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

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).

#### **3.1 Color**

284 Food Industrial Processes – Methods and Equipment

N+ S

8-hydroxy-5-methyldihydrothiazolo (3,2 alpha) pyridinium-3-carboxylate (MRX)

OH

CH3

C CH2 NH Lys

N-(carboxymethyl)lysine (CML)

NH2

O

N

OH NH N CH3

Argpyrimidine

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

CH3

OH

H O O

COOH

N

Glyoxal-lysine dimmer (GOLD)

Lys

N+

Lys

**Fluorescence/crosslinked**

N

N H NH Arg Lys

Pentosine

Pentosidine

OH

N+ Lys

OH OH OH

Crossline

**Non-fluorescence/Non-crosslinked**

O

<sup>2</sup>CH CH CH CH2OH OH OH

<sup>N</sup> HOH2C CHO Lys Pyrraline

N(H)

NH Arg

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

N

3DG-imidazolones

N+

N

Lys

OH

OH

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

N+

OH

N

Methylglyoxal-lysine dimmer (MOLD)

3CH

Lys

N+

Lys

OH OH

> C CH NH Lys

H O O

N-(carboxyethyl)lysine (CEL)

3CH

Lys

Vesperlysine A

N N

NH

Glucosepane

N

MG-Imidazolones

CH2OH OH

GA-pyridine

N+

HOOC NH2

O

3CH

NH N(H)

Arg

<sup>N</sup> NH2

OH

O

N

OH 2NH O

Lys

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 formed by sugar caramelization without the participation of amino groups.

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 documented in the literature.

#### **3.2 Flavor and aroma**

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

Although Maillard reaction effects on food color, flavor and aroma are well understood and used by the food industry, its effects on food texture has attracted less attention from the

281

MRPs are present in the diet and many authors have highlighted their health benefits and risks. For that reason, it is of most importance characterizing and quantifying MRPs in common foods, to get the best balance between benefits and potential risks, and then to

Development of sophisticated analytical techniques made it possible to isolate, characterize, and quantify several specific non browning reaction compounds formed *in vitro* and *in vivo,*  both at the early and advanced stages of Maillard reaction. Among them the most common are: Amadori compounds (indirectly analyzed as furosine), Nε-(carboxymethyl)lysine (CML) and some intermediate derivatives of the reaction, such as hydroxymethylfurfural. Measurement of fluorescent compounds formed during the reaction is also a reliable tool to evaluate the extension and ratio of nutritional loss due to thermal processing of foods

Traditionally, Maillard reaction monitoring in food processing was based on the spectrophotometric evaluation of color development at 420 nm. More recently, the evaluation of fluorescent compounds generated by the Amadori rearrangement product undergoing dehydration and fission has become usual. Besides its use in food systems, fluorescence measurement is also employed to evaluate Maillard reaction at physiological conditions, meaning, AGEs generation, and also to access AGEs correlated pathologies

Fluorescent compounds (FC) are precursors for the brown pigments formed in the Maillard reaction, which present different chemical structures. Evaluation changes in fluorescence intensity helps evaluating the extent of the Maillard reaction in food products (Morales & Van Boekel, 1998; Rufiàn-Henares & Delgado-Andrade, 2009) and biological systems. Fluorescence was first used to evaluate the formation of MRPs in milk and now it is used to monitor the processing of cereals, cookies, soybeans, infant formula, cooked salmon and

Fluorescent compounds may be free in the matrix or linked to te protein fraction. Total FC (free + linked to protein) determination demands a previous enzymatic hydrolysis, which requires the use of a nonspecific protease (pronase) (Delgado-Andrade *et al*., 2008). Free and total FC have been tested in foods such as milk, breakfast cereals, cooked salmon, roasted

Fluorescence of Advanced Maillard products and Soluble Tryptophan (FAST) is a well established method used to evaluate the nutritional and lysine damages. FAST is based on the quantification of protein denaturation using fluorescence: (1) fluorescent of the advanced Maillard products (FAMP), such as pyrrole and imidazole derivatives, at excitation/emission 330/420 nm; and (2) tryptophan fluorescence (FTrp) at excitation/emission 290/340 nm at pH 4.6. The FAST index is calculated as follows:

soy and enteral formula (Delgado-Andrade *et al.,* 2008; Rufiàn-Henares *et al.,* 2002).

scientific community. However this is a promising tool for texture development.

establish guidelines for food health (Delgado-Andrade *et al.,* 2009).

**4. Analyses of Maillard reaction products** 

(Delgado-Andrade *et al.*, 2009; Friedman, 1996).

development (Delgado-Andrade *et al.,* 2006).

(100\*FAMP/FTrp) (Birlouez-Aragon *et al.,* 2002).

**4.1 Fluorescent compounds** 

bakery products.

The volatile products of the Maillard reaction can be classified according to the sugar dehydration/fragmentation products as furans, pyrones, cyclopentenes, carbonyls and acids; the amino acid degradation products as aldehydes and sulfur compounds; and the volatiles produced by further interactions as pyrroles, pyridines, imidazoles, pyrazines, oxazoles, thiazoles, and others. Pyrazines and alkylpyrazines are associated with the flavor and aroma of cooked (roasted) and nutty, respectively. Alkylpyridines confer to foods flavor and aroma of green, bitter, astringent and burnt, and furans, furanones and pyranones of sweet, burnt, pungent and caramel-like flavors/aromas.

Compounds that are essential to the characteristic flavor and aroma of food products are generally present at trace levels. The oxygen-containing aroma compounds 2,3-butanedione, 2,3-pentanedione, methylpropanal, 3-methylbutanal, phenylacetaldehyde, 3-hydroxy-4,5 dimethyl-2(3*H*) furanone and 2,5-dimethyl- 4-hydroxy-3(2*H*)-furanone occur in concentration ranging from 1μg/kg up to 100 mg/kg. The nitrogen-containing aroma compounds 2-ethyl-3,5dimethylpyrazine, 2,3-diethyl-5-methylpyrazine and 2-acetyl-1 pyrroline are present in food in an order of magnitude of 0.001–10 mg/kg. On the whole, sulfur containing Maillard odorants constitute the most powerful aroma compounds and often play, although at trace levels, a dominant role in the flavor of cooked meats. These volatile compounds are responsible for the flavor and aroma to stewed beef juice, boiled trout, french fries, bread crust, cooked chicken, roasted chicken, boiled beef, cocoa powder, peanuts, pilsner, roasted beef, popcorn and coffee (Cerny, 2008).

The meat-related flavor compounds are mainly sulphur containing compounds, derived from cysteine and ribose (coming from nucleotides), while the amino acid proline gives rise to typical bread, rice and popcorn flavors. Cysteine-containing mixtures seem to have the most intense meat-like and sulphur smell. The other amino acid-containing sulphur, methionine, generates a highly intense smell of potatoes and it is employed, in the food industry, to enhance the soft flavor of potatoes. Mixtures containing amino acids other than cysteine or methionine in a combination with reducing sugars are characterized mostly by caramel and jammy smell (Stanimirova *et al*., 2011; Van Boekel, 2006).

The food industry invests great effort trying to create synthetic flavors and aromas by reconstituting combinations of these compounds. The process of creating synthetic flavors is limited since the subtleties of flavor perception are many and varied, and although Electronic Noses may detect these compounds, human sensory perception is considered essential to validate instrumental data, (Gerrard, 2002a; Schaller et al., 1998).

#### **3.3 Texture**

Texture definition is complex and a general agreement has been reached which evolved from the efforts of a number of researchers. According to Szczesniak (2002), ''texture is the sensory and functional manifestation of the structural, mechanical and surface properties of foods detected through the senses of vision, hearing, touch and kinesthetics''.

Maillard reaction influences the texture of food via protein cross-linking. Manipulation of the extent and nature of such protein cross-linking during food processing offers a means by which the food industry can modify the functional properties of food. Despite of this, the extent of how protein cross-linking affects food texture in processed foods and how to control this parameter to maximize food quality is not yet known (Gerrard, 2002b). Protein cross-linking by the Maillard reaction will affect not only texture, but the protein digestibility as well.

Although Maillard reaction effects on food color, flavor and aroma are well understood and used by the food industry, its effects on food texture has attracted less attention from the scientific community. However this is a promising tool for texture development.
