DSC and TGA studies of the behavior of water in native and crosslinked gelatin (original) (raw)
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We present a systematic investigation of hydration and gelation of the polypeptide gelatin in water–glycerol mixed solvent (glycerol solutions). Raman spectroscopy results indicated enhancement in water structure in glycerol solutions and the depletion of glycerol density close to hydration sheath of the protein molecule. Gelation concentration (cg) was observed to decrease from 1.92 to 1.15% (w/v) while the gelation temperature (Tg) was observed to increase from 31.4 to 40.7 °C with increase in glycerol concentration. Data on hand established the formation of organogels having interconnected networks, and the universal gelation mechanism could be described through an anomalous percolation model. The viscosity of sol diverged as η (1 – cg/c)−k as cg was approached from below (c < cg), while the elastic storage modulus grew as G′ (c/cg – 1)t (for c > cg). It is important to note that values determined for critical exponents k and t were universal; that is, they did not depend on the microscopic details. The measured values were k = 0.38 ± 0.10 and t = 0.92 ± 0.17 whereas the percolation model predicts k = 0.7–1.3 and t = 1.9. Isothermal frequency sweep studies showed power-law dependence of gel storage modulus (G′) and loss modulus (G′′) on oscillation frequency ω given as G′(ω) ωn′ and G′′(ω) ωn′′, and consistent with percolation model prediction it was found that n′ ≈ n′′ ≈ δ ≈ 0.73 close to gelation concentration. We propose a unique 3D phase diagram for the gelatin organogels. Circular dichroism data revealed that the gelatin molecules retained their biological activity in these solvents. Thus, it is shown that the thermomechanical properties of these organogels could be systematically tuned and customized as per application requirement.
Molecular configuration of gelatin–water suspensions at low concentration
Food Hydrocolloids, 2014
Direct assessment of gelatin molecular configuration has been difficult due the complexity of the molecule and limitations of analytical techniques. The objective of this work was the molecular characterization of bovine gelatin as a function of temperature, concentration and time. Diluted suspensions were prepared at different concentrations (7.5 Â 10 À5 to 1.5 g/l) and kept at temperatures above (S1) (40 C) and below (S2) (5 C) the gelling point as determined by Differential Scanning Calorimetry. Circular dichroism measurements showed the secondary structure of a polyproline II like spectra similar to native collagen at S1 and a denaturated configuration at S2 conditioning. Atomic Force Microscopy (AFM) at S1, using a HOPG substrate, showed single strands segments with two marked distributions in heights, w0.6 nm and w1.6 nm, indicating possible helical configurations even at high temperatures. At S2, AFM showed only one height distribution in the range of w0.9 nm but wider (0.3e1.6 nm range). At increasing gelatin concentration (12 g/l) and annealing time (48 h), a well-defined network was detected with narrow height distribution (w1.0 nm) featuring aggregates and highly ordered structure zones. An analysis of gelatin strand interactions, showed a network linked by knots with a coordination number Z ¼ 3 (strands) with a bond length of w50 nm. The gel network formation and aggregation was consistent with molecular size increase observed by Dynamic Light Scattering, showing variations in hydrodynamic dimensions from w10 nm (S1) to w100 nm (S2). This experimental approach has allowed to pinpoint differences in molecular configuration of gelatin, which may be applied in the study the structuring pathway of other biopolymers and the association kinetics during storage for a wide range of temperatures.
Polymer, 2009
a b s t r a c t 10, 20, and 40 wt.% aqueous gelatin gels were prepared under isothermal (annealing at 15, 20, and 25 C for 15 to 120 min) and nonisothermal (cooling at 1 C min À1 ) conditions. Isoconversional kinetic analysis of DSC data on gel melting (gel-sol transition) of all types of gels revealed significant variations in the activation energy throughout the process. Activation energy barrier to melting of isothermally prepared gels was in the range 160-190 kJ mol À1 and found to increase with increasing the annealing temperature that was the major effect discovered. Activation energy barrier to melting of nonisothermally prepared gels was determined to be around 120-140 kJ mol À1 and increase with increasing the concentration. Local reversibility of the gel melting was demonstrated by using temperature modulated DSC.
The effect of origin of the gelatine and ageing on the secondary structure and water dissolution
Food Hydrocolloids, 2017
Gelatin is used to make hard capsules and has to meet strict dissolution specifications to guarantee homogeneous drug delivery. The aging of gelatin induces a decrease of its dissolution rate due to the formation of intra-and intermolecular cross-links. Cross-link formation has shown to be dependent on the environmental conditions of production, such as temperature and humidity, and therefore on the geographic origin of production. Gelatin structure consists of an amorphous phase (coil structure) and a crystal phase (triple-helixes) and is very sensitive to environmental changes. The present work aims at understanding the role and effects of structural changes during aging on gelatin dissolution, taking into account the origin of production. The molecular structure of pig skin gelatins from three different production sites of the same company were studied by differential scanning calorimetry, synchrotron circular dichroism and 1 H nuclear magnetic resonance. It turned out that aging induced the melting of triple-helixes, an increase of random coils and, probably, b-turn conformations. The gelatin structure varied with the origin of production and thus affected the dissolution rate. Gelatins with non-compliant dissolution rates exhibited a higher amount of amorphous phase after aging than compliant gelatins. Although it was not possible to formulate any certain interpretation of the synchrotron circular dichroism results regarding gelatin dissolution, this technique is able to differentiate compliant dissolutions from non-compliant ones, even before gelatin aging.
Water Sorption Isotherms and Glass Transition Properties of Gelatin
Drying Technology, 2002
The water sorption isotherms of gelatin of different molecular weights (317,700, 228,900, and 197,400) were determined at 50 C using an isopiestic method. The sorption isotherms were modeled using the Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-deBoer (GAB) equations. The BET and GAB equations were able to predict the equilibrium moisture content (EMC) with a mean relative error of 5.2 and 5.0%, respectively. The BET monolayer moisture content varied from 4.81 to 5.70% (d.b.) while modeling with the GAB equation predicted monolayer moisture content of
The effects of low molecular weight diluents (namely water and glycerol) on the nanostructure and thermodynamic state of low water content gelatin matrices are explored systematically by combining positron annihilation lifetime spectroscopy (PALS) with calorimetric measurements. Bovine gelatin matrices with a variation in the glycerol content (0-10 wt.%) are equilibrated in a range of water activities (a w = 0.11-0.68, T = 298 K). Both water and glycerol reduce the glass transition temperature, T g , and the temperature of dissociation of the ordered triple helical segments, T m , while having no significant effect on the level of re-naturation of the gelatin matrices. Our PALS measurements show that over the concentration range studied, glycerol acts as a packing enhancer and in the glassy state it causes a nonlinear decrease in the average hole size, v h , of the gelatin matrices. Finally, we report complex changes in v h for the gelatin matrices as a function of the increasing level of hydration. At low water contents (Q w ⇠ 0.01-0.10), water acts as a plasticizer, causing a systematic increase in v h .
Enthalpy relaxation of gelatin in the glassy state
International Journal of Biological Macromolecules, 2005
The enthalpy relaxation during the ageing of gelatin in the glassy state was studied for partially crystalline or amorphous materials at different water contents and ageing temperatures. The extent and rate of this relaxation associated with physical ageing were found to increase when the shifted temperature parameter (T a − T g) increased. This parameter was able to account for the effects of structure and water content (through T g) and ageing temperature (T a).
Atypical gelation in gelatin solutions probed by ultra-fast calorimetry
Soft Matter, 2012
This paper describes the first application of ultrafast scanning calorimetry to the process of gelation in concentrated (40 wt%) aqueous gelatin solutions. It demonstrates that the regular gelation that occurs on cooling can be outrun at sufficiently fast cooling rates ($500 K s À1 ) so that the solution reaches a metastable supercooled liquid state. Successive heating of the supercooled solution reveals an atypical process of gelation that takes place while continuously raising the temperature. Isoconversional kinetic analysis is applied to treat the atypical gelation process and to compare it with regular gelation on cooling. Although atypical and regular gelation occur on significantly different time scales they appear to have common dynamics.