Rosario Zamora

Coordinate Contribution of Lipid Oxidation and Maillard Reaction to the Nonenzymatic Food Browning

Abstract Lipid oxidation and the Maillard reaction are probably the two most important reactions in Food Science. Both include a whole network of different reactions in which an extraordinary complex mixture of compounds are obtained in very different amounts and produce important changes in food flavor, color, texture, and nutritional value, with positive and negative consequences. This article analyzes the interactions between both reactions, with special emphasis in nonenzymatic browning development, by discussing the influence of lipid oxidation products in the Maillard pathway and vice versa, as well as the existence of common intermediates and polymerization mechanisms in both reactions. The existing data suggest that both reactions are so interrelated that they should be considered simultaneously to understand the products of the Maillard reaction in the presence of lipids and vice versa, and should be included in one general pathway that can be initiated by both lipids and carbohydrates.

Contribution of lipid oxidation products to acrylamide formation in model systems.

The reactions of asparagine with methyl linoleate ( 1), methyl 13-hydroperoxyoctadeca-9,11-dienoate ( 2), methyl 13-hydroxyoctadeca-9,11-dienoate ( 3), methyl 13-oxooctadeca-9,11-dienoate ( 4), methyl 9,10-epoxy-13-hydroxy-11-octadecenoate ( 5), methyl 9,10-epoxy-13-oxo-11-octadecenoate ( 6), 2,4-decadienal ( 7), 2-octenal ( 8), 4,5-epoxy-2-decenal ( 9), and benzaldehyde ( 10) were studied to determine the potential contribution of lipid derivatives to acrylamide formation in heated foodstuffs. Reaction mixtures were heated in sealed tubes for 10 min at 180 degrees C under nitrogen. The reactivity of the assayed compounds was 7 >> 9 > 4 > 2 >> 8 approximately 6 >> 10 approximately 5. The presence of compounds 1 and 3 did not result in the formation of acrylamide. These results suggested that alpha,beta,gamma,delta-diunsaturated carbonyl compounds were the most reactive compounds for this reaction followed by lipid hydroperoxides, more likely as a consequence of the thermal decomposition of these last compounds to produce alpha,beta,gamma,delta-diunsaturated carbonyl compounds. However, in the presence of glucose this reactivity changed, and compound 1/glucose mixtures showed a positive synergism (synergism factor = 1.6), which was observed neither in methyl stearate/glucose mixtures nor in the presence of antioxidants. This synergism is proposed to be a consequence of the formation of free radicals during the asparagine/glucose Maillard reaction, which oxidized the lipid and facilitated its reaction with the amino acid. These results suggest that both unoxidized and oxidized lipids are able to contribute to the conversion of asparagine into acrylamide, but unoxidized lipids need to be oxidized as a preliminary step.

Model Reactions of Acrylamide with Selected Amino Compounds

The reaction of acrylamide with amines, amino acids, and polypeptides was studied in an attempt to understand the role of amino compounds on acrylamide fate. The obtained results showed that amino compounds are added to acrylamide by means of a Michael addition to produce the corresponding 3-(alkylamino)propionamides. Although 3-(alkylamino)propionamides can also be added to a new molecule of acrylamide to produce a new adduct, this last adduct was not detected under the employed conditions in which the concentration of acrylamide was much lower than the concentration of the amino compounds. The produced 3-(alkylamino)propionamides were not stable, and the addition reaction was easily reversed by heating. Thus, acrylamide was produced from 3-(alkylamino)propionamides by means of an elimination reaction. However, the activation energies (E(a)) of both reactions are not the same. In fact, acrylamide seems to be converted into its Michael adduct with a lower activation energy than the elimination reaction of the Michael adduct. For this reason, when acrylamide was stored in the presence of glycine at 60 degrees C, acrylamide disappeared after 14 days. However, when these samples were heated again for 20 min at 180 degrees C, the equilibrium was reestablished and a significant amount of acrylamide was detected. All of these results suggest that amino compounds may play a significant role in the changes observed in acrylamide content in foods upon storage. In addition, they also point to 3-(alkylamino)propionamides as possible compounds in which acrylamide might be potentially hidden.

Interplay between the Maillard Reaction and Lipid Peroxidation in Biochemical Systems

Abstract: The Maillard reaction and lipid peroxidation are two of the most important chemical reactions that take place in biochemical systems. Both include a whole network of different reactions in which an extraordinarily complex mixture of compounds is produced in very different amounts, with both positive and negative consequences. In addition, both reactions are intimately interrelated, and the products of each reaction influence the other. Furthermore, there are common intermediates and products in both pathways; these products are usually known as advanced glycation end products (AGEs) and advanced lipoxidation end products (ALEs). Moreover, other AGE/ALEs are analogous and participate similarly in both amino acid degradation and amino phospholipid/protein polymerization by identical mechanisms. All these data suggest that the Maillard reaction and lipid peroxidation are so closely interrelated that both reactions should be considered simultaneously to understand the reaction mechanisms, kinetics, and products in the complex mixtures of carbohydrates, lipids, and proteins occurring in biochemical systems. In these systems, lipids and carbohydrates are competing in the chemical modification of amino phospholipids and proteins. Therefore, although there are significant differences between the Maillard reaction and lipid peroxidation, many aspects of both reactions can be better understood if they are included in only one general carbonyl pathway that can be initiated by both lipids and carbohydrates.

Asparagine Decarboxylation by Lipid Oxidation Products in Model Systems

The decarboxylation of asparagine in the presence of alkanals, alkenals, and alkadienals, among other lipid derivatives, was studied in an attempt to understand the reaction pathways by which some lipid oxidation products are able to convert asparagine into acrylamide. Asparagine was converted into 3-aminopropionamide in the presence of lipid derivatives as a function of reaction conditions (pH, water content, time, and temperature), as well as the type and amount of lipid compound involved. Alkadienals (and analogous ketodienes) were the most reactive lipids followed by hydroperoxides and alkenals. Saturated carbonyls and polyunsaturated fatty acids, or other polyunsaturated derivatives, also exhibited some reactivity. On the other hand, saturated lipids or monounsaturated alcohols did not degrade asparagine. A mechanism for the decarboxylation of asparagine in the presence of alkadienals based on the deuteration results obtained when asparagine/2,4-decadienal model systems were heated in the presence of deuterated water was proposed. The activation energy (E(a)) of asparagine decarboxylation by 2,4-decadienal was 81.0 kJ/mol, which is higher than that found for the conversion of 3-aminopropionamide into acrylamide in the presence of 2,4-decadienal. This result points to the decarboxylation step as the key step in the conversion of asparagine into acrylamide in the presence of alkadienals. Therefore, any inhibiting strategy for suppressing the formation of acrylamide by alkadienals should be mainly directed to the inhibition of this step.

Identification and classification of olive oils by high-resolution13C nuclear magnetic resonance

Unsaponifiable matter from 19 olive and olive pomace oils were studied by high-resolution13C nuclear magnetic resonance spectroscopy. Their spectra showed characteristic peaks that corresponded to molecular substructures rather than the individual constituents present in the unsaponifiable matter. The presence of squalene and other hydrocarbons, sterols and triterpenic alcohols, in addition to other groups of minor compounds, were observed. Based on the analysis of these spectra, it was possible to distinguish among different grades of olive oils by using stepwise discriminant analysis. This direct method of analysis is suggested to be used in artificial neural networks to define oil identity and quality.

Modification of lysine amino groups by the lipid peroxidation product 4,5(E)-Epoxy-2(E)-hepteal

The reaction between 4,5(E)-epoxy-2(E)-heptenal (EH) andl-lysine was studied to characterize some of the compounds that may be produced when proteins react with peroxidizing lipids. A mixture of EH and lysine was incubated overnight at room temperature and then fractionated by high-performance liquid chromatography (HPLC). Fractions were freeze-dried and characterized by1H and13C nuclear magnetic resonance (NMR) and mass spectrometry. Four major pyrrole derivatives were obtained, namely 1-(5′-amino-1′-carboxypentyl)-pyrrole (3), 1-(5′-amino-1′-carboxypentyl)-2-(1″-hydroxypropyl)pyrrole (diastereomers 5 and 8), 1-(5′-amino-5′-carboxypentyl)pyrrole (7), and 1-(5′-amino-5′-carboxypentyl)-2-(1″-hydroxypropyl)pyrrole (9). In addition, several lysine complexes were detected. A polymer (1b) that was responsible for the color and the fluorescence produced in the reaction was isolated by gel filtration chromatography from a fraction obtained by HPLC. Formation of pairs of analogs (5 and 3, 9 and 7) with and without a substituent in position 2 of the pyrrole ring suggested that the compounds were produced by the same mechanism, with the formation of the 2-unsubstituted pyrroles corresponding to the loss of the 2-substituent as propanal; propanal was detected by headspace capillary gas chromatography. A reaction mechanism is proposed based on the NMR data obtained when the reaction was monitored in real time in an NMR tube. The results suggest that pyrrolic amino acids 7 and 9 may be present in proteins that have been damaged by peroxidizing lipids.

Strecker type degradation of phenylalanine by 4-hydroxy-2-nonenal in model systems.

The reaction of 4-hydroxy-2-nonenal, an oxidative stress product, with phenylalanine in acetonitrile-water (2:1, 1:1, and 1:2) at 37, 60, and 80 degrees C was investigated to determine whether 4-hydroxy-2-alkenals degrade amino acids, analogously to 4,5-epoxy-2-alkenals, and to compare the reactivities of both hydroxyalkenals and epoxyalkenals for production of Strecker aldehydes. In addition to the formation of N-substituted 2-pentylpyrrole and 2-pentylfuran, the studied hydroxyalkenal also degraded phenylalanine to phenylacetaldehyde with a reaction yield of 17%. The reaction mechanism is suggested to be produced through the corresponding imine, which is then decarboxylated and hydrolyzed. This reaction also produced a conjugated amine, which both may be one of the origins of the produced 2-pentyl-1H-pyrrole and may contribute to the development of browning in these reactions. 4-Hydroxy-2-nonenal and 4,5-epoxy-2-decenal degraded phenylalanine in an analogous extent, which is likely a consequence of the similarity of the degradation mechanisms involved. These results suggest that different lipid oxidation products are able to degrade amino acids; therefore, the Strecker type degradation of amino acids produced by oxidized lipids may be quantitatively significant in foods.

Model Studies on the Degradation of Phenylalanine Initiated by Lipid Hydroperoxides and Their Secondary and Tertiary Oxidation Products

The reaction of methyl 13-hydroperoxyoctadeca-9,11-dienoate (MeLOOH), methyl 13-hydroperoxyoctadeca-9,11,15-trienoate (MeLnOOH), methyl 13-hydroxyoctadeca-9,11-dienoate (MeLOH), methyl 13-oxooctadeca-9,11-dienoate (MeLCO), methyl 9,10-epoxy-13-hydroxy-11-octadecenoate (MeLEPOH), and methyl 9,10-epoxy-13-oxo-11-octadecenoate (MeLEPCO) with phenylalanine was studied to determine the comparative reactivity of primary, secondary, and tertiary lipid oxidation products in the Strecker degradation of amino acids. All assayed lipids were able to degrade the amino acid to a high extent, although the lipid reactivity decreased slightly in the following order: MeLEPCO > or = MeLCO > MeLEPOH > or = MeLOH > MeLOOH approximately = MeLnOOH. These data confirmed the ability of many lipid oxidation products to degrade amino acids by a Strecker-type mechanism and suggested that, once the lipid oxidation is produced, a significant Strecker degradation of surrounding amino acids should be expected. The contribution of different competitive mechanisms to this degradation is proposed, among which the conversion of the different lipid oxidation products assayed into the most reactive MeLEPCO and the fractionation of long-chain primary and secondary lipid oxidation products into short-chain aldehydes are likely to play a major role.

Conversion of 3‐aminopropionamide and 3‐alkylaminopropionamides into acrylamide in model systems

Carbonyl compounds have been shown to play a major role in the conversion of asparagine into acrylamide. However, it is unclear at this point if its role is only restricted to the decarboxylation of the amino acid or if carbonyl compounds also play a role in the deamination reaction of the decarboxylated intermediates 3-aminopropionamide and 3-(alkylamino)propionamides. This study describes the deamination reaction of 3-aminopropionamide and 3-(alkylamino)propionamides (benzyl, phenylethyl, butyl, and octyl) in model systems and in the presence, or not, of different carbonyl compounds (alkadienals, alkenals, and alkanals). All these reactions were mainly produced at almost neutral or basic pH values. In addition, the reaction yields and the activation energies not only depended on the type of aminopropionamide involved but also on the water activity (a(w)) and in the presence, or not, of carbonyl compounds. However, there was not a clear correlation among the activation energies calculated for the different deamination reactions and the yields of acrylamide obtained; therefore, suggesting the existence of diverse pathways by which 3-aminopropionamide and 3-(alkylamino)propionamides are converted into acrylamide. In addition, these reactions are also competing with other carbonyl-amine reactions when carbonyl compounds are present. All these results suggest that the type of the intermediate aminopropionamide involved is going to play a major role in both the amount of acrylamide produced and the conditions required for its formation. On the other hand, the role of carbonyl compounds in the acrylamide produced, but not in the activation energy of the reactions implicated, seems to be more limited than either the type of amine or the a(w). A detailed analysis of the type of the intermediate aminopropionamide formed in foods may help to define strategies for mitigating the formation of this food toxicant.