Water states and displacements can be investigated with thermogravimetry (TG) either in its classical or in the Knudsen version (where standard pans are replaced with Knudsen cells). The case of wheat flour dough is considered in various steps of bread making, namely, mixing, proofing, baking, staling. The split of DTG signals into various components (gaussian functions) support the assumption that the overall dough water is partitioned into various fractions. Few comments are devoted to water displacements during freezing.
Authors:M. Iijima, T. Hatakeyama, M. Takahashi and H. Hatakeyama
Application of thermomechanometry to the measurement of hydrogels containing a large amount of water was carried out by static
and dynamic methods. A thermomechanical analyzer (TMA) equipped with a quartz compression probe immersed in water was used.
Polysaccharide hydrogels containing ca 98% water were measured. Creep of hydrogels in water was analyzed in a stress range
from 1.04⋅103 to 5.2⋅103 Pa and loading rate from 0.3⋅103 to 3.0⋅103 Pa min−1.Stress relaxation was measured in compressed ratio range from 0.02 to 0.45 m m−1 and in compressing rate was 0.09 to 0.15 m m−1 min−1. Dynamic viscoelasticity was measured by TMA when dynamic Young’s modulus which was larger than 1⋅104 Pa in frequencies ranging from 0.02~0.2 Hz.
Authors:Y. H. Roos, K. Jouppila and Bettina Zielasko
An exotherm, observed in differential scanning calorimetry (DSC) scans of amorphous food materials above their glass transition temperature,Tg, may occur due to sugar crystallization, nonenzymatic browning, or both. In the present study, this exothermal phenomenon in initially anhydrous skim milk and lactose-hydrolyzed skim milk was considered to occur due to browning during isothermal holding at various temperatures above the initialTg. The nonenzymatic, Maillard browning reaction produces water that in amorphous foods, may plasticize the material and reduceTg. The assumption was that quantification of formation of water from theTg depression, which should not be observed as a result of crystallization under anhydrous conditions, can be used to determine kinetics of the nonenzymatic browning reaction. The formation of water was found to be substantial, and the amount formed could be quantified from theTg measured after isothermal treatment at various temperatures using DSC. The rate of water formation followed zero-order kinetics, and its temperature dependence well aboveTg was Arrhenius-type. Although water plasticization of the material occurred during the reaction, and there was a dynamic change in the temperature differenceT−Tg, the browning reaction was probably diffusioncontrolled in anhydrous skim milk in the vicinity of theTg of lactose. This could be observed from a significant increase in activation energy. The kinetics and temperature dependence of the Maillard reaction in skim milk and lactose-hydrolyzed skim milk were of similar type well above the initialTg. The difference in temperature dependence in theTg region of lactose, but above that of lactose-hydrolyzed skim milk, became significant, as the rate in skim milk, but not in lactose-hydrolyzed skim milk, became diffusion-controlled. The results showed that rates of diffusion-controlled reactions may follow the Williams-Landel-Ferry (WLF) equation, as kinetic restrictions become apparent within amorphous materials in reactions exhibiting high rates at the same temperature under non-diffusion-controlled conditions.
Precipitation of trehalose dihydrate in water is observed at room temperature for trehalose concentrations higher than 47.5%w/w.
Direct observations of crystal melting in water and measures of the solution density determine the thermal variations of trehalose
saturationS(T) (mM) in water: ln(S(T))=ln(0.1223)-(1330/T) withR2=0.9982. The glass transition (Tg) curve measured by DSC is lower at low concentrations and higher at high concentrations than previously reported.Tg is also measured as a function of the cooling/warming rates. Analysis of specific heat changes atTg and associated activation energy leads to identify a most stable glassy state around the second eutectic concentration.
Authors:A. Zumailan, G. Denis, E. Dargent, J. Saiter and J. Grenet
PET films uniaxially drawn in hot water are studied by means of conventional DSC and modulated DSC (MDSC).Glass transition
is studied by MDSC which allows to access the glass transition temperature Tg and the variations of ΔCp=Cp1−Cpg (difference between thermal capacity in the liquid-like and glassy states at T=Tg). Variations of Tg with the water content (which act as plasticizer) and with the drawing (which rigidifies the amorphous phase) are discussed
with regard to the structure engaged in these materials. The increments of ΔCp at Tg are also interpreted using a three phases model and the 'strong-fragile’ glass former liquid concept. We show that the ‘fragility’
of the medium increases due to the conjugated effects of deformation and water sorption as soon as a strain induced crystalline
phase is obtained. Then, ‘fragility’ decreases drastically with the occurring rigid amorphous phase.
To reveal the denaturation mechanism of lysozyme by dimethyl sulfoxide (DMSO), thermal stability of lysozyme and its preferential
solvation by DMSO in binary solutions of water and DMSO was studied by differential scanning calorimetry (DSC) and using densities
of ternary solutions of water (1), DMSO (2) and lysozyme (3) at 298.15 K. A significant endothermic peak was observed in binary
solutions of water and DMSO except for a solution with a mole fraction of DMSO (x2) of 0.4. As x2 was increased, the thermal denaturation temperature Tm decreased, but significant increases in changes in enthalpy and heat capacity for denaturation, ΔHcal and ΔCp, were observed at low x2 before decreasing. The obtained amount of preferential solvation of lysozyme by DMSO (∂g2/∂g3) was about 0.09 g g−1 at low x2, indicating that DMSO molecules preferentially solvate lysozyme at low x2. In solutions with high x2, the amount of preferential solvation (∂g2/∂g3) decreased to negative values when lysozyme was denatured. These results indicated that DMSO molecules do not interact directly
with lysozyme as denaturants such as guanidine hydrochloride and urea do. The DMSO molecules interact indirectly with lysozyme
leading to denaturation, probably due to a strong interaction between water and DMSO molecules.
The molar heat capacities of the binary mixture composed of water and n-butanol were measured with an adiabatic calorimeter in the temperature range 78–320 K. The functions of the heat capacity with respect to thermodynamic temperature were established. A glass transition, solid–solid phase transition and solid–liquid phase transition were observed. The corresponding enthalpy and entropy of the solid–liquid phase transition were calculated, respectively. The thermodynamic functions relative to a temperature of 298.15 K were derived based on the relationships of the thermodynamic functions and the function of the measured heat capacity with respect to temperature.