J. M. V. Blanshard

The glass transition of amylopectin measured by DSC, DMTA and NMR

Abstract The glass transition in amorphous amylopectin derived from waxy maize starch, and containing between 10 and 22% water, has been studied using differential scanning calorimetry, dynamic mechanical thermal analysis, the Instron texturometer and nuclear magnetic resonance (both pulsed and solid state NMR). The plasticizing effect of water broadly follows the predictions of the Couchman-Karasz equation. The results of the different techniques are compared and the effect of 2–11% crystallinity on the glass transition is investigated.

Diffusion in gels

Abstract The diffusion of solutes in gels is comprehensively reviewed. Because it has been a source of confusion, precise definitions of the gel diffusion coefficient are presented and discussed. Theories as to the effect of the gel substance on the course of diffusion are critically evaluated. These include the obstruction effect, hydrodynamic drag and other frictional couplings, alteration of solvent properties and (for homogeneous gels) the free volume theory. A large proportion of the data on diffusion in gels to be found in the literature is displayed, with the exception of those systems where binding of the solute is a major factor. The success of the theories in accounting for these results is examined. It is concluded that for heterogeneous gels the obstruction effect is prevalent, for organic solvent-polymer systems the free volume theory has had some success while diffusion of both macromolecules and micromolecular solutes in homogeneous gels is not well understood and deserves more experimental effort.

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.

A study of the glass transition of amylopectin—sugar mixtures

Abstract Amylopectin-sugar mixtures in the ratio of 10:1 have been studied using fructose, glucose, sucrose and xylose. Samples of amylopectin containing glucose in the ratio of 5:1 and fructose in the ratio of 2:1 (amylopectin-sugar) were also prepared. The glass transition as a function of water content was studied using d.m.t.a., d.s.c., pulsed n.m.r. and a three-point bend test. Small amounts of sugar were found to reduce the glass transition temperature of starch in accordance with or in excess of the predictions of a Couchman-Karasz equation. For the sample containing the greatest amount of sugar, less plasticization than predicted was observed; this appeared to be due to a substantial degree of phase separation.

The effect of fructose and water on the glass transition of amylopectin

Mechanical methods (dynamic mechanical thermal analysis and three-point bend tests) and differential scanning calorimetry have been used to study the effect of variations in the amylopectin:fructose ratio on properties in the region of the glass transition temperature (Tg). The effects of 0–50% (dry basis) fructose content have been studied over a range of water contents between 7 and 27% (of total weight). At and below 20% fructose, the Tg is reduced in excess of the predictions of a three-component Couchman-Karasz equation, whereas, at higher fructose contents, the sugar glass transition (at that water content) begins to dominate the mechanical properties.

A study of the effect of water on the glass transition of 1:1 mixtures of amylopectin, casein and gluten using DSC and DMTA

Abstract The glass transition temperature (Tg) and mechanical properties of binary mixtures of the food biopolymers amylopectin, casein and gluten have been studied in the ratio 1:1 in the presence of water. In general these polymers appear to be immiscible, showing two glass transitions due to the two polymers when there is sufficient difference between the Tgs of the two components. Increasing the water content reduces the Tg of both components.

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 1H 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.

Study of diffusion and permeation of gases in undrawn and uniaxially drawn films made from potato and rice starch conditioned at different relative humidities

Abstract The permeability coefficients of potato and rice starches were studied at different relative humidities and related to the diffusion and solubility rate over a range of temperatures (0–90°C). Inflections in the permeability and diffusivity plots versus 1/ T (K) provide an indirect indication of the glass transition; these values are in satisfactory agreement with the results of differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) measurements. Both uniaxially drawn and undrawn films were evaluated and the permeability values were correlated with the % crystallinity (determined by wide angle X-ray scattering and DSC) that developed on storage. The incorporation of sugars (fructose, glucose and sucrose) in the starches and their subsequent storage at 20°C was related to the process of crystallization. Fructose and glucose, for example, induce an increase in the rate of crystallization but decrease the permeability, diffusivity and solubility, whereas sucrose, by retarding the crystallization, has precisely the contrary effects. The synergistic action of sugars and water in lowering the glass transition ( T g ) by plasticizing the potato and rice starch films is explicable in terms of the Couchman-Karasz equation if not complicated by phase separation. The values of the activation energies determined by experiment coincide with those calculated theoretically from given models where the % crystallinity is an important factor, which confirms the significance of crystallinity in the permeability of such films.

Simultaneous determinations of the molecular weight distributions of amyloses and the fine structures of amylopectins of native starches

Abstract Native A (wheat and waxy rice), B (potato), and C (cassava and sweet potato) types of starches were each debranched with isoamylase, and separated into amylose and amylopectin fractions by HPLC on size exclusion columns coupled on-line to multi-angle-laser-light-scattering and differential refractometer detectors. The absolute molecular weights of amyloses and chain length distributions of amylopectins were determined simultaneously, and pre-isolation of the amylopectin was not necessary. The molecular weights of debranched amylose from starches that have not been fractionated to separate amylose and amylopectin are significantly higher than published values for the undebranched fractionated amylose. The polymodal profiles of the refractive index chromatograms showed the complexity of the amylopectin structure of starches. The chain length distribution of amylopectin depends critically on the method for analysing the broad chromatogram when determined by either noting the minima/inflections or deconvoluting the overlapping amylopectin fraction into numerous normal/Gaussian distributions. Although the results from the former (conventional) method of analysis were comparable with the literature values, they did not appear to be as sensitive a technique for detecting differences as the multiple Gaussian approach. Overall, the study suggested that the amylopectin chain units might be more complex than originally envisaged and that different degrees of chain packing for the molecules can be inferred from this multiple component analysis.

The application of polymer crystal growth theory to the kinetics of formation of the B-amylose polymorph in a 50% wheat-starch gel

Abstract Gelatinized wheat-starch gels containing 50% water (w/w) were prepared by extrusion and submitted to storage at varying temperatures between 2 and 37°C. The development of crystallinity was followed by X-ray diffraction over a period of three weeks by which time all perceptible postgelatinization changes were complete. The results were analysed by fitting to the Avrami equation (optimum fit occuring where the Avrami exponent n = 1). The derived rate of crystallization at the different temperatures was then analysed in terms of the Lauritzen-Hoffman theory of growth of polymer crystals. Although not all parameters had experimental values, it proved possible to evaluate a series of trial values of two parameters until the best fit to the growth rate data was obtained as judged by the correlation coefficient. All parameters so determined were deemed satisfactory in terms of comparable figures obtained with synthetic polymers. The theoretical Tg for the above 50% aqueous wheat-starch gel was calculated to be approximately 200 K which accorded well with a value calculated by free-volume theory. The practical implications with regard to retrogradation of starch gels at different temperatures and different water contents are discussed.