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.     

向日葵籽壳热解反应动力学的研究

采用热重分析法(TG)在不同升温速率下(5、10、20、30℃/min)对粒径为0.154～0.280 mm向日葵籽壳热解的热失重行为进行了研究。结果表明,向日葵籽壳热解分为四个阶段,随着升温速率的提高,各个阶段的起始和终止温度向高温侧稍微移动,并且主反应区间也略有增加。Ozawa法和Starink法计算得出的向日葵籽壳在热解过程中不同失重率下的活化能(E)都集中在140～178 kJ/mol范围内。用积分法Coats-Redfern方程、微分法Achar方程以及热分析动力学三因子求算的比较法得出该反应过程的机理函数表达式。经过对41种常用机理函数一一代入得出Jander方程能较好地描述向日葵籽壳热解反应过程,机理为三维扩散,球形对称,反应级数n=2。该研究可为生物质热解装置的工艺参数优化提供参考。     

Free radicals in electron donor-acceptor reactions between a soil humic acid and photosynthesis inhibitor herbicides

Products obtained by interaction of a soil humic acid with a number of s-triazines and substituted ureas, well known to act as photosynthesis inhibitor herbicides, were studied by infrared (IR) and electron spin resonance (ESR) spectrometries. Analysis of spectroscopic results showed that single-electron transfer processes take place reasonably between ring or chain donor groups of the herbicide molecule and acceptor quinone units of the humic acid, thus producing additional unpairing of electrons in the resulting charge-transfer complexes. Experimental and theoretical considerations suggested the hypothesis that the investigated herbicides may react chemically with humic compounds by simulating analogous processes that take place on biological scale in chloroplasts, on the basis of similar electron donor-acceptor reactions involving free radicals.     

Kinetic study of oxalic acid inhibition on enzymatic browning

Oxalic acid has a strong antibrowning activity. The inhibitory pattern on catechol-PPO model system appeared to be competitive, with a K(i) value of 2.0 mM. When the PPO was incubated with oxalic acid, the activity was not recovered via dialysis, but the inactivated enzyme partially recovered its activity when cupric ion was added. Comparing the relative antibrowning effectiveness of oxalic acid with other common antibrowning agents, oxalic acid with I(50) value of 1.1 mM is as effective as kojic acid and more potent than cysteine and glutathione.     

A comparative study of the antioxidant power of flavonoid catechin and its planar analogue

The antioxidant ability of the flavanol catechin and its planar derivative, catechin 1 (PC1), was explored using the DF/B3LYP theoretical approach. Their potentiality in the hydrogen abstraction and electron transfer reactions, the main working mechanisms of antioxidants, was evaluated by computing the values of two key parameters, which are the OH bond dissociation energy and the ionization potential. Results indicated that the effect of a planar arrangement in the catechin molecule is small in the case of the hydrogen abstraction but greater for the electron transfer, since the in vacuo ionization potential value decreases by about 3 kcal/mol. The reaction of these molecules with the hydroperoxyl radical (*)OOH indicated that the H(*) abstraction is faster with the planar catechin.     

LC-ESI-MS study of the flavonoid glycoside malonates of red clover (Trifolium pratense)

High-performance liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) was applied to the analysis of the flavonoids and their glycoside malonates of the flowers and leaves of red clover (Trifolium pratense). Through LC-MS comparative studies on the plant extracts and their malonate-free extracts, approximately 20 flavonoid glycoside malonates were detected in the flower extract. Eight were identified as genistin 6' '-O-malonate (39), formononetin 7-O-beta-D-glucoside 6' '-O-malonate (40), biochanin A 7-O-beta-D-glucoside 6' '-O-malonate (41), trifoside 6' '-O-malonate (42), irilone 4'-O-beta-D-glucoside 6' '-O-malonate (43), pratensein 7-O-beta-D-glucoside 6' '-O-malonate (44), isoquercitrin 6' '-O-malonate (45), and 3-methylquercetin 7-O-beta-D-glucoside 6' '-O-malonate (46). About 15 other flavonoids and clovamides were proved to be present in this extract. The study also found that the flowers contained flavones as the major flavonoids, whereas the leaves had isoflavones as the major flavonoids. This is the first detection of the six malonates (39 and 42-46) in the extracts of red clover, and among them, 42, 43, and 46 are new compounds.     

Kinetic study of transformations of arsenic species during heat treatment

The combination of temperatures and pH levels applied in domestic or industrial cooking and in the sterilization of seafood might cause the transformation of certain species of arsenic into other more toxic species, which could pose a risk to the consumer. To clarify the effect of the temperatures traditionally used in cooking or sterilization on the stability of the various species of arsenic, a kinetic study was carried out, using standards of arsenobetaine (AB), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), trimethylarsine oxide (TMAO), tetramethylarsonium ion (TMA(+)), and arsenocholine (AC) heated at different temperatures (85--190 degrees C) and for different treatment times. Various pH levels (4.5, 5.5, 6.5, and 8.0) were applied during the heating process. The results obtained indicated that there were no transformations of arsenic species after temperature treatments up to 120 degrees C. However, when temperatures between 150 and 190 degrees C were used, a partial decomposition of AB was achieved, producing TMAO at 150 degrees C and TMAO and TMA(+) at temperatures of 160 degrees C or above, in proportions that varied according to the temperature and duration of the heat treatment.     

Kinetic study of the acid hydrolysis of various oligofructose samples

The kinetic of acid hydrolysis of five commercially available oligofructose samples used as food ingredients has been investigated as a function of the dry matter concentration, reaction pH, and temperature. The initial fructose release rate is found to be roughly proportional to the inverse of the average polymerization degree in number. A pseudo first order kinetic is found with respect to the fructosyl chain end concentration and to the proton concentration. Arrhenius plot is found to reasonably fit the data in a relatively wide temperature range (7-130 degrees C). The results allow the estimation of the fructose release rate in many foodstuff processing conditions.     

Kinetic study of the oxidation of quercetin by mushroom tyrosinase

The kinetic behavior of mushroom tyrosinase in the presence of the flavonol quercetin was studied. This flavonol was oxidized by mushroom tyrosinase and the reaction was followed by recording spectral changes over time. The spectra obtained during the reaction showed two isosbectic points, indicating a stable o-quinone. When quercetin was oxidized by tyrosinase in the presence of cysteine and 3-methyl-2-benzothiazolone hydrazone (Besthorn's hydrazone, MBTH) isosbestic points were also observed indicating a definite stoichiometry. From the data analysis of the initial rate in the presence of MBTH, the kinetic parameters: = (16.2 +/- 0.6) microM/min, = (0.12 +/- 0.01) mM, (/) = (V(max)/K(S)(')()) = (13.5 +/- 1.4) x 10(-)(2) min(-)(1), = (6.2 +/- 0.6) s(-)(1) were determined. We propose that quercetin acts simultaneously as a substrate and a rapid reversible inhibitor of mushroom tyrosinase, depending on how it binds to the copper atom of the enzyme active site. Thus, if the binding occurs through the hydroxylic groups at the C3' and C4' positions, quercetin acts as a substrate, while if it occurs through the hydroxylic group at the C3 position of the pyrone ring, quercetin acts as an inhibitor.     

Two types of radicals in whole milk powder. Effect of lactose crystallization, lipid oxidation, and browning reactions

Whole milk powder was stored in closed vials at 60 degrees C to induce crystallization of lactose within a short time scale. After an induction period of 3-4 days simultaneous crystallization of lactose, increase of water activity, formation of browning products, and increase of radical content took place. Radicals detected before lactose crystallization were characterized by a narrow ESR spectrum (g = 2.006) and could be depleted by removal of oxygen and therefore were assigned to oxidation processes. Late-stage radicals present after crystallization of lactose gave much wider spectra (g = 2.0048) and were independent of oxygen availability and assigned to late-stage Maillard reaction products. The study indicates that the processes of lactose crystallization, browning, and formation of radical species (g = 2.0048) are strongly coupled, while lipid oxidation is less dependent on the other processes.     

Role of hydroxyl radicals and singlet oxygen in the formation of primary radicals in unsaturated lipids: a solid state electron paramagnetic resonance study

Primary radicals were generated by UV photolysis of samples of trilinolein, at 77 K and under a controlled atmosphere. The resulting EPR spectra clearly show that the amount of radicals is dependent on the purity of the lipid, the exposure to visible light in the presence of a photosensitizer and oxygen, and, finally, the presence of an antioxidant. These solid state EPR experiments indicate that if all of the elements for the production of singlet oxygen (Rose Bengal, molecular oxygen, and visible light) are not present, primary radicals are practically not generated. They also point out the various steps of the oxidation mechanism: formation of singlet oxygen, which reacts with the lipid to form a hydroperoxide; and photolytic formation of the hydroxyl radical, which reacts with the frozen lipid to generate primary lipidic radicals. This constitutes a new method for investigating lipid oxidation and studying the influence of photosensitizers and molecules that are likely to react with singlet oxygen.     

Kinetic study of the thermal hydrolysis of Agave salmiana for mezcal production

The kinetics of the thermal hydrolysis of the fructans of Agave salmiana were determined during the cooking step of mezcal production in a pilot autoclave. Thermal hydrolysis was achieved at different temperatures and cooking times, ranging from 96 to 116 °C and from 20 to 80 h. A simple kinetic model of the depolymerization of fructans to monomers and other reducing sugars and of the degradation of reducing sugars to furans [principally 5-(hydroxymethyl)furfural, HMF] was developed. From this model, the rate constants of the reactions were calculated, as well as the pre-exponential factors and activation energies of the Arrhenius equation. The model was found to fit the experimental data well. The tradeoff between a maximum fructan hydrolysis and a critical furan concentration in allowing for the best ethanol yield during fermentation was investigated. The results indicated that the thermal hydrolysis of agave was optimal, from the point of view of ethanol yield in the ensuing fermentation, in the temperature range of 106-116 °C and the cooking range time of 6-14 h. The optimal conditions corresponded to a fructan hydrolysis of 80%, producing syrups with furan and reducing sugar concentrations of 1 ± 0.1 and 110 ± 10 g/L, respectively.     

Regiospecific flavonoid 7-O-methylation with Streptomyces avermitilis O-methyltransferase expressed in Escherichia coli

O-Methylation, commonly found in synthesis of secondary metabolites of plants and micro-organisms, appears to transfer a methyl group to the hydroxyl group of the recipient which increases the hydrophobicity of the recipient. O-Methyltransferase (OMT), , was isolated and characterized from Streptomyces avermitilis MA-4680. Its amino acid sequence showed 68% similarity with antibiotic C-1027 OMT and 53% similarity with the carminomycin 4-OMT. was expressed in E. coli as a His-tag fusion protein and showed that the methyl was transferred onto the 7-hydroxyl group of the isoflavones, daidzein and genistein, and the flavones, kaempferol and quercetin, as well as the flavanone naringenin. NMR and liquid chromatography-mass spectrometry were used to confirm the location of the methyl group on the recipient compound of naringenin, which was biotransformed into sakuranetin by E. coli transformant expressing (E. coli Sa-2). Therefore, E. coli Sa-2 would be used for the synthesis of the antifungal flavonoid, sakuranetin, through biotransformation.     

Free radicals scavenging efficiency of a few naturally occurring flavonoids: a comparative study

The interaction of antiperoxidative flavonoids artocarpin (AR), cycloartocarpin (CAR), dalspinin (DP), dalspinosin (DPO), and dalspinin-7-O-galactoside (DPG) with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS(.+)) and O2(-.) was studied in phosphate buffer (pH 7.4). The ability of these compounds to inhibit lipid peroxidation and DNA scission was also investigated. The radical scavenging efficiency of flavonoids is demonstrated by the reduction of nitrogen-centered radical cation (ABTS(.+)). The reduction of ABTS(.+) follows the order quercetin > morin > Trolox > AR > DPO > CAR > DP. Inhibition of lipid peroxidation was studied by following Mb(IV) reduction, induced by lipid, arachidonic acid. These results are compared with those obtained for well-known antioxidants such as quercetin, morin, and Trolox. The structure-activity relationships between chemical structures of the flavonoids and their radical scavenging activities are anlayzed. The scavenging of O2(-.), inhibition of lipid peroxidation, and DNA damage depend on the oxidation potential of the flavonoids. The possible mechanism for radical scavenging activities of flavonoids in relation to their structure is also outlined.     

A comparative study of flavonoid compounds, vitamin C, and antioxidant properties of baby leaf Brassicaceae species

A comparative study of antioxidant compounds, flavonoids and vitamin C, and also antioxidant activity was carried out in four species of Brassicaceae vegetables used for salads: watercress ( Nasturtium officinale R. Br.), mizuna [ Brassica rapa L. subsp. nipposinica (L.H. Bailey) Haneltand], wild rocket [ Diplotaxis tenuifolia (L.) DC.], and salad rocket [ Eruca vesicaria (L.) Cav.]. The characterization of individual phenolic compounds by HPLC-DAD-MS/MS-ESI in watercress and mizuna completes the polyphenol study previously reported for wild rocket and salad rocket. The qualitative study of flavonoids in watercress leaves showed a characteristic glycosylation pattern with rhamnose at the 7 position. Isorhamnetin 3,7-di- O-glucoside was identified in mizuna leaves and may be considered a chemotaxonomical marker in some B. rapa subspecies. Brassicaceae species showed differences in the quantitative study of flavonoids, and the highest content was detected in watercress leaves. Watercress and wild rocket leaves had the highest content of vitamin C. The antioxidant activity evaluated by different methods (ABTS, DPPH, and FRAP assays) showed a high correlation level with the content of polyphenols and vitamin C. In conclusion, the Brassicaceae leaves studied, watercress, mizuna, wild rocket, and salad rocket, presented a large variability in the composition and content of antioxidant compounds. These baby leaf species are good dietary sources of antioxidants with an important variability of bioactive compounds.     

Quantitation of flavonoid constituents in citrus fruits.

Twenty-four flavonoids have been determined in 66 Citrus species and near-citrus relatives, grown in the same field and year, by means of reversed phase high-performance liquid chromatography analysis. Statistical methods have been applied to find relations among the species. The F ratios of 21 flavonoids obtained by applying ANOVA analysis are significant, indicating that a classification of the species using these variables is reasonable to pursue. Principal component analysis revealed that the distributions of Citrus species belonging to different classes were largely in accordance with Tanaka's classification system.     

Equilibrating stable Cd isotopes with acidic soils

ABSTRACT

Cadmium (Cd) in the soil solution is in dynamic equilibrium with the reservoir of bioavailable Cd attached to the solid phase, i.e. the labile pool (Cd_E). Traditionally, Cd_E is estimated using the radioisotope ¹⁰⁹Cd, which has severely restricted access to estimates of Cd_E. Using stable isotope dilution and isotope ratio measurement by inductive coupled plasma-quadrupole mass spectroscopy (ICP-QMS) would increase access to estimates of Cd_E; however, detail remains scant about the optimal conditions for equilibration and measurement. We report optimal conditions for spiking with ¹¹⁰Cd, batch equilibration and ICP-QMS measurement of the ratio of ¹¹⁰Cd to ¹¹¹Cd using results for six acidic soils with total Cd concentrations of 0.19–6.4 mg Cd kg^?1, suspended in three background electrolytes (10 mM CaCl₂, 1 M NH₄NO₃, and 1 M NH₄Cl). Our optimised procedure produces robust estimates of Cd_E. Application of this approach will greatly increase access to estimates of Cd_E and to the investigation of its role in Cd uptake by plants.     

玉米苞叶中总黄酮提取及其体外抗氧化性能的研究

运用超声波技术提取玉米苞叶中的总黄酮,用亚硝酸钠-硝酸铝比色法测定总黄酮含量,得出玉米苞叶中总黄酮提取的最佳工艺条件:超声波功率700W,温度60℃,料液比(g/mL)1∶50,乙醇浓度为60%,提取时间为35min,玉米苞叶中总黄酮最大提取率为1.225%。体外抗氧化试验表明:玉米苞叶中的总黄酮对超氧阴离子自由基、羟基自由基清除能力较好,其IC50分别为1.5μg/mL左右和0.14mg/mL,清除能力均高于Vc;对DPPH自由基体系的清除率达到50%时,玉米苞叶中总黄酮的IC50为0.2mg/mL左右,Vc的IC50为0.075mg/mL,说明玉米苞叶中的总黄酮清除DPPH自由基的能力低于Vc,但在所选浓度剂量范围内,其最大清除率高于Vc。试验表明,玉米苞叶中的总黄酮是一种较好的天然自由基清除剂。     

Identification of new flavonoid glycosides and flavonoid profiles to characterize rocket leafy salads (Eruca vesicaria and Diplotaxis tenuifolia)

"Rocket" is a collective name used to term some species within the Eruca and Diplotaxis genera, whose leaves are characterized by a more or less pungent taste. Different approaches have been carried out to differentiate both genera that have similar leaf morphologies. Following our research in flavonoid profiling of the Brassicaceae family using high-performance liquid chromatography/ultraviolet-diode array detection/electrospray ionization mass spectroemtry, we have investigated Eruca vesicaria and Diplotaxis tenuifolia leaf samples as new ingredients of fresh salads. The MS/MS study allowed the identification of new naturally occurring quercetin mono- and diacyl-tri-O-glucosides and the elucidation of the flavonoid glycosylation and acylation patterns. Important differences between flavonoid profiles of E. vesicaria and D. tenuifolia were observed. E. vesicaria contained kaempferol derivatives as principal compounds whereas D. tenuifolia instead accumulated quercetin derivatives. The exhaustive study of the profiling of these species could help further studies concerning the bioavailability of these flavonoids for epidemiological or clinical intervention studies because these species have considerable potential as healthy leafy salads because of the bioactive phytochemicals.     

Re-examination of chromogenic quantitative assays for determining flavonoid content

Flavonoids in plants have gained worldwide attention because of their benefits for human health. This study compared three analytical procedures commonly used for determining flavonoid content in plant samples in terms of chromogenic relationships and the reaction products of different flavonoid structures by means of using flavonoid standards with flavone, flavonol, flavanone, flavanol, and isoflavone and analytes such as phenolic acids commonly found in plant extracts. Procedure A produced a stable color reaction between 3-hydroxy-4-keto-flavonoids (flavonols) and 5-hydroxyflavones and was highly sensitive. Procedure B produced color reactions among most of the flavonoids, but the reaction products had different colors and faded over time. Procedure B also produced a color reaction with caffeic and chlorogenic acid. Procedure C was the most sensitive. It produced a color reaction and, like procedure A, could be used to quantify flavonols and 5-hydroxyflavones, but also showed color reaction toward caffeic and chlorogenic acid. On the basis of the results, the current three procedures are not satisfactory for determining all of the types of flavonoid. Two issues needed to be clarified before a promising determination of flavonoid content could be performed with chromogenic assays. The first is a survey of the literature to screen the possible predominant component of flavonoid in analytes. The other is guided by the predominant flavonoid; a promising calibration curve for flavonoid detection can be established on the basis of the selection of an appropriate method and a chemical standard with an equivalent dose response to the predominant flavonoid.