John Lelievre

A study of starch gelatinization using differential scanning calorimetry, X-ray, and birefringence measurements

Abstract The order-disorder transition that occurs on heating an aqueous suspension of starch granules has been investigated using differential scanning calorimetry (d.s.c.), X-ray crystallinity, and birefringence methods. Starches from wheat, corn, rice, and waxy maize starches were used for the study. Measurements on dilute suspensions (2wt.% starch), showed that decreases in crystallinity occur both before the birefringence of granules starts to disappear and after all birefringence is lost. At such concentrations the ordered domains in single granules melt over a temperature span of about 10°, indicating that the order-disorder transition is not a highly cooperative process. The crystallinity values of more-concentrated suspension (∼ 50wt.% starch), suggest that a melting process accounts for the two main peaks evident in the corresponding d.s.c. traces. The X-ray data do not support the concept that the specific heat change in the d.s.c. traces is attributable to a glass transition in the initial stages of gelatinization. Firstly, the X-ray measurements do not show that a significant endothermic transition occurs without a corresponding change in crystallinity. Secondly, the X-ray data suggest that the volume expansion functions measured by other investigators using thermomechanical analysis are attributable to the increase in the quantity of amorphous starch polymer with temperature rather than to a glass transition followed by a melt.

A differential scanning calorimetry study of melting transitions in aqueous suspensions containing blends of wheat and rice starch

Differential scanning calorimetry (dsc) has been used to study the transitions that take place when an aqueous suspension containing a blend of wheat and rice granules is heated. These starch types gelatinize over distinctly different temperature ranges. The positions of the wheat and rice endotherms in the dsc traces are governed by the melting of granules of different stabilities at different volume fractions of diluent. When the total concentration of starch in the suspensions is relatively low, that is less than about 30% wt, the dsc traces for the blends are simply the sum of the outputs for each of the components in the mixture. However, at higher starch concentrations competition for water occurs and non-additive behaviour is found. Under these conditions the wheat starch, since it gelatinizes first, has access to more water and melts at about the same temperature as controls. The rice granules give an endotherm at higher temperatures than controls since the gelatinized wheat particles have already absorbed much of the aqueous phase.

A differential scanning calorimetry study of glass and melting transitions in starch suspensions and gels

Abstract Differential scanning calorimetry (d.s.c.) has been used to study the glass and melting transitions that take place when an aqueous suspension of starch granules, or a partially crystalline starch gel, is heated. A baseline shift was found to accompany both the single major d.s.c. endotherms given by relatively dilute suspensions and gels and the biphasic endotherms obtained from more concentrated suspensions. The shift, which indicates a change in the heat capacity, starts at the same temperature as the endotherm commences and continues to increase in size until the endotherm is completed. This finding seems to differ in certain respects with other reports suggesting the heat capacity change occurs at the leading edge of the first melting peak for suspensions and does not take place during the melting of starch gels. The change in heat capacity of the suspensions and gels does not appear to be due to a glass transition since this transition was detected at lower temperatures. Hence the present study does not support the suggestion that the melting of the polymers in these starch systems is controlled by a glass transition.

Stress-strain relationships for gellan gels in tension, compression and torsion

Abstract Formulated foods often contain small amounts of polymeric ingredients that interact to form three-dimensional networks which stabilize structure and provide desirable textural quality. One goal of food engineers is to understand how gels are formed and to be able to predict their mechanical properties. We have studied calcium-crosslinked gels of gellan, a polysaccharide recently introduced to the food industry, in order to understand their stress-strain behavior in tension, compression and torsion. This information will be applied when designing stability and texture for a variety of food systems. Gels were prepared by dispersing gellan polymer in 90 °C water, adding calcium chloride and cooling in molds to form the test specimens. Four levels of polymer, ranging from 0.6 to 1.8% w/v, were used with seven calcium concentrations between 1.5 and 60 mM. Molds of differing construction provided specimen shapes that were cylindrical with enlarged ends for tensile tests, cylindrical for compression tests and capstan-shaped for torsion tests. Specimens for tension and torsion testing were modified with plastic adapters to facilitate attachment in mechanical testing machines; whereas compression was carried out between lubricated parallel Teflon plates. Gel properties were influenced strongly by the content and combination of polysaccharide and ion. Shear stress relationships in small deformations were almost identical in the three testing modes for a given gellan and calcium concentration. In large deformations, gels were more rigid in tension than in compression and torsion, but they failed at the same maximum shear stress regardless of testing mode. Stress-strain responses were analyzed using constitutive equations based on energy functions. Mooney-Rivlin constitutive equations were applicable for stress-strain relationships for small strains; however, equations based on three-term energy functions fitted the data more accurately at larger strains.

A model of starch gelatinization linking differential scanning calorimetry and birefringence measurements

Abstract Suspensions of wheat, corn or rice starch, containing 5% polymer, were heated to temperatures in the range 20–145°C in a differential scanning calorimeter (DSC). Once the desired temperature was reached, the sample was immediately cooled at a rate of 320°C min −1 to room temperature. The numbers of native and gelatinized granules in the sample were then determined by microscopy. The variation in the fraction of gelatinized particles with temperature was found to follow a normal distribution function. A mathematical model based on this function was developed in an attempt to understand the DSC traces. The model predicts the shape of the gelatinization endotherm and accounts for the fact that the temperature band over which the endotherm takes place is broader than that for the birefringence loss. The model also suggests that the heat flow into an individual granule decreases as gelatinization proceeds therein.

Transitions in frozen gelatinized-starch systems studied by differential scanning calorimetry

Abstract Differential scanning calorimetry (d.s.c.) has been used to study the transitions that occur when gelatinized-starch systems are heated from −20°C to +20°C. The samples contained from 10 to 70% polymer. With gelatinized suspensions containing less than about 30% starch, a glass transition due to plasticization of amorphous polymer was detected at about −5°C. The ice in the specimen melted at about the same temperature as pure ice. In the case of samples of intermediate concentration, containing, say, 45% starch, an exotherm due to crystallization of unfrozen water occurred once the glass-transition temperature had been exceeded. This exotherm only had a significant area when the sample was frozen at a relatively high rate. The melting of the ice in these specimens was characterized by a biphasic endotherm. It was established that, when starch is gelatinized in a d.s.c. pan, water evaporates and can condense on the pan lid. Division of moisture between the sample and the lid was found to be responsible for the twin ice-melting endotherms. Experiments showed that concentrated amorphous polymer caused the melting temperature of the ice in the specimen to decrease below that of pure ice, whereas the crystalline domains had less effect. When the concentration of gelatinized polymer was increased still further, for example, to 70%, polysaccharide-water interactions inhibited freezing, and no transitions were detected in the d.s.c. trace.

Failure Testing of Gellan Gels

Abstract Gels, formed using a 1% gellan gum solution containing 7 m m calcium ions, were tested to failure in compressive, tensile and torsional modes. The gels were brittle and fractured at relatively small strains. In the compression experiments samples failed in shear at low deformation rates, and in a combination of compression and shear at higher strain rates. Tensile fracture was always evident in uniaxial tension measurements, and in the torsion experiments at low strain rates. At high deformation rates the torsional samples underwent a combination of shear and tensile fracture. The three tests were compared on the basis of the shear stress and strain at failure. The shear stress agreed in all cases. The strain only agreed in the compressive and torsional modes and was an order of magnitude lower in tension.

A Theoretical Analysis of Stress Concentrations in Gels Containing Spherical Fillers

Stress conditions in a model biocomposite system undergoing uniaxial compression have been theoretically analyzed. The composite structure considered was that of an incompressible isotropic gel system containing a dilute dispersion of spherical filler particles. The analysis was based on the theory of elasticity and the assumption that particle-particle interactions were negligible. The stress concentration in the vicinity of the filler was calculated for adhesive and nonadhesive interfaces between the filler and the medium. The analysis indicates that localized stress concentrations are induced in the medium within two radii from the filler particle. The level of stress concentration depends upon the condition of the interface and the ratio of the shear moduli of the two materials. The stress concentration factor may be as high as 3 at the interface in the case where rigid filler particles are loosely embedded in a soft gel. The minimum stress concentration occurs at a bonded interface when the two materials have similar elastic moduli.