Kinetic modeling of reactions in heated monosaccharide-casein systems

In the present study, a kinetic model of the Maillard reaction occurring in heated monosaccharide-casein systems was proposed. Its parameters, the reaction rate constants, were estimated via multiresponse modeling. The determinant criterion was used as the statistical fit criterion instead of the familiar least squares to avoid statistical problems. The kinetic model was extensively tested by varying the reaction conditions. Different sugars (glucose, fructose, galactose, and tagatose) were studied regarding their effect on the reaction kinetics. This study has shown the power of multiresponse modeling for the unraveling of complicated reaction routes as occur in the Maillard reaction. The iterative process of proposing a model, confronting it with experiments, and criticizing the model was passed through four times to arrive at a model that was largely consistent with all results obtained. A striking difference was found between aldose and ketose sugars as suggested by the modeling results: not the ketoses themselves but only their reaction products were found to be reactive in the Maillard reaction.     

On the effect of tetraborate ions in the generation of colored products in thermally processed glycine-carbohydrate solutions

The effect of tetraborate ions on Maillard browning was investigated in a series of monosaccharide-glycine reactions in aqueous bis-tris buffer at pH 7.2. Addition of borax (sodium tetraborate) in catalytic amounts led to enhanced browning measured by absorbance at 420 nm in the order xylose > arabinose > galactose approximately = fructose > ribose > mannose > rhamnose, and the degree of browning with borax was uniformly greater than that produced by phosphate on an equimolar basis. A mechanism is proposed for borax catalysis in which monosaccharide-borate complexation shifts carbohydrate equilibria to favor open-chain (carbonyl) forms, thereby enhancing the rate of the Maillard reaction.     

The effect of high pressure-high temperature processing conditions on acrylamide formation and other Maillard reaction compounds

The effect of high pressure-high temperature (HPHT) processing on the formation of acrylamide and other Maillard-type reaction compounds was investigated in order to elucidate the impact of HPHT conditions on the different stages of the Maillard reaction. This study was performed in equimolar asparagine-glucose model systems that were treated at various HP/HT conditions (100-115 °C, 400-700 MPa, 0-60 min), and, for comparison, the model system was also heat-treated at ambient pressure. On the treated samples, the concentration of acrylamide, reactants, hydroxymethylfurfural, organic acids, and melanoidins was determined and the pH prior to and after treatment was measured. Based on the measured responses, the retarding effect of high pressure on the overall Maillard reaction was demonstrated; no or little differences were observed between 400 and 700 MPa. The study was conducted in two types of buffer, i.e. phosphate and MES buffer. In case of acrylamide, aspartic acid and browning, a higher concentration was generated in the MES buffer system, but these differences with the phosphate buffer system could be ascribed to pH changes resulting from the application of combined high pressure and high temperature. Based on the results, acrylamide formation is not expected to pose a major hazard to HPHT-treated products.     

Modification of ovalbumin with a rare ketohexose through the Maillard reaction: effect on protein structure and gel properties

The effect of nonenzymatic glycation on the structural changes and gelling properties of hen ovalbumin (OVA) through the Maillard reaction was studied. OVA was incubated at the dry state with a rare ketohexose (D-psicose, Psi) and two alimentary sugars (D-fructose, Fru; D-glucose, Glc) at 55 degrees C and 65% relative humidity. To evaluate the modification of OVA by different reducing sugars during the glycation process, the extent of the Maillard reaction, aggregation processes, structural changes, and gelling behaviors were investigated. Reactivity of Psi with the protein amino groups was much lower than that of both Fru and Glc, whereas Psi induced production of browning and fluorescent substances more strongly than the two alimentary sugars did. Furthermore, OVA showed an increased tendency toward multimeric aggregation upon modifying with Psi through covalent bond. The modified OVAs with reducing sugar were similar to nonglycated control sample in Fourier transform infrared (FT-IR) characteristics, but significantly decreased in intensity of tryptophan-related fluorescence. The results indicate that although glycation brought about similar changes in the secondary structure without great disruption of native structure, its influence on the side chains of protein in tertiary structure could be different. Breaking strength of heat-induced glycated OVA gels with Psi was markedly enhanced by the Maillard reaction. These results suggest that Psi had a strong cross-linking activity with OVA; consequently, the glycated OVA with Psi could improve gelling properties under certain controlled conditions.     

Reactions of monosaccharides during heating of sugar-casein systems: building of a reaction network model

The Maillard reaction is important during the heating and processing of foods for its contribution to food quality. To control a reaction as complex as the Maillard reaction, it is necessary to study the reactions of interest quantitatively. In this paper the main reaction products in monosaccharide-casein systems, which were heated at 120 degrees C and pH 6.7, were identified and quantified, and the reaction pathways were established. The main reaction routes were (i) sugar isomerization, (ii) degradation of the sugar into carboxylic acids, and (iii) the Maillard reaction itself, in which not only the sugar itself but also its reaction products react with the epsilon-amino group of lysine residues of the protein. Significant differences in reaction mechanism between aldose and ketose sugars were observed. Ketoses seemed to be more reactive in the sugar degradation reactions than their aldose isomers, and whereas the Amadori product was detected as a Maillard reaction intermediate in the aldose-casein system, no such intermediate could be found in the ketose-casein system. The reaction pathways found were put together into a model, which will be evaluated by kinetic modeling in a subsequent paper.     

Effects of cationic species on visual color formation in model Maillard reactions of pentose sugars and amino acids

Effects of cationic species on Maillard browning were examined after heating (ca. 100 degrees C) aqueous pH 7.2 buffered solutions of amino acids and pentose sugars. Metallic ions of Group I metals (Li, Na, K, Rb and Cs) produced a small increase in browning (A420), but somewhat greater effects were observed with ions of Group II metals Ca and Mg. Browning was suppressed by triethylammonium ion, but unaffected by a salt of the stronger base, guanidine. The quaternary amine salt choline chloride produced enhanced browning and served as a model for phospholipid involvement in Maillard reactions. With alpha,omega-diamino acids increases in browning were observed which related to lowered pK2 values resulting from positively charged omega-substituents in these molecules.     

Thermal decomposition of specifically phosphorylated D-glucoses and their role in the control of the Maillard reaction

One of the main shortcomings of the information available on the Maillard reaction is the lack of knowledge to control the different pathways, especially when it is desired to direct the reaction away from the formation of carcinogenic and other toxic substances to more aroma and color generation. The use of specifically phosphorylated sugars may impart some elements of control over the aroma profile generated by the Maillard reaction. Thermal decomposition of 1- and 6-phosphorylated glucoses was studied in the presence and absence of ammonia and selected amino acids through pyrolysis/gas chromatography/mass spectrometry using nonpolar PLOT and medium polar DB-1 columns. The analysis of the data has indicated that glucose-1-phosphate relative to glucose undergoes more extensive phosphate-catalyzed ring opening followed by formation of sugar-derived reactive intermediates as was indicated by a 9-fold increase in the amount of trimethylpyrazine and a 5-fold increase in the amount of 2,3-dimethylpyrazine, when pyrolyzed in the presence of glycine. In addition, glucose-1-phosphate alone generated a 6-fold excess of acetol as compared to glucose. On the other hand, glucose-6-phosphate enhanced retro-aldol reactions initiated from a C-6 hydroxyl group and increased the subsequent formation of furfural and 4-cyclopentene-1,3-dione. Furthermore, it also stabilized 1- and 3-deoxyglucosone intermediates and enhanced the formation of six carbon atom-containing Maillard products derived directly from them through elimination reactions such as 1,6-dimethyl-2,4-dihydroxy-3-(2H)-furanone (acetylformoin), 2-acetylpyrrole, 5-methylfurfural, 5-hydroxymethylfurfural, and 4-hydroxy-2,5-dimethyl-3-(2H)-furanone (Furaneol), due to the enhanced leaving group ability of the phosphate moiety at the C-6 carbon. However, Maillard products generated through the nucleophilic action of the C-6 hydroxyl group such as 2-acetylfuran and 2,3-dihydro-3,5-dihydroxy-4H-pyran-4-one were retarded, due to the blocked nucleophilic atom at C-6.     

Progress of the Maillard reaction and antioxidant action of Maillard reaction products in preheated model systems during storage

The progress of the Maillard reaction and the effect of Maillard reaction products (MRPs) on lipid oxidation in preheated model systems containing pregelatinized starch, glucose, lysine, and soybean oil have been studied during storage. The samples, either containing all components or excluding one or more of them, were heated at 100 degrees C for 90 min and then stored for up to 180 days at 25 degrees C. Browning indices and lipid oxidation were measured, and the results showed that, in samples containing oil, the Maillard reaction had a significant rate also at room temperature and confirmed the ability of MRPs to retard peroxide formation. Under the conditions adopted the rate of the Maillard reaction was increased by the presence of the oil and its oxidation products. The antioxidant action of the MRPs was also evaluated using a peroxide scavenging test based on crocin bleaching. The results demonstrated that antioxidant activity developed with increased browning of the samples.     

Rye Flour Extraction Rate Affects Maillard Reaction Development,Antioxidant Activity,and Acrylamide Formation in Bread Crisps

This report shows the effect of rye flour extraction rate on Maillard reaction, antioxidant activity, and acrylamide formation during toasting of rye bread crisps. Four rye flours with extraction rates of 70, 85, 95, and 100% were tested. Maillard reaction development was studied by measuring browning development, hydroxymethylfurfural (HMF), and glucosilisomaltol (GIM) formation, as well as antioxidant activity. Results showed that HMF and GIM concentrations in toasted bread crisps were higher as the flour extraction rate increases. Antioxidant activity increased during toasting as a consequence of antioxidant Maillard reaction product formation. Acrylamide concentration was clearly affected by free asparagine content of flour, while no effect of dietary fiber and natural antioxidant content of flours had an effect on acrylamide formation. Overall data suggest that the rate of Maillard reaction is higher in whole flours because of their higher free amino acid and protein content.     

Effect of maillard reaction on allergenicity of scallop tropomyosin

Scallop tropomyosin (TM), the major allergen of shellfish, was prepared from adductor muscles and reacted with four reducing sugars to investigate the effect of the Maillard reaction on the allergenicity of TM. The IgE-binding ability of TM increased significantly with the progress of the reaction with glucose, ribose, and maltose, but not with maltotriose. The allergenicity was enhanced at the early stage of the Maillard reaction, and the trend of the effect depended on the type of reducing sugar used. 2,4,6-Trinitrobenzenesulfonic acid treatment of the lysine residues in TM showed that the protein surface charge resulting from the Maillard reaction had no effect on the enhancement of the allergenicity. Thus, the change in the allergenicity would be closely related to the structural change caused by the Maillard reaction.     

Chemical and physical changes in milk protein concentrate (MPC80) powder during storage

The solubility and chemical changes due to the Maillard reaction were investigated in milk protein concentrate powder containing 80% protein (MPC80) during storage at temperatures and relative humidities in the ranges of 25-40 °C and 44-84%, respectively. The Maillard reaction was studied by measuring furosine (a product of lactosylated protein after digestion with acid) and free hydroxymethylfurfural (HMF) contents by HPLC and L*, a*, b* values with a color-meter. Furosine, free HMF, and browning in MPC80 increased during storage, whereas the solubility decreased. The correlation between the Maillard reaction and solubility loss was explored in modified MPC80 to which glucose was added to enhance the rate of the Maillard reaction. More furosine and brown pigments were observed in the glucose-containing MPC80 than in MPC80 with added lactose. The opposite trend occurred for solubility, suggesting that the Maillard reaction may be a cause of solubility loss in MPC powder.     

Changes in free amino acids and sugars in potatoes due to sulfate fertilization and the effect on acrylamide formation

To examine how sulfur deprivation may affect acrylamide formation in cooked potatoes, three varieties of potato were grown under conditions of either severe sulfur deprivation or an adequate supply of sulfur. In all three varieties sulfur deprivation led to a decrease in acrylamide formation, even though the levels of sugars, which are acrylamide precursors, were higher in tubers of the sulfur-deprived plants. In one variety the concentration of free asparagine, the other precursor for acrylamide, was also higher. There was a very close correlation between the concentration of asparagine in the tubers expressed as a proportion of the total free amino acid pool and the formation of acrylamide upon cooking, whereas sugars were poorly correlated with acrylamide. In potatoes, where concentrations of sugars are usually limiting, competition between asparagine and other amino acids participating in the Maillard reaction may be a key determinant of the amount of acrylamide that is formed during processing.     

Allergenicity of Maillard reaction products from peanut proteins

It is known that peanut allergy is caused by peanut proteins. However, little is known about the impact of roasting on the allergenicity of peanuts. During roasting, proteins react with sugars to form Maillard reaction products, which could affect allergenicity. To determine if the Maillard reaction could convert a nonallergenic peanut protein into a potentially allergenic product, nonallergenic lectin was reacted with glucose or fructose at 50 degrees C for 28 days. Browning products from heat-treated peanuts were also examined. The products were analyzed in immunoblot and competitive assays, using a pooled serum (i.e., IgE antibodies) from patients with peanut anaphylaxis. Results showed that the products were recognized by IgE and had an inhibitory effect on IgE binding to a peanut allergen. Thus, the findings suggest that these Maillard reaction products are potentially allergenic and indicate the need to verify whether the Maillard reaction products formed in peanuts during roasting increase their allergenicity.     

Impact of harvest year on amino acids and sugars in potatoes and effect on acrylamide formation during frying

Acrylamide is formed via the Maillard reaction between reducing sugars and asparagine in a number of carbohydrate-rich foods during heat treatment. High acrylamide levels have been found in potato products processed at high temperatures. To examine the impact of harvest year, information on weather conditions during growth, that is, temperature, precipitation, and light, was collected, together with analytical data on the concentrations of free amino acids and sugars in five potato clones and acrylamide contents in potato chips (commonly known as crisps in Europe). The study was conducted for 3 years (2004-2006). The contents of acrylamide precursors differed between the clones and the three harvest years; the levels of glucose were up to 4.2 times higher in 2006 than in 2004 and 2005, and the levels of fructose were 5.6 times higher, whereas the levels of asparagine varied to different extents. The high levels of sugars in 2006 were probably due to the extreme weather conditions during the growing season, and this was also reflected in acrylamide content that was approximately twice as high as in preceding years. The results indicate that acrylamide formation is dependent not only on the content and relative amounts of sugars and amino acids but also on other factors, for example, the food matrix, which may influence the availability of the reactants to participate in the Maillard reaction.     

Qualitative determination of specific protein glycation products by matrix-assisted laser desorption/ionization mass spectrometry Peptide mapping

The nonenzymatic reaction between reducing sugars and proteins, known as the Maillard reaction, has received increased recognition from nutritional science and medical research. The development of new analytical techniques for the detection of protein-bound Maillard products is therefore crucial. In this study, we applied peptide mapping by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to investigate the formation of structurally specific Maillard products on glycated lysozyme (AGE-lysozyme), produced upon incubation with D-glucose. In parallel, we synthesized N(epsilon)-(carboxymethyl)lysine-modified lysozyme (CML-lysozyme) and N(epsilon)-(carboxyethyl)lysine-modified lysozyme, two well-described glycation products, as model substances. 3-Deoxyglucosone-modified lysozyme and methylglyoxal-modified lysozyme were prepared as examples of glycation products incubated with dicarbonyl compounds. We were able to detect specific modifications on AGE-lysozyme, which were assigned to CML, imidazolone A, and the Amadori product.     

Influence of the maillard reaction on the allergenicity of rAra h 2, a recombinant major allergen from peanut (Arachis hypogaea), its major epitopes, and peanut agglutinin

The influence of thermal processing and nonenzymatic browning reactions on the IgE-binding activity of rAra h 2 was studied and compared to findings recently reported for the allergen's natural counterpart. ELISA experiments as well as inhibition assays revealed that thermal treatment of rAra h 2 in the presence of reactive carbohydrates and carbohydrate breakdown products induces a strong increase of the IgE-binding activity, thus collaborating with the data reported for the natural protein isolated from peanuts. To localize the Ara h 2 sequences responsible for the formation of highly IgE-affine glycation sites, model peptides have been synthesized mimicking sequences which contain possible targets for glycation as well as the immunodominant epitopes. Immunological evaluation of these peptides heated in the absence or presence of reducing sugars and carbonyls, respectively, revealed that neither the two lysine residues of Ara h 2 nor its N-terminus are involved in the formation of IgE-affine structures by Maillard reaction. Also, the cysteine-containing major epitope 3 (aa 27-36) was found to lose its IgE-binding capacity upon heating. By contrast, the overlapping major epitopes 6 and 7, which do not contain any lysine or arginine moieties, showed a distinct higher level of IgE binding when subjected to Maillard reaction, thus giving the first evidence that nonbasic amino acids might be accessible for nonenzymatic glycation reactions and that these posttranslational modifications might induce increased IgE binding of the glycated Ara h 2. Analogous experiments were performed with peanut agglutinin, considered in the literature as a minor allergen. ELISA experiments revealed that the majority of tested sera samples from peanut-sensitive patients showed a high level of IgE binding to the lectin even after heat treatment. In contradiction to published data, nonenzymatic browning reactions seem to deteriorate the IgE affinity of the lectin.     

Influence of processing parameters on acrylamide formation during frying of potatoes

Consistent evidence suggests that the probable human carcinogen acrylamide is formed in starch-rich foodstuffs through heat-induced interaction of asparagine and reducing sugars during Maillard browning. However, information regarding the influence of processing parameters on acrylamide formation is scarce. We investigated the impact of temperature, heating time, browning level, and surface-to-volume ratio (SVR) on acrylamide generation in fried potatoes. Acrylamide content was determined by liquid chromatography (LC) and electrospray ionization tandem mass spectrometry (ESI-MS/MS). In potato shapes with low SVR, acrylamide content consistently increased with increasing temperature and processing times. By contrast, in shapes with intermediate to high SVR, maximal acrylamide formation occurred at 160-180 degrees C, while higher temperatures or prolonged processing times caused a decrease of acrylamide levels. Moreover, browning levels were not a reliable measure of acrylamide content in large-surface products.     

Color formation in dehydrated modified whey powder systems as affected by compression and T(g)

Whey powders have attracted attention for use in the food industry. The Maillard reaction is a major deteriorative factor in the storage of these and other dairy food products. The objective of the present work was to further study the Maillard reaction as related to the physical structure of the matrix, either porous or mechanically compressed, or to storage above the T(g) of anhydrous whey systems. Sweet whey (W), reduced minerals whey (WRM), whey protein isolate (WPI), and whey protein concentrate (WPC) were stored in ovens at selected temperatures. Colorimetric measurements were performed with a spectrocolorimeter, thermal analyses (TGA) by means of a thermobalance, and glass transition temperature studies by DSC. The browning order in the vials and in the compressed systems followed the order W > WRM> WPC > WPI. k(w2), the slope of the second linear segment of the TGA curve, was related to the loss of water due to nonenzymatic browning (NEB). Browning development was in good relationship with this loss of weight. In the glassy state, the compressed systems developed higher rates of browning and weight loss (assigned to NEB reactions) than the porous systems. Reaction rates in both (porous and compressed) systems became similar as (T - T(g)) increased.     

Formation of sulfur aroma compounds in reaction mixtures containing cysteine and three different forms of ribose

The headspace volatiles produced from buffered and unbuffered cysteine model systems, containing inosine 5'-monophosphate, ribose 5-phosphate, or ribose, were examined by GC-MS. Sulfur compounds dominated the volatiles of all systems and included mercaptoketones, furanthiols, and disulfides. The inosine monophosphate systems produced much lower quantities of volatiles than ribose phosphate or ribose systems. In the systems buffered with phosphate or phthalate buffers, both ribose and ribose phosphate systems gave similar quantities of sulfur volatiles. However, in the absence of buffer, the ribose system was relatively unreactive, especially for volatiles formed via the 2,3-enolization route in the Maillard reaction, where 4-hydroxy-5-methyl-3(2H)-furanone is a key intermediate. A number of keto-enol tautomerisms, which are known to be acid-base-catalyzed, occur in the 2,3-enolization route. This may explain the catalysis of the ribose systems by the buffers. In the ribose phosphate systems, however, Maillard mechanisms probably played a less important role, because ribose 5-phosphate readily dephosphorylated to give 4-hydroxy-5-methyl-3(2H)-furanone on heating and thus provided an easier route to aroma compounds than the Maillard reaction.     

Nonenzymatic browning of lactose and caseinate during dry heating at different relative humidities

Dry mixtures of lactose and caseinate were heated at 60 degrees C for up to 96 h at different relative humidities (RHs) ranging from 29 to 95%. The resulting nonenzymatic browning was studied by determining lactulosyl lysine formation in the caseinate (as measured by the conversion to furosine), amount of reacted lactose, loss of lysine, color formation, and fluorescent intensity. For each measurement, the maximum reaction occurred at intermediate RHs. While there is general agreement between the results obtained by different methods, discrepancies are understandable given the complex nature of nonenzymatic browning. It was shown that the degradation of the Amadori product, lactulosyl lysine, increased with RH. Moreover, the Maillard reaction, as opposed to caramelization of lactose, was the major pathway at all RHs. Visible browning occurred when the destruction of Amadori product became dominant, and interactions between sugar fragments and caseinate were not the rate-limiting steps in the nonenzymatic browning.