*, 2004; Nass *et al.*, 2007).

Fig. 1. Main stages in Maillard reaction proposed by Hodge (adapted from Nursten, 2005)

MRPs/AGEs generated in food and biological systems are shown in Figures 2 and 3. Nguyen (2006) describes the MRP/AGE content of selected popular foods, such as roasted almonds (66.5 kU/g), oil (120.0 kU/g), butter (94.0 kU/g), mayonnaise (265.0 kU/g), broiled chicken for 15 minutes (58.0 kU/g), fried chicken for 15 minutes (61.0 kU/g), homemade pancakes (10.0 kU/g), bread (0.5 kU/g). It is noteworthy that temperature and cooking process are more relevant for the formation of MRPs than time of cooking or other parameters (Nguyen, 2006).

accumulation of these compounds in tissues and, thus favoring the onset and progression of

279

Up to 90% of pyrraline free and Amadori products excreted by the kidneys are from food source. Free pentosidine, for example, presents about 60 to 80% bioavailability (Forster *et al*., 2005). Somoza *et al*. (2006) administered casein-linked lysinoalanine (LAL), Nεfructoselysine (FL) and Nε-(carboxymethyl)lysine (CML) to rats and revealed that the kidneys are the main organs in which AGEs are accumulated and excreted. In this study, the

In other words, the rate of absorption and renal excretion of MRPs depends on dietary intake

Maillard reaction is one of the most important reaction which results from food processing. Maillard reaction products (MRPs) greatly influence essential food quality attributes such as flavor, aroma, color and texture. Actually, this reaction can be used to design foods that present sensory attributes demanded by the consumer (Ames, 1990; Yu & Zang, 2010).

Color formation is the primary characteristic of the Maillard reaction. In the last decade, efforts have been driven to detect Maillard reaction kinetics and the formation ratio of colored compounds, mainly with the use of model systems. Brown color development during processing and storage is desirable for many products such as baked foods, coffee, cookies while undesirable in some kinds of food products orange juice, white chocolate, milk and powder egg. Predicting and controlling food color development are particularly important for companies to satisfy consumer preference, since a complex array of melanoidins produced by the Maillard reaction is strongly dependent on the food matrix composition as well as the technological conditions of the reaction (Wang *et al.*, 2011). Melanoidin can also be

The presence of melanoidins, brown nitrogen-containing high molecular weight pigments, responds for the characteristic color of roasted foods such as coffee, cocoa, bread and malt. Although the chemical structures and health effects of these compounds produced both in food and model systems have been investigated for over 30 years, the health effects are not well understood, mainly because their bioavailability depends on several parameters that include gut microbiota metabolism. Despite of the lack of general knowledge, the positive correlation between melanoidins content in food and antioxidant activity is well

Flavor and aroma development due to the Maillard reaction depends on the reaction temperature, time, pH, water content and on the type of sugars and amino acids involved (Yu & Zhang, 2010; Van Boekel, 2006). In most cases, the first factor mentioned influences the kinetics parameters, while the second factor determines the type of flavor compounds formed. The intermediate and final stages of the Maillard reaction are the most important to flavor development, especially the so-called Strecker degradation step, in which amino acids are degraded by dicarbonyls formed previously in the reaction,leading to the aminoacids

dietary LAL, FL and CML excreted in the urine was 5.6, 5.2 and 29%, respectively.

or the presence of pathologies as well as the amount and type of compound ingested.

**3. The role of Maillard reaction products in food acceptability** 

formed by sugar caramelization without the participation of amino groups.

deamination and decarboxylation (Ames, 1990; Rizzi, 2008).

metabolic complications (Barbosa *et al*., 2008).

**3.1 Color** 

documented in the literature.

**3.2 Flavor and aroma** 

#### **Fluorescence/crosslinked**

Fig. 2. Main chemical structures of Fluorescence MRPs/AGEs

Fig. 3. Main chemical structures of non-fluorescence MRPs/AGEs

Sato *et al*. (2006a) proposed a scheme of formation of six distinct AGEs *in vivo* (AGE-1, AGE-2, AGE-3, AGE-4, AGE-5 and AGE-6). AGE-1 is formed from glucose through Schiff base and Amadori products, AGE-2 from glyceraldehyde, AGE-3 from glycoaldehyde, AGE-4 from methylglyoxal, AGE-5 from glyoxal, and AGE-6 from 3-deoxyglucosone. AGE-2 and AGE-3 are considered toxic AGEs by contributing to the neuronal cell toxicity. They also proposed that pentosidine, Nε-(carboxymethyl)lysine, pryrraline and crossline are nontoxic AGEs, however, other studies are needed to validate these conclusions (Nguyen, 2006).

AGEs formation occurs slowly and naturally in the body of healthy people, however, this process is accelerated under certain conditions, such as hyperglycemia and oxidative stress. MRPs dietary are added to the intra and extracellular AGEs produced, contributing to the accumulation of these compounds in tissues and, thus favoring the onset and progression of metabolic complications (Barbosa *et al*., 2008).

Up to 90% of pyrraline free and Amadori products excreted by the kidneys are from food source. Free pentosidine, for example, presents about 60 to 80% bioavailability (Forster *et al*., 2005). Somoza *et al*. (2006) administered casein-linked lysinoalanine (LAL), Nεfructoselysine (FL) and Nε-(carboxymethyl)lysine (CML) to rats and revealed that the kidneys are the main organs in which AGEs are accumulated and excreted. In this study, the dietary LAL, FL and CML excreted in the urine was 5.6, 5.2 and 29%, respectively.

In other words, the rate of absorption and renal excretion of MRPs depends on dietary intake or the presence of pathologies as well as the amount and type of compound ingested.
