Paul D. Orford

The roles of amylose and amylopectin in the gelation and retrogradation of starch

Abstract The retrogradation of starch gels has been studied by using X-ray diffraction, differential scanning calorimetry, and measurements of the shear modulus. Starch gels were considered as composites containing gelatinised granules embedded in an amylose matrix. The short-term development of gel structure and crystallinity in starch gels was found to be dominated by irreversible (T

The gelation and crystallisation of amylopectin

Abstract A range of physical and chemical techniques, including viscometry, rheological measurements, dilatometry, turbidity measurements, X-ray diffraction, and differential scanning calorimetry, has been used to study the gelation of amylopectin. Gels form on cooling concentrated aqueous solutions to 1°. The development of gel stiffness is closely related to the association of amylopectin chains, as monitored by dilatometry and differential scanning calorimetry. X-Ray diffraction studies suggest that intermolecular association involves a crystallisation process. The association of amylopectin chains in the gel is substantial and is thermo-reversible at temperatures below 100°. Heterogenous acid hydrolysis of the gel followed by examination of the residue by gel-permeation chromatography showed that the associated regions contained branched fragments, the individual chains of which had a d.p. of 15. The combined data suggest that the amylopectin molecules associate by crystallisation of the branches with d.p. 15 to form a network.

Effect of water as a diluent on the glass transition behaviour of malto-oligosaccharides, amylose and amylopectin

The glass transition behaviour of amorphous malto-oligomers from dimer to hexamer was investigated as a function of diluent (water) concentration using differential scanning calorimetry. The glass transition temperatures of the pure compounds ranged from 364 K for maltose to 448 K for maltohexaose. At low diluent concentrations the addition of water strongly depressed Tg. From the measurement of Tg and the heat capacity increment, delta Cp, of the transition for the pure compounds it was possible to predict the Tg of the malto-oligomer/water mixtures using a thermodynamic approach developed by Couchman. From the measurements on the malto-oligomers it was possible to obtain, by extrapolation, the high DP limits of delta Cp and Tg, which are appropriate to amylose and amylopectin. The predicted variation of Tg with diluent concentration for these materials was compared with the experimentally observed behaviour.

Aspects of the glass transition behaviour of mixtures of carbohydrates of low molecular weight

The glass transition temperature (Tg) values for carbohydrates alone and in binary mixtures have been determined using differential scanning calorimetry. The Tg of the carbohydrate depends strongly on molecular weight and less on structure. The Tg of the binary mixtures was dependent on composition and, in most instances, was linear with respect to mole fraction. The effect of water on depressing the Tg of the carbohydrates and their binary mixtures was also evaluated. The observed behaviour was compared with that predicted from a thermodynamic approach.

The retrogradation and gelation of amylopectins from various botanical sources

Abstract The distribution of the chain lengths of amylopectins from wheat, barley, maize, pea, canna, and potato starches were characterized by size-exclusion chromatography. The cereal amylopectins had the shorter average chain-length. The gelation and retrogradation behaviour of the amylopectins, examined using measurements of shear modulus and differential scanning calorimetry, together with studies of the structure of the associated regions in amylopectin gels, indicated that association of the chains in cereal amylopectin gels occurred over shorter segment lengths. The cereal amylopectins have a reduced rate of retrogradation which is linked to their shorter average chain-lengths.

Aqueous dissolution of crystalline and amorphous amylose-alcohol complexes

Complexes of amylose, the linear starch polysaccharide, with linear alcohols having chain lengths varying from 4 to 8 carbon atoms, were prepared. Either crystalline or amorphous complexes could be formed depending on preparation conditions. Crystalline complexes gave sharp X-ray diffraction patterns, characteristic of the VH form of amylose, whereas no observable pattern was obtained from the amorphous form. Thermal dissociation of the complexes occurred at increasing temperatures with increasing alcohol chain length. Crystalline complexes dissociated at temperatures approximately 23 degrees C higher than their amorphous counterparts and the enthalpy of dissociation was also greater for the crystalline samples. Enthalpy values were independent of alcohol chain length. Differences in thermal behaviour of the two types of complex may be described in terms of the polymer crystal lattice energy and may explain the variability of reported results for complex dissociation in the literature.

The incompatibility of concentrated aqueous solutions of dextran and amylose and its effect on amylose gelation

Abstract At 75°C concentrated aqueous solutions of dextran and amylose exhibit immiscibility. The presence of dextran has a major effect on the microstructure and mechanical behaviour of amylose gels formed on cooling. At low dextran concentrations the modulus of the gel increases with increasing dextran concentration, whereas at higher dextran concentrations, polymer incompatibility and segregation of dextran-rich droplets reduces the firmness of the gel.