Toward a kinetic model for acrylamide formation in a glucose-asparagine reaction system

A kinetic model for the formation of acrylamide in a glucose-asparagine reaction system is pro-posed. Equimolar solutions (0.2 M) of glucose and asparagine were heated at different temperatures (120-200 degrees C) at pH 6.8. Besides the reactants, acrylamide, fructose, and melanoidins were quantified after predetermined heating times (0-45 min). Multiresponse modeling by use of nonlinear regression with the determinant criterion was used to estimate model parameters. The proposed model resulted in a reasonable estimation for the formation of acrylamide in an aqueous model system, although the behavior of glucose, fructose, and asparagine was slightly underestimated. The formation of acrylamide reached its maximum when the concentration of sugars was reduced to about 0. This supported previous research, showing that a carbonyl source is needed for the formation of acrylamide from asparagine. Furthermore, it is observed that acrylamide is an intermediate of the Maillard reaction rather than an end product, which implies that it is also subject to a degradation reaction.     

农药降解动力学模型的改进研究

研究构造出生物和非生物非线性影响因子函数1-a(c/ c0 ) θ,提出一种通过参数调整兼容指数衰减、线性和非线性作用等各种影响机制的自适应的农药降解非线性动力学模型,该改进模型直观且适用性广,可描述降解曲线凹凸有拐点的情形,用于预测时间间隔相同、连续施药的累积残留量及其极限值,实例应用表明其拟合精度和拟合效果明显优于其他模型     

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.     

Glucosylisomaltol,a new indicator of browning reaction in baby cereals and bread

Glucosylisomaltol is proposed as a new indicator of the browning reaction in baby cereals and bread. The glucosylisomaltol was synthesized from maltose and proline, purified by semipreparative HPLC, and characterized by NMR, high-resolution mass spectrometry, and GC-MS analysis. Analysis of glucosylisomaltol, previously separated from cereals by centrifugation, was carried out by reversed-phase HPLC with UV detection in isocratic elution with water/acetonitrile (95:5). Mean recovery of glucosylisomaltol by the standard addition method was 96.9%. The relative standard deviation and detection limit were 1.56% and 0.14 mg/kg, respectively. This compound was identified in samples by the similarity of the t(R) and UV spectra to those of synthesized glucosylisomaltol. Moreover, the glucosylisomaltol from samples, previously separated by semipreparative HPLC, was acetylated and then separated and confirmed by GC-MS. Glucosylisomaltol was determined in baby cereals stored at 32 and 55 degrees C for 1 year and at 25 and 55 degrees C for 1 month at a water activity of 0.65. The amount of this indicator increased during storage from 0.48 to 7.7 mg/kg. The glucosylisomaltol was also determined in prebaked bread by heating at 190 degrees C for 30 min. The amount of this compound increased from nondetectable to 20.9 mg/kg after 30 min of baking. Glucosylisomaltol is a useful indicator to control the browning reaction during baby cereal storage and the baking of bread.     

Nonenzymatic browning reaction of essential amino acids: effect of pH on caramelization and Maillard reaction kinetics.

The interaction between glucose and essential amino acids at 100 degrees C at pH values ranging from 4.0 to 12.0 was investigated by monitoring the disappearance of glucose and amino acids as well as the appearance of brown color. Lysine was the most strongly destroyed amino acid, followed by threonine which induced very little additional browning as compared with that undergone by glucose. Around neutrality, the nonenzymatic browning followed pseudo-zero-order kinetics after a lag time, while the glucose and amino acid losses did not follow first-order kinetics at any of the pH values tested. Glucose was more strongly destroyed than all of the essential amino acids, the losses of which are really small at pH values lower than 9.0. However, glucose was less susceptible to thermal degradation in the presence of amino acids, especially at pH 8.0 with threonine and at pH 10.0 with lysine. The contribution of the caramelization reaction to the overall nonenzymatic browning above neutrality should lead to an overestimation of the Maillard reaction in foods.     

Ascorbic acid-induced browning of (+)-catechin in a model wine system

The ability of ascorbic acid to induce browning of (+)-catechin in a model wine system has been studied. A significant increase in absorbance at 440 nm was observed over 14 days when ascorbic acid was incubated at 45 degrees C with (+)-catechin in a model wine base. The onset of browning was delayed for about 2 days, although the length of the lag period was dependent on the amount of molecular oxygen in the headspace of the reaction system. The lag period was not observed when a preoxidized solution of ascorbic acid was used, suggesting that a product of ascorbic acid oxidation is responsible for the onset of browning. Hydrogen peroxide, when added directly to (+)-catechin in the model system, was not capable of producing the same degree of browning as that generated by ascorbic acid. Liquid chromotography evidence is presented to show that different reaction products are produced by ascorbic acid and hydrogen peroxide.     

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.     

应用多级反应动力学模型研究土壤甲氨基酚吸附

Metolachlor retention on a Sharkey clay soil was quantified using a kinetic batch method for different initial solution concentrations.Time-dependent adsorption was carried out by monitoring solution concentration at different reaction times.Adsorption was kinetic multireaction model which includes reverible and irreversible retention processes of the equilibrium and kinetic types,The predictive capability of the model for the dexcription of experimental results for metolachlor retention was examined and proved to be adequate。     

Maillard reaction and enzymatic browning affect the allergenicity of Pru av 1, the major allergen from cherry (Prunus avium)

The influence of thermal processing and nonenymatic as well as polyphenoloxidase-catalyzed browning reaction on the allergenicity of the major cherry allergen Pru av 1 was investigated. After thermal treatment of the recombinant protein rPru av 1 in the absence or presence of carbohydrates, SDS-PAGE, enzyme allergosorbent tests, and inhibition assays revealed that thermal treatment of rPru av 1 alone did not show any influence on the IgE-binding activity of the protein at least for 30 min, thus correlating well with the refolding of the allergen in buffer solution as demonstrated by CD spectroscopic experiments. Incubation of the protein with starch and maltose also showed no effect on IgE-binding activity, whereas reaction with glucose and ribose and, even more pronounced, with the carbohydrate breakdown products glyceraldehyde and glyoxal induced a strong decrease of the IgE-binding capacity of rPru av 1. In the second part of the study, the effect of polyphenoloxidase-catalyzed oxidation of polyphenols on food allergen activity was investigated. Incubation of rPru av 1 with epicatechin in the presence of tyrosinase led to a drastic decrease in IgE-binding activity of the protein. Variations of the phenolic compound revealed caffeic acid and epicatechin as the most active inhibitors of the IgE-binding activity of rPru av 1, followed by catechin and gallic acid, and, finally, by quercetin and rutin, showing significantly lower activity. On the basis of these data, reactive intermediates formed during thermal carbohydrate degradation as well as during enzymatic polyphenol oxidation are suggested as the active chemical species responsible for modifying nucleophilic amino acid side chains of proteins, thus inducing an irreversible change in the tertiary structure of the protein and resulting in a loss of conformational epitopes of the allergen.     

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.     

阻尼喷头动能分布试验及模型

针对阻尼喷头外流场水力学特性研究较少的问题,该文研究了单位体积动能、动能强度与有无散水齿、不同喷嘴直径之间的关系。在工作压力分别为175、200、250和300 k Pa,喷嘴直径分别为3.6、4.0、4.4和4.8 mm下,采用激光雨滴谱仪对Nelson R33阻尼喷头的水滴直径、速度和数目等参数进行试验测试,并对试验结果分析及模型建立。结果表明:有无散水齿条件下的单位体积动能均有逐渐增大的趋势;距喷头相同测点处,单位体积动能随喷嘴直径的增大而减小;给出了有无散水齿下不同喷嘴阻尼喷头单位体积水滴动能分布模型,相关系数均在0.94以上;在距喷头0~4 m,不同喷嘴直径的动能强度较小且差值不超过0.002 W/m2,在距喷头较远处,不同喷嘴直径的动能强度差值较大,最大差值达到0.006 W/m2。有散水齿时,动能强度在不同压力下波动均比较小,差值不超过5%。该结果为进一步研究阻尼喷头外流场水力学特性提供理论依据。     

A kinetic study on the isomerization of hop alpha-acids

In this article, a detailed study on hop alpha-acid isomerization kinetics is presented. Because of the complex wort matrix and interfering interactions occurring during real wort boiling (i.e., trub formation and alpha-acids/iso-alpha-acids complexation), this investigation on alpha-acid isomerization kinetics was performed in aqueous buffer solution as a function of time (0-90 min) and heating temperature (80-100 degrees C). Rate constants and activation energies for the formation of individual iso-alpha-acids were determined. It was found that iso-alpha-acid formation follows first-order kinetics and Arrhenius behavior. Differences in activation energies for the formation of trans- and cis-isomers were noticed, the activation energy for the formation of trans-iso-alpha-acids being approximately 9 kJmol (-1) lower.     

pH胁迫下尖孢镰刀菌生长动力学模型

尖孢镰刀菌(Fusarium oxysporum)是兼性寄生真菌,引起农作物枯萎病,防治困难。研究来自不同寄主的尖孢镰刀菌在不同pH马铃薯蔗糖(PS)琼脂培养基上的生长情况,构建动力学模型,以了解培养基的初始pH差异对尖孢镰刀菌生长特性的影响。测定了尖孢镰刀菌菌株在不同pH培养基、25℃培养条件下的生长速度,构建生长动力学模型。供试6个尖孢镰刀菌菌株在pH为3~10的液体培养基中均有不同程度生长,不同菌株对培养基pH的影响规律相似,在不同pH的PS液体培养基中培养14 d后,液体培养基的pH均有靠近6~7的趋势。在pH为3~9的培养条件下,尖孢镰刀菌菌落可以分成4类:菌丝型、粘滑层型、菌丝粘滑层型和菌丝带粉状物,各菌株的菌丝和培养基色泽也有所差异。同一菌株在培养基不同pH下生长速度不同,其生长的最适pH为6~8,菌落平均直径最大(45~49 mm),亚适pH为4、5和9;而在pH为3、10和11时菌落直径明显变小(17~21 mm)。培养14 d后培养基pH对菌落形态、色泽有一定的影响,不同菌株在不同pH培养基培养下产孢量也存在差异。pH在4~8时其平均产孢量最大,达(223.8~273.3)×104cfu.mL 1,其中pH为6.38(自然pH)时产孢量最高,pH为11时产孢量最低。供试菌株在不同pH条件下的菌落生长速度和产孢量变化动力学模型均符合二次曲线方程。     

棉籽粕固态发酵过程及其动力学模型构建

为了研究枯草芽孢杆菌Bs-1在工业化条件下发酵棉籽粕的过程,该文采用槽式发酵方式,测定了发酵过程中物料堆温、含水率、粗蛋白、酸溶性蛋白和游离棉酚质量分数;并采用Logistic方程建立芽孢杆菌Bs-1生长模型,在其与粗蛋白、酸溶性蛋白和游离棉酚质量分数相关性分析的基础上,构建了固态发酵动力学模型并进行了发酵成本分析。结果表明:棉籽粕槽式发酵过程中的温度和含水率均呈现一定的变化规律;粗蛋白质量分数、酸溶性蛋白质量分数分别较发酵前提高了9.28%和46.51%,游离棉酚质量分数降低了42.31%。Bs-1的生长与粗蛋白质量分数(R2=0.831)和酸溶性蛋白质量分数(R2=0.867)呈正相关,与游离棉酚质量分数(R2=0.976)呈负相关;建立了棉籽粕固态发酵动力学模型。分析表明,该模型计算值与试验值能较好吻合,且发酵成本低。表明该模型可应用于工业化生产,对实现发酵棉籽粕的产业化生产,具有较好的参考价值。     

A one-parameter model for the soil water characteristic

The two-parameter model In φ= a+b ln θis widely accepted in the literature as providing a good fit to the soil moisture characteristic for a wide variety of soil types. This model is examined and it is shown that the parameters a and b are not independent, and hence that the SMC may be represented by a one-parameter model with little loss of accuracy. Thus it is possible to define the SMC from one paired measurement of soil water content against potential. Because this single paired measurement is all that is required it also avoids the necessity of classification of the soil type–indeed it may provide an additional means of soil identification.     

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.     

Characterization of reaction products formed in a model reaction between pentanal and lysine-containing oligopeptides

Aldehydes formed as a result of lipid oxidation form fluorophores after binding to proteins. The structure of the fluorophores formed by reaction between saturated aldehydes and lysine has not yet been identified. The reaction products formed in the reaction between pentanal and oligopeptides were studied by fluorescence spectroscopy and mass spectrometry. The emission spectra showed an increase in fluorescence intensity with incubation time, and the rates were linear with the concentration of pentanal. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analyses of the reaction products suggested a molar relation for peptide:pentanal of 1:4. Further tandem mass spectrometry analysis of one of the modified peptides (Pro-Thr-His-Ile-Lys-Trp-Gly-Asp) strongly suggested binding of one pentanal molecule to the amino terminal proline and three pentanal molecules bound to the lysine residue. The latter species is suggested to be the actual fluorophore, through the formation of conjugated double bonds, and a possible reaction pathway through a combination of aldol condensation of pentanal and Schiff base formation with the lysine is suggested.     

Tyrosinase inhibitory activity of cucumber compounds: enzymes responsible for browning in cucumber

The inhibition of mushroom tyrosinase by cucumber extracts was evaluated. The inhibitory effect was measured by both polarographic and spectrophotometric methods. The commercial aldehyde, trans,cis-2,6-nonadienal, described as a major volatile compound of cucumber, was characterized as a noncompetitive inhibitor against 4-tert-butylcatechol oxidation by mushroom tyrosinase. The K(I) obtained was 3.4 mM. Polyphenol oxidase (PPO) activity was not detected in cucumber skin extracts. However, the presence of PPO was revealed by Western blot; a single band was found with a M(r) of 53 kDa. These results support the assumption that the enzyme PPO is present in the cucumber skin, but its activity is inhibited. Peroxidase (PO) was also found in cucumber skin extracts. This enzyme was detected in the soluble fraction but not in the membrane fraction. The kinetic characterization of PO was carried out. Native isoelectric focusing revealed several acidic PO isoenzymes with a pI in the range between 5 and 6, a basic isoenzyme, and one principal neutral isoenzyme of pI = 7.2.     

硫自养反硝化对含盐水体脱氮及其动力学模型

以闭合循环养殖系统去除硝酸盐为目的,研究了填料床硫自养反硝化反应器对含盐水体的NO₃^--N去除效果及动力学特性。结果表明,反应器对NO₃^--N浓度为22.5～368 mg/L的含盐水体具有良好的反硝化性能。（29±1）℃条件下,进水NO₃^--N负荷0.052～1.088 kg/(m³·d)为最适进水负荷范围,NO₃^--N去除率大于95%,出水NO2--N浓度小于1 mg/L。进水NO₃^--N负荷2.171 kg/(m³·d)时,达到最大NO₃^--N体积负荷去除率,为1.65 kg/(m³·d)。动力学研究结果表明反应器填料表面生物膜对污染物NO₃^--N的去除呈半级反应速率关系,反应器单位体积半级动力学常数K^1/2v为7.84～ 8.5 mg^1/2/(L^1/2·h)。建立的动力学模型采用该值的计算结果可以预测出水NO₃^--N的浓度,预测值与实际值采用统计软件SAS 8.0做方差分析表明,Pr＞F值分别为0.9732和 0.8845,模型预测值与实际值无显著性差异。     

多酚氧化酶高强度脉冲磁场灭活及动力学模型

为了找到一种有效控制果蔬中多酚氧化酶（PPO）活性的方法,该文研究了高强度脉冲磁场（PMF）对PPO活性的影响,并进行了灭酶动力学模型的研究。结果表明,当PPO于磁场强度2.5、3.5和4.5特斯拉（T）分别处理5至40个脉冲时,酶的残余活性随着磁场强度和脉冲数的增加而逐渐降低。在4.5 T处理40个脉冲时,酶的灭活率最高达到93.10%。对灭活动力学曲线分别用Bigelow模型、Weibull模型和Hülsheger模型进行拟合,发现Weibull模型对PMF下PPO的灭活的拟合度最好。可见,高强度脉冲磁场可以作为一种有效杀灭果蔬中多酚氧化酶的非热技术,且酶的灭活过程符合Weibull模型,该模型可以为实际应用提供参考。