The retrogradation of waxy maize starch extrudates: effects of storage temperature and water content.

The effects of water content and storage temperature on the kinetics of the retrogradation of nonexpanded waxy maize starch extrudates were studied using (1)H pulsed NMR and wide-angle X-ray diffraction. The increase in crystallinity observed by XRD was accompanied by a decrease in the relaxation times of the solid-like component of the NMR free induction and the spin-echo decays, and an increase in the contribution of the solid-like component to the total signal. The dependence of the rate of starch retrogradation on the storage temperature showed the typical "bell-shaped" behavior, which was successfully modeled using the Lauritzen-Hoffman theory of crystallization of chain-folded polymers. This theory was extended to model the effect of water content on the rate of isothermal crystallization by exploiting the ten-Brinke and Karasz, and the Flory equations to describe the dependence of the glass-transition and the melting temperatures on water content.

Effect of moisture and aging on gutta-percha.

The purpose of this article was to study the plasticizing effect of controlled levels of different humidities on native and commercial gutta-percha samples. It was expected that the results might provide the practicing endodontist with invaluable insight into the mechanical properties of commercial gutta-percha cones (i.e. retrievability, malleability, flexibility, torsional, and bending ability). The "conditioning" of the gutta-percha samples depended greatly on the percentage of compatible and incompatible substances. Mixtures of native gutta-percha with compatible substances (e.g. wax) showed greater elongation, tensile strength, and number of rotations but lower deformation angle than mixtures of native gutta-percha with incompatible substances (e.g. salts, metal oxides). The observed lower values for tensile strength and torsional strain and higher value for elongation and number of rotations for wet samples compared with the dry ones were attributed to the plasticizing effect on gutta-percha of the insertions of water molecules in the polymer chains.

Glass transition of gluten. 2: The effect of lipids and emulsifiers.

The effects of lipids and emulsifiers on the glass transition temperature (Tg) of gluten have been studied as a function of water content, using differential scanning calorimetry, dynamic mechanical thermal analysis, a three point bend test and pulsed nuclear magnetic resonance. Lipids and emulsifiers were found to have little or no effect on Tg, with the exception of the lower molecular weight caproic and hydroxycaproic acids, which had a plasticizing effect. In the case of hydroxycaproic acid this was found to be in accordance with the Couchman-Karasz equation. The magnitude of the drop in modulus at Tg was increased in the presence of these additives at low water contents. The effect of lipids and emulsifiers on the Tg of gluten and their compatibility with gluten appears to depend both on their hydrophobicity and on their molecular weight.

Glass transition of gluten. 1: Gluten and gluten-sugar mixtures.

The glass transition in hydrated wheat gluten has been studied using dynamic mechanical thermal analysis, differential scanning calorimetry, pulsed nuclear magnetic resonance and a three point bend test. The results for gluten alone are in good agreement with results obtained by other workers for gluten and glutenin. In contrast to their effect on the Tg of amylopectin, a gluten:sugar ratio of 10:1 (where the sugar is amorphous fructose, sucrose or glucose) has little effect on the glass transition temperature, as a function of water content. A sample containing gluten and fructose in the ratio 2:1 showed plasticization due to the sugar.

Thermal analysis of commercial Gutta-percha using differential scanning calorimeter and dynamic mechanical thermal analysis.

Gutta-percha is the most widespread root canal filling material and knowledge of its chemical and thermomechanical properties is of a great interest to its use in clinical dentistry. In the present investigation, the results from differential scanning calorimeter were compared with those obtained by dynamic mechanical thermal analysis. A significant correlation was established between these results and the chemical composition of six different commercial samples of gutta-percha.

Crystallization from concentrated sucrose solutions.

When concentrated aqueous solutions of sucrose are cooled, ice or sucrose crystals separate, depending on the concentration and temperature. The state diagram of the sucrose-water system describes both the equilibrium and non-equilibrium behaviour. A detailed account is given of the effect of sucrose concentration and various, principally polysaccharide, stabilizers on the processes of homogeneous and heterogeneous nucleation, as well as the kinetics of crystal growth. Increasing concentrations of sucrose reduce the diffusion coefficient of water molecules and the linear growth velocity of ice crystals. Polysaccharide stabilizers have relatively little effect until incipient gelation occurs. Thereafter, there is a gross change in crystal form and growth rate. A careful analysis of the impact of sucrose concentration on the rate of ice crystal growth has not permitted satisfactory matching of theoretical predictions with actual behaviour. Recent studies of the rate of growth of sucrose crystals in sucrose glasses of varying water contents are recorded. There is no simple relationship between the moisture content (in the 2-12 w/w % H2O range) and the rate of growth of the sucrose crystals.

Electrophoretic differentiaion f myofibrillar proteins in the pig.

1. Starch-gel electrophoretograms of myosin and tropomyosin preparations in 8m-urea, from longissimus dorsi and psoas muscles of the pig, were characterized by laser densitometry. 2. The typical pattern for freshly prepared myosin from both muscles was similar, there being five electrophoretically distinct components. 3. The number of electrophoretically distinct components in both muscles increased after freeze-drying, but the effect of freeze-drying was more marked in longissimus dorsi. 4. Extraction with 8m-urea containing 2% beta-mercaptoethanol decreased the number of major electrophoretically distinct components of the fresh myosin of both muscles to four. 5. Although there was also some simplification of the patterns after freeze-drying the greater susceptibility of the myosin from longissimus dorsi was still evident. 6. The typical pattern for freshly prepared tropomyosin in 8m-urea differed in the two muscles: in each case it was more complex than that of the corresponding myosins. 7. The pattern of tropomyosin from neither longissimus dorsi nor psoas was altered significantly after freeze-drying. 8. Electrophoretograms of pig longissimus dorsi tropomyosin in 8m-urea differed from those of longissimus dorsi tropomyosin from sheep, ox and rabbit. 9. Extraction of the tropomyosins in 8m-urea and 2% beta-mercaptoethanol simplified the electrophoretic pattern to two major components with samples from pig, sheep and ox, and to one major component with samples from rabbit. 10. It was concluded that classification of skeletal muscles as ;red' or ;white' is insufficient to account for the degree of functional specialization which the electrophoretograms suggest.