R. Parker

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.

An experimental study of screw configuration effects in the twin-screw extrusion-cooking of maize grits

Abstract The effects of screw configuration on the product properties, die pressure and screw torque in the twin-screw extrusion-cooking of maize grits is described. Extrudate bulk density, water solubility and water absorption were measured for a set range of extrusion moisture contents and barrel temperatures for each of four screw configurations of different conveying efficiencies. It is shown that screw configuration is a potent variable in determining product properties. The variation in water solubility with specific mechanical energy input conforms to Meusers model. The screw configuration controls the specific mechanical energy input range. Screw configurations of low conveying efficiency give rise to low-viscosity melts which expand into low-bulk-density products. Dead-stopped runs showed that the conveying efficiency controls the degree of fill in the extruder barrel. Typically, the mean residence time was increased from 56 to 94 s by decreasing the conveying efficiency of the screws.

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.

Deformation and fracture behaviour of wheat starch plasticized with glucose and water

The mechanical properties of wheat starch and the effects of plasticizers upon them are studied in flexure at 293 K. For compositions low in water and glucose the material is glassy, with a flexural modulus between 0.7 and 5.0 GPa. The addition of water and glucose to wheat starch plasticizes the material through its glass transition into a rubbery state. The flexural moduli of the rubbery samples are in the range 50 to 200 MPa, which is indicative of a partially crystalline polymer. For starch-water mixtures the glass transition occur in the water content range 18 to 20%. The addition of glucose progressively shifts the glass transition to lower water contents. At strains below 0.04 brittle failure is only observed in the glassy samples. The surface morphology of the fractured samples shows features typical of pure synthetic glassy polymers.

The deformation and failure behaviour of wheat starch plasticized with water and polyols

The mechanical properties of starch plasticized with water and glycerol or xylitol have been studied in flexure at room temperature. The addition of a polyol lowers the glassy modulus and the glass transition temperature of the starch. With the addition of polyols, the fall in modulus with increasing water content at the glass transition becomes less sharp and shifts to lower water contents. The high strain deformation shows that all samples fracture at a water content below 10% (wet weight basis), a proportion of which tears at water contents in the range 9%–10% and no fracture occurs in samples above about 10% water. Microscopy shows that there is evidence of permanent plastic deformation in fractured samples as the water content is increased to 10% water, although there is a negligible change in fracture stress and strain.

Microstructural changes during the twin-screw extrusion cooking of maize grits

Abstract Microstructural changes which occur during the extrusion cooking of maize grits were studied using optical microscopy and X-ray diffraction. The complete disruption of the maize grit particles and their constituent starch granules was found to be related to changes in extruder die pressure, screw torque and product solubility and water absorption. The disruption was found to depend upon extrusion moisture, barrel temperature and screw configuration. It occurred over a narrow range of specific mechanical energy input, 115–145 W h kg −1 . The E-type form of the crystalline amylose-lipid complex was found to occur throughout the temperature range 90–150°C when the maize grit was extruded with low conveying efficiency screws.

A comparative study of the conveying performance of screws in a twin-screw co-rotating extrusion-cooker

Abstract A study of extruder performance has been made using both self-wiping and closely intermeshing screws for the extrusion-cooking of wheat starch in a twin-screw co-rotating extruder. The axial pressure profile, die pressure, screw torque and residence time distribution were measured over a range of screw speeds (75–300 rev min −1 ) for each screw geometry. The residence time distributions for the closely intermeshing and self-wiping screws were similar. The mean residence time decreased from about 80 s at 75 rev min −1 to 40–50 s at 300 rev min −1 . The axial pressure profile and dead-stopped runs showed that there were different distributions of fill within the extruder barrel for the two screw geometries. Whereas the mean time in the solids-conveying section was shorter for the self-wiping screws the time in the fully filled section was correspondingly longer. The results demonstrated that the closely intermeshing screws had a greater melt pumping action than the self-wiping screws.

A comparison of the kinetics of in vitro starch digestion in smooth and wrinkled peas by porcine pancreatic alpha-amylase

This study describes the impact of crop genetics and processing in two pea lines (Pisum sativum L.) on starch digestion kinetics. Mutation at the rugosus (r) locus leads to wrinkled pea seeds, a reduction in starch content and a lower extent of in vitro starch digestibility. The Logarithm of Slope (LOS) kinetic model was used to analyse digestion curves obtained using porcine pancreatic α-amylase for a range of particle size fractions. Changes in starch structure induced by the r mutation led to clear differences in starch digestion kinetics for purified starches and pea flours. Larger particle size fractions showed slower starch digestion relative to the purified starch, but significant differences still existed between r and wild type pea lines. It is expected that this work will help inform the design of future studies where both starch structure and food structure are important determinants of digestion behaviour.

Mechanical Properties of Wheat Starch Plasticized with Glucose and Water

Publisher Summary This chapter focuses on the mechanical properties of wheat starch plasticized with glucose and water. The deformation and fracture behavior of systems containing food biopolymers is important both to food processing operations and to product texture. Much work has concentrated on food systems that contain high proportions of water making them soft and rubbery. In terms of modulus-temperature behavior, these materials are in the rubbery plateau region. Relatively little work exists on systems containing low proportions of water (