Minimizing sample evaporation in the environmental scanning electron microscope

The ElectroScan environmental scanning electron microscope (ESEM) enables wet samples to be observed by eliminating air but allowing water vapour into the sample chamber. However, evaporation from, and condensation on, the sample may occur during the pumpdown sequence used to reach this state, which means that the sample may not be in its natural state when viewed if due care is not taken. In this paper, the pumping system of the ESEM is described mathematically and expressions are derived for the evaporation and condensation. This treatment is then used to calculate the optimum pumpdown sequence. The importance of using the optimized procedure is illustrated by micrographs of fat emulsions.     

Prediction Equations for Gases in Flexible Modified-Atmosphere Packages of Respiring Produce are Different Than Those for Rigid Packages

Total pressure generally decreases in a sealed rigid package containing respiring produce, whereas total pressure is essentially constant and free volume decreases in a flexible package. We found that predicted O₂, CO₂, and N₂ partial pressures were different for similarly designed (same surface area, thickness, film permeabilities and produce mass) flexible and rigid packages at “quasi steady state” and steady state, respectively. Predicted O₂ and CO₂ partial pressures were slightly higher in a flexible package than in a rigid package and were a function of the film permeability ratio of N₂ to O₂ and of CO₂ to O₂. They also related to the ratio of product CO₂ production rate to O₂ uptake rate, target steady-state O₂ partial pressure, and respiration characteristics. Differences were slight for films such as low-density polyethylene.     

Variation in the Levels of Sodium and Other Minerals of Nutritional Importance in Louisiana Oysters (Crassostrea Virginica)

The concentrations of nine individual minerals were determined in oysters harvested over a 10-month period. Significant (p > 0.01) monthly variations were observed in the levels of all the minerals determined. Salinity of the growing waters was significantly (p > 0.01) correlated to levels of sodium, magnesium, iron, chromium, and nickel in the oysters. Levels of potassium, copper and chromium were related to pH. Zinc and iron levels were correlated (p > 0.05) to water temperature. In this study, variations in the mineral content of oysters were often directly influenced by the growing water. This variation must be taken into consideration, should sodium labeling be required for fresh oysters.     

Thermofluids Considerations and the Dynamic Behavior of a Finger Seal Assembly

Finger seals represent a compliant seal configuration. What differentiates and makes them preferable to the brush seals is their potential hydrodynamic lifting capabilities, and thus their noncontacting nature. The fingers' compliance allows both axial and radial adjustment to rotor excursions without damage to the integrity of the seal. The work to be presented here concerns the mapping of the thermofluid and dynamic behavior of a repetitive section of the newly proposed design of a two-layer finger seal. The assembly contains four high-pressure and four low-pressure fingers arranged axially in a staggered configuration and subject to an axial pressure drop. The numerical three-dimensional temperature and pressure results were obtained using a customized Navier-Stokes–based commercial package, CFD-ACE+. The results were obtained in a parametric fashion where the high-pressure side, the speed of rotation, and the heat transfer coefficient are the controlling parameters; the gas compressibility and the viscosity are also considered in the model of the thermofluids seal behavior. The stiffness and damping characteristics of the padded/unpadded fingers and the fluid were obtained through numerical simulation and were then used to model the interaction with the motion of the shaft. It is shown that the proposed geometry provides satisfactory lifting capability for the fingers. The fingers follow the motion of the shaft and their stiffness is small when compared to that of the fluid; thus, the displacement transmissibility is in most cases close to 1. Lifting forces and seal leakages, as well as the interaction between the profiled backplate and the low-pressure fingers through Coulomb damping/friction forces, are also parametrically studied.     

PRODUCTION OF FLAVOR COMPOUNDS: ALDEHYDES AND ALCOHOLS FROM LEUCINE BY MICROENCAPSULATED CELL-FREE EXTRACTS OF STREPTOCOCCUS LACTIS VAR. MALTIGENES

Cell-free extract (CFE) from Streptococcus lactis var. maltigenes was examined for its ability to catalyze the formation of 3-methyl butanal and 3-methyl butanol from leucine as affected by pH, substrate concentration and gradual addition of NADH. Pseudo K_m and v_max, determined for CFE were 6.5 × 10^-3 M and 7.1 μmol/liter-min when 3-methyl butanal was measured and 9.4 × 10^-3 M and 2.4 μmol/liter-min for 3-methyl butanol. The CFE, leucine and cofactors were encapsulated in milkfat and incubated for 40 h at 22°, Rate of formation and final concentration of 3-methyl butanal and 3-methyl butanol were related to substrate concentration, indicating that flavor intensity can be controlled with encapsulated enzymes by limiting substrate concentration. Diffusion of 3-methyl butanol and acetic acid from microcapsules was affected by concentration and their lipophilic/hydrophilic character. Altering the pH of fluid surrounding microcapsules containing CFE had limited effect on the internal pH, as indicated by enzyme probes, demonstrating that capsules provide an environment isolated from the bulk phase of foods.     

MODELING FREQUENCY DISTRIBUTION of STEADY-STATE O2 PARTIAL PRESSURES IN MODIFIED-ATMOSPHERE PACKAGES

A mathematical model was developed to predict the frequency distribution of steady-state O₂ partial pressures in modified-atmosphere packages of fresh and minimally processed fruits and vegetables. Variation in O₂ uptake was predicted to have relatively larger influence on the package O₂ distribution than variation in film permeability to O₂. the model also predicted that the range of O₂ partial pressures in the packages would be larger for the products with lower K_1/2 (a constant in product O₂ uptake equation) values. the percent deviation of package O₂ partial pressures from [O₂]_med increased with [O₂]_med up to about 3 kPa and gradually decreased thereafter. Package O₂ distribution was symmetrical at higher [O₂]_med but became gradually asymmetrical with decreasing [O₂]_med from 6 kPa. the model was used to predict target [O₂]_med for the design of modified-atmosphere packages for different probability levels of having package O₂ being equal to or less than a minimum tolerable level of the product.     

Partial Purification and Thermal Characterization of Pectinmethylesterase from Red Grapefruit Finisher Pulp

Four forms of pectinmethylesterase have been partially purified from red grapefruit finisher pulp by chromatography on Heparin-Sepharose CL-6B. Three forms bound to the heparin-sepharose and could be eluted with a sodium chloride gradient. One form (RGPME3) exhibited high thermal stability retaining 66.7% relative activity after 2 min incubation in an 80°C water bath, and 45.2% of its relative activity after 60 set incubation in a 95°C water bath. RGPME3 had a K_m of 0.106 mg * mL^?1 citrus pectin and a pH optimum between 6 and 7. The most abundant form (RGPMEZ) had a K_m of 0.079 mg * mL^?1 citrus pectin. Denaturing electrophoresis and gel filtration chromatography suggested molecular weights between 20 and 40 kDa for the various forms.     

Simulation and Control of an Active Tilting-Pad Journal Bearing

This study developed an active tilting-pad journal bearing with a feedback control system to regulate the orbit of a rotating shaft. The control is implemented by means of linear actuators installed behind the pivot of each pad, which allow the radial motion of the pads in real time. The control design uses the linear feedback of the state variables of the bearing-rotor system, with the feedback gains determined by the optimization of a quadratic performance index. The optimization is based on a linear spring-mass model that incorporates the direct stiffness and damping elements associated with each of the bearing pads. This linear model is found by the simulation of the system under small perturbations using a nonlinear Reynolds equation model. The nonlinear model is capable of simulating the radial motions of the pads by the actuators and is used to verify the effectiveness of the feedback control. It is shown that certain design parameters in the quadratic performance index may be used to determine both the stiffness and the damping of the closed-loop bearing system and that the shaft orbit can be thereby suitably regulated.