Strecker degradation of phenylalanine initiated by 2,4-decadienal or methyl 13-oxooctadeca-9,11-dienoate in model systems

The reaction of 2,4-decadienal and methyl 13-oxooctadeca-9,11-dienoate with phenylalanine was studied to determine if alkadienals and ketodienes are able to produce the Strecker-type degradation of amino acids to the corresponding Strecker aldehydes. When reactions were carried out at 180 degrees C, both carbonyl compounds degraded phenylalanine to phenylacetaldehyde, among other compounds. The yield of the phenylacetaldehyde produced depended on the reaction pH and increased linearly with both the amount of the lipid and the reaction time. The yield of this conversion was approximately 8% when starting from decadienal and approximately 6% when starting from methyl 13-oxooctadeca-9,11-dienoate, and the reaction rate was lower for the ketone than for the aldehyde. Simultaneous to these reactions, the lipid was converted into pyrrole, pyridine, or aldehyde derivatives as a result of several competitive reactions. In particular, 9-14% of the decadienal was converted into hexanal under the assayed conditions. All these reactions are suggested to be produced as a consequence of the oxidation of the alkadienal or the ketodiene to the corresponding epoxyalkenal or unsaturated epoxyketone, which were identified in the reaction mixtures by GC-MS. All these results suggest that alkadienals and ketodienes, which are quantitatively important secondary lipid oxidation products, can degrade amino acids to their corresponding Strecker aldehydes. Therefore, under appropriate conditions, these products are not final products of the lipid oxidation and can participate in carbonyl-amine reactions analogously to other lipid oxidation products with two oxygenated functions.     

Chemical conversion of alpha-amino acids into alpha-keto acids by 4,5-epoxy-2-decenal

The comparative formation of phenylalanine and phenylpyruvic acid in the reaction of 4,5-epoxy-2-decenal with phenylalanine was studied to determine whether epoyalkenals may also degrade amino acids without producing their decarboxylation. Both compounds were produced in the reaction to an extent that depended on the reaction pH, the amount of lipid oxidation product, and the reaction time and temperature. The optimum pH was 3 for producing both carbonyl derivatives, and the amount of both compounds increased linearly with the amount of epoxyalkenal present in the reaction mixture. In addition, phenylpyruvic acid was produced to a higher extent than phenylacetaldehyde at 37 degrees C. However, at 60 degrees C the degradation of phenylpyruvic acid was observed and phenylacetaldehyde was usually found to a higher extent than the alpha-keto acid in the overnight-incubated reaction mixtures. The degradation of phenylpyruvic acid produced benzaldehyde and phenylacetaldehyde. All these results suggest that epoxyalkenals can not only degrade amino acids by a Strecker-type mechanism but convert them into their corresponding alpha-keto acids. This new reaction may be an alternative chemical route for the formation in foods of alpha-keto acids, which can later participate in the generation of important amino acid-derived flavor compounds.     

Strecker-type degradation produced by the lipid oxidation products 4,5-epoxy-2-alkenals

Strecker degradation is one of the most important reactions leading to final aroma compounds in the Maillard reaction. In an attempt to clarify whether lipid oxidation products may be contributing to the Strecker degradation of amino acids, this study analyzes the reaction of 4,5-epoxy-2-alkenals with phenylalanine. In addition to N-substituted 2-(1-hydroxyalkyl)pyrroles and N-substituted pyrroles, which are major products of the reaction, the formation of both the Strecker aldehyde phenylacetaldehyde and 2-alkylpyridines was also observed. The aldehyde, which was produced at 37 degrees C-as could be determined by forming its corresponding thiazolidine with cysteamine-and pH 6-7, was not produced when the amino acid was esterified. This aldehyde is suggested to be produced through imine formation, which is then decarboxylated and hydrolyzed. This reaction also produces a hydroxyl amino derivative, which is the origin of the 2-alkylpyridines identified. All these data indicate that Strecker-type degradation of amino acids is produced at 37 degrees C by some lipid oxidation products. This is a new proof of the interrelations between lipid oxidation and Maillard reaction, which are able to produce common products by analogue mechanisms.     

Strecker-type degradation of phenylalanine by methyl 9,10-epoxy-13-oxo-11-octadecenoate and methyl 12,13-epoxy-9-oxo-11-octadecenoate

The reaction of methyl 9,10-epoxy-13-oxo-11(E)-octadecenoate, methyl 12,13-epoxy-9-oxo-11(E)-octadecenoate, 4,5(E)-epoxy-2(E)-heptenal, and 4,5(E)-epoxy-2(E)-decenal with phenylalanine in acetonitrile-water (2:1, 1:1, and 1:2) at 80 degrees C and at different pHs and carbonyl compound/amino acid ratios was investigated both to determine if epoxyoxoene fatty esters were able to produce the Strecker-type degradation of the amino acid and to study the relative ability of oxidized long-chain fatty esters and short chain aldehydes with identical functional systems to degrade amino acids. The studied epoxyoxoene fatty esters degraded phenylalanine to phenylacetaldehyde. The mechanism of the reaction was analogous to that described for epoxyalkenals and is suggested to be produced through the corresponding imine, which is then decarboxylated and hydrolyzed. This reaction also produced a conjugated hydroxylamine, which was the origin of the long-chain pyridine-containing fatty ester isolated in the reaction and characterized as methyl 8-(6-pentylpyridin-2-yl)octanoate. Epoxyoxoene fatty esters and epoxyalkenals exhibited a similar reactivity for producing phenylacetaldehyde, therefore suggesting that nonvolatile lipid oxidation products, which are produced to a greater extent than volatile products, should be considered for determining the overall contribution of lipids to Strecker degradation of amino acids produced during nonenzymatic browning. In addition, the obtained data confirm that, analogously to carbohydrates, lipid oxidation products are also able to produce the Strecker degradation of amino acids.     

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.     

Influence of lipids in the generation of phenylacetaldehyde in wort-related model systems

The effect of lipids on the formation of the Strecker aldehyde phenylacetaldehyde during wort boiling was studied to determine the role that small changes in the lipid content of the wort have in the production of significant flavor compounds in beer. Wort was treated with 0-2.77 mmol per liter of glucose, linoleic acid, or 2,4-decadienal and heated at 60-98 degrees C for 1 h. After this time, the amount of the Strecker aldehyde phenylacetaldehyde increased in the samples treated with linoleic acid or decadienal but not in the samples treated with glucose. Thus, the amount of phenylacetaldehyde produced in the presence of linoleic acid was 1.1-2.5 times the amount of the Strecker aldehyde produced in the control wort, and this amount increased to 3.6-4.6 times when decadienal was employed. The higher reactivity of decadienal than linoleic acid for this reaction decreased with temperature and was related to the oxidation of linoleic acid that occurred to a higher extent at higher temperatures. The above results suggest that lipids can contribute to the formation of Strecker aldehydes during wort boiling and that changes in the lipid content of the wort will produce significant changes in the formation of Strecker aldehydes in addition to other well-known consequences in beer quality and yeast metabolism. On the other hand, because of the high glucose content in wort, small changes in its content are not expected to affect the amount of Strecker aldehydes produced.     

Chemical conversion of phenylethylamine into phenylacetaldehyde by carbonyl-amine reactions in model systems

The chemical conversion of phenylethylamine into phenylacetaldehyde in the presence of lipid oxidation products (LOPs) was studied to investigate the possibility that biogenic amines can be converted into Strecker aldehydes upon processing. Model systems of phenylethylamine and methyl 13-hydroperoxyoctadeca-9,11-dienoate (HP), 2,4-decadienal (DD), 4,5-epoxy-2-heptenal (EH), 4,5-epoxy-2-decenal (ED), 4-oxo-2-hexenal (OH), 4-oxo-2-nonenal (ON), or 4-hydroxy-2-nonenal (HN) were heated for 1 h at 180 °C and pH 3. Although HN and EH did not produce more phenylacetaldehyde than when phenylethylamine was heated alone, all other lipid oxidation products assayed increased the amount of phenylacetaldehyde produced by 300-900%, with ON being the most reactive compound for this reaction. The reaction was mainly produced at acidic pH values (<6) and was dependent upon the concentration of the LOPs involved, and the phenylacetaldehyde produced increased linearly as a function of the time and temperature. The E(a) values for the reactions between phenylethylamine and DD and ON were 54.8 and 53.8 kJ/mol, respectively. The reaction is proposed to take place by the formation of an imine between the phenylethylamine and the LOPs, which is later converted into another imine by an electronic rearrangement. This new imine is the origin of phenylacetaldehyde by hydrolysis. These results show a new pathway for Strecker aldehyde formation. This route provides a potential way to reduce biogenic amine content in foods when they can be thermally processed before consumption.     

Gas chromatographic investigation of acrylamide formation in browning model systems

Acrylamide formed in browning model systems was analyzed using a gas chromatograph with a nitrogen-phosphorus detector. Asparagine alone produced acrylamide via thermal degradation at the level of 0.99 microgram/g of asparagine. When asparagine was heated with triolein-which produced acrolein at the level of 1.82 +/- 0.31 (n = 5) mg/L of headspace by heat treatment-acrylamide was formed at the level of 88.6 microgram/g of asparagine. When acrolein gas was sprayed onto asparagine heated at 180 degrees C, a significant amount of acrylamide was formed (114 microgram/g of asparagine). On the other hand, when acrolein gas was sprayed onto glutamine under the same conditions, only a trace amount of acrylamide was formed (0.18 microgram/g of glutamine). Relatively high levels of acrylamide (753 microgram/g of ammonia) were formed from ammonia and acrolein heated at 180 degrees C in the vapor phase. The reaction of acrylic acid, which is an oxidation product of acrolein and ammonia, produced a high level of acrylamide (190 000 microgram/g of ammonia), suggesting that ammonia and acrolein play an important role in acrylamide formation in lipid-rich foods. Acrylamide can be formed from asparagine alone via thermal degradation, but carbonyl compounds, such as acrolein, promote its formation via a browning reaction.     

Lipid peroxidation and coupled vitamin oxidation in simulated and human gastric fluid inhibited by dietary polyphenols: health implications

The Western diet contains large quantities of oxidized lipids, because a large proportion of the food in the diet is consumed in a fried, heated, processed, or stored form. We investigated the reaction that could occur in the acidic pH of the stomach and accelerate the generation of lipid hydroperoxides and cooxidation of dietary vitamins. To estimate the oxygen content in the stomach after food consumption, oxygen released from masticated bread (20 g) into deoxygenated water (100 mL) was measured. Under these conditions, the oxygen concentration rose by 250 microM and reached a full oxygen saturation. The present study demonstrated that heated red meat homogenized in human gastric fluid, at pH 3.0, generated hydroperoxides and malondialdehyde. The cross-reaction between free radicals produced during this reaction cooxidized vitamin E, beta-carotene, and vitamin C. Both lipid peroxidation and cooxidation of vitamin E and beta-carotene were inhibited at pH 3.0 by red wine polyphenols. Ascorbic acid (44 mg) at a concentration that represented the amount that could be ingested during a meal inhibited lipid peroxidation only slightly. Red wine polyphenols failed to prevent ascorbic acid oxidation significantly but, in conjunction with ascorbic acid, did inhibit lipid peroxidation. In the presence of catechin, a well-known polyphenol found in red wine, ascorbic acid at pH 3.0 works in a synergistic manner preventing lipid peroxidation and beta-carotene cooxidation. The present data may explain the major benefits to our health and the crucial role of consuming food products rich in dietary antioxidants such as fruits, vegetables, red wines, or green tea during the meal.     

Comparative methyl linoleate and methyl linolenate oxidation in the presence of bovine serum albumin at several lipid/protein ratios

The oxidation of methyl linoleate (LMe) and methyl linolenate (LnMe) in the presence of bovine serum albumin (BSA) in the dark at 60 degrees C was studied to analyze the role of the type of fatty acid and the protein/lipid ratio on the relative progression of the processes involved when lipid oxidation occurs in the presence of proteins. The disappearance of the fatty acid, the formation of primary and secondary products of lipid peroxidation, the loss of amino acid residues, the production of oxidized lipid/amino acid reaction products, and the development of color and fluorescence were studied as a function of incubation time in protein/lipid samples at 10:1, 6:1, and 3:1 w/w ratios. The incubation of LMe and LnMe in the presence of BSA at 60 degrees C rapidly produced lipid peroxidation and protein damage. Although reaction rates were much faster for LnMe than for LMe, both fatty acids had similar behaviors, and LnMe seemed to be only slightly more reactive than LMe for BSA by producing a higher increase of protein pyrroles in the protein and the development of increased browning and fluorescence. The protein/lipid ratio also influenced the relative progress of the reactions implicated. Thus, a lower protein/lipid ratio increased sample oxidation and protein damage. This also produced an increased browning, in accordance with the mechanisms proposed for browning production by oxidized lipid/protein reactions. On the contrary, browning of extracted lipids increased at higher protein/lipid ratios. This opposite tendency allowed evaluation of the overall significance of the different browning processes implicated in the final colors observed, concluding that color changes observed in BSA/lipid samples were mostly a consequence of oxidized lipid/protein reactions.     

Methyl linoleate oxidation in the presence of bovine serum albumin

The oxidation of methyl linoleate (LMe) in the presence of bovine serum albumin (BSA) was studied to analyze both the processes involved when lipid oxidation occurs in the presence of proteins and the relative progression of the several reactions implicated. The disappearance of LMe, the formation of primary and secondary lipid oxidation products, the loss of essential amino acids, and the production of oxidized lipid/amino acid reaction products (OLAARPs) were studied as a function of incubation time. During the first steps of lipid oxidation, LMe was converted quantitatively to methyl linoleate hydroperoxides, which were very rapidly degraded to either secondary products of lipid oxidation or OLAARPs. No significant differences were identified in the major lipid oxidation products formed in incubations with or without proteins, indicating that mechanisms for formation of these compounds are similar in both cases. In addition, no significant differences were observed between the time-courses of formation of secondary oxidation products and OLAARPs, suggesting that hydroperoxide decomposition and OLAARP formation occur simultaneously when the lipid oxidation process takes place in the presence of proteins. Furthermore, OLAARP formation seems to be an unavoidable process that should be considered as a last step in the lipid peroxidation process.     

Studies on radical intermediates in the early stage of the nonenzymatic browning reaction of carbohydrates and amino acids

Determination of the color intensity of heated mixtures of L-alanine and carbohydrate degradation products revealed furan-2-carboxaldehyde and glycolaldehyde as by far the most effective color precursors. EPR studies demonstrated that furan-2-carboxaldehyde generated colored compounds exclusively via ionic mechanisms, whereas glycolaldehyde led to color development accompanied by intense radical formation. In agreement with literature data, these radicals were also detected in heated mixtures of L-alanine and pentoses or hexoses, respectively, and were identified as 1,4-dialkylpyrazinium radical cations by EPR as well as LC/MS measurements. Studies on the mechanisms of radical formation revealed that under the reaction conditions applied, glyoxal is formed as an early product in hexose/L-alanine mixtures prior to radical formation. Reductones then initiate radical formation upon reduction of glyoxal and/or glyoxal imines, formed upon reaction with the amino acid, into glycolaldehyde, which was found as the most effective radical precursor. LC/MS measurements gave evidence that these pyrazinium radicals cations are not stable but are easily transformed into hydroxylated 1,4-dialkyl-1, 4-dihydropyrazines upon oxidation and hydrolysis of intermediate diquarternary pyrazinium ions. Besides other types of color precursors, these intermediates might be involved in the formation of colored compounds in the Maillard reaction.     

Kinetics of interaction of vanillin with amino acids and peptides in model systems

Model systems were used to study the reaction kinetics of vanillin and pentalysine, lysine, glutathione, cysteine, aspartame, or phenylalanine (molar ratio 1:1) in phosphate buffer. The buffer pH was adjusted to the pK(a)(2) of the available alpha-amino group of each amino acid or peptide. Reductions of vanillin followed first-order kinetics at 55, 65, and 75 degrees C in the presence of each of the amino acids or peptides used. The reaction rates were accelerated as the temperature increased. The rate constants were highest for pentalysine followed by lysine, phenylalanine, glutathione/cysteine, and aspartame. The reduction of phenylalanine followed first-order kinetics, whereas the formation of its reaction product followed zero-order kinetics. The activation energy (E(a)) for the reaction ranged from 5.6 to 14.5 kcal/mol.     

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.     

Formation of aroma-active strecker-aldehydes by a direct oxidative degradation of Amadori compounds

alpha-Dicarbonyls, generated by sugar degradation, catalyze the formation of the so-called Strecker aldehydes from alpha-amino acids. To check the effectiveness of Amadori compounds (suggested as important intermediates in alpha-dicarbonyl formation from carbohydrates) in Strecker aldehyde formation, the amounts of phenylacetaldehyde (PA) formed from either an aqueous solution of L-phenylalanine/glucose or the corresponding Amadori compound N-(1-deoxy-D-fructosyl-1-yl)-L-phenylalanine (ARP-Phe) were compared. The results revealed the ARP-Phe as a much more effective precursor in PA generation. On the contrary, a binary mixture of glucose/phenylalanine yielded preferentially phenylacetic acid, in particular, when reacted in the presence of oxygen and copper ions. Further model experiments gave evidence that a transition-metal-catalyzed oxidation of the ARP-Phe by air oxygen into the 2-hexosulose-(phenylalanine) imine is the key step responsible for the favored formation of phenylacetaldehyde from the Amadori compound. This mechanism might explain differences in the ratios of Strecker aldehydes and the corresponding acids depending on the structures of carbohydrate degradation products involved.     

Model studies on the oxygen-induced formation of benzaldehyde from phenylacetaldehyde using pyrolysis GC-MS and FTIR

Benzaldehyde, a potent aroma chemical of bitter almond, can also be formed thermally from phenylalanine and may contribute to the formation of off-aroma. To identify the precursors involved in its generation during Maillard reaction, various model systems containing phenylalanine, phenylpyruvic acid, phenethylamine, or phenylacetaldehyde were studied in the presence and absence of moisture using oxidative and nonoxidative Py-GC-MS. Analysis of the data indicated that phenylacetaldehyde, the Strecker aldehyde of phenylalanine, is the most effective precursor and that both air and water significantly enhanced the rate of benzaldehyde formation from phenylacetaldehyde. Phenylpyruvic acid was the most efficient precursor under nonoxidative conditions. Phenethylamine, on the other hand, needed the presence of a carbonyl compound to generate benzaldehyde only under oxidative conditions. On the basis of the results obtained, a free radical initiated oxidative cleavage of the carbon-carbon double bond of the enolized phenylacetaldehyde was proposed as a possible major mechanism for benzaldehyde formation, and supporting evidence was provided through monitoring of the evolution of the benzaldehyde band from heated phenylacetaldehyde in the presence and absence of 1,1'-azobis(cyclohexanecarbonitrile) on the ATR crystal of an FTIR spectrophotometer. In the presence of the free radical initiator, the enol band of the phenylacetaldehyde centered at 1684 cm(-1) formed and increased over time, and after 18 min of heating time the benzaldehyde band centered at 1697 cm(-1) formed and increased at the expense of the enol band of phenylacetaldehyde, indicating a precursor product relationship.     

Formation of strecker aldehydes from polyphenol-derived quinones and alpha-amino acids in a nonenzymic model system

Fruits and vegetables contain naturally occurring polyphenolic compounds that can undergo enzyme-catalyzed oxidation during food preparation. Many of these compounds contain catechol (1,2-dihydroxybenzene) moieties that may be transformed into o-quinone derivatives by polyphenoloxidases and molecular oxygen. Secondary reactions of the o-quinones include the Strecker degradation of ambient amino acids to form flavor-important volatile aldehydes. The purpose of this work was to investigate the mechanism of the polyphenol/o-quinone/Strecker degradation sequence in a nonenzymic model system. By using ferricyanide ion as the oxidant in pH 7.17 phosphate buffer at 22 degrees C, caffeic acid, chlorogenic acid, (+) catechin, and (-) epicatechin were caused to react with methionine and phenylalanine to produce Strecker aldehydes methional and phenylacetaldehyde in 0.032-0.42% molar yields (0.7-10 ppm in reaction mixtures). Also, by employing l-proline methyl ester in a reaction with 4-methylcatechol, a key reaction intermediate, 4-(2'-carbomethoxy-1'-pyrrolidinyl)-5-methyl-1,2-benzoquinone (7), was isolated and tentatively identified.     

Reaction of folic acid with reducing sugars and sugar degradation products

The reaction of folic acid with reducing sugars (nonenzymatic glycation) under conditions that can occur during food processing and preparation was studied by high-performance liquid chromatography with diode array detection. N-(p-Aminobenzoyl)-L-glutamic acid, a well-established oxidation product, was detected in the reaction mixtures. Furthermore, a new product was isolated and identified as N2-[1-(carboxyethyl)]folic acid (CEF). CEF was the main product that was formed by the nonenzymatic glycation of folic acid. For preparation, N2-[1-(carboxyethyl)]folic acid was obtained in high yields when folic acid and dihydroxyacetone (DHA), a sugar degradation product, were heated at 100 degrees C in phosphate buffer. Mixtures of folic acid and different sugars or DHA were heated under variation of reaction time and temperature, and CEF was quantified. Up to 50% of the vitamin was converted to CEF, with highest yields formed from maltose (49%) and lactose (43%).     

Formation of nitric oxide during tobacco oxidation

The sources of NO during biomass oxidation, and in particular tobacco oxidation, have been disputed. Literature results range from decomposition of nitrate to the oxidation of atmospheric nitrogen. To rectify these discrepancies, this study focuses on the sources of nitric oxide (NO) during the oxidation of tobacco samples. When Burley tobacco was heated in a partially oxidized atmosphere, NO was produced at two distinct temperature ranges, namely 275-375 degrees C (the low-temperature range) and 425-525 degrees C (the high-temperature range). The formation of NO at the low-temperature range with Burley tobacco was found to be unaffected by oxygen, while the formation of NO at the high-temperature range required an oxygen atmosphere. With Bright and Oriental tobacco, NO was produced only within the higher-temperature range. To understand the formation processes and the sources of NO formation within these two temperature ranges, several endogenous nitrogenous tobacco compounds were examined. These were mixed with non-nitrogenous biomass model materials, namely cellulose, pectin, xylan, or lignin, which also occur naturally in tobacco, and the mixtures were heated in a flow tube reactor under a partially oxidative atmosphere. A commercial gas analyzer was used to monitor the formation of NO during heating. Nitrate ion was determined to be the source of NO formation in the range of 275-375 degrees C. This ion was decomposed in a carbonaceous surrounding to produce NO. For NO formation at the higher temperature range, amino acids and proteins were shown to be the sources. The interaction between nitrogenous organic compounds (amino acids and proteins) and pectin first produced a nitrogen-containing char at a temperature below 350 degrees C. Oxidation of this char at the higher temperatures produced NO.