RELAXATION BEHAVIOR AND THE APPLICATION OF INTEGRAL CONSTITUTIVE EQUATIONS TO WHEAT DOUGH

The small amplitude rheological properties of doughs of different compositions were measured using a dynamic rheometer. In the first study, the effect of water, flour, and added gluten on the dynamic properties was investigated. Frequency sweep was carried out in the range of 0. 1 to 100 s⁻¹ at an applied strain of 0. 1%. Storage (G') and loss (G") moduli for each composition were recorded. The storage modulus versus frequency plots were a series of parallel curves indicating that the moisture-to-flour ratio was the most important parameter. The loss modulus versus frequency plot showed a falling-off after a frequency of 40 s⁻¹. A plot of tan δ versus frequency indicated a more elastic dough at shorter time     

An optical tweezer-based study of antimicrobial activity of silver nanoparticles

Understanding and characterizing microbial activity reduction in the presence of antimicrobial agents can help in the design and manufacture of antimicrobial drugs. We demonstrate the use of an optical tweezer setup in recording the changes in bacterial activity with time, induced by the presence of foreign bodies in a bacterial suspension. This is achieved by monitoring the fluctuations of an optically trapped polystyrene bead immersed in it. Examining the changes in the fluctuation pattern of the bead with time provides an accurate characterization of the reduction in the microbial activity. Here, we report on the effect of addition of silver nanoparticles on bacterial cultures of Pseudomonas aeroginosa, Escherichia coli and Bacillus subtilis. We observe a decrease in the bacterial activity with time for the investigated bacterial samples. This method in our opinion, enables one to track changes in bacterial activity levels as a function of time of contact with the antibacterial agent with greater efficacy than traditional cell counting methods.     

Electrical breakdown in very thin Al2O3 films

Electrical breakdown in very thin A1₂O₃ films of thickness less than 100 Å and sandwiched between Al and Au metal electrodes has been studied experimentally. Recovery of high resistance of the oxide insulator film has been observed (after breakdown) on the application of either a reverse voltage or a high current pulse or just by resting the junction for a few hours after the removal of the applied external voltage. An explanation for the effect is suggested. The breakdown field is found to depend on the insulator thickness, the temperature and the nature of the electrode metals. On the basis of experimental results the mechanisms for electrical break down at room temperatures and above, in very thin Al₂O₃ films, can be attributed to electronic modified thermal breakdown.     

FLOW CURVES, STRESS RELAXATION AND CREEP MEASUREMENTS OF STARCH GELS

Flow curves, stress relaxation, and creep compliance were measured for various types of corn starch gel that underwent different degrees of modification. The four starches were native corn starch, medium hydroxypropylated distarch phosphate, highly hydroxypropylated distarch phosphate, and distarch phosphate. All the starches exhibited pseudo-yield stress behavior at low shear rates and shear-thinning at higher shear rates. Stress relaxation data at different strains indicated strain-softening phenomenon for all starches. Higher degrees of cross-linking decreased the half relaxation time due to the addition of rigid filler into the matrix. Increasing the concentration increased the relaxation modulus for all starches. Creep compliance and recovery measurements were conducted over the range of 3.2 Pa-320 Pa. In the linear viscoelastic region, the compliance data were not dependent on the magnitude of stress applied. As concentration increases, higher stress magnitude leads to nonlinear behavior. The creep compliance data were fitted into the Ninomiya and Ferry relation from dynamic viscoelastic data and the Maxwell-Voigt model. Ninomiya and Ferry overpredicted the relaxation data while the Maxwell-Voigt model fitted the data reasonably well.     

Textural Properties of Extruded Plant Protein Blends

Blends of corn gluten meal and soy protein concentrate in the ratio of 25:75, 50:50 and 75:25 w/w were made up. The blends at 20, 30 and 40% moisture content (db) were extruded at 145°C through 4.24, 3.0, and 2.12 mm diam dies at screw speeds of 60, 120, and 180 rpm to generate different shear environments. Multiple nonlinear regression analysis was used to develop models of bulk density, puff ratio, water-holding capacity and shear strength as a function of moisture content and shear rate. Shear rate affected the dependent variables most. Cooked and uncooked viscosity and product color were also measured. Electron micrographs confirmed some of the findings.     

Investigation of colour generation mechanism in silicate glasses containing CdSxSe1−x

Abstracts are not published in this journal     

The role of lattice water in determining the reactivity of various crystalline aluminas

The thermal decomposition of alkali metal chlorides was studied on a series of colloidal and particulate aluminas with different crystalline phases. Decomposition was monitored by differential thermal and thermogravimetric analysis and the resulting phase development by X-ray diffraction. It was found that the presence of strongly bound lattice water in colloidal gel bohemite alumina caused significantly lower decomposition temperatures and increased reactivity towards compound formation. This has been attributed to the simultaneous loss of water via a hydrolysis reaction with the metal chloride, and subsequent phase transformation of the pseudo-bohemite into γ-alumina, forming the aluminate at a much reduced temperature. This revised version was published online in November 2006 with corrections to the Cover Date.     

EFFECT OF GUM ARABIC ON THE RHEOLOGY OF CORN FLOUR DOUGHS AND FRIED PRODUCT QUALITY

Small deformation stress relaxation studies were performed on corn flour dough at 48–62%, wet basis moisture and 0–2% gum Arabic contents. Corn doughs were sensitive to moisture contents that were reflected by the values of the elastic and viscous components. The instrumental stickiness of the dough increased markedly when the moisture content was increased. The stress decay curves were modeled by using 3-element spring-dashpot models as evidenced by low values of root mean square error. Sensory assessment of the dough as well as fried products were conducted to relate the parameters obtained from stress relaxation studies.