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.     

Formation of pyrazines and a novel pyrrole in Maillard model systems of 1,3-dihydroxyacetone and 2-oxopropanal

Alkylpyrazines are a very important class of Maillard flavor compounds, but their mechanism of formation is complex and consists of different pathways. The model reaction of 20 different amino acids with 1,3-dihydroxyacetone, as a precursor of 2-oxopropanal, was studied by means of SPME-GC-MS to investigate the involvement of the amino acid side chain in the substitution pattern of the resulting pyrazines. 2,5-Dimethylpyrazine was quantitatively the most important pyrazine formed from all of the amino acids. The amino acid side chain is not involved in its formation. The substituents of other less abundant pyrazines resulted mainly from the incorporation of the Strecker aldehyde or aldol condensation products in the intermediate dihydropyrazine. The importance of different reaction mechanisms was evaluated, taking into account the pattern of pyrazines identified. In the solvent extracts of aqueous model reactions of 2-oxopropanal with amino acids, the main reaction product was not a pyrazine but a novel pyrrole. This pyrrole was identified as 2,5-diacetyl-3-methyl-1 H-pyrrole by means of spectral analysis, secured by chemical synthesis. A reaction mechanism for its formation was proposed and evaluated. The influence of various reaction conditions on the formation of 2,5-diacetyl-3-methyl-1 H-pyrrole and 2,5-dimethylpyrazine in the model reaction of alanine with 2-oxopropanal was studied. These results underscore the importance of the ratio of the different reagents and the presence of water in the resulting flavor formation in the Maillard reaction.     

Mechanisms of alkylpyrazine formation in a potato model system containing added glycine

The use of glycine to limit acrylamide formation during the heating of a potato model system was also found to alter the relative proportions of alkylpyrazines. The addition of glycine increased the quantities of several alkylpyrazines, and labeling studies using [2-13C]glycine showed that those alkylpyrazines which increased in the presence of glycine had at least one 13C-labeled methyl substituent derived from glycine. The distribution of 13C within the pyrazines suggested two pathways by which glycine, and other amino acids, participate in alkylpyrazine formation, and showed the relative contribution of each pathway. Alkylpyrazines that involve glycine in both formation pathways displayed the largest relative increases with glycine addition. The study provided an insight into the sensitivity of alkylpyrazine formation to the amino acid composition in a heated food and demonstrated the importance of those amino acids that are able to contribute an alkyl substituent. This may aid in estimating the impact of amino acid addition on pyrazine formation, when amino acids are added to foods for acrylamide mitigation.     

Role of water upon the formation of acrylamide in a potato model system

The moisture sorption isotherms of a commercial potato powder were investigated at 20 degrees C for water activities ranging from 0.11 to 0.97. The sorption isotherms were typical type-II sigmoidal curves, with a steep increase in moisture content for water activities above 0.9 and exhibiting hysteresis over the whole water activity range. On the basis of the isotherms, the influence of the initial water activity and moisture content on both Maillard browning and acrylamide formation was determined by heating oil containing potato powder mixtures in a closed stainless-steel tubular reactor. The Maillard browning, as determined spectrophotometrically, showed an optimum at intermediate water activities. The yields of acrylamide, expressed relatively to the molar amount of asparagine, remained constant below 0.8 aw and below moisture contents of about 20% (on a dry basis). For the more intense heat treatments, an increased acrylamide yield was however observed at higher moisture contents, with an optimum at water contents of about 100% (on a dry basis). However, this increase and optimum was not observed at less intense heat treatments. At moisture contents above 100%, a significant decrease in acrylamide yields was assessed, although the water activity increased only marginally in this area of the sorption isotherms. It was thus observed that the acrylamide content was rather dependent upon the moisture content than upon the water activity in the high-moisture potato powder model system.     

不同油炸时间的马铃薯片油脂吸收规律

为了表征不同油炸时间下马铃薯片的油脂吸收规律,该文结合索氏抽提、染色法、激光共聚焦扫描镜观察、低场核磁共振成像技术等方法对马铃薯片的油脂吸收规律进行描述和解释,获得了油炸温度180℃、油料比10:1时不同油炸时间下马铃薯片油脂含量、油脂类型以及油脂分布的变化规律,并结合其孔隙特征解释油脂的吸收行为。结果表明:油炸时间(0~12 min)显著影响马铃薯片的总油脂质量分数、表面油脂、结构油脂和表面渗透油脂质量分数(P0.05);随着油炸时间的延长,马铃薯片的总油脂质量分数逐渐上升,当油炸10 min后,总油脂质量分数达到47.09%,之后逐渐趋于稳定;马铃薯片的表面油脂质量分数最低,约1.30%~2.06%;随油炸时间的延长,结构油脂质量分数逐渐上升,而油炸6 min后马铃薯片的表面渗透油脂质量分数呈下降趋势;激光共聚焦扫描镜观察显示,油炸过程中,随着油炸时间的延长,马铃薯片红色区域面积增大,表明油脂质量分数增加,且油脂从表面逐渐向中心渗透,黏附于细胞壁上,充盈细胞间质;当油炸时间为0~8 min时,马铃薯片的孔体积和孔隙率随油炸时间而增大,对应的油脂质量分数也逐渐提高,研究结果可为油脂吸收的表征,以及低油马铃薯片的开发提供参考。     

Influence of thermal processing conditions on acrylamide generation and browning in a potato model system

Fried potato products such as French fries and chips may contain substantial amounts of acrylamide. Numerous efforts are undertaken to minimize the acrylamide content of these products while sensory properties such as color and flavor have to be respected as well. An optimization of the frying process can be achieved if the basic kinetic data of the browning and acrylamide formation are known. Therefore, heating experiments with potato powder were performed under controlled conditions (moisture, temperature, and time). Browning and acrylamide content both increased with heating time at all temperatures and moisture contents tested. The moisture content had a strong influence on the activation energy of browning and acrylamide formation. The activation energy strongly increased at moisture contents below 20%. At higher moisture contents, it was very similar for both parameters. At low moisture contents, the activation energy of acrylamide formation was larger as compared to the one for browning. This explains why the end of the frying process is very critical. Therefore, a lower temperature toward the end of frying reduces the acrylamide content of the product while color development is still good.     

Effect of citric acid and glycine addition on acrylamide and flavor in a potato model system

Acrylamide levels in cooked/processed food can be reduced by treatment with citric acid or glycine. In a potato model system cooked at 180 degrees C for 10-60 min, these treatments affected the volatile profiles. Strecker aldehydes and alkylpyrazines, key flavor compounds of cooked potato, were monitored. Citric acid limited the generation of volatiles, particularly the alkylpyrazines. Glycine increased the total volatile yield by promoting the formation of certain alkylpyrazines, namely, 2,3-dimethylpyrazine, trimethylpyrazine, 2-ethyl-3,5-dimethylpyrazine, tetramethylpyrazine, and 2,5-diethyl-3-methylpyrazine. However, the formation of other pyrazines and Strecker aldehydes was suppressed. It was proposed that the opposing effects of these treatments on total volatile yield may be used to best advantage by employing a combined treatment at lower concentrations, especially as both treatments were found to have an additive effect in reducing acrylamide. This would minimize the impact on flavor but still achieve the desired reduction in acrylamide levels.     

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.     

Changes in macroscopic viscosity do not affect the release of aroma aldehydes from a pectinaceous food model system of low sucrose content

The effects of pectin and viscosity on the release of a systematic series of aldehydes (alkanals, methyl-alkanals, alkenals, and alkandienals) were studied in a food model system of low sucrose content (10% w/w). The viscosity was varied by adding different amounts of Ca(2+) (0, 13.5, and 27 mg/g pectin) to the model system of constant pectin concentration (0.4% w/w). Air-liquid partition coefficients, K (37 degrees C), of the aroma compounds were determined in aqueous and pectin-thickened solutions. Diffusivities of the aroma compounds in water and three pectin-thickened solutions were estimated from release rate constants that were obtained via timed collection of volatiles in the gas phase and quantifications by dynamic headspace-gas chromatography. The partition coefficients increased as the carbon chain increased within each homologous series. Overall, no significant difference was found between partition coefficients of aldehydes in water and in pectin solutions except for 2-methyl-propanal and butanal that showed higher K values when pectin was present. Furthermore, the diffusional properties of the model system with a constant pectin level (0.4% w/w) remained constant when the viscosity was increased from 0.001 to 150 Pa s. It was concluded that neither pectin nor alterations in macroscopic viscosity as such influenced the release of aldehydes from the pectin-thickened food model system.     

Index for potato nitrogen status diagnosis in a three-phase hydroponic system

Abstract

Up to now, there has not been any research that has tried to establish nitrogen (N) status diagnosis in basic seed potatoes, propagated by sprout in a three-phase hydroponic system. The objective of this study was to develop a potential index to be used in the diagnosis of N status in potato, using a three-phase hydroponic system, with the potato cultivar Agata. The treatments consisted of four N concentrations: 0, 7.8, 13 and 26?mmol L^?1. The experiment was conducted in a randomized block design with 12 replications in a non-acclimatized greenhouse. It was evaluated in a destructive and non-destructive way, and in real-time or space–time, 18 biometric characteristics of the reference leaf and of plant organs besides other variables related to the fourth leaf green color. Among the non-destructive characteristics measured, length, and diameter of the internodes and thickness of the fourth leaf showed the highest correlation coefficient (p?相似文献   

Formation and stability of foam made with various potato protein preparations

In the present study, foam-forming and -stabilizing properties of potato proteins were studied using whipping and sparging tests. The soluble potato proteins are mainly composed of patatin and protease inhibitors. The whipping tests showed that less foam was formed from untreated patatin than from the protease inhibitors, but patatin foam was much more stable. The foam-forming properties of patatin could be strongly improved by partial unfolding of the protein. Whipping tests, at both low (0.5 mg/mL) and high (10 mg/mL) protein concentration, also indicated that foams made with an ethanol-precipitated protein isolate were more stable than those made with beta-casein and beta-lactoglobulin. More generally, it can be concluded that when proteins are used as a foaming agent, a high concentration is required, because the protein available is inefficiently used. Also, there are several variables that may all, in different ways, affect both foam formation (amount of foam, bubbles size distribution) and foam stability. These variables include the type and concentration of protein, solvent conditions (pH, I), and the method used to make the foam.     

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.     

Impact of the N-terminal amino acid on the formation of pyrazines from peptides in Maillard model systems

Only a minor part of Maillard reaction studies in the literature focused on the reaction between carbohydrates and peptides. Therefore, in continuation of a previous study in which the influence of the peptide C-terminal amino acid was investigated, this study focused on the influence of the peptide N-terminal amino acid on the production of pyrazines in model reactions of glucose, methylglyoxal, or glyoxal. Nine different dipeptides and three tripeptides were selected. It was shown that the structure of the N-terminal amino acid is determinative for the overall pyrazine production. Especially, the production of 2,5(6)-dimethylpyrazine and trimethylpyrazine was low in the case of proline, valine, or leucine at the N-terminus, whereas it was very high for glycine, alanine, or serine. In contrast to the alkyl-substituted pyrazines, unsubstituted pyrazine was always produced more in the case of experiments with free amino acids. It is clear that different mechanisms must be responsible for this observation. This study clearly illustrates the capability of peptides to produce flavor compounds such as pyrazines.     

Formation of amines and aldehydes from parent amino acids during thermal processing of cocoa and model systems: new insights into pathways of the strecker reaction

A method based on a derivatization with dansyl chloride and LC-MS-MS determination was developed for the quantitation of 2-methylbutyl-, 3-methylbutyl-, 2-phenylethyl-, 3-(methylthio)propyl-, and 2-methylpropylamine. Its application on unfermented, fermented, and roasted cocoas from Ghana and Sulawesi revealed an increase of all amines, except the 3-(methylthio)propylamine, during cocoa fermentation, suggesting an enzymic formation from the parent amino acids isoleucine, leucine, phenylalanine, and valine. However, a much more pronounced formation of most of the amines was measured after roasting of the cocoa, leading to concentrations in the milligrams per kilogram range. This result suggested a new "thermogenic" formation pathway of "biogenic amines". A comparison of the amounts of the amines and the aldehydes in roasted cocoa revealed similar concentrations, for example, for 2- and 3-methylbutanal and the respective amines, whereas the amounts of 2-phenylethylamine were much higher as compared to the amounts of phenylacetaldehyde. Strecker-type model systems, in which each parent amino acid was reacted with 2-oxopropanal, revealed the formation of both the amine and the aldehyde; however, in contrast to cocoa, the concentrations of the aldehydes were always much higher as compared to the amines. The results showed for the first time the thermally induced generation of "biogenic amines" from amino acids. Possible reasons for the different ratios of amines versus aldehydes formed during the roasting of cocoa or the model systems, respectively, are discussed.     

Characterization of pyrazines in some Chinese liquors and their approximate concentrations

Pyrazines are very important impact aroma compounds in Chinese liquors. The identification of pyrazine derivatives was carried out by liquid-liquid extraction (LLE). The liquor sample was adjusted to the H(+) concentration of 1 N with 12 N HCl and then concentrated by rotatory evaporator under vacuum condition. The concentrated liquor was extracted by diethyl ether, and the residual aqueous phase was adjusted to pH 10. The basic compounds were detected and identified by gas chromatography (GC)-mass spectrometry (MS). A total of 27 pyrazines were identified in Chinese liquors, mainly alkyl- and acetylpyrazines. A method for determining pyrazines in Chinese liquors was developed. It involves extraction by headspace (HS) solid phase microextraction (SPME) and determination using GC-flame thermionic detector (FTD). The optimum method was that the sample alcohol concentration was diluted to 12% vol by freshly redistilled-deionized water, and the diluted samples were saturated with NaCl and equilibrated at 50 degrees C for 15 min and extracted for 30 min at the same temperature. The developed method enabled detection limits of <200 ng/L. Linearity (R(2) > 0.99) and recovery rate were satisfied in all cases. Pyrazines of 12 commercial typical Chinese liquors were quantified by HS-SPME followed by GC-FTD and had a wide range of concentration.     

Chitin decomposition in a model soil system

The effect of various organisms on the decompositon of chitin in a gnotobiotic soil system was investigated. Chitin decomposers were isolated from the short grass prairie in Colorado and selected by their ability to use chitin as a source of both C and N. Three bacteria, a fungus, and an actinomycete were grown for 45 days in sterile chitin amended (3 mg g^?1 chitin-C) and unamended soil microcosms. Net mineralization of ammonium was greatest in the chitin-ainended microcosms. The greatest increases in N mineralization occurred in chitin-amended microcosms containing the fungus and the actinomycete. A second series of sterile soil microcosms amended with chitin (3 mg g^?1 chitin-C) were inoculated with decomposers, a fungus and a bacterium, and a nematode and an amoeba (bacteriophagic grazers) in various combinations. Bacterial and grazer populations, NH₄⁺ CO₂ evolution, and residual chitin were measured periodically for 80 days. Bacterial grazing reduced bacterial populations, increased N mineralization, but had no effect on the decomposition of chitin.     

Evidence of Strecker aldehyde excretion by yeast in cold contact fermentations

A medium containing labeled leucine-d(10) has been used to show that Saccharomyces cerevisiae was able to produce deuterated 3-methylbutanal in a cold contact fermentation. Whereas residual unreduced Strecker aldehydes bound to polyphenols were considered until now as the main defect of alcohol-free beers, yeast emerges from this work as an additional source of worty flavor. When the same experiment was conducted without leucine, 3-methylbutanal excretion also occurred, indicating that valine could be used by yeast as precursor.