Biomedical subjects

Geoffrey J Brownsey

Publications and source records attributed to Geoffrey J Brownsey.

5 recordsLinked to original sources

PubMed2005

Physical aging of starch, maltodextrin, and maltose.

The physical aging of low water content, amorphous starch/water, maltodextrin/water, and maltose/water mixtures in the glassy state was examined using mechanical testing and calorimetry. Stress relaxation measurements showed that upon storage of the glassy materials there was a time-dependent increase in both flexural modulus and mechanical relaxation time. The mechanical relaxation time increased with depth of quench below the calorimetric glass transition temperature and with aging time at the quench temperature. Calorimetry of the aged materials showed an overshoot in heat capacity in the vicinity of the glass transition. The logarithm of the mechanical relaxation time showed a simple linear relationship with the size of the overshoot expressed as an enthalpy change. The calorimetric behavior could be modeled using the Tool-Narayanaswamy-Moynihan method.

Calorimetry, Differential Scanning↗

PubMed2005

Pectin-chitosan multilayer formation.

The deposition of alternating layers of pectin and chitosan at a solid surface was studied using surface plasmon resonance. The binding of biopolymer to the surface was irreversible over the time scales examined. The deposition was dependent on the flow rate through the measurement cell with mass transport limitation at lower flow rates. The thickness of the deposited layer was dependent on the biopolymer concentration and was particularly marked for pectin. This was consistent with a process of initial attachment, followed by a slower structural rearrangement, which was inhibited at high initial surface concentrations of adsorbed biopolymer. Sequential deposition resulted in the formation of multilayers with an essentially linear growth rate.

Adsorption↗

PubMed2005

The nonequilibrium phase and glass transition behavior of beta-lactoglobulin.

Concentrated solutions of bovine beta-lactoglobulin were studied using osmotic stress and rheological techniques. At pH 6.0 and 8.0, the osmotic pressure was largely independent of NaCl concentration and could be described by a hard sphere equation of state. At pH 5.1, close to the isoelectric point, the osmotic pressure was lower at the lower NaCl concentrations (0 mM, 100 mM) and was fitted by an adhesive hard sphere model. Liquid-liquid phase separation was observed at pH 5.1 at ionic strengths of 13 mM and below. Comparison of the liquid-liquid and literature solid-liquid coexistence curves showed these solutions to be supersaturated and the phase separation to be nonequilibrium in nature. In steady shear, the zero shear viscosity of concentrated solutions at pH 5.1 was observed at shear rates above 50 s(-1). With increasing concentration, the solution viscosity showed a progressive increase, a behavior interpreted as the approach to a colloidlike glass transition at approximately 60% w/w. In oscillatory shear experiments, the storage modulus crossed the loss modulus at concentrations of 54% w/w, an indication of the approaching glass transition. Comparison of the viscous behavior with predictions from the Krieger-Dougherty equation indicates the hydrodynamic size of the protein decreases with increasing concentration, resulting in a slower approach to the glass transition than a hard sphere system.

Computer Simulation↗

PubMed2003

The glass transition behavior of the globular protein bovine serum albumin.

The glass-like transition behavior of concentrated aqueous solutions of bovine serum albumin was examined using rheological techniques. At mass fractions >0.4, there was a marked concentration dependence of viscosity with a glass-like kinetic arrest observed at mass fractions in the region of 0.55. At mass fractions >0.6 the material behaved as a solid with a Young's modulus rising from approximately 20 MPa at a mass fraction of 0.62-1.1 GPa at 0.86. The solid was viscoelastic and exhibited stress relaxation with relaxation times increasing from 33 to 610 s over the same concentration range. The concentration dependence of the osmotic pressure was measured, at intermediate concentrations, using an osmotic stress technique and could be described using a hard sphere model, indicating that the intermolecular interactions were predominantly repulsive. In summary, a major structural relaxation results from the collective motion of the globules at the supra-globule length scale and, at 20 degrees C, this is arrested at water contents of 40% w/w. This appears to be analogous to the glass transition in colloidal hard spheres.

Animals↗

PubMed2002

Structural relaxation and physical ageing of starchy materials.

The structural relaxation during the ageing of an amorphous maltose and a starch-sorbitol mixture was examined using a range of physical techniques. Heat capacity, measured by differential scanning calorimetry, showed an overshoot in the glass-transition region, the size of which was temperature and time dependent. Volume relaxation measurements were made at different ageing temperatures in the range T(g) -15 to -30 K. The volume decreased with increasing ageing time, in an essentially linear fashion with log time. The mechanical behaviour of the materials showed a progressive embrittlement on ageing. For both materials, the mechanical relaxation time increased with ageing, and the material became stiffer. Investigation of the effect of physical ageing on transport properties was also performed using conductivity measurements on a maltose-water-KC1 mixture. A decay in conductivity, which was almost linear with log time, was observed. The structural relaxation was modelled using the Tool-Narayanaswamy approach to describe the calorimetric data.

Calorimetry, Differential Scanning↗