Actividad A3.2: Elaboración del plan seguridad de la información

Nonenzymatic browning is the most complex reac- tion in food chemistry due to the large number of food components able to participate in the reaction

through different pathways, giving rise to a complex mixture of products (Olano and Martínez-Castro 1996). It is referred to as the Maillard reaction when it takes place between free amino groups from amino acids, peptides, or proteins and the carbonyl group of a reducing sugar.

The Maillard reaction is one of the main reactions causing deterioration of proteins during processing and storage of foods. This reaction can promote nutritional changes such as loss of nutritional qual- ity (attributed to the destruction of essential amino acids) or reduction of protein digestibility and amino acid availability (Malec et al. 2002).

The Maillard reaction covers a whole range of complex transformations (Fig. 4.4) that lead to the formation of numerous volatile and nonvolatile compounds. It can be divided into three major phas- es, the early, intermediate, and advanced stages. The early stage (Fig. 4.5) consists of the condensation of primary amino groups of amino acids, peptides, or

proteins with the carbonyl group of reducing sugars (aldose), with loss of a molecule of water, leading, via formation of a Schiff’s base and Amadori rear- rangement, to the scalled Amadori product (1- amin1-deoxy-2-ketose), a relatively stable interme- diate (Feather et al. 1995). The Heyns compound is the analogous compound when a ketose is the start- ing sugar. In many foods, the
-amino group of the lysine residues of proteins is the most important source of reactive amino groups, but due to block- age these lysine residues are not available for diges- tion, and consequently the nutritive value decreases (Brands and van Boekel 2001, Machiels and Istasse 2002). Amadori compounds are precursors of num- erous compounds important in the formation of characteristic flavors, aromas, and brown polymers. They are formed before the occurrence of sensory changes; therefore, their determination provides a very sensitive indicator for early detection of quality changes caused by the Maillard reaction (Olano and Martínez-Castro 1996).

The intermediate stage leads to breakdown of Amadori compounds (or other products related to the Schiff’s base) and the formation of degradation prod- ucts, reactive intermediates (3-deoxyglucosone), and volatile compounds (formation of flavor). The 3- deoxyglucosone participates in cross-linking of pro- teins at much faster rates than glucose itself, and fur- ther degradation leads to two known advanced products: 5-hydroxymethyl-2-furaldehyde and pyr- raline (Feather et al. 1995).

The final stage is characterized by the production of nitrogen-containing brown polymers and copoly- mers known as melanoidins (Badoud et al. 1995). The structure of melanoidins is largely unknown, and to date, there are several proposals about it. Me- lanoidins have been described as low molecular weight (LMW) colored substances that are able to cross-link proteins via
-amino groups of lysine or arginine to produce high molecular weight (HMW) colored melanoidins. Also, it has been postulated that they are polymers consisting of repeating units of furans and/or pyrroles formed during the ad- vanced stages of The Maillard reaction and linked by polycondensation reactions (Martins and van Boekel 2003).

In foods, predominantly glucose, fructose, malt- ose, lactose, and to some extent reducing pentoses are involved with amino acids and proteins in form- ing fructoselysine, lactuloselysine, or maltulosely-

sine. In general, primary amines are more important than the secondary ones because the concentration of primary amino acids in foods is usually higher than that of secondary amino acids (an exception is the high amount of proline in malt and corn prod- ucts) (Ledl 1990).

Factors Affecting the Maillard Reaction

The rate of the Maillard reaction and the nature of the products formed depend on the chemical envi- ronment of food including the water activity (aw),

pH, and chemical composition of the food system, temperature being the most important factor (Carabasa-Giribert and Ibarz-Ribas 2000). In order to predict the extent of chemical reactions in pro- cessed foods, a knowledge of kinetic reactions is necessary to optimize the processing conditions. Since foods are complex matrices, these kinetic studies are often carried out using model systems in which sugars and amino acids react under simplified conditions. Model system studies may provide guid- ance regarding the directions in which to modify the food process and to find out which reactants may produce specific effects of the Maillard reaction (Lingnert 1990).

The reaction rate is significantly affected by the pH of the system; it generally increases with pH (Namiki et al. 1993, Ajandouz and Puigserver 1999).

Figure 4.6. Effect of phosphate buffer concentration on the loss of glycine in 0.1M glucose/glycine solutions at pH 7 and 25°C (Bell 1997).

also has been studied more recently (Carabasa- Giribet and Ibarz-Ribas 2000, Mundt and Wedzicha 2003).

Studies on the effect of time and temperature of treatment on Maillard reaction development have been also conducted in different model systems, and it has been shown that an increase in temperature increases the rate of Maillard browning (Ryu et al. 2003, Martins and van Boekel 2003).

Concentration and ratio of reducing sugar to amino acid have a significant impact on the reaction. Browning reaction increased with increasing gly- cine:glucose ratios in the range 0.1:1 to 5:1 in a model orange juice system at 65°C (Wolfrom et al. 1974). In a model system of intermediate moisture (aw, 0.52), Warmbier et al. (1976) observed an in-

crease of browning reaction rate when the molar ratio of glucose to lysine increased from 0.5:1 to 3.0:1.

Water activity (aw) is another important factor

influencing Maillard reaction development; thus, this reaction occurs less readily in foods with high aw values. At high awvalues, reactants are diluted,

while at low awvalues the mobility of reactants is

Figure 4.7. Brown color development in aqueous solutions containing glucose alone or in the presence of an essen- tial amino acid when heated to 100°C at pH 7.5 as a function of time (Ajandouz and Puigserver 1999).

Bell (1997) studied the effect of buffer type and con- centration on initial degradation of amino acids and formation of brown pigments in model systems of glycine and glucose stored for long periods at 25°C. The loss of glycine was faster at high phosphate buffer concentrations (Fig. 4.6), showing the catalyt- ic effect of the phosphate buffer concentration on the Maillard reaction.

The type of reducing sugar has a great influence on Maillard reaction development. Pentoses (e.g., ribose) react more readily than hexoses (e.g., glu- cose), which, in turn, are more reactive than dis- accharides (e.g., lactose) (Ames 1990). A study on brown development (absorbance 420 nm) in a heat- ed model of fructose and lysine showed that brown- ing was higher than in model systems with glucose (Ajandouz et al. 2001).

Participation of amino acids in the Maillard reac- tion is variable; lysine was the most reactive amino acid (Fig. 4.7) in the heated model system of glu- cose and lysine, threonine, and methionine in buffer phosphate at different pH values (4–12) (Ajandouz and Puigserver 1999). The influence of type of amino acid and sugar in the Maillard reaction development

limited, despite their presence at increased concen- trations (Ames 1990). Numerous studies have de- monstrated a browning rate maximum at awvalues

from 0.5 to 0.8 in dried and intermediate-moisture foods (Warmbier et al. 1976, Tsai et al. 1991, Buera and Karel 1995).

Due to the complex composition of foods, it is unlikely that the Maillard reaction involves only sin- gle compounds (mono- or disaccharides and amino acids). For this reason, several studies on factors (pH, T, aw) that influence the Maillard reaction

development have been carried out using more com- plex model systems: heated starch-glucose-lysine systems (Bates et al. 1998), milk-resembling model systems (lactose or glucose-caseinate systems) (Morales and van Boekel 1998), and a lactose- casein model system (Malec et al. 2002). Brands and van Boekel (2001) studied the Maillard reaction using heated monosaccharide (glucose, galactose, fructose, and tagatose)-casein model systems to quantify and identify the main reaction products and to establish the reaction pathways.

Studies on mechanisms of degradation, via the Maillard reaction, of oligosaccharides in a model system with glycine were performed by Hollnagel and Kroh (2000, 2002). The reactivity of di- and tri- saccharides under quasi water-free conditions de- creased in comparison with that of glucose due to the increasing degree of polymerization.

Study of the Maillard Reaction in Foods

During food processing, the Maillard reaction pro- duces desirable and undesirable effects. Processes such as baking, frying, and roasting are based on the Maillard reaction for flavor, aroma, and color forma- tion (Lignert 1990). Maillard browning may be de- sirable during manufacture of meat, coffee, tea, chocolate, nuts, potato chips, crackers, and beer and in toasting and baking bread (Weenen 1998, Bur- dulu and Karadeniz 2003). In other processes such as pasteurization, sterilization, drying, and storage, the Maillard reaction often causes detrimental nutri- tional (lysine damage) and organoleptic changes (Lingnert 1990). Available lysine determination has been used to assess Maillard reaction extension in different types of foods: breads, breakfast cereals, pasta, infant formula (dried and sterilized), and so on (Erbersdobler and Hupe 1991); dried milks (El

and Kavas 1997); heated milks (Ferrer et al. 2003); and infant cereals (Ramírez-Jimenez et al. 2004).

Sensory changes in foods due to the Maillard reaction have been studied in a wide range of foods including honey (Gonzales et al. 1999), apple juice concentrate (Burdulu and Karadeniz 2003), and white chocolate (Vercet 2003).

Other types of undesirable effects produced in processed foods by Maillard reaction may include the formation of mutagenic and cancerogenic com- pounds (Lingnert 1990, Chevalier et al. 2001). Frying or grilling of meat and fish may generate low (ppb) levels of mutagenic/carcinogenic heterocyclic amines via Maillard reaction. The formation of these compounds depends on cooking temperature and time, cooking technique and equipment, heat, mass transport, and/or chemical parameters. Recently, Tareke et al. (2002) reported their findings on the carcinogen acrylamide in a range of cooked foods. Moderate levels of acrylamide (5–50 g/kg) were measured in heated protein-rich foods, and higher levels (150–4000 _{g/kg) were measured in carbohy-} drate-rich foods such a potato, beet root, certain heated commercial potato products, and crisp bread. Ahn et al. (2002) tested different types of commer- cial foods and some foods heated under home cook- ing conditions, and they observed that acrylamide was absent in raw or boiled foods, but it was present at significant levels in fried, grilled, baked, and toasted foods. Although the mechanism of acry- lamide formation in heated foods is not yet clear, several authors have put forth the hypothesis that the reaction of asparagine (Fig. 4.5), a major amino acid of potatoes and cereals (Mottram et al. 2002, Weis- shaar and Gutsche 2002), or methionine (Stadler et al. 2002) with reducing sugars (glucose, fructose) via Maillard reaction could be the pathway. In 2003, a lot of research was conducted to study the mecha- nism of acrylamide formation, to develop sensible analytical methods, and to quantify acrylamide in different types of foods (Becalski et al. 2003, Jung et al. 2003, Roach et al. 2003, Yasuhara et al. 2003).

Beneficial properties of Maillard products have been also described. Resultant products of the reac- tion of different amino acid and sugar model sys- tems presented different properties: antimutagenic (Yen and Tsai 1993), antimicrobial (Chevalier et al. 2001), and antioxidative (Manzocco et al. 2001, Wagner et al. 2002). In foods, antioxidant effects of

Maillard reaction products have been found in hon- ey (Antoni et al. 2000) and in tomato purees (Anese et al. 2002).

Control of the Maillard Reaction in Foods

For a food technologist, one of the most important objectives must be to limit nutritional damage of food during processing. In this sense, many studies have been performed find useful heat-induced mark- ers derived from the Maillard reaction, and most of them have been proposed to control and check the heat treatments and/or storage of foods. There are many indicators of different stages of the Maillard reaction, but this review cites one of the most recent indicators proposed to control the early stages of the Maillard reaction during food processing: the 2- furoylmethyl amino acids as an indirect measure of Amadori compound formation.

Determination of the level of Amadori com- pounds provides a very sensitive indicator for early detection (before detrimental changes occur) of quality changes caused by the Maillard reaction as well as a retrospective assessment of the heat treat- ment or storage conditions to which a product has been subjected (Olano and Martínez-Castro 1996, del Castillo et al. 1999).

Evaluating for Amadori compounds can be car- ried out through furosine [
_{-N-(2-furoylmethyl)-} L-lysine] measurement. This amino acid is formed by acid hydrolysis of the Amadori compound
-N- (1-deoxy-D-fructosyl)-L-lysine. It is considered a useful indicator of the damage in processed foods or foods stored for long periods: milks (Resmini et al. 1990, Villamiel et al. 1999), eggs (Hidalgo et al. 1995), cheese (Villamiel et al. 2000), honey (Vil- lamiel et al. 2001), infant formulas (Guerra- Hernandez et al. 2002), jams and fruit-based infant foods (Rada-Mendoza et al. 2002), fresh filled pasta (Zardetto et al. 2003), prebaked breads (Ruiz et al., 2004), and cookies, crackers, and breakfast cereals (Rada-Mendoza et al. 2004).

In the case of foods containing free amino acids, free Amadori compounds can be present, and acid hydrolysis gives rise to the formation of the corre- sponding 2-furoylmethyl derivatives. For the first time, 2-furoylmethyl derivatives of different amino acids (arginine, asparagine, proline, alanine, glutam- ic acid, and -amino butyric acid) have been detect-

ed and have been used as indicators of the early stages of Maillard reaction in stored dehydrated orange juices (del Castillo et al. 1999). These com- pounds were proposed as indicators to evaluate quality changes either during processing or during subsequent storage. Later, most of these compounds were also detected in different foods: commercial orange juices (del Castillo et al. 2000), processed tomato products (Sanz et al. 2000), dehydrated fruits (Sanz et al. 2001), and commercial honey samples (Sanz et al. 2003).

CARAMELIZATION

During nonenzymatic browning of foods, various degradation products are formed via caramelization of carbohydrates, without amine participation (Aj- andouz and Puigserver 1999, Ajandouz et al. 2001). Caramelization occurs when surfaces are heated strongly (e.g., during baking and roasting), during the processing of foods with high sugar content (e.g., jams and certain fruit juices) or in wine pro- duction (Kroh 1994). Caramelization is desirable to obtain caramel-like flavor and/or development of brown color in certain types of foods. Caramel fla- voring and coloring, produced from sugar with dif- ferent catalysts, is one of the most widely used addi- tives in the food industry. However, caramelization is not always a desirable reaction due to the possible formation of mutagenic compounds (Tomasik et al. 1989) and the excessive changes in sensory attrib- utes that could affect the quality of certain foods.

Caramelization is catalyzed under acidic or alka- line conditions (Namiki 1988), and many of the products formed are similar to those resulting from the Maillard reaction.

Caramelization of reducing carbohydrates starts with the opening of the hemiacetal ring followed by enolization, which proceeds via acid- and base-cat- alyzed mechanisms, leading to the formation of iso- meric carbohydrates (Fig. 4.8). The interconversion of sugars through their enediols increases with increasing pH and is called the Lobry de Bruyn- Alberda van Ekenstein transformation (Kroh 1994). In acid media, low amounts of isomeric carbohy- drates are formed; however, dehydration is favored, leading to furaldehyde compounds: 5-(hydroxyme- thyl)-2-furaldehyde (HMF) from hexoses (Fig. 4.9) and 2-furaldehyde from pentoses. With unbuffered

acids as catalysts, higher yields of HMF are pro- duced from fructose than from glucose. Also, only the fructose moiety of sucrose is largely converted to HMF under the unbuffered conditions that produce the highest yields. The enolization of glucose can be greatly increased in buffered acidic solutions. Thus, higher yields of HMF are produced from glucose and sucrose when a combination of phosphoric acid and pyridine is used as the catalyst than when phos- phoric acid is used alone (Fenemma 1976).

In alkaline media, dehydration reactions are slow- er than in neutral or acid media, but fragmentation products such as acetol, acetoin, and diacetyl are detected. In the presence of oxygen, oxidative fis- sion takes place, and formic, acetic, and other organ- ic acids are formed.

All of these compounds react to produce brown polymers and flavor compounds (Olano and Mar- tínez-Castro 1996).

In general, caramelization products consist of volatile and nonvolatile (90–95%) fractions of low and high molecular weights that vary depending on temperature, pH, duration of heating, and starting material (Defaye et al. 2000). Although it is known that caramelization is favored at temperatures higher

than 120°C and at a pH greater than 9 and less than 3, depending on the composition of the system (pH and type of sugar), caramelization reactions may also play an important role in color formation in sys- tems heated at lower temperatures. Thus, some stud- ies have been conducted at the temperatures of accelerated storage conditions (45–65°C) and pH values from 4 to 6 (Buera et al. 1987a,b). These authors studied the changes of color due to car- amelization of fructose, xylose, glucose, maltose, lactose, and sucrose in model systems of 0.9 aw and

found that fructose and xylose browned much more rapidly than the other sugars and that lowering the pH inhibited caramelization browning of sugar solu- tions.

In a study on the kinetics of caramelization of several monosaccharides and disaccharides, Diaz and Clotet (1995) found that at temperatures of 75– 95°C, browning increased rapidly with time and to a higher final value with increasing temperature, this effect being more marked in the monosaccharides than in the disaccharides. In all sugars studied, increase of browning was greater at aw  1 than at

aw  0.75.

The effect of sugars, temperature, and pH on caramelization was evaluated by Park et al. (1998). Reaction rate was highest with fructose, followed by sucrose. As reaction temperature increased from 80 to 110°C, reaction rate was greatly increased. With respect to pH, the optimum value for caramelization was 10.

Although most studies on caramelization have been conducted in model systems of mono- and di- saccharides, a number of real food systems contain oligosaccharides or even polymeric saccharides; therefore, it is also of great interest to know the con- tribution of these carbohydrates to the flavor and color of foods. Kroh et al. (1996) reported the break-

Figure 4.8. The Lobry de Bruyn-Alberda van Ekenstein transformation.

down of oligo- and polysaccharides to nonvolatile reaction products. Homoki-Farkas et al. (1997) stud- ied, through an intermediate compound (methylgly- oxal), the caramelization of glucose, dextrin 15, and starch in aqueous solutions at 170°C under different periods of time. The highest formation of methyl- glyoxal was in glucose and the lowest in starch sys- tems. The authors attributed the differences to the number of reducing end groups. In the case of glu- cose, when all molecules were degraded, the con- centration of methylglyoxal reached a maximum and began to transform, yielding low and high molecular weight color compounds. Hollhagel and Kroh (2000, 2002) investigated the degradation of maltoligosaccharides at 100°C through-dicarbonyl compounds such as 1,4-dideoxyhexosulose, and they found that this compound is a reactive interme- diate and precursor of various heterocyclic volatile compounds that contribute to caramel flavor and color.

Perhaps, as mentioned above, the most striking feature of caramelization is its contribution to the color and flavor of certain food products under con- trolled conditions. In addition, it is necessary to con- sider other positive characteristics of this reaction such as the antioxidant activity of the caramelization products. Kirigaya et al. (1968) suggested that high molecular weight and colored pigments might play an important role in the antioxidant activity of caramelization products. However, Rhee and Kim (1975) reported that caramelization products from glucose have antioxidant activity that consists main- ly of colorless intermediates, such as reductones and