Kinetics of the pectin methylesterase catalyzed de-esterification of pectin in frozen food model systems

The pectin methylesterase (PME) catalyzed de-esterification of pectin was studied in four frozen food model systems based on sucrose, fructose, maltodextrin, and carboxymethylcellulose (CMC) in a temperature range from -24 to 20 degrees C, with the aim of elucidating the applicability of the theory of "food polymer science" on the kinetics. The rate substantially decreased around the glass transition temperature in the case of CMC, while very low rates were observed far above the glass transition temperature in the case of maltodextrin, fructose, and sucrose model systems. In general, the kinetics of this reaction was found to be influenced more by factors such as the characteristics of the component solutes, freeze concentration, the possible viscosity enhancement due to a particular combination of solutes, and the molecular size of the substrate molecule rather than the glass transition process. The Arrhenius equation described the temperature dependence of kinetics both in the liquid state of all the systems studied (r(2) > or = 0.97) and the glassy state of CMC (r(2) = 0.95). A clear break in the Arrhenius plot was observed as the temperature decreased to subfreezing temperatures. The Arrhenius equation could describe the kinetics reasonably well in the rubbery state for fructose and sucrose model systems (r(2) > 0.992). In the case of maltodextrin and CMC, the Arrhenius plots showed a slight curvature followed by a break at the glass transition temperature for CMC. The WLF equation with system-dependent coefficients better described the kinetics in the rubbery state of the CMC and part of the maltodextrin system. A linear relationship between the logarithm of the rate and T - Tg' described the kinetics in the sucrose as well as fructose model systems (r(2) = 0.9928 and 0.993, respectively).     

Kinetics of the alkaline phosphatase catalyzed hydrolysis of disodium p-nitrophenyl phosphate in frozen model systems

The alkaline phosphatase catalyzed hydrolysis of disodium-p-nitrophenyl phosphate was studied in four model systems comprising sucrose, maltodextrin, carboxymethylcellulose (CMC), and CMC-lactose in a temperature range of -28 to 20 degrees C. In the maltodextrin and CMC-lactose model systems, the reaction rate decreased to a very low value as the glass transition temperature was approached. In the CMC and CMC-lactose systems with low initial solute concentration, as a consequence of freeze-concentration, a rate maximum around the initial freezing temperature was observed. The Arrhenius equation described the temperature dependence of the reaction rate both in the liquid and the glassy states in all systems studied, while a slightly curved Arrhenius plot was observed in the "rubbery" state of the CMC and CMC-lactose systems. The WLF equation with system-dependent coefficients described the kinetics in the rubbery state of all the model systems except sucrose, excluding the short temperature range where reaction rate enhancement with decreasing temperature was observed.     

Building on the WLF/free volume framework: utilization of the coupling model in the relaxation dynamics of the gelatin/cosolute system

The onset of softening in the glass transition dispersion of the gelatin/cosolute system at 78% solids was examined using the stress relaxation modulus and dynamic oscillatory data on shear. Measurements were made between 5 and -70 degrees C, and isothermal runs were reduced to a master curve covering 21 orders of magnitude in the time domain. The sharpness with which the mechanical properties of our system changed with temperature was reflected in the shift factor a(T) used to pinpoint the glass transition temperature (T(g)). The prevalent analytical framework traditionally employed to follow the transition from the rubbery to glasslike consistency in biomaterials is that of the free volume theory in conjunction with the WLF equation. Increasingly, the combined WLF/free volume approach is challenged by the coupling model, which is able to provide additional insights into the physics of intermolecular interactions in synthetic materials at the vicinity of T(g). The model in the form of the Kohlrausch-Williams-Watts function described well the spectral shape of the local segmental motions of gelatin/cosolute at T(g). The analysis provided the intermolecular interaction constant and apparent relaxation time, parameters which depend on chemical structure. Results appear to be encouraging for further explorations of the dynamics of densely packed biomaterials at the glass transition region.     

Glassy State and Seed Storage Stability: The WLF Kinetics of Seed Viability Loss at T>Tgand the Plasticization Effect of Water on Storage Stability

The relationship between the glassy state in seeds and storage stability was examined, using the glass transition curve and a seed viability database from previous experiments. Storage data for seeds at various water contents were studied by Williams–Landel–Ferry (WLF) kinetics, whereas the glass transition curves of seeds with different storage stability were analysed by the Gordon–Taylor equation in terms of the plasticization effect of water on seed storage stability. It was found that the critical temperatures (T_c) for long-term storage of three orthodox seeds were near or below their glass transition temperatures (T_g), indicating the requirement for the presence of the glassy state for long-term seed storage. The rate of seed viability loss was a function of T-T_gat T>T_g, which fitted the WLF equation well, suggesting that storage stability was associated with the glass transition, and that the effect of water content on seed storage was correlated with the plasticization effect of water on intracellular glasses. A preliminary examination suggested a possible link between the glass transition curve and seed storage stability. According to the determined WLF constants, intracellular glasses in seeds fell into the second class of amorphous systems as defined by Slade and Levine (Critical Reviews in Food Science and Nutrition30: 115–360, 1991). These results support the interpretation that the glassy state plays an important role in storage stability and should be a major consideration in optimizing storage conditions.     

Viscometric study of pectin. Effect of temperature on the hydrodynamic properties

Hydrodynamic properties are important parameters affecting the performance of pectin. This polysaccharide is used as a thickening and gelling agent in food and pharmaceutical industries. The most common and economical of the hydrodynamic properties is the determination of viscosity, in which are determined the intrinsic viscosity and the diffusion coefficient. They indirectly measure the molecular weight (M(w)); hydrodynamic radius (R(H)); number of Simha, (ν(a/b)); Perrin parameter (P); Scheraga-Mandelkern parameter (β); and Flory parameters (?(0) and P(0)). All the hydrodynamic parameters are dependent on temperature. Normally these parameters are reported at a temperature of 25°C, which limits their application to different temperatures. This work studies pectin dependence on temperature, finding that this biopolymer in aqueous solution presents a conformation of rod-like with ν(a/b)=10.5, and a value from 0.8232 to 0.8129. Pectin behavior in this system indicates that it behaves like a colloidal particle that tends to compact with increasing temperature (R(H) decrease). The molecular weight calculated for pectin is 180,000 g/mol. Mark-Houwink-Sakurada (M-H-S) equation constants, a and k, for pectin in water solvent-temperature systems have been already reported.     

Mechanical, microstructural and solubility properties of pectin/poly(vinyl alcohol) blends

Citrus pectin was blended and cast into films with poly(vinyl alcohol) (PVOH). PVOH and pectin were miscible in all proportions. Dynamic mechanical analysis revealed that pectin controls exhibited no thermal transitions, whereas PVOH controls exhibited a glass transition temperature (T_g) over a broad temperature range commencing at about 0 °C and ending about 50 °C. A mixture of 49% pectin, 21% PVOH and 30% glycerol exhibited lower storage moduli and more flexibility than comparable mixtures of either pectin/PVOH or pectin/glycerol. Scanning electron microscopy and phase contrast optical microscopy indicated that the mixture was biphasic and a compatible composite either of PVOH in pectin or pectin in PVOH depending on which macromolecule was in excess. Elongation to break measurements revealed that pectin/PVOH films underwent a brittle to ductile transition with increasing PVOH composition. The addition of glycerol to pectin/PVOH films increased ductility significantly when films were relatively brittle. Initial moduli (IM) as a function of composition gave complex curves which exhibited either one or two local maxima depending on such factors as degree of hydrolysis and molar mass of the PVOH in addition to the moisture content of the film. Solubility studies in water revealed that, at 30 and 50 °C, only films with 30% PVOH or less were soluble. At 70 °C, all compositions were soluble but films containing pectin dissolved more rapidly than those without. The solution kinetics of pectin/PVOH films with 30% or less PVOH were approximated with zero-order kinetics and activation energies were about 3–5 kcal mol⁻¹. In general, addition of PVOH to pectin films resulted in films with more PVOH-like properties and addition of pectin to PVOH films resulted in films with more pectin-like properties.     

Temperature dependence of thermal inactivation rate constants of bacterial spores in a glassy state

Summary Differential scanning calorimetry data obtained from corn embryos is consistent with the hypothesis of their glassy state. This work extends that hypothesis to explain the speculation about the high heat resistance of bacterial spores. By considering the protoplast to be in a glassy solid-state, it can be assumed that the configurational rearrangements of the key life dependent polymer chain backbones (DNA, etc.) are extremely slow, thereby ceasing thermal motions. It is assumed that at the glass transition temperature, the spore protoplast undergoes a discontinuity in the thermal expansion coefficient, and above this critical temperature, the rate of thermal inactivation of spores is free volume dependent and can be described adequately by the William, Landel and Ferry (WLF) equation. Glass transition temperatures forBacillus stearothermophilus andClostridium botulinum spores, obtained by fitting the inactivation rate data to the WLF equation, indicate a decrease in the inactivation rates with increasing glass-transition temperatures.     

Thermal behavior of native and hydrophobized wheat gluten, gliadin and glutenin-rich fractions by modulated DSC

The glass transition temperature (T(g)) of hydrophobized and native wheat gluten and its protein fractions, with water mass fraction from 0 to 0.2, was studied using modulated differential scanning calorimetry. The T(g) values of unplasticized products were approximately 175 degrees C whatever the treatment (hydrophobization) or the fraction tested, except for the gliadin-rich fraction (162 degrees C). Experimental change in heat capacity at the glass transition (DeltaC(p)) ranged from 0.32 to 0. 50 J/g/ degrees C depending on the gluten fractions. The Gordon-Taylor fit of T(g) evolution as a function of water content showed that glutenin-rich fractions were more sensitive to water plasticization than the gliadin-rich fraction. The Kwei equation gave better fit to experimental data and demonstrated that the water plasticization of gluten and its fractions is influenced by secondary interactions. However, the application of the Couchman-Karasz equation without fitting predicts satisfactorily the plasticization of gluten proteins by water.     

The mercury binding activity of pectin isolated from the seagrass Zostera marina

The kinetics of mercury binding by pectin isolated from the seagrass Zostera marina was described, and its maximum mercury binding activity at pH from 2.0 up to 6.0 was determined. It was shown that mercury binding by pectin in in vitro conditions did not depend on concentration of hydrogen ions in the environment. The maximum mercury binding activity estimated from the Langmuir equation was 2.64 mmol/g of dry mass of the pectin.     

Protein stability in the amorphous carbohydrate matrix: relevance to anhydrobiosis

The formation of intracellular glass is proposed to be relevant to protein stabilization and survival of anhydrobiotic organisms in the dry state. The stability of proteins in the amorphous carbohydrate matrix and its relevance to seed survival have been investigated in the present study. Glucose-6-phosphate dehydrogenase (G6PDH) was preserved in the amorphous glucose/sucrose (1:10, w/w) matrix by freeze-drying. The stability of freeze-dried G6PDH was examined at temperatures above and below the glass transition temperature (T_g). The rate of G6PDH inactivation in the amorphous carbohydrate matrix deviated significantly from the Arrhenius kinetics, and conformed to the Williams-Landel-Ferry (WLF) relationship. The temperature dependence of G6PDH inactivation in two sets of samples with different T_g values was compared. Identical temperature dependence of G6PDH inactivation was observed after temperature normalization by (T?T_g). Seed survival of Vigna radiata Wilczek (mung bean) showed a similar WLF kinetics at storage temperatures T≥T_g. In situ protein stability in mung bean embryonic axes was studied using differential scanning calorimetry (DSC). Thermal stability of seed proteins exhibited a strong dependence on the T_g of intracellular glass. These results indicate an important role of the glassy state in protein stabilization. Our data suggest an association between protein stability in intracellular glass and seed survival during storage.     

Maillard reaction rate in various glassy matrices

The Maillard Reaction (MR) rate below the glass transition temperature (T(g)) for various model glassy food systems was studied at temperatures between 40 degrees C and 70 degrees C. As a sample, freeze-dried glucose and lysine systems embedded in various glassy matrices (e.g., polyvinylpyrrolodone and trehalose) were used, and the MR rate below the T(g) was compared among the various glassy matrices. The extent of MR was estimated spectrophotometrically from the optical density at 280 nm (OD(280)), and the MR rate (k(280)) was determined as a pseudo zero order reaction rate from the time course of OD(280). Although k(280) was described by the Arrhenius plot, the temperature dependence of k(280) was almost the same and the intercept was different among the matrices. From the comparison of k(280), it was suggested that the MR rate in glassy matrix was affected not only by the T(g), but also by the hydrogen bonding between MR reactants and glassy matrix.     

Viscous solutions, networks and the glass transition in high sugar galactomannan and kappa-carrageenan mixtures

Small-deformation rotational oscillation was used to examine the effect of small additions of galactomannan and kappa-carrageenan on the vitrification of glucose syrup at a total level of solids of 83%. The method of reduced variables allowed construction of composite curves covering the glass transition and glassy state (from 10(5) to 10(9.5) Pa) over a wide frequency range (up to 15 orders of magnitude). The combined WLF/free volume framework was employed to determine the rheological glass transition temperature (T(g)), fractional free volume and thermal expansion coefficient of the samples. It was found that the WLF-predicted glass transition temperature matched the cross over of experimental modulus traces in the passage from the glass transition (GG') to the glassy state (GG"). This coincides with the mechanistic transformation from free volume effects to the Arrhenius-type phenomena, thus ascribing physical significance to the rheological T(g). The T(g) value of 83% glucose syrup at a scan rate of 2 degrees C min(-1) was -25.3 degrees C. Replacing, for example, 1% glucose syrup with guar gum shifted the T(g) of the mixture to -19.7 degrees C. Network formation via the K(+)-supported junction zones of the kappa-carrageenan chains further increased the T(g) to about -1 degrees C. It appears that the low rates of relaxation processes and diffusion mobility in the presence of a polysaccharide network accelerate the collapse of the free volume thus inducing vitrification of the high sugar/polysaccharide mixture at high temperatures.     

The glass transition temperatures of amorphous trehalose-water mixtures and the mobility of water: an experimental and in silico study

Isothermal-isobaric molecular dynamics simulations are used to calculate the specific volume of models of trehalose and three amorphous trehalose-water mixtures (2.9%, 4.5% and 5.3% (w/w) water, respectively) as a function of temperature. Plots of specific volume versus temperature exhibit a characteristic change in slope when the amorphous systems change from the glassy to the rubbery state and the intersection of the two regression lines provides an estimate of the glass transition temperature T(g). A comparison of the calculated and experimental T(g) values, as obtained from differential scanning calorimetry, shows that despite the predicted values being systematically higher (about 21-26K), the trend and the incremental differences between the T(g) values have been computed correctly: T(g)(5.3%(w/w))相似文献   

c,T-dependence of the viscosity and the self-diffusion coefficients in some aqueous carbohydrate solutions

Self-diffusion coefficients for both components are reported for the highly concentrated aqueous solutions of some disaccharides and fructose as a function of temperature and concentration. These data are complemented by viscosity measurements. The disaccharides studied are sucrose, alpha,alpha-trehalose, allosucrose, and leucrose. Up to a sugar concentration of approximately 30% w/w the viscosity and the self diffusion coefficients of the four disaccharides are identical within experimental error for a given concentration and temperature. Water diffusion shows no differences in the four systems studied under these conditions. At higher concentrations significant differences are observed that become more pronounced with increasing temperature. Analysis of the data by the VTF equation yields the result that at a given concentration the self diffusion coefficients of the sugar Dc and the viscosity eta are described by identical ideal glass transition temperatures T0, while the diffusion of the water D(W) molecule decouples from these properties. T0(W) is always lower than T0(c,eta).     

Viscoelastic properties of pectin--co-solute mixtures at iso-free-volume states

Small-deformation oscillatory measurements were performed on pectin-sucrose-glucose syrup systems at a total level of solids of 81%, with the polysaccharide content being fixed at levels of industrial use (1%). The experimental temperature range was between 50 and - 50 degrees C. Analysis of the temperature dependence of viscoelastic processes by the equation of Williams, Landel, and Ferry provides values of fractional free volume for the temperatures covering the glass transition region. The shift factors used in the conversion of mechanical spectra into master curves were normalised at suitably different temperatures so that their temperature dependence becomes coincident. The treatment implies an iso-free-volume state and relates to changes in the monomeric friction coefficient with increasing levels of intermolecular interactions in the mixture. A free-volume related glass transition temperature was defined and manipulated markedly by introducing pectin of variable degrees of esterification to the sucrose-glucose syrup system.     

Dynamic mechanical properties of elastin.

The dynamic mechanical properties of water-swollen elastin under physiological conditions have been investigated. When elastin is tested as a colsed, fixed-volume system, mechanical data could be temperature shifted to produce master curves. Master curves for elastin hydrated at 36°C (water content, 0.46 g water/g protein) and 55°C (water content, 0.41 g/g) were constructed, and in both cases elastin goes through a glass transition, with the glass transition temperatures of -46 and -21°C, respectively. Temperature shift data used to construct the master curves follow the WLF equation, and the glass transition appears to be characteristic of an amorphous, random-polymer network. For elastin tested as an open, variable-volume system free to change its swollen volume as temperature is changed, dynamic mechanical properties appear to be virtually independent of temperature. No glass transition is observed because elastin swelling increases with decreased temperature, and the increase in water content shifts elastin away from its glass transition. It is suggested that the hydrophobic character of elastin, which gives rise to the unusual swelling properties of elastin, evolved to provide a temperature-independent elastomer for the cold-blooded, lower vertebrates.     

Temperature dependency of molecular mobility in preserved seeds

Although cryogenic storage is presumed to provide nearly infinite longevity to cells, the actual timescale for changes in viability has not been addressed theoretically or empirically. Molecular mobility within preserved biological materials provides a first approximation of the rate of deteriorative reactions that ultimately affect shelf-life. Here, temperature effects on molecular mobility in partially dried seeds are calculated from heat capacities, measured using differential scanning calorimetry, and models for relaxation of glasses based on configurational entropy. Based on these analyses, glassy behavior in seeds containing 0.07 g H(2)O/g dm followed strict Vogel-Tamman-Fulcher (VTF) behavior at temperatures above and just below the glass transition temperature (Tg) at 28 degrees C. Temperature dependency of relaxation times followed Arrhenius kinetics as temperatures decreased well below Tg. The transition from VTF to Arrhenius kinetics occurred between approximately 5 and -10 degrees C. Overall, relaxation times calculated for seeds containing 0.07 g H(2)O/g dm decreased by approximately eight orders of magnitude when seeds were cooled from 60 to -60 degrees C, comparable to the magnitude of change in aging kinetics reported for seeds and pollen stored at a similar temperature range. The Kauzmann temperature (T(K)), often considered the point at which molecular mobility of glasses is practically nil, was calculated as -42 degrees C. Calculated relaxation times, temperature coefficients lower than expected from VTF kinetics, and T(K) that is 70 degrees C below Tg suggest there is molecular mobility, albeit limited, at cryogenic temperatures.     

The glass transition temperatures of sugar mixtures

We measured the glass transition temperatures of mono-, di-, and trisaccharide mixtures using differential scanning calorimeter (DSC) and analyzed these temperatures using the Gordon-Taylor equation. We found that the glass transition temperatures of monosaccharide-monosaccharide and disaccharide-disaccharide mixtures could be described by the conventional Gordon-Taylor equation. However, the glass transition temperatures of monosaccharide-disaccharide and monosaccharide-trisaccharide mixtures deviated from the conventional Gordon-Taylor equation and the amount of deviation in the monosaccharide-trisaccharide mixtures was larger than those in the monosaccharide-disaccharide mixtures. From these results, we conclude that the size and shape of the sugars play an important role in the glass transition temperature of the mixtures.     

The glass transition temperature of mixtures of trehalose and hydroxyethyl starch

Although mixtures of HES and sugars are used to preserve cells during freezing or drying, little is known about the glass transition of HES, or how mixtures of HES and sugars vitrify. These difficulties may be due to the polydispersity between HES samples or differences in preparation techniques, as well as problems in measuring the glass transition temperature (T(g)) using differential scanning calorimetry (DSC). In this report, we examine the T(g) of mixtures of HES and trehalose sugar with <1% moisture content using DSC measurements. By extrapolating these measurements to pure HES using the Gordon-Taylor and Fox equations, we were able to estimate the T(g) of our HES sample at 44 degrees C. These results were additionally confirmed by using mixtures of glucose-HES which yielded a similar extrapolated T(g) value. Our approach to estimating the glass transition temperature of HES may be useful in other cases where glass transitions are not easily identified.     

The effect of water on the glass transition of human hair

The glass transition of human hair and its dependence on water content were determined by means of differential scanning calorimetry (DSC). The relationship between the data is suitably described by the Fox equation, yielding for human hair a glass transition temperature of T(g) = 144 degrees C, which is substantially lower than that for wool (174 degrees C). This effect is attributed to a higher fraction of hydrophobic proteins in the matrix of human hair, which acts as an internal plasticizer. The applicability of the Fox equation for hair as well as for wool implies that water is homogeneously distributed in alpha-keratins, despite their complex morphological, semicrystalline structure. To investigate this aspect, hair was rendered amorphous by thermal denaturation. For the amorphous hair neither the water content nor T(g) were changed compared to the native state. These results provide strong support for the theory of a quasi-homogeneous distribution of water within alpha-keratins.