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1.
The influence of water activity on the formation and elimination reactions of acrylamide was examined by means of multiresponse modeling on two different levels of complexity: basic equimolar asparagine-glucose systems and equimolar potato-based asparagine-glucose systems. To this end, model systems were first equilibrated to initial water activities in the range of 0.88-0.99 (corresponding roughly to the moisture gradient observed in French fries) and then heated at temperatures between 120 and 200 degrees C during different reaction times. For each sample, the concentration of acrylamide, glucose, asparagine, and aspartic acid was measured, as well as the extent of browning. A mechanistic model was proposed to model the five measured responses simultaneously. For both types of model systems, the model prediction was quite adequate, with the exception of the extent of browning, especially in the case of the potato-based model system. Moreover, the corresponding estimated kinetic parameters for acrylamide formation and elimination did not change significantly (based on a 95% confidence level) within the range of water activities tested, nor between the systems in the absence or presence of the potato matrix. The only remarkable difference was observed for the activation energy of acrylamide elimination, which was lower in the presence of the potato matrix, although not always significant. In general, these results confirm the generic nature of the model proposed and show that the influence of different moisture levels on acrylamide formation and elimination is minimal and that the addition of a potato matrix has little or no influence on the kinetic model and corresponding kinetic parameters.  相似文献   

2.
The kinetics of acrylamide (AA) was analyzed by heating a simple model system consisting of asparagine and glucose, fructose, or sucrose (0.01 M, pH 6) at temperatures between 140 and 200 degrees C. The AA concentration appeared to be the net result of simultaneous formation and elimination. A general kinetic model describing the AA yield was identified, and kinetic parameters were obtained by nonlinear regression on the nonisothermally derived data. On the basis of kinetic parameters, the AA formation appeared to proceed faster and to be more temperature sensitive in the asparagine-glucose than in the asparagine-fructose model system. The AA elimination kinetics, on the other hand, was similar. Significantly less AA was formed in the asparagine-sucrose model system as compared to the model systems with glucose or fructose.  相似文献   

3.
Soybean lipoxygenase (LOX) inactivation [0.4 mg/mL in Tris-HCl buffer (0.01 M, pH 9)] was studied quantitatively under constant pressure (up to 650 MPa) and temperature (-15 to 68 degrees C) conditions and kinetically characterized by rate constants, activation energies, and activation volumes. The irreversible LOX inactivation followed a first-order reaction at all pressure-temperature combinations tested. In the entire pressure-temperature area studied, LOX inactivation rate constants increased with increasing pressure at constant temperature. On the contrary, at constant pressure, the inactivation rate constants showed a minimum around 30 degrees C and could be increased by either a temperature increase or decrease. On the basis of the calculated rate constants at 102 pressure-temperature combinations, an iso-rate contour diagram was constructed as a function of pressure and temperature. The pressure-temperature dependence of the LOX inactivation rate constants was described successfully using a modified kinetic model of Hawley.  相似文献   

4.
Acrylamide formation from asparagine and glucose in different ratios in neutral glycerol/water mixtures was found to increase with decreasing water activity (0.33 < or = aw < or = 0.71 investigated) and increasing temperature (120 degrees C < or = T < or = 160 degrees C investigated). The initial rate of acrylamide formation was found to be approximately proportional to the asparagine concentration for an excess of asparagine, but less dependent on an excess of glucose. A steady-state concentration of acrylamide was established at 160 degrees C after 1 h for aw = 0.33 (30 microg x L-1 for GLU:ASN = 10:1, 11 microg x L-1 for GLU:ASN = 1:1, and 130 microg x L-1 for GLU:ASN = 1:10) and for aw = 0.47 (15 microg x L-1 for GLU:ASN = 10:1 and 80 microg x L-1 for GLU:ASN = 1:10), suggesting a protection by glucose against acrylamide degradation. The energy of activation, as estimated from the temperature dependence of the initial rate, increased with decreasing aw despite a higher rate of formation of acrylamide at low aw. For high aw, water elimination from a reaction intermediate is suggested to be rate determining. For low aw, the increase in energy of activation (and enthalpy of activation) is accordingly counteracted by a more positive entropy of activation, in agreement with decarboxylation as rate determining at low aw.  相似文献   

5.
The effect of high pressure-high temperature (HPHT) processing on the formation of acrylamide and other Maillard-type reaction compounds was investigated in order to elucidate the impact of HPHT conditions on the different stages of the Maillard reaction. This study was performed in equimolar asparagine-glucose model systems that were treated at various HP/HT conditions (100-115 °C, 400-700 MPa, 0-60 min), and, for comparison, the model system was also heat-treated at ambient pressure. On the treated samples, the concentration of acrylamide, reactants, hydroxymethylfurfural, organic acids, and melanoidins was determined and the pH prior to and after treatment was measured. Based on the measured responses, the retarding effect of high pressure on the overall Maillard reaction was demonstrated; no or little differences were observed between 400 and 700 MPa. The study was conducted in two types of buffer, i.e. phosphate and MES buffer. In case of acrylamide, aspartic acid and browning, a higher concentration was generated in the MES buffer system, but these differences with the phosphate buffer system could be ascribed to pH changes resulting from the application of combined high pressure and high temperature. Based on the results, acrylamide formation is not expected to pose a major hazard to HPHT-treated products.  相似文献   

6.
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.  相似文献   

7.
A kinetic study was conducted on the effect of heat pretreatment in the temperature range of 50-85 degrees C at atmospheric pressure and of high hydrostatic pressure pretreatment (100-700 MPa) at four temperatures (10, 25, 40, and 60 degrees C) on the susceptibility of egg white solutions (10% v/v, pH 7.6) to subsequent enzymatic hydrolysis by a mixture of trypsin and alpha-chymotrypsin at 37 degrees C and pH 8.0. Both heat pretreatment at atmospheric pressure and high-pressure pretreatment resulted in an increase in degree of hydrolysis (DH) after 10 min of enzymatic reaction (DH10) of egg white solutions, as measured using the pH-stat method, which could be described by a fractional conversion model (based on an apparent first-order reaction kinetic model). The temperature dependence of the corresponding rate constants could be described by the Arrhenius equation. At elevated pressure, a negative apparent activation energy was obtained, implying an antagonistic effect of pressure and temperature. The pressure dependence of the rate constants could be described by the Eyring equation, and negative activation volumes were observed, which demonstrates the positive effect of pressure on the susceptibility of egg white solutions to subsequent enzymatic hydrolysis.  相似文献   

8.
The lactosylation of alpha-lactalbumin in aqueous solution was followed at pH(c) = 6.0, 6.3, 7.0, 7.3, and 7.9 and constant ionic strength (I = 0.080) at 50-60 degrees C by reversed-phase high-performance liquid chromatography (RP-HPLC) and electrospray mass spectrometry (MS). The rate of the lactosylation reaction increased with increasing pH and with temperature most significantly at lower pH. The rate of lactosylation could be described by an acid dissociation curve corresponding to pK(a) of the epsilon-amino group of lysine in alpha-lactalbumin. From initial rates for conditions of excess of lactose, pseudo-first-order rate constants were calculated and further transferred into second-order rate constants by dividing with the lactose concentration. Second-order rate constants for protonated and unprotonated lysine in alpha-lactalbumin both showed Arrhenius behavior, and using transition-state theory, DeltaH# = 31 +/- 2 kJ/mol and DeltaS# = -266 +/- 48 J/(mol . K) were determined for the unprotonated form and DeltaH# = 158 +/- 49 kJ/mol and DeltaS# = 80 +/- 150 J/(mol . K) for the protonated form, respectively. On the basis of the marked differences in activation parameters, initial formation of a lactosylamine is suggested as rate-determining for reaction between lactose and a protonated lysine in alpha-lactalbumin, while subsequent water elimination to form a Schiff base becomes rate-determining for the unprotonated form.  相似文献   

9.
The effect of pH and temperature on the microbial reductive transformation of pentachloronitrobenzene (PCNB), an organochlorine fungicide, was investigated with a mixed fermentative/methanogenic culture developed from a contaminated estuarine sediment. Culture series were incubated at a temperature range from 4 to 45 degrees C at pH 6.9+/-0.1 and at a pH range from 2.7+/-0.1 to 7.6+/-0.1 at 22 degrees C. Significant differences were observed in terms of biotransformation rate, extent, and products as a function of temperature. Incubation at different pH values resulted in differences in biotransformation rate and extent, but not in terms of products formed. PCNB (3 microM) was transformed to pentachloroaniline (PCA) in all culture series. However, sequential dechlorination of PCA was observed only at a temperature range from 4 to 35 degrees C and at a pH range from 6.2+/-0.1 to 7.6+/-0.1. The highest PCA dechlorination rate was observed at 22 degrees C and at pH 7.6+/-0.1. The effect of temperature on the PCA dechlorination rate was modeled using an Arrhenius relationship, which accounts for both enzyme activation and deactivation. The dechlorination of PCA and chlorinated aniline intermediates was simulated using a branched-chain Michaelis-Menten model, and kinetic constants were determined.  相似文献   

10.
The formation of acrylamide in crystalline model systems based on asparagine and reducing sugars was investigated under low-moisture reaction conditions. The acrylamide amounts were correlated with physical changes occurring during the reaction. Molecular mobility of the precursors turned out to be a critical parameter in solid systems, which is linked to the melting behavior and the release of crystallization water of the reaction sample. Heating binary mixtures of asparagine monohydrate and anhydrous reducing sugars led to higher acrylamide amounts in the presence of fructose compared to glucose. Differential scanning calorimetry measurements performed in open systems indicated melting of fructose at 126 degrees C, whereas glucose and galactose fused at 157 and 172 degrees C, respectively. However, glucose was the most reactive and fructose the least efficient sugar in anhydrous liquid systems, indicating that at given molecular mobility the chemical reactivity of the sugar was the major driver in acrylamide formation. Furthermore, reaction time and temperature were found to be covariant parameters: acrylamide was preferably formed by reacting glucose and asparagine at 120 degrees C for 60 min, whereas 160 degrees C was required at shorter reaction time (5 min). These results suggest that, in addition to the chemical reactivity of ingredients, their physical state as well as reaction temperature and time would influence the formation of acrylamide during food processing.  相似文献   

11.
In this study different Arabica and Robusta coffee beans from different regions of the world were analyzed for acrylamide after roasting in a laboratory roaster. Due to the complex matrix and the comparably low selectivity of the LC-MS at m/ z 72, acrylamide was analyzed after derivatization with 2-mercaptobenzoic acid at m/ z 226. Additionally, the potential precursors of acrylamide (3-aminopropionamide, carbohydrates, and amino acids) were studied. The highest amounts of acrylamide formed in coffee were found during the first minutes of the roasting process [3800 ng/g in Robusta ( Coffea canephora robusta) and 500 ng/g in Arabica ( Coffea arabica)]. When the roasting time was increased, the concentration of acrylamide decreased. It was shown that especially the roasting time and temperature, species of coffee, and amount of precursors in raw material had an influence on acrylamide formation. Robusta coffee contained significantly larger amounts of acrylamide (mean = 708 ng/g) than Arabica coffee (mean = 374 ng/g). Asparagine is the limiting factor for acrylamide formation in coffee. 3-Aminopropionamide formation was observed in a dry model system with mixtures of asparagine with sugars (sucrose, glucose). Thermal decarboxylation and elimination of the alpha-amino group of asparagine at high temperatures (>220 degrees C) led to a measurable but low formation of acrylamide.  相似文献   

12.
Water-soluble O-acetyl galactoglucomannan (GGM) is a softwood-derived polysaccharide, which can be extracted on an industrial scale from wood or mechanical pulping waters and now is available in kilogram scale for research and development of value-added products. To develop applications of GGM, information is needed on its stability in acidic conditions. The kinetics of acid hydrolysis of GGM was studied at temperatures up to 90 degrees C in the pH range of 1-3. Molar mass and molar mass distribution were determined using size exclusion chromatography with multiangle laser light scattering and refractive index detection. The molar mass of GGM decreased considerably with treatment time at temperatures above 70 degrees C and pH below 2. The molar mass distribution broadened with hydrolysis time. A first-order kinetic model was found to match the acid hydrolysis. The reaction rate constants at various pH values and temperatures were calculated on the basis of the first-order kinetic model. Furthermore, the activation energy, E, was obtained from the Arrhenius plot. The activation energy E was 150 kJ mol (-1) for acid hydrolysis of spruce GGM. The apparent rate constant during acid hydrolysis increased by a factor of 10 with a decrease in pH by 1 unit, regardless of temperature. In addition, gas chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry were applied to study the released GGM monomers and oligomers.  相似文献   

13.
The acrylamide content of heated foodstuffs should be considered to be the net result of complex reactions leading to the formation and elimination/degradation of this compound. The present study, involving primarily homogenized potato heated in an oven, was designed to characterize parameters that influence these reactions, including the heating temperature, duration of heating, pH, and concentrations of various components. Higher temperature (200 degrees C) combined with prolonged heating times produced reduced levels of acrylamide, due to elimination/degradation processes. At certain concentrations the presence of asparagine or monosaccharides (in particular, fructose and also glucose and glyceraldehyde) was found to increase the net content of acrylamide. Addition of other free amino acids or a protein-rich food component strongly reduced the acrylamide content, probably by promoting competing reactions and/or covalently binding acrylamide formed. The dependence on pH of the acrylamide content exhibited a maximum around pH 8; in particular, lower pH was shown to enhance elimination and decelerate formation of acrylamide. In contrast, the effects of additions of antioxidants or peroxides on acrylamide content were small or nonexistent.  相似文献   

14.
Inhibition of the heat-induced cholesterol oxidation at 150 degrees C by incorporation of quercetin was kinetically studied. Results showed that without quercetin, the cholesterol oxidation products (COPs) concentration increased with increasing heating time. A low amount (0.002%, w/w) of quercetin was effective in inhibiting the formation of COPs during the initial heating period (< or =30 min) at 150 degrees C. However, after prolonged heating (30-120 min), a low antioxidant activity was observed because of the degradation of quercetin. When using nonlinear regression models for kinetic study of cholesterol oxidation in the absence of quercetin, the epoxidation showed the highest rate constant (h(-1) = 683.1), followed by free radical chain reaction (h(-1) = 453.5), reduction (h(-1) = 290.3), dehydration (h(-1) = 155.5), triol dehydrogenation (h(-1) = 5.35), dehydrogenation (h(-1) = 0.68), thermal degradation (h(-1) = 0.66), and triol formation (h(-1) = 0.38). However, in the presence of quercetin, the reaction rate constants (h(-1)) for epoxidation (551.4), free radical chain reaction (111.7), and thermal degradation (0.28) were reduced greatly. The kinetic model developed in this study can be used to predict the inhibition of COPs by quercetin during the heating of cholesterol.  相似文献   

15.
The crude extract of the polyphenol oxidase (PPO) enzyme from the Manzanilla cultivar (Olea europaea pomiformis) was obtained, and its properties were characterized. The browning reaction followed a zero-order kinetic model. Its maximum activity was at pH 6.0. This activity was completely inhibited at a pH below 3.0 regardless of temperature; however, in alkaline conditions, pH inhibition depended on temperature and was observed at values above 9.0 and 11.0 at 8 and 25 degrees C, respectively. The thermodynamic parameters of substrate oxidation depended on pH within the range in which activity was observed. The reaction occurred according to an isokinetic system because pH affected the enzymatic reaction rate but not the energy required to carry out the reaction. In the alkaline pH region, browning was due to a combination of enzymatic and nonenzymatic reactions that occurred in parallel. These results correlated well with the browning behavior observed in intentionally bruised fruits at different temperatures and in different storage solutions. The use of a low temperature ( approximately 8 degrees C) was very effective for preventing browning regardless of the cover solution used.  相似文献   

16.
Stability of folic acid and 5-methyltetrahydrofolic acid in phosphate buffer (0.2 M; pH 7) toward thermal (above 65 degrees C) and combined high pressure (up to 800 MPa)/thermal (20 up to 65 degrees C) treatments was studied on a kinetic basis. Residual folate concentration after thermal and high pressure/thermal treatments was measured using reverse phase liquid chromatography. The degradation of both folates followed first-order reaction kinetics. At ambient pressure, the estimated Arrhenius activation energy (E(a)) values of folic acid and 5-methyltetrahydrofolic acid thermal degradation were 51.66 and 79.98 kJ mol(-1), respectively. It was noticed that the stability of folic acid toward thermal and combined high pressure thermal treatments was much higher than 5-methyltetrahydrofolic acid. High-pressure treatments at room temperature or higher (up to 60 degrees C) had no or little effect on folic acid. In the whole P/T area studied, the rate constant of 5-methyltetrahydrofolic acid degradation was enhanced by increasing pressure, and a remarkable synergistic effect of pressure and temperature on 5-methyltetrahydrofolic acid degradation occurred at temperatures above 40 degrees C. A model to describe the combined pressure and temperature effect on the 5-methyltetrahydrofolic acid degradation rate constant is presented.  相似文献   

17.
The reaction of Fe(III) and ascorbic acid (AA) in food products and digestive tracts affects the efficiency and uptake of these two nutrients. We investigated the kinetics of Fe(III) reduction by AA at pH 5 and 6 in a model system at 25 degrees C. The results indicate that the reduction of Fe(III) by AA is of zero order with respect to AA. The reaction order with respect to Fe(III) cannot be represented by a simple kinetic model at pH 5 or 6. The major stage of the reduction (about 80%, stoichiometrically), however, could be represented by a general equation of -d[Fe(III)]/dt = k[Fe(III)],(1. 811) where k is a rate constant and [Fe(III)] is the total ferric concentration. The rate constant decreased 1 order of magnitude as pH increased from 5 to 6. Aging of Fe(III) solution slows its reduction rate at pH 6 but not at pH 5.  相似文献   

18.
The Brassicaceae plant family contains high concentrations of glucosinolates, which can be hydrolyzed by myrosinase yielding products having an anticarcinogenic activity. The pressure and temperature stabilities of endogenous broccoli myrosinase, as well as of the synthetic isothiocyanates sulforaphane and phenylethyl isothiocyanate, were studied in broccoli juice on a kinetic basis. At atmospheric pressure, kinetics of thermal (45-60 degrees C) myrosinase inactivation could be described by a consecutive step model. In contrast, only one phase of myrosinase inactivation was observed at elevated pressure (100-600 MPa) combined with temperatures from 10 up to 60 degrees C, indicating inactivation according to first-order kinetics. An antagonistic effect of pressure (up to 200 MPa) on thermal inactivation (50 degrees C and above) of myrosinase was observed indicating that pressure retarded the thermal inactivation. The kinetic parameters of myrosinase inactivation were described as inactivation rate constants (k values), activation energy (Ea values), and activation volume (Va values). On the basis of the kinetic data, a mathematical model describing the pressure and temperature dependence of myrosinase inactivation rate constants was constructed. The stability of isothiocyanates was studied at atmospheric pressure in the temperature range from 60 to 90 degrees C and at elevated pressures in the combined pressure-temperature range from 600 to 800 MPa and from 30 to 60 degrees C. It was found that isothiocyanates were relatively thermolabile and pressure stable. The kinetics of HP/T isothiocyanate degradation could be adequately described by a first-order kinetic model. The obtained kinetic information can be used for process evaluation and optimization to increase the health effect of Brassicaceae.  相似文献   

19.
The relations between the formation of acrylamide and color, pyrazines, or antioxidants in an asparagine/d-glucose browning model system under various conditions were investigated. The highest level of acrylamide was produced in the asparagine/glucose (1:3) system heated at 170 degrees C for 30 min (2629 microg/g asparagine). Color intensity increased with temperature and heating time. The formation of pyrazines increased steadily with an increase of temperature (140-170 degrees C) and heating time (15-60 min). Antioxidant formation varied among the samples heated under different conditions. A clear correlation between formation of acrylamide and browning color was obtained. The formation of acrylamide was linearly correlated with the formation of total pyrazines during the initial stages of the Maillard reaction. No obvious correlation between formation of acrylamide and antioxidants was observed. However, excess amounts of asparagine increased the formation of antioxidants, whereas excess amounts of glucose reduced its formation.  相似文献   

20.
Kinetics of maneb degradation during thermal treatment of tomatoes   总被引：1，自引：0，他引：1  
The kinetics of maneb degradation in tomato homogenates at high temperatures and at two pH values (4 and 9) and the rate of formation of the toxic metabolite, ethylenethiourea (ETU), were studied. Maneb was measured as carbon disulfide by headspace gas-chromatography and ETU by high-performance liquid chromatography with photodiode array detection. First-order kinetics adequately described the degradation of maneb in tomato homogenates. The degradation rate constants exhibited an Arrhenius temperature dependence in the range from 50 to 90 degrees C and the apparent activation energy (E(a)) was calculated to be 36 KJ mol(-1) in homogenates with natural pH (4). Raising temperature from 60 to 75 and to 90 degrees C, ETU formation was significantly increased. Interestingly, the selectivity toward ETU showed a downward trend when the total conversion increased at longer heating times. When the pH of the tomato homogenates was adjusted to 9, the degradation of maneb proceeded faster at both 60 and 90 degrees C. The combination of alkaline pH and the highest temperature (90 degrees C) resulted in the maximum ETU conversion rates. The results of the present study on the fate of maneb and ETU residues during tomato processing, may prove valuable in estimating potential risk from dietary exposure.  相似文献   

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