Thermal Degradation Rate Constants Research Articles

Thermal Stability of Fructooligosaccharides Extracted from Defatted Rice Bran: A Kinetic Study Using Liquid Chromatography-Tandem Mass Spectrometry.

Thermal degradation kinetics of fructooligosaccharides (FOS) in defatted rice bran were studied at temperatures of 90, 100, and 110 °C. FOS extracted from rice bran and dissolved in buffers at pH values of 5.0, 6.0, and 7.0 were prepared for the thermal treatments. The residual FOS (including 1-kestose (GF2), nystose (GF3), and 1F-fructofuranosylnystose (GF4)) contents were determined using the ultra-performance liquid chromatography-electrospray ionization-tandem mass spectrometry (UPLC-ESI-MS/MS) method. The results showed that the thermal degradation kinetics of GF2, GF3, and GF4 followed a first-order kinetic model. Thermal degradation rate constants (k values) of GF2, GF3, and GF4 at different temperature and pH values were estimated using the first-order kinetic equation and SAS 9.1. As a result, these k values decreased gradually as the pH of the sample increased from 5.0 to 7.0. The Arrhenius model was applied to describe the heat dependence of the k-values. The activation energy (Ea) was calculated for each case of GF2, GF3, and GF4 degradation at pH values of 5.0, 6.0, and 7.0. The result showed that rice bran FOS is very thermostable at neutral pH while more labile at acidic pH.

Highly efficient extraction of mulberry anthocyanins in deep eutectic solvents: Insights of degradation kinetics and stability evaluation

In this work, an efficient strategy for extracting mulberry anthocyanins using deep eutectic solvents (DESs) was developed. Results show that both the hydrogen bond acceptors and hydrogen bond donors profoundly influenced the density, water content, and viscosity of the DESs. Six newly prepared DESs exhibited higher extraction amounts of anthocyanins than that of the acidified ethanol. Influence factors on choline chloride (ChCl)-lactic acid (HL) (1:2)-mediated extractability were optimized by response surface methodology (RSM). The optimal condition was acquired at 57.24 °C, for 31.54 min, and 10.76 mL/g, where 6.84 ± 0.21 mg/g of anthocyanins was extracted. Notably, the decreased thermal degradation rate constant, improved half-life time, and prolonged storage time of the anthocyanins in ChCl/HL demonstrated the superiority of DESs in the extraction technique over conventional solvents, which might be attributed to the wide and strong hydrogen bond network formed between the DES component with solute and water molecular.

Influence of pH, temperature, and light on the stability of melatonin in aqueous solutions and fruit juices

The ability to predict melatonin stability during food processing or storage is important. Therefore, the degradation of melatonin in both aqueous solutions and fruit juice samples was investigated. The pH values of aqueous solutions were set over a pH range from 1 to 13 and at four different temperatures (60, 70, 80 and 90 °C). The highest remaining melatonin (CR) was observed in the lowest pH solution (pH = 1, CR > 65%). Melatonin concentrations decreased with rising pH levels from pH 4 to 13 during storage time. The thermal degradation rate constant of melatonin (k) values obtained followed the order: k90°C (0.175) >k80°C (0.123) >k70°C (0.082) >k60°C (0.027). Thermal degradation kinetics followed the first-order reaction model with a high range of coefficients of determination (0.9744 < R2 < 0.995). The temperature also affected on melatonin degradation in fruit juices which the degradation rate was increased with the presence of light and high temperature. Our results can be used as guidelines to develop a processing method that predicts melatonin degradation during thermal processing of food products.

Cinética y Mecanismo de la descomposición Térmica del Nipecotato de Etilo e Isonipecotato de Etilo

The objective of this study was to carry out the thermolysis of two compounds not yet reported in literature: ethyl nipecotate and ethyl isonipecotate, in order to find the thermal degradation rate constants and the activation parameters and to propose a reaction mechanism. The thermolysis experiments were performed in a chemical minireactor at five temperatures between 523K and 563K with ten degrees difference; the values of the areas were measured with a gas chromatography coupled to mass spectrometry, also used in the identification of products of each reaction. It was found that ethyl nipecotate was degraded faster than ethyl isonipecotate and in both reactions the rate constants increased with the temperature and demonstrated to be unimolecular and of first order. A mechanism is proposed with a concerted, semi- polar six- member cyclic transition state.

Poly(3-hydroxybutyrate) - Organo Modified Montmorillonite Nanohybrid; Preparation and Characterization

Nano-biohybrid, based on biodegradable polyester, poly (3-hydroxybutyrate) (PHB), and a commercial organo modified montmorillonite (MMT), were prepared by solvent casting technique. Morphology and non-isothermal degradation of PHB and PHB/MMT hybrids were characterized by POM and TG-DTG technique. In the composite materials, an increase MMT loading in the PHB decreased the onset temperature (Tonset) of thermal degradation, while it achieved higher values upon increasing the heating rate. Kissinger plots deduced a trend of the degradation activation energy, Ed, which was related to the agglomeration of MMT. The thermal degradation rate constant, k, was related to the MMT loading in PHB. X-ray diffraction analysis confirmed the expansion of MMT silicate layers.

Modelling thermal degradation of zearalenone in maize bread during baking

The thermal degradation of zearalenone (ZEA) was investigated using a crust-like model, representing maize bread, which was prepared with naturally contaminated maize flour. Model samples were heated under isothermal conditions at the temperature range of 100–250°C. No reduction was observed at 100°C. Thermal degradation rate constants (k) were calculated as 0.0017, 0.0143 and 0.0216 min−1 for 150, 200 and 250°C, respectively. Maize bread baked at 250°C for 70 min was used to test the capability of model kinetic data for the prediction of ZEA reduction. The time–temperature history in the crust and crumb parts was recorded separately. Partial degradation of ZEA at each time interval was calculated by means of the corresponding k-values obtained by using the Arrhenius equation, and the total reduction occurring at the end of the entire baking process was predicted. The reduction in the crumb and crust of bread was also experimentally determined and found to be consistent with the predicted values. It was concluded that the kinetic constants determined by means of the crust-like model could be used to predict the ZEA reduction occurring during baking of maize bread.

Rapid Estimation of Glucosinolate Thermal Degradation Rate Constants in Leaves of Chinese Kale and Broccoli (Brassica oleracea) in Two Seasons

Kinetic modeling was used as a tool to quantitatively estimate glucosinolate thermal degradation rate constants. Literature shows that thermal degradation rates differ in different vegetables. Well-characterized plant material, leaves of broccoli and Chinese kale plants grown in two seasons, was used in the study. It was shown that a first-order reaction is appropriate to model glucosinolate degradation independent from the season. No difference in degradation rate constants of structurally identical glucosinolates was found between broccoli and Chinese kale leaves when grown in the same season. However, glucosinolate degradation rate constants were highly affected by the season (20-80% increase in spring compared to autumn). These results suggest that differences in glucosinolate degradation rate constants can be due to variation in environmental as well as genetic factors. Furthermore, a methodology to estimate rate constants rapidly is provided to enable the analysis of high sample numbers for future studies.

Thermal degradation of deoxynivalenol during maize bread baking

The thermal degradation of deoxynivalenol (DON) was determined at isothermal baking conditions within the temperature range of 100–250°C, using a crust-like model, which was prepared with naturally contaminated maize flour. No degradation was observed at 100°C. For the temperatures of 150, 200 and 250°C, thermal degradation rate constants (k) were calculated and temperature dependence of DON degradation was observed by using Arrhenius equation. The degradation of DON obeyed Arrhenius law with a regression coefficient of 0.95. A classical bread baking operation was also performed at 250°C for 70 min and the rate of DON degradation in the bread was estimated by using the kinetic data derived from the model study. The crust and crumb temperatures recorded during bread baking were used to calculate the thermal degradation rate constants (k) and partial DON degradations at certain time intervals. Using these data, total degradation at the end of the entire baking process was predicted for both crust and crumb. This DON degradation was consistent with the experimental degradation data, confirming the accuracy of kinetic constants determined by means of the crust-like model.

Silver sulfide/poly(3-hydroxybutyrate) nanocomposites: Thermal stability and kinetic analysis of thermal degradation

The non-isothermal degradation of poly(3-hydroxybutyrate) (PHB) and silver sulfide/poly(3-hydroxybutyrate) (Ag 2S/PHB) nanocomposites was investigated using thermogravimetric (TG) analysis. In the composite materials, Ag 2S caused the degradation of PHB at a lower temperature as opposed to that of neat PHB. Moreover, an increase Ag 2S loading in the PHB decreased the onset temperature ( T onset) of thermal degradation, whereas it was raised upon augmenting the heating rate. From Kissinger plots, the observed trend of the degradation activation energy, E d, was attributed to polymer–particle surface interactions and the agglomeration of Ag 2S. The thermal degradation rate constant, k, was linearly related to the Ag 2S loading in PHB. Thus, the Ag 2S nanoparticles effectively catalyzed the thermal degradation of PHB in the Ag 2S/PHB nanocomposites. Differential scanning calorimetry (DSC) data also supported the catalytic property of Ag 2S.

A study on the degradation of polylactic acid in the presence of phosphonium ionic liquids

In this overview study, two ionic liquids (IL) with different anions (decanoate, tetrafluoroborate) but with the same phosphonium-based cation that showed promising plasticizing/lubricating behavior in polylactic acid (PLA) were screened for their effects on the polymer degradation under thermomechanical, thermo-oxidative (at 160 °C), hydrolytic (100% humidity, 60 °C), conditions, and during soil immersion. Depending on the particular medium and conditions used, degradation was followed by changes in molecular weight, melt viscosity, sample weight and appearance, morphology, crystallinity, acid number, and pH. The effects of the IL containing a decanoate anion were more pronounced on lubrication and also on degradation as evidenced by reduced melt viscosities and accelerated thermomechanical, isothermal, hydrolytic, and soil degradation. The IL containing the tetrafluoroborate anion showed higher thermal stability compared with the IL containing decanoate anion as also confirmed from thermal degradation rate constants which were calculated from random chain scission statistics. Accelerated hydrolytic degradation was observed in PLA containing the tetrafluoroborate based IL but to a lesser extent than the decanoate based IL. The catalytic role of the decanoate anion in hydrolytic degradation was confirmed through experiments with model compounds. X-ray diffraction (XRD) data on the materials exposed to soil degradation provided evidence that the initially amorphous polymer attained a certain degree of crystallinity as a result of the significant MW reduction.

Nanosize and microsize clay effects on the kinetics of the thermal degradation of polylactides

Polylactide (PLA)–montmorillonite (MMT) micro- and nanocomposites based on semicrystalline and amorphous polymers and unmodified or organomodified clays at 5 wt% content were produced by melt mixing. Based on the three different test methods that were used to follow thermal degradation, different conclusions were obtained. During melt processing, thermomechanical degradation was more pronounced in the presence of all fillers, which apparently acted catalytically, but to different degrees. During isothermal degradation in air from 180 °C to 200 °C, degradation rate constants were calculated from novel equations incorporating changes in intrinsic viscosity (IV). Results show that the thermal degradation rate constants of the amorphous PLA and its composites are lower than those of the semicrystalline PLA and its composites. Due to better filler dispersion in the polymer matrix, the thermal degradation rate constants of the nanocomposites are significantly lower than those of the unfilled polymers and their microcomposites under air. As per dynamic TGA data and thermal kinetic analysis from weight losses and activation energy calculations, organomodified nanofillers have a complex effect on the polymer thermal stability; the unmodified fillers, however, reduce polymer thermal stability. These TGA data and kinetic analysis results also support the findings that the thermal stability of the amorphous PLA and its composites is higher than that of the semicrystalline polymer and its composites and the thermal stability of the nanocomposites is higher than that of the microcomposites. In general, mathematical modeling based on random thermal scission equations was satisfactory for fitting the TGA experimental data.

Thermal degradation of polystyrene in the presence of hydrogen by catalyst in solution

For the first time, low temperature degradation (170–240 °C) of polystyrene in benzene is carried out in the presence of hydrogen using iron(III) oxide catalyst. The effect of temperature, catalyst loading and polymer loading on degradation are studied in hydrogen atmosphere. Degradation is also carried out at different initial hydrogen partial pressure. The time dependent molecular weight is calculated using viscosity average method. It is found that the degradation is enhanced considerably in the presence of hydrogen and followed random degradation chain scission. A random degradation kinetic model of Kelen [Kelen T. Polymer degradation. New York: Van Nostrand Reinhold Company; 1983.] is used to estimate the degradation rate constants. Empirical correlations are proposed to account for the effect of catalyst loading and initial hydrogen partial pressure on degradation. The true thermal degradation rate constants are calculated using these proposed correlations at given catalyst loading and initial hydrogen partial pressure with varying temperature. The frequency factor and activation energy are also determined using Arrhenius equation considering the true thermal degradation rate constants.

Steady shear and dynamic properties of biodegradable polyesters

AbstractThe rheological properties of three biodegradable partially crystalline alipathic polyesters—poly ε carolactone (PCL), polylactic acid (PLA), and polyhydroxybutyrate‐co‐hydroxyvalerate (PHBV)—were investigated. Low shear viscosity (η), first normal stress difference (N1), and dynamic properties (G′ and G″), were measured for each polymer using rotational instruments. High shear viscosity data was obtained using a capillary rheometer. Gel permeation chromatography indicates that both PLA and PHBV undergo degradation during processing. Thermal degradation rate constants for PLA and PHBV were determined from the time‐sweep data. Data indicates that material functions are proportional to the molecular weight for PCL and PLA samples, and directly related to the HV content in the PHBV samples. The relaxation spectra were calculated from the G′ and G″ data using the regularization method and the applicability of the nonlinear Wagner equation was checked on these materials. Results are discussed in terms of variations in the molecular characteristics on rheological properties and their possible influence on the processability.

Kinetics of hydrolysis and thermal degradation of polyester melts

AbstractPolyester melts exhibit two discrete degradation rates depending on the moisture content in the solid state. The initial fast rate is very sensitive to the moisture content while the later slow rate is not sensitive to the moisture content. The former is attributed to hydrolysis and the latter is attributed to thermo‐oxidative degradation. A combined kinetic equation which represents both hydrolysis and thermo‐oxidative degration of the polyester melt is proposed. The model equation agrees well with the experimental data. A method to determine hydrolytic and thermal degradation rate constants, moisture content, molecular weight for polyesters by means of a melt viscometry is discussed.